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Discover our blog on the Metrohm website

Discover our blog on the Metrohm website

by | Mar 28, 2022 | Company

Metrohm Discover: A new platform

From now on, new «Analyze This» posts will only be featured on the Metrohm website rather than WordPress.

We invite you to come check out our articles on the Metrohm Discover platform, where you will find all kinds of helpful material organized by industry, matrix, analyte, and measuring technology. Find everything you need easily with our new platform.

Check out the blog on Metrohm Discover

Thank you!

Thanks to all of our loyal readers who have made this analytical chemistry blog such a success. Keep reading our articles, and visit your local Metrohm website for other articles in your native language!

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Find the newest blog articles and more on our website

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Post written by Dr. Alyson Lanciki, Scientific Editor at Metrohm International Headquarters, Herisau, Switzerland.
Best practice for separation columns in ion chromatography (IC) – Part 3

Best practice for separation columns in ion chromatography (IC) – Part 3

by | Mar 21, 2022 | Fundamentals, Lab hacks

This is the third and final post in our series providing you with tips and tricks on the proper use of ion chromatography columns. In the first part, we mostly discussed the standard operation conditions as well as operational limits for columns, while in the second post, we focused on application related topics and what elution parameters can be changed to modify the separation performance. In the conclusion of this series we will take a closer look at the ways to assess the column performance during its lifetime and offer some troubleshooting tips which can help in fixing issues that may appear. 
Click on a topic below to go directly to each section:
    Did you miss Parts 1 and 2? Read them here!
    Best practice for separation columns in ion chromatography (IC) – Part 1
    Best practice for separation columns in ion chromatography (IC) – Part 2

    Essential column parameters

    Let’s begin by looking at the main parameters that can be used to judge the column performance. Most of these reference values can be extracted from the certificate of analysis. The separation column certificates can be found in the Metrohm Certificate Finder. Reproducing the certificate chromatogram periodically to verify the column performance can be helpful to detect changes in performance early and to avoid irreversible damage of the column.

    Metrohm Certificate Finder
    Below, we will consider the most important parameters one by one.
    Retention time and column capacity

    The retention time of the individual analytes is a good measure of the column selectivity and capacity. When the retention times of the analytes do not match the retention times of the certificate, there are many possible reasons for this behavior.

    Possibly, the eluent composition is wrong, e.g., because the eluent components are not present in the correct concentrations. In case the eluent strength is too high, the retention times will be reduced. This phenomenon usually occurs for all ions simultaneously and shifts all the peaks closer together. Thereby, the multivalent ions are accelerated to a stronger extent than the monovalent ions. This issue can be fixed by preparing a fresh eluent with the correct composition.

    The eluent may not be fresh or not sufficiently protected from the surrounding atmosphere, e.g., with a CO2 adsorber. Carbon dioxide from the ambient air can change the eluent composition (i.e., strength, pH, etc.) over time and this will affect the retention times of the analytes. Weak hydroxide eluents are particularly affected by this issue, since the elution strength of carbonate ions is much stronger than the one of hydroxide ions, leading to strong shifts in retention times. Therefore, multivalent ions are more affected by this than monovalent ones. This issue can be fixed by preparing a fresh eluent and by using a CO2 adsorber. When working with hydroxide eluents, it is also important to verify the status of the hydroxide stock solution as it can absorb CO2 from the air. Figure 1 shows the effect of CO2 uptake from the air on the retention times of common anions.

    Effect of eluent absorption of carbon dioxide in the air on ion separation.
    Figure 1. Example chromatograms showing the effect of eluent absorption of carbon dioxide in the air for seven standard anions measured on a Metrosep A Supp 5 – 250/4.0 column under standard conditions. Red: unprotected eluent. Black: eluent protected with a carbon dioxide adsorber.
    Some samples may contain constituents that occupy the ion exchange groups of the column material. In that case, the number of available ion exchange groups on the column is reduced compared to the initial state, leading to an apparent smaller column capacity and shorter retention times. Depending on the type of contaminant(s), there are possibilities to reverse this effect and wash them from the column surface. These regeneration procedures are adapted individually to the different stationary phases and should be used as a last option to save a column.
    The procedures as well as the operation limits are described in the respective leaflets of the column and can be found on our website. Please consult your Metrohm dealer before performing a regeneration procedure.
    Leaflet for Metrosep A Supp 17
    • In case the contamination is from a multivalent ion, it is usually possible to wash them off by flushing the column with an eluent of a higher concentration. When doing so, do not forget the operation limits of the column, particularly regarding the eluent pH.
    • Sometimes, organic molecules can adsorb to the column stationary phase because of their strong affinity to it. This can lead to a blockage of the ion exchange groups and reduced retention times along with increased operating pressure. When this happens, it can help to wash the stationary phase with an eluent containing a fraction of an organic modifier. This will increase the affinity of the contaminant to the mobile phase and help to detach it from the stationary phase.

    Usually, after such a regeneration procedure, the original column capacity can be restored.

    An even better way to extend the lifetime of the separation column is by regularly exchanging the guard column. One of its purposes is to protect the separation column from contaminants by retaining them.

    Make sure to exchange the guard column three to four times over the lifetime of the separation column, or even more frequently when dealing with very complex sample matrices such as dyes or food matrices. In these situations, we suggest using the Metrosep RP 2 Guard/3.5.

    Metrosep RP 2 Guard/3.5
    The chromatogram overlay in Figure 2 shows the effect of a defective guard column on the peak shape.
    Effect of eluent absorption of carbon dioxide in the air on ion separation.
    Figure 2. Example chromatograms showing the effect of a damaged guard column. Black: analysis of standard anions when using a defective guard column. Red: identical analysis without a guard column installed. In this case, the replacement of the guard column is strongly recommended.
    In some cases, the column capacity is reduced due to chemical modification of the stationary phase. This happens when a column is used in conditions outside of the operation limits—e.g., extreme pH situations. Extreme pH conditions can alter the chemical bonds of the ion exchange groups to the base particle substrate. This irreversible process leads to permanent column capacity loss. In this case, no regeneration procedure can restore the original column capacity.

    Please note that this loss of ion exchange groups also happens slowly during the regular use of ion exchange columns. The strength of the chemical bond of the ion exchange groups is thoroughly tested during the development of Metrohm columns to provide a long column lifetime.

    Theoretical plates and peak symmetry

    The number of theoretical plates (TP) can be a useful tool to judge the column packing status. The higher the TP value, the better the packing of the column. For a meaningful judgement of the column packing bed, choose an analyte that does not elute too early in the chromatogram (these peaks can be affected by extra-column effects, in particular on 2 mm systems) and that is not affected by secondary interactions (i.e., avoid using nitrate). For cations, potassium is a good choice, while for anions, sulfate is a suitable choice.

    Although the columns are packed at pressures higher than the normal operating pressure, the packing bed continues to further densify throughout the column lifetime with the continuous application of flowing mobile phase (eluent). While the packing bed of the stationary phase improves due to this effect, it may also lead to some dead volume at the entrance of the separation column. This dead volume can be responsible for peak broadening and reduced theoretical plates, particularly at the beginning of the chromatogram.

    While this is a regular aging process of the column, it is possible to slow it down by taking good care of the column. Preventive actions include the slow initiation of the eluent flow rate and temperature at start-up as well as the proper and complete shutdown of the high-pressure pump before removing the column, as mechanical stress can be detrimental to the packing bed. Exchanging the guard column can often have a positive impact on the theoretical plates and peak shapes (see Figure 2).

    Issues with the column packing bed often also become visible in the asymmetry factor values and the overall peak shapes. More pronounced fronting as well as peak broadening can be warning signs for channeling in the column or the guard column. Unfortunately, this kind of damage on the column is irreversible and requires an exchange of the separation column.

    Before column replacement, it is important to check if the issues originated from the column or another connection in the IC system whenever peak broadening is observed in the chromatogram. Make sure that all capillaries in the high-pressure path have a diameter of ≤0.25 mm and that all capillaries have been installed and connected correctly without additional dead volume. Systems equipped with small inner diameter (2 mm, microbore) columns are more strongly affected by dead volume than those with 4 mm columns. This means that when using microbore columns, less dead volume is required for the peak broadening effect to become visible compared to when using 4 mm columns.

    Column pressure

    Another important parameter to regularly check throughout the column lifetime is the system pressure. High pressure is among the most frequent reasons leading to a column replacement. Whenever a pressure increase is noted, it is important to verify what part of the IC system is the cause.

    If the sample contains particles and insufficient sample preparation is applied, the particles will accumulate at the entrance of the protective guard column and eventually lead to increased system pressure. In this case, the guard column performs its intended use of protecting the separation column and needs to be replaced. If the guard column is not replaced soon enough, then particulate contamination may break through and load onto the separation column. While there is no regeneration procedure for a guard column loaded with particles, separation columns contaminated with particles may be regenerated by rinsing the column at low flow rates in the reverse flow direction.

    Please note that these regeneration procedures may not always be successful.

    Avoiding particles from being injected into the high-pressure path of the IC system is a good way of protecting the guard and separation columns. Metrohm provides several automated sample preparation techniques which result in a positive impact on the column lifetime. The most common techniques for this purpose are Inline Ultrafiltration (Figure 3) and Inline Dialysis.

    Effect of eluent absorption of carbon dioxide in the air on ion separation.
    Figure 3. Inline Ultrafiltration is a useful automated sample preparation technique that protects the separation column from particle accumulation.
    Learn more about Metrohm Inline Ultrafiltration in our related blog post.
    When do I have to exchange the filtration membrane with Inline Ultrafiltration?

    Column end of life

    All of the parameters described above should be considered to judge the column (and the IC system) performance. Many of these parameters can be monitored closely in the MagIC Net software so that a potential issue can be detected as early as possible.

    Even though Metrohm IC columns are designed and manufactured to have very long lifetimes, at some point performance will decrease and even regeneration procedures may not be able to restore the required column performance to solve the application needs. This represents the end of the column life and makes the exchange of the separation column unavoidable.

    Metrohm separation columns cannot be recycled and can be disposed with normal waste. However, depending on the samples measured as well as the used chemical types and their associated hazards, it may be necessary to consider a proper disposal option.

    Irrespective of the sample nature, do not open the column at any time. 

    Troubleshooting overview

    Table 1 gives an overview about certain troubleshooting strategies when looking at your column performance behavior.

    Table 1. Preventing and correcting performance loss in IC columns

     

    Indicator Cause Preventive and corrective measures
    Increasing counterpressure Particles on the guard column Replace the guard column.
    Particles on the separation column

    Rinse out the separation column in the reverse flow direction

    • Place the column inlet (i.e., the opening next to the intelligent chip) in a beaker, as this is now the outlet path for the contaminants.
    • Rinse out the separation column for approx. one hour.
    • Reinstall the separation column in the flow direction.
    Particles in the sample Sample preparation, e.g., remove particles through lnline Ultrafltration
    Shortened retention time Carbonate in the eluent

    Carbon dioxide from the air affects the carbonate/hydrogen carbonate balance in the eluent. A carbonate/hydrogen carbonate eluent weakens over time; a hydroxide eluent strengthens.

    • Always tightly seal eluent bottles and bottles containing eluent concentrate.
    • Always use a CO2 adsorber.
    Air bubbles in the eluent

    Air bubbles make the eluent flow unstable. The counterpressure is an indicator of unstable flow. lt should remain stable
    within a range of ± 0.1 MPa.

     

    • Deaerate the high-pressure pump.
    • Use an eluent degasser.
    Capacity loss in the column due to high- valency ions Regenerate the column as per the column leaflet to remove any inorganic deposits.
    Resolution loss Eluent too old or produced incorrectly Eluents should be freshly prepared. Make sure that they are produced correctly and particularly that carbonate and hydrogen carbonate are not confused.
    Adsorptive effect of the contamination deposited in the guard column Replace the guard column.
    Adsorptive effect of the contamination deposited in the separation column Regenerate the column as per the column leaflet to remove any organic or inorganic deposits.
    Loss of theoretical plates Guard column contaminated Replace the guard column.
    Separation column contaminated Regenerate the column as per the column leaflet to remove any organic or inorganic deposits.
    Separation column overloaded

    The separation column can be overloaded by factors such as a high salt content in the sample matrix.

    • Dilute the sample.
    • Inject less sample.
    Dead volume in the IC system
    • Check that all capillaries have a diameter ≤0.25 mm; if they don’t, replace the capillaries.
    • Check that all of the capillaries have been installed correctly. The installation process is described step by step in the «IC Maintenance» multimedia guide.
    Asymmetry Dead volume or contamination on the guard column Replace the guard column.
    Separation column contaminated Regenerate the column as per the column leaflet to remove any organic or inorganic deposits.

     

    Conclusion

    This article explained how IC column performance can be assessed and monitored throughout the column lifetime and what measures can be taken to ensure a long column lifetime. With that, we now conclude the series «Best practice for separation columns in ion chromatography». If you have more questions, do not hesitate to contact your local Metrohm IC salesperson.

    Get free tips and tricks

    for your IC columns 
    Read our blog article here!
    Post written by Dr. Vincent Diederich (Product Manager IC Columns) and Dr. Anne Katharina Riess (Head of Column Division) at Metrohm International Headquarters, Herisau, Switzerland.
    NIR spectroscopy: a 21 CFR Part 11 compliant tool for QC and product screening

    NIR spectroscopy: a 21 CFR Part 11 compliant tool for QC and product screening

    by | Mar 7, 2022 | Analysis, Fundamentals

    Pharmacology: a brief history

    Our search for medicines is nearly as old as humanity itself. Medicinal ingredients from plant, mineral, and animal sources were used for healing purposes since the earliest of advanced civilizations. Herbal remedies from China date back to a couple of thousand years ago, while indigenous populations have been relying on environmental sources for healing for several millennia. Systematic descriptions of medicines have been handed down to us from the ancient Greeks and the Roman Empire, laying a foundation for contemporary pharmacology. It was not until the 16th century that the science began its departure from the models passed down from antiquity.

    The path to developing synthetic drugs began with the emergence of organic chemistry at the beginning of the 19th century. Although drug therapies had been limited to naturally occurring substances and inorganic compounds up to that point, this changed with the targeted production of organic synthetic drugs based on substances isolated from medicinal plants. Within a very short period, this led to a vast number of synthesized active pharmaceutical ingredients (APIs). Researchers had finally come to understand the relationship linking the action of these substances to their chemical structures.

    Pharmaceutical analysis provides information about the identity, purity, content, and stability of starting materials, excipients, and APIs. Medicinal products come in various forms (e.g., ointments, tinctures, pills, lotions, suppositories, infusions, sprays, etc.) and consist of the active substance and at least one pharmaceutical excipient. Impurities are mainly introduced during the synthesis of the active ingredients.
    According to the World Health Organization (WHO), specifications and test methods for commonly used active ingredients and excipients are outlined in detail in monographs contained in the national pharmacopoeias of more than 38 countries. The pharmacopoeias are official compendia containing statutory requirements pertaining to identity, content, quality, purity, packaging, storage, and labeling of active pharmaceutical ingredients and other products used for therapeutic purposes. They are essential for anyone seeking to produce, test, or market medicinal products.

    Near-infrared spectroscopy—a 21 CFR Part 11 compliant tool to assess the quality of pharmaceuticals

    Near-infrared spectroscopy (NIRS) has been an established method for fast and reliable quality control within the petrochemical industry for more than 30 years. However, many companies still do not consistently consider the implementation of NIRS in their QA/QC labs. The reasons could be either limited experience regarding application possibilities or a general hesitation about implementing new methods.

    However, the advantages of NIRS are numerous, such as the ability to measure multiple parameters in just 30 seconds with no sample preparation! The non-invasive light-matter interaction used by NIRS, influenced by physical as well as chemical sample properties, makes it an excellent method for the determination of both property types.

    In the remainder of this post, we will indicate where NIRS technology can be used in the pharmaceutical production process and what parameters can be analyzed. Furthermore, we present proven applications developed according the NIRS implementation guidelines of ASTM E1655 (quantitative method development) and ASTM E1790 (qualitative method development).

    For more detailed information about NIRS as a secondary technique and NIRS as a QC tool for pharmaceuticals, please read our previous blog posts.

    Benefits of NIRS: Part 1
    Benefits of NIRS: Part 2
    Benefits of NIRS: Part 3
    Benefits of NIRS: Part 4
    Moisture Analysis – Karl Fischer Titration, NIRS, or both?

    NIRS compliance with international pharmacopoeias

    As a secondary test method, NIRS is recommended in all the key pharmacopoeias – European (Ph. Eur. 2.2.40) as well as American (USP<856/1856>) and Japanese pharmacopoeias (Chapter 6, since 2007).

    All Metrohm NIRS instruments are fully compliant with USP <856/1856> and all the other key pharmacopoeias. Furthermore, all Metrohm NIRS instruments meet the standards for wavelength precision, reproducibility, and photometric noise. Numerous reference standards and user-friendly software make it easy to check the instrument requirements specified in the pharmacopoeias. The pharmaceutical version of the Vision Air software is fully validated and compliant with 21 CFR Part 11.

    Vision Air spectroscopy software
    Find out more about Metrohm NIRS instruments for laboratory and process analysis below.
    Laboratory NIRS instruments
    Process NIRS instruments

    Where can NIRS be used in pharmaceutical manufacturing processes and what parameters can be analyzed?

    NIRS is an indispensable analysis technique that can be used along the entire production chain: from checking the incoming materials to the production line, even to the quality control of finished products. A typical pharmaceutical tablet production process is shown in Figure 1 with markers noting where NIRS can be implemented.

    Figure 1. Illustration of a pharmaceutical tablet production plant with NIRS used all along the process.
    Typical NIRS applications at the different stages of the pharmaceutical production process are outlined in Figure 2.
    Figure 2. NIRS analysis can be used at several points in the pharmaceutical manufacturing process for many types of applications.
    Relevant application notes can be found on the Metrohm website.
    NIRS applications for the pharma industry

    Applications and parameters for NIRS in pharmaceutical production

    NIR spectroscopy is the fast solution for determining myriad parameters simultaneously in almost any sample matrix. In addition, this technique requires no sample preparation, and it is non-destructive. Table 1 contains a list of different administered forms of medications given with commonly measured parameters and related materials to download for more information.

    Table 1. NIRS can be used to determine several parameters in many kinds of administered forms of pharmaceuticals.
    Substance form Parameter Conventional method Related NIRS Application Notes
    Powders or granulates API, Moisture HPLC / GC / KF titration AN-NIR-018

    AN-NIR-014
    Tablets and capsules API, Content uniformity, Moisture content, Dissolution profile HPLC / GC / KF titration AN-NIR-073

    AN-NIR-066

    AN-NIR-063

    AN-NIR-017
    Creams or gels API, Moisture content HPLC / GC / KF titration AN-NIR-020
    Solutions or suspensions API HPLC / GC AN-NIR-088

    AN-NIR-076
    Injectables API HPLC / GC AN-NIR-042
    Lyophilized products Moisture content KF titration AN-NIR-078
    Following are a few examples of successfully implemented NIRS applications at numerous pharmaceutical companies worldwide.
    Incoming (raw) material inspection

    Raw material identification and qualification, shown in Figure 3, is one of the most mature applications in the pharmaceutical industry. Good manufacturing practice requirements demand that every single package unit that arrives in the warehouse needs to be checked.  Therefore, a rapid analytical technology as NIR spectroscopy is needed to measure many samples in a short period of time. NIRS is a general accepted alternative method by the USP.

    Figure 3. Raw material inspection by NIR spectroscopy.
    NIRS is an ideal analytical method for raw material inspection:

    • identify tests of APIs, excipients, packaging
    • quantitative control of materials (e.g., moisture)
    • GMP Requirement to test 100% of raw materials
    • quality control of materials (e.g., supplier, particle size, dyes)
    Blending

    Blending of APIs and excipients can be easily monitored without sample destruction by NIRS (Figure 4). This application focuses on the standard deviation between consecutive spectra. As the blending proceeds, the differences between consecutive spectra will become smaller and smaller, approaching unity. This parameter indicates when the blend is homogeneous. This information is important for operators to optimize the blending time and consequently increase the blending capacity of a facility.

    Figure 4. Blending of APIs and excipients is easier to monitor with NIRS.
    NIRS is an ideal analytical method for blending processes:

    • determination of blend homogeneity
    • sample spectra statistically compared to library of well blended material
    • optimized blending for improved operation of next production steps
    Learn more about the Metrohm NIRS DS2500 Analyzer here.
    Brochure: DS2500 Analyzer – Boosting efficiency in the QC laboratory with Near-Infrared Spectroscopy (NIRS)  
    Lyophilized products

    Moisture determination in lyophilized products is an ideal application for NIRS (Figure 5). Typical water content of lyophilizates is somewhere between 0.5% and 3.0%. For many pharmaceutical lyophilized products, this value must be lower than 2.0%. Water is a strong NIR absorber, so detection limits are low and the demands of the pharmaceutical industry can be easily met.

    Primary methods like loss on drying (LOD) or Karl Fischer titration are typically time-consuming, especially compared to NIRS (data in seconds). These samples have a relatively high value combined with a low moisture content. NIRS analysis only illuminates the sample so the same sample can be used for compendial analysis without any damage. This application has a quick return on investment (ROI) due to the high sample frequency.

    Figure 5. Moisture determination in lyophilized products with NIRS is fast and cost-effective compared to other techniques.
    NIRS is an ideal analytical method for measuring moisture in lyophilized products:

    • samples can be easily altered to provide calibration sets with varying amounts of moisture
    • complete analysis takes less than one minute and is non-destructive
    Tablet analysis

    When analyzing intact tablets with a NIR spectrometer, it is important to measure them in diffuse transmission mode. This illuminates a larger sample big portion and investigates the inner composition of the tablet (Figure 6). Measuring the same tablet in reflectance mode will gather the information from the outside coating of the tablet due to the low penetration depth. Content uniformity parameters are measured within a minute, and the tablet trays allow unattended measurement of up to 30 tablets.

    Figure 6. NIRS allows fast determination of API content in intact tablets.
    Learn more about the differences in NIRS measurement modes in our previous blog post.
    Benefits of NIR spectroscopy: Part 1

    Summary

    Near-infrared spectroscopy has long been one of the most important and versatile analytical techniques in the pharmaceutical industry. The biggest benefit of using NIRS is the possibility of obtaining reliable analysis results in just seconds without any sample preparation or reagents required.

    The pharmaceutical version of Vision Air software from Metrohm is 21 CFR Part 11 compliant and compatible with third party method development software like Unscrambler.

    For more information

    About spectroscopy solutions provided by Metrohm, visit our website!

    We offer NIRS for lab, NIRS for process, as well as Raman solutions

    Visit our website
    Post written by Wim Guns, International Sales Support Spectroscopy at Metrohm International Headquarters, Herisau, Switzerland.
    Ion-selective electrodes: General tips – Part 1

    Ion-selective electrodes: General tips – Part 1

    by | Feb 21, 2022 | Analysis, Fundamentals

    Ions – we encounter these tiny charge carriers constantly. Depending on the concentration of certain anions (negative ions) and cations (positive ions), they can have a significant impact on humans and the environment. Thanks to ongoing quality control in several industries such as food and beverage, the metallurgical industry, and water management, defined limits are neither exceeded nor undercut.
    So, how can these small, ubiquitous ions be determined? Mistakenly, I thought at first that ion measurement is only possible by means of more costly analytical methods such as ion chromatography (IC), inductively coupled plasma optical emission spectrometry (ICP-OES), or atomic absorption spectroscopy (AAS). A promising cost-efficient alternative to these techniques is to use so-called ion-selective electrodes (ISE).
    If you are interested in one or more of these subjects:

    …then you have come to the right place.

    Types of ion-selective electrodes

    If you want to determine, for example, the fluoride concentration in your toothpaste, the ammonium content of the water in your aquarium, or perhaps how much calcium is really in a fruit juice, then there are many suitable ion-selective electrodes for your application needs.

    AB-082: Determination of fluoride with an ion-selective electrode
    AB-113: Determination of cadmium, lead and copper in foodstuffs, waste water and sewage sludge by anodic stripping voltammetry after digestion
    AB-125: Simultaneous determination of calcium, magnesium, and alkalinity by complexometric titration with potentiometric or photometric indication in water and beverage samples
    Membrane material

    The very first ion-selective electrode was the pH electrode. However, this article will not discuss pH electrodes—you can find more information in our other posts pertaining to this special ISE.

    «Analyze This»: Avoiding the most common mistakes in pH measurement
    «Analyze This»: FAQ: All about pH calibration
    Aside from the glass membrane used for pH electrodes, there are other membrane materials available for selective measurement of myriad ions. The most widely applied types are listed in Table 1.
    Table 1. Various types of ion-selective electrodes.
    Membrane material Description Ions Close-up view
    Crystal membrane Crystal lattice containing defined gaps for the ion to be measured. Ag+, Cu2+, Pb2+, Br–, Cl–, CN–, F–, I–, S2-
    Polymer membrane Polymer membrane containing a molecule (ionophore) that only binds the ion to be measured. Ca2+, K+, Na+, surfactants, NO3–
    Glass membrane Framework of silicate glass with interstitial sites for H+ and Na+. Na+, H+
    Gas permeable membrane Membrane acts as a permeable barrier through which only specific substances can move across. NH4+
    The membrane material can limit the possible matrices in which the ISE can be used. For example, electrodes with a polymer membrane cannot be used to measure ions in organic solvents. For more information on the specific restrictions, check the user manual of your ISE.
    Leaflet: Manual for ion-selective electrodes

    Basic theory behind ISEs

    Measuring range

    Each electrode type has its own specific measuring range (see Table 2). Before beginning any ion measurement, first ensure that the ion-selective electrode is able to measure in the concentration range of the sample.

    Table 2. Each ion-selective electrode has its specific measuring range. Note: The given measuring ranges only apply to ion-selective electrodes from Metrohm.
    Cation of interest Measuring range
    Ag+ 1×10–7 – 1 mol/L
    Ca2+ 5×10–7 – 1 mol/L
    Cd2+ 1×10–7 – 10–1 mol/L
    Cu2+ 1×10–8 – 10–1 mol/L
    H+ 1×10–14 – 1 mol/L
    K+ 1×10–7 – 1 mol/L
    Na+ (Polymer)

    Na+ (Glass)

    		 5×10–6 – 1 mol/L

    1×10–5 – 1 mol/L
    NH4+ 5×10–6 – 10–2 mol/L
    Pb2+ 1×10–6 – 10-1 mol/L
    Anion of interest Measuring range
    Br– 1×10–6 – 1 mol/L
    Cl– 1×10–5 – 1 mol/L
    CN– 8×10–6 – 10–2 mol/L
    F– 1×10–6 – sat. mol/L
    I– 5×10–8 – 1 mol/L
    NO3– 1×10–6 – 1 mol/L
    S2- 1×10–7 – 1 mol/L
    However, something more important than the measuring range is the linear range. Figure 1 illustrates a measuring range which also includes a linear range. Within this linear range the Nernst equation applies, and the signal is proportional to the analyte concentration. By performing an ion measurement within the linear range, the most precise and reproducible results will be obtained. Find out more about the Nernst equation in our previous blog post.
    «Analyze This»: FAQ: All about pH calibration
    Figure 1. The measuring range of an electrode.
    Outside of the linear range, the curve becomes flatter, and the potential difference becomes smaller, preventing a reliable measurement by standard addition. Even in this non-linear, flattened range, it is possible to determine the ion concentration by means of direct measurement – provided that your ion-selective electrode is also calibrated for this range.

    If the concentration is too low or the sensor is saturated, this situation is considered outside of the measuring range. Potential changes can no longer be determined.

    Interfering ions

    Compared to a pH electrode with a linear range over 14 decades, the sensitivity of ISEs is limited since interfering ions reduce the linear and measuring ranges (see Figure 1).

    There are two different types of interfering ions, both outlined in Table 3.

    Table 3. Types of interfering ions and their impact on measurement with ISEs.
    Interfering ion Description Impact
    Irreversible
    • Binds to the membrane material and reacts with it or
    • Forms complexes and precipitates with the measuring ion
    		 Destruction of the ISE since the irreversible interfering ion reacts with the membrane and is unavailable for further analysis.
    Reversible
    • Exhibits cross-sensitivities
    		 Erroneous results since the reversible interfering ion binds to the membrane material and contribute to the signal.
    Nowadays, the most important interfering ions for an ISE are known, and information about them is provided by the sensor manufacturer. For the measurement, the impact of interfering ions is considered in a so-called selectivity coefficient, which in turn is used in the Nikolsky equation—an expanded version of the Nernst equation.
    Table 4. Each ion to be measured has at least one interfering ion that must be considered when setting up an experiment.
    Measuring cation Interfering ion(s)
    Ag+ Hg2+, proteins
    Ca2+ Na+, Pb2+, Fe2+, Zn2+, Cu2+, Mg2+
    Cu2+ Ag+, Hg2+, S2-, Cl–, Br–, I–, Fe3+, Cd2+
    K+ Na+, NH4+, Cs+, Li+, H+
    Na+ (Polymer)

    Na+ (Glass)

    		 SCN–, acetate

    H+, Li+, K+, Ag+
    Pb2+ Ag+, Hg2+, Cu2+, Fe3+, Cd2+
    Measuring anion Interfering ion(s)
    Br– Hg2+, I–, S2-, CN–, NH4+, S2O32-
    Cl– Hg2+, Br–, I–, S2-, CN–, NH4+, S2O32-
    CN– S2-, Ag+ complexing substances, I–, Cl–, Br–
    F– OH–
    I– Hg2+, S2-, CN–, Cl–, Br–, S2O32-
    NO3– Br–, NO2–, Cl–, acetate
    S2- Hg2+, proteins
    Some examples for the most important interfering ions of ISEs are listed in Table 4. For more information on the theoretical background of pH as well as ion-selective electrodes, download our free monograph below.
    Monograph: Electrodes in Potentiometry

    Ionic strength adjustment

    The measurement depends on the activity of the measuring ion in solution, which in turn depends on the ionic strength. For this reason, ion-selective measurements are always carried out in solutions with approximately the same ionic strength. By addition of ionic strength adjuster (ISA) or total ionic strength adjustment buffers (TISAB), a constant ionic background can be achieved.

    ISA and TISAB are chemically inert respective to the measurement, and they contain a relatively high concentration of salt so that the ionic strength of the sample solution can be neglected. Some examples can be found in Table 5. Check the user manual of your ISE to find its ideal ISA or TISAB solution.

    Table 5. Recommended ISA and TISAB solutions for ion-selective electrodes and further information about each procedure.
    Ion to be measured ISA / TISAB Further information about procedure
    Fluoride (F–) NaCl / glacial acetic acid / CDTA Application Bulletin AB-082
    Potassium (K+) c(NaCl) = 0.1–1 mol/L Application Bulletin AB-134
    Sodium (Na+), glass membrane c(Tris(hydroxymethyl)aminoethane) = 1 mol/L Application Bulletin AB-083
    Sodium (Na+), polymer membrane c(CaCl2) = 1 mol/L
    Ammonium (NH4+) c(NaOH) = 10 mol/L Application Bulletin AB-133

    Electrode maintenance and lifetime

    Cleaning the ISE
    DO:
    • After each measurement or titration, the ISE has to be rinsed thoroughly with distilled water.
    DON’T:
    • Never use organic solvents for cleaning. They may attack or irreversibly destroy the polymer membrane ISE or reduce the lifetime of your crystal membrane ISE.
    Conditioning the ISE

    Conditioning steps must be performed before the first use as well as in between measurements. This step activates the measuring membrane and provides a stable equilibrium of the measuring ion in the membrane. By doing so, an accurate ion measurement is possible. An ion standard solution with a concentration of c(ion) = 0.01 mol/L is recommended as the conditioning solution.

    Storing the ISE

    An overview of proper storage instructions for your ion-selective electrode is shown in Table 6. For more detailed information, check the ISE manual.

    Table 6. Recommended storage conditions for different ISE types.
    Membrane material Short storage period Long storage period
    Crystal membrane In c(ion) = 0.1 mol/L Dry, with protective cap
    Polymer membrane Dry Dry
    Polymer membrane, combined In c(ion) = 0.01–0.1 mol/L Dry, with some residual moisture
    Glass membrane In c(ion) = 0.1 mol/L In deionized water
    Lifetime of an ISE

    The lifetime of an ion-selective electrode depends on several influencing parameters including membrane type, sample matrix, and electrode maintenance. Don’t forget to regularly exchange the electrolyte of your combined ISE or – in case of a separate ISE – your reference electrode. Furthermore, never touch the membrane with bare fingers.

    In general, the following can be said:

    • Polymer membrane electrodes: Limited lifetime of about half a year since the membrane ages, resulting in a loss of performance.
    • Crystal membrane electrodes: Lifetime of several years – the membrane can be regenerated by polishing using an appropriate polishing material. Watch the video below for more details.

    Summary

    • If you decide to perform an ion measurement using an ion-selective electrode, you must consider the measuring range and any interfering ions that may be present in advance.
    • In addition to the membrane type and the sample matrix, the cleaning, storage, and conditioning all have an influence on the lifetime of your ion-selective electrode.
    Hungry for more information regarding direct measurement and standard addition? Check out Part 2 (coming soon!) where we will discuss the different determination methods.

    Can’t wait? Download our free White Paper: «Overcoming difficulties in ion measurement: Tips for standard addition and direct measurement».

    Free White Paper:

    Overcoming difficulties in ion measurement: Tips for standard addition and direct measurement
    Download it here
    Post written by Doris Hoffmann, Product Manager Titration at Metrohm International Headquarters, Herisau, Switzerland.
    Five myths about online dispersive NIR spectroscopy, FT-NIR, and FT-IR – Part 1

    Five myths about online dispersive NIR spectroscopy, FT-NIR, and FT-IR – Part 1

    by | Feb 7, 2022 | Analysis, Evaluating results, Fundamentals

    Spectroscopy is not just spectroscopy—or is it?

    When talking with our project partners and customers, the topic of near-infrared (NIR) spectroscopy is often automatically associated with FT-NIR spectroscopy. So, why isn’t it just called NIR? What is the difference between IR and NIR? Some of you might even wonder: “Can I replace an old IR analyzer with NIR hardware?” And additionally: “Why should I replace the IR with a NIR process analyzer?”

    This two-part series aims to explain the differences between these techniques and dispel some myths.

    Click below to jump directly to a section:

    A brief historical overview

    Wavelengths

    The NIR wavelength range has a long history. As early as the 1880s, organic components were investigated in the NIR range and the strong –OH band relating to the presence of water was discovered as a very important piece of information. Shortly after, measurement of oils from the agricultural industry and investigations into various polymers followed. Some of the first industrial applications of dispersive NIR spectrometers were in the food and agricultural industries. In such applications, parameters including moisture, protein content, and fat content were analyzed quantitatively.

    On the other hand, some strong advantages came from using the infrared (IR) wavelength range—high structural sensitivity and specificity—making it possible to obtain precise fingerprints for structural identification.

    Figure 1. Historical punch card for assigning various spectral features to acetyl chloride in the infrared wavelength region [1]. (Click to enlarge image)
    For more information about the differences between IR and NIR spectroscopy, read our previous blog posts.
    Benefits of NIR spectroscopy: Part 2
    Frequently asked questions in near-infrared spectroscopy analysis – Part 1
    Hardware

    The hardware for NIR and IR analysis was fundamentally different. At that time, even though the evaluation of NIR spectra seemed to be too difficult and ambiguous due to the broad overlapping peaks, there was one major advantage: robust and cheap materials could be used for NIRS (e.g., PbS detectors, tungsten lamps, and simple glass materials for the optics). Since the NIR-bands were broad and overlapped, users were limited to only the essential information and therefore did not need higher resolution, so simple dispersive gratings (monochromator gratings) were sufficient.

    For IR, Fourier transform infrared spectrometers (FT-IR) were used which operated based on Michelson interferometers. This was necessary to obtain the spectral resolution needed for structural interpretation (e.g., distinguishing the isomers 1-propanol and 2-propanol at about 2700 nm for the first time). These spectrometers were introduced to the market in the 1960s. Due to the high costs for interferometers, special optics, and lasers, they were mainly used for research purposes.

    Software

    Significant progress has been made in the field of spectroscopy due to the development of more powerful computers in the 1980s and 1990s in combination with chemometrics. While IR spectroscopy, due to its high data point density, was still far behind in the application of computer-based chemometric methods, NIR spectrometers were already able to benefit from fast evaluation methods.

    Chemometric tools combined with the hardware benefits of NIR technology led many manufacturers to transfer their existing FT-IR measurement technology to the NIR range. And other companies? They just used and improved their already existing measurement technology to achieve perfect synergy between spectrometer and chemometrics.

    Now that some of the background has been laid out, it’s time to answer some myths about NIR, FT-NIR, and FT-IR spectroscopy.

    Myth 1: NIR spectroscopy always means FT-NIR

    This is a persistent myth that you could easily miss unless you look closely. What exactly does the «FT» mean and why doesn’t everyone use it to describe NIR spectroscopy?

    When using a FT-NIR spectrometer, first an interferogram is generated—not a spectrum in that sense. The conversion of the interferogram into a spectrum is done by applying a mathematical operation, the Fourier transform (FT). This transforms the path-dependent information (e.g., relative mirror position of two mirrors in the spectrometer) into a frequency-dependent function. This means FT-NIR is nothing more than the methodology of generating the spectrum in the NIR wavelength range.

    As before, it is NIR spectroscopy that provides the same information as dispersive NIR spectroscopy or diode array spectroscopy. FT-NIR uses the interferences produced by an interferometer to extract single wavelengths from white light (halogen lamp), while dispersive spectrometers use gratings. Gratings are produced by very modern lithographic techniques and offer the highest precision (of wavelength accuracy).

    Table 1. Comparison of FT-NIR and dispersive NIR spectroscopy

    Parameter FT-NIR NIR
    Wavelength splitting Mathematical calculation (Fourier transformation) from the phase shift of two incident light beams (interferogram) Diffraction or dispersion, movement by a digital encoder
    Resolution depends on… Setting the maximum offset of the moving mirror Number of lines of the monochromator grid, slit width, encoder quality
    Moving elements Yes (motor of the interference mirror) Yes (motor of the grid)
    Wavelength range 12500 cm-1–4000 cm-1

    (800–2500 nm)

    		 800–2500 nm

    (expandable to 400 nm)
    Noise Depending on the resolution, higher than dispersive NIR with comparable setup Depending on the resolution, lower than FT-NIR with comparable setup
    Method transferability (i.e., to other spectrometers) Yes Yes (due to the Metrohm calibration concept)

     
    Consumables Laser source, halogen lamp, and desiccant Halogen lamp
    Compared to a dispersive analyzer, a FT-NIR spectrometer uses a laser to control the position of the interferometer mirror. This laser must be changed periodically, however this task is generally not done by the end-user themselves compared to halogen lamp, which is easily replaceable.

    Looking a little bit deeper into the details of Table 1, it is clear that dispersive spectroscopic instruments are more suitable for industrial process applications. Why? A low acquisition time is critical for real-time measurements with the least time loss. Compared to a FT-NIR instrument, the acquisition time is lower for a dispersive analyzer (leading to faster results) at the same resolution.

    If you have ever wondered why you might need to calculate a Fourier transform, you will know after the next myth is answered.

    Myth 2: Method transfer is only possible with FT-NIR spectrometers and not with dispersive spectrometers

    Where does this myth originate?

    Visualize the internal structure of an interferometer. A He-Ne laser is used as the reference measurement for the precise determination of the mirror position and thus also obtains an exact spectrum with high wavelength reproducibility at the same spatial coordinate by the Fourier transformation.

    What is different about the dispersive spectrum?

    In this case, the spectrum is not calculated in a mathematically complex way but is recorded directly via the dispersing element (the monochromator) on the detector. Here, a high-resolution state of the art grating and an accurate digital encoder that is precisely matched with the detector play an important role. NIRS XDS Process Analyzers from Metrohm Process Analytics (Figure 2) use wavelength standards to achieve both high wavelength precision and reproducibility and to ensure the transferability of the developed method.

    Figure 2. The NIRS XDS Process Analyzer from Metrohm Process Analytics. 
    Wavelength Precision <0.015 nm Valid
    975.880 nm 0.0012 nm Yes
    1221.342 nm 0.0005 nm Yes
    1678.040 nm 0.0012 nm Yes
    Figure 3. Results of the wavelength reproducibility test of a NIRS XDS Process Analyzer during a performance test. The precision meets the very narrowly defined testing specifications.
    By using a wavelength and reference standard built into the spectrometer, additional diagnostics can be carried out at routine intervals (either as part of maintenance or automated within regular process operation) to check the wavelength accuracy and precision.

    Due to the standardization concept with reference standards and wavelength standards, methods can be transferred without much effort even when changing or adjusting accessories (e.g., longer optical fibers, partially changed probes).

    To summarize: a good standardization concept with NIST certified reference and wavelength standards as well as internally installed standards allow robust method transfers to other spectrometers and excellent long-term stability in the production process.

    Myth 3: Many applications cannot be measured with dispersive NIRS, but require well-resolved FT-NIR spectroscopy

    The Michelson interferometer and the monochromator grating were both developed in the 1800s. Both of these technologies have been used industrially since the advancement of computer technology and utilize the same light sources, detectors, optical fibers, and probes.

    Monochromator gratings now consist of, e.g., a holographic concave diffraction grating with an optimized image plane to avoid aberrations and stray light. Holographic gratings are created by etching interference lines via laser into a photoresist layer. An advantage of this is very high spectral resolution, which together with a detailed adjustable encoder (and other components of the monochromator), provides very good resolution with the NIR spectrometer. For example, the NIRS XDS Process Analyzer (Figure 2) has a real resolution of 8.75 nm.

    In comparison, higher resolutions can be achieved with interferometers, but this can also decrease the signal-to-noise (S/N) ratio. Usually, resolutions of approximately 8 cm-1 or 16 cm-1 are used, which corresponds to 10–25 nm at 2500 nm.

    Parameter FT-NIR Dispersive NIR (Metrohm)
    Wavelength range (nm) 800–2200

    (12,500–4545 cm-1)

    		 800–2200

    400–2200 (optional)
    Wavelength precision (nm) ~0.01 ~0.005
    Wavelength accuracy (nm) ~0.05–0.2 ~0.05
    Figure 4. Noise spectra recorded with a Metrohm dispersive NIRS analyzer and a typical FT-NIR spectrometer.
    Higher resolutions are usually not required for the majority of applications since harmonics/overtones and combination bands of pure substances in the NIR wavelength range have a broad bandwidth. The absorption peak with the smallest bandwidth currently known in the NIR region is talc at slightly more than 10 nm.

    Very similar overlapping information (e.g., –OH bands or –COOH bands) are separated by using chemometric methods and are evaluated individually and specifically.

    Another example that shows how powerful dispersive NIR spectroscopy is compared to FT techniques and IR spectroscopy can be seen in the separation of xylene isomers in a mixture of several aromatics/hydrocarbons (Figure 5).

    Figure 5. Xylene isomers present in a mixture of aromatics and other hydrocarbons. (L) Raw spectra of all components from 800–2200 nm. (R) Raw spectra of xylene isomers show significant molecular vibration variations in the –CH region from 1700–1850 nm.
    Figure 5 shows that the three xylene isomers can be clearly distinguished spectroscopically. Applying chemometrics elaborates the information even more and ultimately all six components can be determined individually and quantitatively. In a production process, the real-time reaction monitoring can be performed for all six components (Figure 6).
    Figure 6. Graph showing real-time monitoring during a feed separation process of a mixture including three xylene isomers.
    The application shown in Figures 5 and 6 has often been implemented with IR photometers in the past. We have now shown that the application can not only be transferred to the NIR wavelength range, but that even minimal structural differences between the functional groups of molecules can be detected by dispersive NIR spectroscopy.

    To summarize: dispersive spectrometers have very good spectral resolution—in some cases better than FT-NIR spectrometers—and can even distinguish between different isomers in complex mixtures, or between very similar components like –OH and –COOH functional groups.

    Summary

    In the first part of this series, we went more into detail about the practical differences between FT-NIR and dispersive NIR spectroscopy. Three myths were discussed: that NIR spectroscopy always means FT-NIR—False, method transfer is only possible with FT-NIR spectrometers and not with dispersive spectrometers—False, and many applications cannot be measured with dispersive NIRS, but require well-resolved FT-NIR spectroscopy—False.

    Some myths should no longer be kept alive because they are not facts!

    We have also compared FT-NIR spectrometers to dispersive spectrometers when used in a process environment. Some critical points to remember: the dispersive analyzer is less sensitive to vibrations; less maintenance is necessary, and the grating is post-dispersive and therefore less prone to pollution due to lower ambient light.

    To prove those arguments, Part 2 will show that, contrary to some expectations, it is possible to replace IR measurement techniques in industrial processes with easy-to-implement NIR measurement techniques. We will dispel some more myths and go into detail of an extremely low water content measurement in a process with the support of our primary analysis methods.

    Reference

    [1] Baker, A. W.; Wright, N.; Opler, A. Automatic Infrared Punched Card Identification of Mixtures. Anal. Chem. 1953, 25 (10), 1457–1460. doi:10.1021/ac60082a011

    Find your process application

    Our Applications Book contains hundreds of sold process applications from our more than 10,000 installed analyzers over several industries!
    Click here to download

    Post written by Sabrina Hakelberg, Product Manager NIRS Process Analyzers, Deutsche Metrohm Prozessanalytik (Germany).

    NIR spectroscopy in the petrochemical and refinery industry: The ASTM compliant tool for QC and product screening – Part 5

    NIR spectroscopy in the petrochemical and refinery industry: The ASTM compliant tool for QC and product screening – Part 5

    by | Jan 24, 2022 | Analysis, Fundamentals

    History of ASTM International

    The American Society for Testing and Materials (ASTM) is an organization that currently provides over 12,500 international standards. Its roots date back to 1898, when ASTM was formed by a group of scientists and engineers to address the frequent rail breaks affecting the fast-growing railroad industry. The group developed a standard for the steel used to fabricate rails.

    Originally, this organization was called the «American Society for Testing Materials» (1902) and was changed to the «American Society for Testing and Materials» in 1961. In 2001, ASTM officially changed its name to «ASTM International» and added the tagline «Standards Worldwide». This tagline was modified in 2014 to «Helping our world work better». Now, aside from the US, ASTM International also has offices in Belgium, Canada, China, and Peru.

    ASTM International aims to ensure that quality and standard requirements are met when using materials for engineering projects. Therefore, they had to agree upon a single language for engineers and technicians to enhance compatibility, and ultimately developed a system grouped according to industries in the form of letters A–G. Currently, there are over 12,500 ASTM standards used by about 150 countries. This has increased trade in different markets by instilling and strengthening consumer confidence.

    ASTM Committee D02 on Petroleum Products, Liquid Fuels, and Lubricants

    Formed in 1904, ASTM Committee D02 currently meets twice per year for five days of technical meetings, attended by approximately 1000 members (out of around 2500). D02 has jurisdiction over 814 standards with a prominent role in all aspects relating to the standardization of petroleum products and lubricants, which are published in the Annual Book of ASTM Standards (Volumes 05.01 through 05.06).

    Near-infrared spectroscopy—an ASTM compliant tool to assess the quality of petrochemical products

    Near-infrared spectroscopy (NIRS) has been an established method for fast and reliable quality control within the petrochemical industry for more than 30 years. However, many companies still do not consistently consider the implementation of NIRS in their QA/QC labs. The reasons could be either limited experience regarding application possibilities or a general hesitation about implementing NIRS as an alternative technology.

    Many companies are not aware that now there are many ASTM standards about how to implement NIR spectroscopy as an alternative to conventional methods. Several NIRS-related ASTM guidelines are shown in Figure 1.

    ASTM E1655 (method development quantitative NIR analysis) and ASTM E1790 (method development qualitative NIR analysis) are applicable for all industries (e.g., polymer, chemical, petrochemical, etc.).

    ASTM D6122 (method validation), ASTM D8321 (development and validation of multivariate analysis), and ASTM D8340 (performance qualification) are dedicated for the petrochemical industry. These three standards were released recently in 2020, and ASTM D8340 was updated at the end of 2021.

    Figure 1. Overview of NIRS-related ASTM guidelines.

    Method development

    ASTM E1655: Standard Practices for Infrared Multivariate Quantitative Analysis

    «These practices cover a guide for the multivariate calibration of infrared spectrometers used in determining the physical or chemical characteristics of materials. These practices are applicable to analyses conducted in the near infrared (NIR) spectral region (roughly 780 to 2500 nm).»

    ASTM E1790: Standard Practice for Near Infrared Qualitative Analysis

    «This practice covers the use of near-infrared (NIR) spectroscopy for the qualitative analysis of liquids and solids. The practice is written under the assumption that most NIR qualitative analyses will be performed with instruments designed specifically for this purpose and equipped with computerized data handling algorithms.»

    Method validation

    ASTM D6122: Standard Practice for Validation of the Performance of Multivariate Online, At-Line, Field and Laboratory Infrared Spectrophotometer, and Raman Spectrometer Based Analyzer Systems

    «This practice covers requirements for the validation of measurements made by laboratory, field, or process (online or at-line) infrared (near- or mid-infrared analyzers, or both), and Raman analyzers, used in the calculation of physical, chemical, or quality parameters (that is, properties) of liquid petroleum products and fuels.»

    These requirements include the following topics:

    • Analyzer calibration
    • Correlation of NIRS vs. lab method (treated and untreated samples)
    • Probationary validation
    • General & Continual validation

    Results validation

    ASTM D8321: Standard Practice for Development and Validation of Multivariate Analyses for Use in Predicting Properties of Petroleum Products, Liquid Fuels, and Lubricants based on Spectroscopic Measurements

    «This practice covers a guide for the multivariate calibration of infrared (IR) spectrophotometers and Raman spectrometers used in determining the physical, chemical, and performance properties of petroleum products, liquid fuels including biofuels, and lubricants. This practice is applicable to analyses conducted in the near infrared (NIR) spectral region (roughly 780 nm to 2500 nm).»

    The main purpose for this standard is to establish the validity of test results during calibration.

    ASTM D8340: Standard Practice for Performance-Based Qualification of Spectroscopic Analyzer Systems

    «This practice covers requirements for establishing performance-based qualification of vibrational spectroscopic analyzer systems intended to be used to predict the test result of a material that would be produced by a Primary Test Method (PTM) if the same material is tested by the PTM.»

    Furthermore, it includes the prescriptive requirements regarding multivariate models, including Multi Linear Regression (MLR), Partial Least Square (PLS), Principal Component Regression (PCR), Cross validation, and Outlier statistics, as well as instrument considerations.

    Regarding the required accuracy of the NIRS method the expected agreement and user requirements is that the Standard Error of Prediction for the NIRS value should be equal to or smaller than the laboratory method reproducibility.

    ASTM D8340 compliance with the NIRS DS2500 Petro Analyzer and Vision Air

    Temperature stability, section 5.4:

    There are prescriptive requirements included in this norm regarding temperature stability of the NIRS Analyzer. Section 5.4 requires sample temperature to be carefully controlled in the analyzer system hardware or that effects of temperature change be compensated in the modeling or software. Metrohm’s solution to this issue for the NIRS DS2500 Petro Analyzer is shown in Figure 2.

    Figure 2. Temperature stability for the NIRS DS2500 Petro Analyzer.
    Learn more about the possibilities of petrochemical analysis with Metrohm NIRS DS2500 Petro Analyzers in our free brochure.
    Brochure: NIRS DS2500 Petro Analyzer
    Analyzer wavelength accuracy and precision, section 6.3:

    Section 6.3 requires that the analyzer shall include a means of demonstrating that it is operating within the vendor’s specification. Therefore, the analyzer shall incorporate instrument performance tests to demonstrate that it is operating within historically expected limits. Furthermore, the analyzer shall have a means of validating wavelength/frequency precision and accuracy. Also, the wavelength precision must be sufficient to allow spectra to be collected and used in creating a multivariate model that meets or exceeds user’s specifications. The wavelength precision of the analyzer used for calibration and the analyzer-to-analyzer wavelength accuracy and reproducibility must be sufficient to allow analyzers to be validated by Practice D6122. Metrohm’s solution to this is the Vision Air software shown in Figure 3.

    Figure 3. Analyzer wavelength accuracy and precision for ASTM D8340 section 6.3 (click to enlarge image).
    Vendor created global multivariate model, section 8.2.2:

    The multivariate model can be that of a standardized test method, a user/vendor-created global multivariate model, or a user-created site-specific multivariate model. A global multivariate model is one developed by use of samples and data that may represent materials produced at multiple facilities or locations. Some locations may start with a global model and add site-specific sample to it. Metrohm offers several pre-calibrations for the NIRS DS2500 Petro Analyzer, listed in Figure 4.

    Figure 4. Available NIRS pre-calibrations for the petrochemical industry.
    Discover our selection of NIRS pre-calibrations available for the petrochemical industry in our free brochure.
    Brochure: Quality control of fuels – Fast results with NIR pre-calibrations
    Outlier statistics, sections 9.3, 9.4, and 9.5:

    The requirements of outlier handling are described in these sections. Identification and handling of outliers is important to the success of meeting this performance-based practice. It is permissible for the identification and handling of outliers to be performed by the same or separate software used for generating predictions from spectra. For analysis of a sample for the purposes of determining property values, the software shall indicate whether the spectrum is identified as an outlier, based on the criteria set by the user. The sample analysis may indicate that expected performance is not reached for a sample identified as an outlier.

    Vision Air is Metrohm’s universal software for Vis-NIR spectroscopy. Vision Air accounts for the unique needs of each type of instrument user. It offers specific interfaces optimized for the most common tasks – simplifying routine measurement, method development, outlier handling, and both data and instrument management. Vision Air is also compatible with third-party software like Unscrambler (Figure 5).

    Figure 5. Vision Air—Metrohm’s universal software for lab Vis-NIR spectroscopy.

    Summary

    In the petrochemical industry, typical requirements from management are usually that quality control should be quick, operating costs should be low, feedback should be fast, and operation should be safe.

    Typical requirements from the user are that the analysis should be accurate and precise, and the instrumentation should be easy to use, fast, and safe.

    NIR spectroscopy is compliant with all of these requirements, and as shown in this blog series, the technology is supported by several ASTM guidelines for method development, method validation, and results validation.

    By utilizing NIRS in the petrochemical industry, manufacturers not only improve the efficiency of screening and quality control of petrochemical products, but also fully adhere to internationally accepted standards.

    For more information

    About spectroscopy solutions provided by Metrohm, visit our website!

    We offer NIRS for lab, NIRS for process, as well as Raman solutions

    Visit our website
    Post written by Wim Guns, International Sales Support Spectroscopy at Metrohm International Headquarters, Herisau, Switzerland.
    USP – simple automated analysis of ultrapure water

    USP <645> – simple automated analysis of ultrapure water

    by | Jan 10, 2022 | Analysis, Evaluating results, Fundamentals

    H2O – two simple elements, oxygen (O) and hydrogen (H), fuse together to form one of the most important molecules in the world: water. Water is everywhere on Earth and it is vital for our health and survival. It often contains other ions like calcium, magnesium, and chloride which are essential for the human body to function. However, in specific situations, ultrapure water (UPW) is needed to prepare e.g., injections or other solutions used in hospitals. How is the quality of UPW ensured so that it is always suitable for such medical purposes? The answer to this comes from USP <645>. This standard explains how the water quality can be determined and how this analysis must be performed.

    Instruments

    For this analysis, a measuring device capable of measuring the conductivity and the pH value is required. If a combined device is not available, then using two separate ones is also fine. Then, a pH electrode that is especially suitable for the determination of the pH value of water and a conductivity cell for measurement of low conductivities is necessary. In this case, the Aquatrode plus and the stainless steel conductivity cell are recommended.

    The Aquatrode plus responds very quickly in ion-deficient matrices (such as ultrapure water) and thanks to its double junction system, the bridge electrolyte can be chosen freely.
    Aquatrode plus
    The stainless steel conductivity cell has been specially designed for measurements in samples with low conductivity. With a cell constant of c = 0.1 cm-1, it is ideal for conductivities ranging from 0–300 µS/cm. The built-in temperature sensor makes handling even easier as no additional sensor is needed for the temperature measurement.
    Stainless steel conductivity cell

    USP <645> procedure

    Now that the necessary instrumentation has been introduced, it’s time to take a look at the standard procedure itself. At first glance this looks a bit difficult as it is a three-step analysis, or actually a four-step analysis if you count the calibration as well.

    Step 1: First, calibrate the pH electrode and the stainless steel conductivity cell (sensor). The pH electrode is calibrated with pH 4 and 7 solutions, whereas the stainless steel sensor is calibrated with a 100 µS/cm standard.

    Find the standard solutions you need here:

    Buffer solution pH 4
    Buffer solution pH 7
    Conductivity standard
    Step 2: After calibrating the sensors, both the temperature of the water and the conductivity are measured without temperature compensation. If the measured conductivity is lower than the value mentioned in the table of USP <645>, then the requirement for the conductivity is met and the water can be used for medical purposes. If this is not the case, then step 3 must be performed.

    Step 3: 100 mL water is transferred to an external titration vessel where its temperature is adjusted to 25 ± 1 °C. The water is stirred vigorously to incorporate carbon dioxide present in the air. If the conductivity does not change by more than 0.1 µS/cm per 5 minutes, the value is noted for further evaluation. If this value is below 2.1 µS/cm, then the water is usable for medical purposes. If not, then proceed with step 4.

    Step 4: The solution is tempered to 25 ± 1 °C. Once the temperature is stable, 0.3 mL of saturated KCl solution is added and the pH of the water is measured. The pH value must lie between pH 5 and 7. If this is not the case, the water does not meet the requirements and must be discarded. If the pH is measured between 5 and 7, then the conductivity must additionally be lower as mentioned in the USP <645> table. If this is the case, the analysis passes and the water can be used for medical purposes.

    Automation of USP <645>

    The analysis can be quite time-consuming and therefore Metrohm has provided a solution to make this process much easier. Our system combines all of these steps into one method, allowing you to perform walk-away automation and focus on more important tasks.

    To expand the capabilities of the measuring system, the 856/867 modules can be exchanged with a modular OMNIS Titrator which can also be used for other standard potentiometric titrations.

    Take a closer look at our automated analysis solution

    Proper electrode immersion depth
    • Aquatrode plus for accurate pH measurement in ion-deficient matrices
    • Stainless steel conductivity cell for low conducting samples
    • Special holder for performing step 2 of the standard procedure
    • Thermostated vessel (for step 2)
    • DIS-Cover lid to prevent the sample from CO2 uptake (before step 3)
    • Rod stirrer to saturate the solution with carbon dioxide (for step 3)

    Conclusion

    With this complete system, standard analysis of UPW quality according to USP <645> is performed in a fully automatic and reliable manner. At the end of each analysis a clear message is received if the deionized water (UPW) has passed the test or not. Handling is very easy and allows users to check if an analysis passes or not with just a glance.

    Download our Application Bulletin

    Automatic conductometry in water samples with low electrical conductivity in accordance with USP<645>
    Click here!
    Post written by Iris Kalkman (Product Specialist Titration) and Heike Risse (Product Manager Titration – Automation), Metrohm International Headquarters, Herisau, Switzerland.
    NIR spectroscopy in the petrochemical and refinery industry: The ASTM compliant tool for QC and product screening – Part 4

    NIR spectroscopy in the petrochemical and refinery industry: The ASTM compliant tool for QC and product screening – Part 4

    by | Dec 13, 2021 | Analysis, Fundamentals

    What is a lubricant?

    A lubricant is defined as a petroleum-derived product used to control and reduce the friction and wear of moving machinery parts (e.g., in engines and turbines). The main purpose of lubricants  is to help protect and prolong the lifetime of the equipment.

    Machinery and lubricants go hand in hand, as shown here.
    These goals are accomplished in the following ways:

    Lubrication by reducing friction and wear. The lubricant forms a film between the mechanical moving parts of the equipment. In this way the metal-to-metal contact and, thus, the wear is reduced.

    Cooling by acting as a heat sink. This causes the heat to dissipate away from critical parts of the equipment so that deformation due to increased temperature is prevented.

    Protection by building a film. This film is unaffected by oxygen or corrosive substances and therefore prevents metal damage and oxidation (rust) and therefore also prevents wear.

    Types of lubricants

    For the most part, lubricants consist of oils to which additives and other chemical substances are added. There are two common types of lubricants which are based on the origin of the oil:

    1. Lubricants based on mineral oils (Figure 1a) are the most commonly used type. They are comprised of petroleum products (base stock) to which synthetic additives are added. These types of lubricants are used in applications where there are no high temperature requirements. Typical areas where mineral oil-based lubricants are used include: engines, hydraulics, gears, and bearings.
    2. Lubricants based on synthetic oils (Figure 1b) are artificially developed substitutes for mineral oils. They are less common and more expensive. Synthetic oils are specifically developed to create lubricants with superior properties to mineral oils. For example, heat-resistant synthetic oils are used in high performance machinery operating at high temperatures.
    a)
    b)
    Figure 1. Difference in the molecular structure found in lubricants: a) mineral oil and b) synthetic oil.
    In the following table, different lubricant types with sub-classes are listed.
    Table 1. Different lubricating oil types.

    Lubricant type

    Sub-classes
    Automotive oil Engine oil

    Gear oil

    Transmission fluids
    Industrial oil Hydraulic oil

    Turbine oil
    Greases
    Metal working fluids Forming fluids

    Cutting fluids
    The physical properties of a lubricant (such as viscosity and density) mostly depend on the oil base stock, whereas the additives fine-tune the chemical properties, e.g., the acid number or base number. For each application, the oil is typically formulated to meet the physical and chemical properties required by the customer. Therefore, various types of oil exist (Table 1).

    Near-infrared spectroscopy—an ASTM compliant tool to assess the quality of lubricants

    Near-infrared spectroscopy (NIRS) has been an established method for fast and reliable quality control within the petrochemical industry for more than 30 years. However, many companies still do not consistently consider the implementation of NIRS in their QA/QC labs. The reasons could be either limited experience regarding application possibilities or a general hesitation about implementing new methods.

    There are several advantages of using NIRS over other conventional analytical technologies. For one, NIRS is able to measure multiple parameters in just 30 seconds without any sample preparation! The non-invasive light matter interaction used by NIRS, influenced by physical as well as chemical sample properties, makes it an excellent method for the determination of both property types.

    In the remainder of this post, available solutions for lubricants are discussed which have been developed according the NIRS implementation guidelines of ASTM E1655 (method development), ASTM D6122 (method validation), and ASTM D8340 (results validation).

    Did you miss the first parts in this series about NIRS as a QC tool for the petrochemical industry? Find them all below!

    NIRS in the petrochemical and refinery industry: The ASTM compliant tool for QC and product screening – Part 1
    NIRS in the petrochemical and refinery industry: The ASTM compliant tool for QC and product screening – Part 2
    NIRS in the petrochemical and refinery industry: The ASTM compliant tool for QC and product screening – Part 3
    Read our previous blog posts to learn more about NIRS as a secondary technique.
    Benefits of NIRS: Part 1
    Benefits of NIRS: Part 2
    Benefits of NIRS: Part 3
    Benefits of NIRS: Part 4

    Applications and parameters for lubricant analysis with NIRS

    The main NIRS application for lubricants is to easily monitor the oil condition, i.e., checking if the oil is still of suitable quality for proper lubrication of the equipment. Reducing unnecessary oil changes means significant cost savings. On the other hand, changing the oil too infrequently can result in possible damage of the equipment, leading to costly repairs. Therefore optimizing the usage of the lubricating oil is very important.

    The following parameters can be correlated between NIRS and the values from a primary method: kinematic viscosity, viscosity index, color, density, water content, TAN (total acid number), and TBN (total base number). A large set of samples provided by several different companies was used to develop working NIRS models of these parameters, including hydraulic oil, gear oil, and others. In some cases, it was not clear for what application the lubricant was used, so the exact identity of the oil was unknown.

    The most relevant application notes for NIRS analysis of lubricants are listed below in Table 2.

    Table 2. Metrohm’s NIRS solutions for lubricants including application details and benefits.
    Parameter Reference method Norm NIRS Application Notes NIRS benefits
    Acid number Titration ASTM D664 AN-NIR-071

    AN-NIR-041

    		 All parameters are measured simultaneously within a minute, without requiring any sample preparation or chemical reagents.
    Kinematic viscosity at 40 °C Viscosimeter ASTM D445
    Kinematic viscosity at 100 °C Viscosimeter ASTM D445
    Viscosity index Calculation ASTM D2270
    Color number Colorimeter ASTM D1500
    Moisture content Karl Fischer titration ASTM D6304
    Base number Titration ASTM D2896
    Density Density meter ASTM D4052

     

    Solutions by means of starter models—expedite and simplify quality control of lubricants

    Lubricants keep our modern lives running smoothly. During use, the oil needs to be monitored to check if it still of good enough quality or whether it needs to be exchanged.

    The data obtained here indicate that lubricants vary per application and per supplier. This means that there is still not sufficient information for each oil type and subtype to prepare a model robust enough to transform into a pre-calibration. However, if a partner provides the samples, a feasibility study can quickly indicate if the NIR spectra are able to be correlated to the primary method values.

    Typically, several key parameters such as the acid and base numbers (AN and BN), viscosity, moisture content, color, and density are determined in the laboratory by various chemical and physical methods. These methods not only incur high running costs, they are also quite time consuming to perform.

    NIRS on the other hand requires neither chemicals nor sample preparation, and provides results in less than a minute. This spectroscopic technique is also easy enough to be used by non-chemists. Furthermore, multiple chemical and physical parameters can be determined simultaneously. The combined benefits of this technology make NIRS the ideal solution for many daily QA/QC measurements or ad-hoc atline analysis.

    Application example: starter model for lubricants with the NIRS DS2500 Liquid Analyzer

    For lubricant analysis, determination of the acid number (ASTM D664), viscosity (ASTM D445), moisture content (ASTM D6304), and color number (ASTM D1500) require the use of multiple analytical technologies and, in part, large volumes of chemicals. The time to result can therefore be quite a long and costly process.

    In this example, different lubricant samples were measured with a Metrohm NIRS DS2500 Liquid Analyzer in transmission mode over the full wavelength range (400–2500 nm). The built-in temperature controlled sample chamber was set to 40 °C to provide a stable sample environment. For convenience reasons, disposable vials with a pathlength of 8 mm were used, which made a cleaning procedure obsolete.

    Learn more about the possibilities of petrochemical analysis with Metrohm NIRS DS2500 Analyzers in our free brochure.
    DS2500 Analyzer – Boosting efficiency in the QC laboratory with Near-Infrared Spectroscopy (NIRS)
    Figure 2. Quality control of lubricants as performed by the Metrohm NIRS DS2500 Liquid Analyzer.
    The obtained Vis-NIR spectra (Figure 2) were used to create prediction models for the determination of key lubricant parameters (such as those in Table 2). The quality of the prediction models was evaluated using correlation diagrams, which display the correlation between the Vis-NIR prediction and primary method values. The respective figures of merit (FOM) display the expected precision of a prediction during routine analysis (Figure 3).
    Figure 3. Correlation plots and figures of merit (FOM) for different parameters measured in lubricants.
    This solution demonstrates that NIR spectroscopy is excellently suited for the analysis of multiple parameters in lubricants in less than one minute without sample preparation or using any chemical reagents.

    In case a large series of samples must be analyzed, there is also the possibility to measure lubricant samples in a fully automated way, as detailed in our free Application Note below.

    Quality Control of Lubricants – Unassisted, rapid determination of the Acid Number by automated NIR spectroscopy according to ASTM E1655
    Here, the samples were measured in transmission mode over the full wavelength range (400–2500 nm) using a NIRS XDS RapidLiquid Analyzer in combination with an 815 Robotic USB Sample Processor, which can carry a total of 141 samples (Figure 4).
    Figure 4. Metrohm NIRS XDS RapidLiquid Analyzer equipped with a with 5.0 mm flow cell (left) and the 815 Sample Processor (right).

    Summary

    Near-infrared spectroscopy is very well suited for lubricant analysis. Available starter models are developed and validated in accordance with the ASTM guidelines. Positive aspects of using NIRS as an alternative technology to primary methods are the short time to result (less than one minute), no chemicals or other expensive equipment needed, and ease of handling so that even shift workers and non-chemists can perform these analyses in a safe manner.

    Future installments in this series

    This blog article was dedicated to the topic of lubricants and how NIR spectroscopy can be used as the ideal QC tool for the petrochemical / refinery industry. The final installment will be dedicated to:

     

    For more information

    About spectroscopy solutions provided by Metrohm, visit our website!

    We offer NIRS for lab, NIRS for process, as well as Raman solutions

    Visit our website
    Post written by Wim Guns, International Sales Support Spectroscopy at Metrohm International Headquarters, Herisau, Switzerland.
    Side reactions in Karl Fischer titration

    Side reactions in Karl Fischer titration

    by | Dec 6, 2021 | Analysis, Evaluating results, Fundamentals

    Many chemists that utilize Karl Fischer titration are nervous about the presence of side reactions because they know that the water determination in their samples can only be specific without any side reactions. Other KFT users do not know what the possible side reactions are and therefore may obtain incorrect results.

    What are side reactions?

    These are reactions with substances in the sample that:

    • interfere with the stoichiometry of the KF reaction
    • change the pH value of the KF reagent
    • either produce or use up water themselves
    • oxidize on the anode of the generator electrode
    • reduce on the cathode of the generator electrode
    • react with the ingredients of the KF reagent

    Recognizing side reactions

    One of the worst things that can happen with KFT is not knowing that a side reaction is falsifying your results. Below are some characteristic signs of side reactions.

    Titration time and titration curve

    Some indications of side reactions include longer titration times compared to the titration of a water standard, slow endpoint detection, and a higher drift value after the titration finishes than at the titration start. Comparing the titration curves of the sample and a water standard with a similar water quantity makes it easier to evaluate the situation. Just plot a graph of the volume against time (or µg water against time, in the case of coulometry). If the graph exhibits a curve that increases steadily as illustrated in Figure 1 (in orange), this can indicate a side reaction.

    Figure 1. Side reactions can often be identified from checking the titration time and the titration curve, as shown in this graph.
    Linearity

    If you notice that the water content depends on the sample weight or the titrant consumption (µg water for coulometry), then you can check the slope of a regression line after plotting water content against titrant consumption (µg water).

    Ideally, the slope (b) should be 0. Significantly positive or negative values can indicate a side reaction, as shown in Figure 2.

    Figure 2. If the slope of the regression line for the water content / titrant consumption value pairs deviates significantly from 0, this indicates a side reaction.
    Spiking

    If the water recovery value found after spiking the samples is not within 100 ± 3%, this can indicate a side reaction. Depending on the type and speed of the side reaction, the recovery may be too high or too low. For example, samples which contain DMSO (dimethyl sulfoxide) change the stoichiometry of the Karl Fischer reaction and therefore result in false low readings.

    Please note that a recovery rate of almost 100% does not guarantee the absence of a side reaction. Side reactions that take place very rapidly will not be detected, since the side reaction is already complete when the spiking process begins. A spiking procedure is described in detail in chapter 2.5.12 of the European Pharmacopoeia.

    Preliminary tests

    The oxidation of iodide or reduction of iodine leads to incorrect results.

    How can you check whether your sample is undergoing a side reaction with iodine or iodide? A simple preliminary test can clarify the situation. Dissolve the sample in a weakly acidic (alcoholic) solution and then add some drops of iodine or potassium iodide solution. Based on the coloring (a discoloration of iodine or the formation of brown iodine), a side reaction can be detected.

    Evaluating redox potentials

    Comparing the redox potentials of the redox pairs of sample substances with the redox potential of iodine/iodide can be helpful to assess whether an undesired redox reaction may occur.

    If the standard potential is higher than that of iodine/iodide, as in the case of e.g., chlorine, the oxidation of the iodide may result in false low readings.

    If it is lower (e.g., lead), the reduction of the iodine may result in values that are too high.

    Element changing oxidation state oxidized form + x e– → reduced form Standard electrode potential E°
    Cl Cl2 + 2e– ⇌ 2 Cl– +1.36 V
    I I2 + 2e– ⇌ 2 I– +0.54 V
    Pb Pb2+ + 2e– ⇌ Pb -0.13 V

    Avoiding side reactions

    Most side reactions can be suppressed by taking suitable measures, such as those listed here.

    • For ketones and substances that react with the methanol present in the KF reagent: Use methanol-free reagents.
    • For samples that lower the pH range of the KF reagent: Add buffer solution for acids or a stoichiometric excess of imidazole.
    • For samples that increase the pH value (e.g., aminic bases): Add buffer solution for bases or a stoichiometric excess of salicylic acid / benzoic acid.
    • High drift after titration: Postdrift correction may help. This is done by stopping the titration at a defined time and recording the additional consumption over several minutes. This allows the calculation of the drift after the titration. This postdrift is then used to correct the water quantity found.
    • Samples that reduce iodine: Subtract the iodine consumption of the reductant in the sample from the overall iodine consumption of the sample.
    • Samples that oxidize iodide: Reduce the oxidant, e.g., Cl2, in advance with an excess of SO2, for example, by treating the sample with the solvent of a two-component reagent.
    • General: Carry out the titration in a thermostatically controlled cell connected to a circulation thermostat at, e.g., -20 °C in order to slow down the side reaction. Note that the titration parameters should be adjusted to the low temperatures.
    • General: Extract the water with the KF oven method if the interfering components are thermally stable at oven temperature.
    • General: Mask or eliminate the interfering component, e.g., by adding N-ethylmaleimide in the case of thiols.
    Find out more about the Karl Fischer oven method in our blog article.
    «Analyze This»: Oven method for sample preparation in Karl Fischer titration

    Summary

    Side reactions can negatively influence and falsify your results. Recognizing and avoiding side reactions in KF titration is therefore crucial for the most accurate determinations.

    For more information, check out our blog series about frequently asked questions in Karl Fischer titration.

    «Analyze This»: Frequently asked questions in Karl Fischer titration – Part 1
    «Analyze This»: Frequently asked questions in Karl Fischer titration – Part 2

    Download our free monograph:

    Water determination by Karl Fischer Titration
    Click here!
    Post written by Michael Margreth, Sr. Product Specialist Titration (Karl Fischer Titration) at Metrohm International Headquarters, Herisau, Switzerland.
    Ph. Eur. 2.2.48 Raman Spectroscopy: How Raman instruments from Metrohm comply with the 2022 update

    Ph. Eur. 2.2.48 Raman Spectroscopy: How Raman instruments from Metrohm comply with the 2022 update

    by | Nov 29, 2021 | Analysis, Company

    The European Pharmacopoeia (Ph. Eur.) is a single reference work for the quality control of medicines. Ph. Eur. contains norms, suggests analytical methods, and lists many properties that define quality control (QC) during the production of medicines, the raw materials used, and the instruments required to perform such tests. These official standards are legally binding in several countries – not only in Europe, but worldwide.

    Raman spectrometers—especially handheld and portable instruments—are increasingly used for QC of medicines and raw materials (RMID). Instrument interfaces are user-friendly, requiring little technical expertise, and they provide flexible sampling options for most sample types with rapid, non-destructive measurements.

    Metrohm’s Raman systems exhibit great flexibility—from see-through to standoff to immersion sampling.
    An excerpt from the Ph. Eur. 2.2.48 Raman Spectroscopy chapter says:
    «Raman spectroscopy is commonly used for qualitative and quantitative applications and can be applied to solid, liquid, and gaseous samples. Raman spectroscopy is a rapid and non-invasive analytical method and can be performed off-line, at-line, on-line, or in-line[…] Raman spectrometers can be situated far from the point of measurement using long-distance optical fibres to collect the Raman signal.»

    For a look at RMID from start to finish, download our free four-part tutorial series.
    Handheld Raman for RMID in pharma and other regulated industries
    Technological developments and their increasing adoption in the pharmaceutical industry prompted a revision of Ph. Eur. 2.2.48 which ensures the reliability of Raman results. The updated chapter 2.2.48 was published in the Ph. Eur. Supplement 10.7 (October 2021) and will ultimately take effect in April 2022.

    While much of the Ph. Eur. 2.2.48 chapter has remained the same, the latest revision features:

    • new requirements for spectral resolution for qualitative Raman analysis using a suitable reference material
    • updated requirements for the Raman response-intensity scale
    • detailed procedures for the comparison of spectra

    We will address these new requirements across our Raman spectroscopy product lines in the rest of this article.

    Spectral Resolution

    «Spectral resolution is the ability of a spectroscopic system to separate adjacent bands, which makes it possible to characterise complex samples (e.g., brand analysis, crystallinity, polymorphism).

    […] For identity tests, unless otherwise prescribed in a monograph, the spectral resolution must be less than or equal to 15cm-1 (measured in the wavenumber range between 1000cm-1 and 1100cm-1).

    The spectral resolution is verified using a suitable reference material. The instrument parameters used for the test, such as laser, slit-width and grating for dispersive instruments and circular aperture […] for FT-instruments, must be the same as those applied for sample measurements. For example record the Raman spectrum of calcium carbonate for equipment qualification CRS, and determine the full width at half height (W1085) of the band located at 1085 cm-1. The spectral resolution (R) using calcium carbonate is then given by the following relation:»

    Handheld Raman instruments: MIRA P and NanoRam

    All MIRA P and NanoRam devices (including both NanoRam and NanoRam-1064) for the pharmaceutical industry are designed and tested to meet stringent resolution requirements. During QC, the resolution of each instrument is tested to be less than 15 cm-1 against a secondary USP (US Pharmacopeia) reference standard of calcium carbonate according to ASTM E2529, which is the same procedure recommended in this newly released Ph. Eur. chapter.

    The measured spectral resolution value for each instrument, along with its identifying serial number, is included in the instrument final test report. A certificate or final test report is packaged with the device and sent to the customer (starting in April 2022 for MIRA P). This resolution is fixed by the optical design of the instrument and is stable over time.

    Portable Raman instruments: i-Raman series, QTRam, STRam, and PTRam

    The instrument resolution for all of Metrohm’s portable Raman instruments from B&W Tek are factory-tested with calcium carbonate and displayed on final instrument test reports. The spectral resolution is dependent on the instrument design and defined for each specific instrument configuration. Depending on the instrument model, the spectral resolution is between 3.5–11 cm-1. Additionally, the instrument control softwares Vision and BWAnalyst have the performance test function that verifies spectral resolution using the 1001.4 cm-1 peak of polystyrene.

    Handheld and portable Raman instruments from B&W Tek.

    Response-Intensity Scale

    «The verification of the response-intensity scale is principally performed for quantitative methods.

    Appropriate acceptance criteria will vary with the application. A maximum variation of ± 10 per cent in band intensities compared to the previous instrument qualification is achievable in most cases. Response calibration may involve the use of white-light standards or luminescent glass (e.g., NIST SRM 2241).»

    Handheld Raman instruments: MIRA P, MIRA M-3, and NanoRam series

    MIRA P, MIRA M-3, and NanoRam systems are designed for qualitative analysis, not for quantitative purposes. Therefore, this criterion is not a strict requirement for handheld Raman products.

    However, the relative intensity response of MIRA P and NanoRam series instruments is calibrated with a NIST standard SRM calibration material (SRM 2241, SRM 2242) or NIST SRM 2241-traceable calibration standard to achieve better uniformity from instrument-to-instrument.

    The NanoRam series instruments have an acceptance criterion for relative intensity response in the instrument performance validation in alignment with Ph. Eur. 2.2.48 and USP<858>. To pass the performance validation, <10% relative intensity error is required using the factory-supplied polystyrene cap.

    Portable Raman instruments: i-Raman series, QTRam, STRam, and PTRam

    The relative intensity response of these portable Raman instruments is calibrated using a proper NIST standard SRM calibration material to achieve better uniformity from instrument-to-instrument. Additionally, the Vision instrument control software includes the performance test function that verifies the intensities of several Raman peaks of polystyrene relative to its 1001.4 cm-1 peak, to a maximum variation of ±10% compared to the previous instrument qualification.

    Comparison Procedures

    For qualitative methods, additional information for identification has been defined.

    «Several comparison procedures may be used, and the analyst must document and justify the method used and the specific acceptance criteria that allow a conclusion for identification. The spectra can be compared by either overlaying the spectra (in the whole spectral range or in the region of interest specified in the monograph) or by using mathematical calculations of the software. It is possible for example to perform:

    • visual comparison based on band positions and relative intensities unless otherwise specified[…]
    • a statistical determination of the similarity between the spectra of the material to be examined and the reference standard[…]
    • evaluation by chemometric methods[…]»

    While an experienced Raman spectroscopist can certainly compare spectra visually and assess sample validity based on peak location, fluorescence, saturation, and signal-to-noise ratio, the widespread implementation of Raman in the real world means that complex analysis must be done by the device and not the user. Statistical comparison methods are used primarily for identification of unknowns through correlation of a sample spectrum with library spectra. The software performs library searches and returns a Hit Quality Index (HQI) value indicating the level of correlation as defined by a user-defined threshold.

    Chemometric methods rely on dimensionality-reduction methods that are performed by the software, such as Principal Component Analysis (PCA), where new sample data is compared within a multivariate model created from representative samples. This permits highly accurate verification of known materials according to how well a spectrum fits into the model limits, which are determined by a confidence interval. In the analysis of medicines and raw materials, chemometric methods are used to distinguish the quality and consistency of a material. MIRA P (and its dedicated software, MIRA Cal P) and NanoRam instruments use both statistical and chemometric methods for sample identification and verification based on the needs of the end-user.

    For more information, download our free technical and application notes as well as a White Paper below.

    Pros and Cons of Using Correlation vs. Multivariate Algorithms for Material Identification via Handheld Spectroscopy
    Method Development with NanoRam®-1064
    Simplified RMID Model Building – Mira Cal P and Model Expert
    White Paper: Verification, p-values, and Training Sets for the Mira P

    Wavenumber accuracy requirements of Ph. Eur. 2.2.48

    «Verify the wavenumber scale for Raman shifts using a suitable standard that has characteristic maxima at the wavenumbers under investigation, for example an organic substance such as polystyrene, paracetamol or cyclohexane[..]

    A minimum of 3 wavenumbers covering the working range of the instrument intended for measurements should be selected.[…]»

    This chapter maintains the same requirements for Raman wavenumber accuracy and is consistent with the USP <858> and JP 2.26. All of Metrohm’s handheld Raman instruments meet these requirements. Users are recommended to run performance validation tests at regular intervals using polystyrene or another ASTM Raman shift calibration material.

    Download our free White Paper below to learn more about instrument calibration, system verification, and performance validation.

    Instrument Calibration, System Verification, and Performance Validation for Metrohm Instant Raman Analyzers (Mira)
    The Calibrate/Verify Attachment (CVA) shown here is a dual-ended accessory containing a toluene/acetonitrile ASTM standard for calibration/verification of the wavenumber axis and polystyrene for a second wavenumber verification according to Ph. Eur. 2.2.48.
    Performance tests for Raman wavenumber accuracy are included the Vision and BWAnalyst softwares for the i-Raman series and other portable Raman products (STRam, QTRam, PTRam), with acceptance criteria in accordance with the pharmacopeial requirements.

    Metrohm’s unique way of compliance with Ph. Eur. 2.2.48

    Better representation of the material

    «When using Raman spectroscopy[…] care must be taken to ensure that the measurement is representative. This can be achieved by, for example rotation of the sample, performing multiple measurements on different preparations of the sample, using orbital raster scanning (ORS), increasing the area of illumination by reducing the magnification, by demagnification of the laser beam or by changing the focal length between measurements to scan at different depths.»

    ORS™ is Metrohm Raman’s proprietary method for moving the excitation laser in a pattern over a sample in order to collect more representative data from a larger area of the sample, especially on heterogeneous samples. All MIRA and MISA instruments are equipped with ORS.
    Learn more about ORS by downloading the related Application Note.
    Improving verification with Orbital Raster Scan technology
    For more details about how we comply, please check the U.S. Pharmacopeia Raman Chapters Updates page on the B&W Tek website. For more general information, download this General Compliance Statement for MIRA handheld Raman systems
    B&W Tek: U.S. Pharmacopeia Raman Chapters Updates
    MIRA General Compliance Statement
    For a more comprehensive look at raw material identification and verification in the pharmaceutical industry, there is a significant amount of information on this topic in our related blog post.
    Real World Raman: Simplifying Incoming Raw Material Inspection

    Post written by Dr. Melissa Gelwicks (Technical Writer at Metrohm Raman, Laramie, Wyoming), and Dr. Xiangyu (Max) Ma (Handheld Raman Product Manager) and Dr. Jun Zhao (R&D Director) at B&W Tek, Plainsboro, New Jersey.