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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.

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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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

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. 

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.

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.
  • 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.

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.

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 
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

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.

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.

Find out more about Metrohm NIRS instruments for laboratory and process analysis below.

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.

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.
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.

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

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

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).

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.

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.

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.

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.
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.

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».

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Overcoming difficulties in ion measurement: Tips for standard addition and direct measurement
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

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.
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

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