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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

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.
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.
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 5Vision 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

Post written by Wim Guns, International Sales Support Spectroscopy at 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

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!

Read our previous blog posts to learn more about NIRS as a secondary technique.

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.
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.

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:

 

  • ASTM Norms

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.
Side reactions in Karl Fischer titration

Side reactions in Karl Fischer titration

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.

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.

Download our free monograph:

Water determination by Karl Fischer Titration
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

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.»

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.

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.

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.
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
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.
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, Newark, Delaware.
Green hydrogen generation: A cross-disciplinary challenge rooted in electrochemistry

Green hydrogen generation: A cross-disciplinary challenge rooted in electrochemistry

Green hydrogen, produced from water electrolysis using renewable energy sources, is being explored as a strategy to reduce the dependence on fossil fuels and decarbonize chemical processes. From an environmental standpoint, this approach is extremely attractive given that mild conditions are used during electrolysis and there are no greenhouse gases produced when using the hydrogen in a fuel cell.

However, the economics of electrolysis and fuel cell systems for energy conversion relies heavily on the costs of electricity and of metals like nickel, platinum, iridium, and titanium. Electrolyzer operating expenses must be minimized for green hydrogen to become an economically viable option. The electricity input contributes heavily to cost. Thus, decreasing the cost of renewable energy is a necessary step. Solar panels becoming more efficient and affordable within the past decades is cause for optimism in this regard [1], but there is much more that can be done to increase the success of green hydrogen. More efficient electrolyzers could make better use of the input electricity and the development of cheaper and more durable components can reduce both the capital and operational costs.

Check out our other blog articles about green hydrogen and decarbonization of chemical processes below!

Cross-disciplinary interest in green hydrogen

Electrolyzers are primarily electrochemical devices with electrocatalysts responsible for water splitting (Figure 1). The scientific challenges related to optimizing electrolyzers are attracting the attention of researchers that are not traditionally trained in electrochemistry. The search for efficient HER (Hydrogen Evolution Reaction) and OER (Oxygen Evolution Reaction) electrocatalysts also piques the interest of inorganic chemists and physicists. Development of better membranes calls for expertise in organic and polymer chemistry. Optimization of catalyst inks and their interaction with substrates requires the know-how of a materials scientist. Heat and mass flow management within the fuel cell stack and balance of plant are engineering endeavors. Clearly, the ongoing development of green hydrogen technologies has encouraged the collaboration of scientists and engineers across many disciplines. The result is an influx of creativity and insight, as well as development of exciting new materials and techniques.

Figure 1. Diagram of the electrolysis of water (water splitting) with respective half reactions at the cathode and anode in alkaline and acidic media.

Back to basics

Working in an unfamiliar domain means there is a need for quickly getting up to speed with best practices and learning a new scientific vocabulary. For many institutions, education on electrochemical principles and laboratory skills was not a key focus area until recent years.

In some cases, the deficiency of fundamental electrochemical training has led to inconsistencies in the reporting of important performance indicators. The electrochemical community has taken note of this and called for a more rigorous approach. As a result, experts have stepped up and provided practical guidance for quantifying and reporting in this domain.

When investigating electrocatalyst materials it is necessary to have benchmarks and well-defined performance indicators. In 2013, a comprehensive benchmarking protocol for evaluating and reporting figures of merit for OER electrocatalysts was published.

This JACS article [2] provides practical advice on how to interpret the catalyst surface in terms of roughness and geometric surface area and how to perform and analyze measurements for valid comparisons of electrocatalytic performance.

A common source of confusion and inconsistency in electrochemical measurements is the use of various reference electrodes (RE). Electrocatalytic activity is judged by the overpotential needed for a specified production rate (i.e., the current density for the HER or OER process, Figure 1). A three-electrode setup is needed to measure the potential, and the RE is crucial for situating this potential on a relative scale, allowing comparison of measurements carried out by different groups and in various conditions.

Find out more about reference electrodes and their usage in our free Application Note.

A 2020 Viewpoint article in ACS Energy Letters [3] provides a detailed explanation of how to report the overpotential of an electrocatalyst, focusing on commonly used reference electrodes like Hg/HgO, Hg/Hg2Cl2 (SCE), and Ag/AgCl.

The reversible hydrogen electrode (RHE) is another commonly used RE that is extremely well-suited for HER and OER studies. A recent ACS Catalysis article [4] explains why the RHE is the ideal reference electrode for electrolysis research and explains how to prepare and work with an RHE. By convention, all standard redox potentials are reported versus the standard hydrogen electrode (SHE). The RHE is a pH-dependent extension of the SHE and refers to the reduction of a proton under non-standard conditions as described by the Nernst equation.

Electrolyzers operate under both acidic and alkaline conditions, thus, the HER and OER are studied across the pH scale (Figure 1). The RHE is suitable for use at any pH and it shares the same dependency on pH as the HER and OER.

A common ground to stand on

Finding common language and understanding between these different fields is vital. This JOC synopsis article [5] clarifies electrochemical concepts for organic chemists. The article is highly visual, providing schematics that link concepts like free energy, redox potential, and overpotential. Equilibrium thermodynamics helps to provide a common point of reference that all chemists can relate to.

Thermodynamic analysis is often applied to quantify the energy efficiency of electrolysis cells and stacks. A recent review article in the Journal of Power Sources [6] highlights diverging definitions for the energy efficiency coefficient from academic and industrial literature. The article provides derivations in various conditions and reminds readers that both electricity and heat must be accounted for in the analysis.

Summary

The articles highlighted in this blog post represent just a small fraction of the many resources available for building a common understanding and better collaboration among all researchers working on the improvement of green hydrogen technologies. When the COVID pandemic shut down laboratory work and travel for many people, the research community carried on with enthusiasm.

Online seminars and working groups held openly and without cost have brought scientists together across disciplines and from around the world. For example, the Electrochemical Online Colloquium was started in 2021. This ongoing series of lectures addresses essential topics in electrochemistry by providing educational content alongside the personal perspective of expert speakers.

The electrochemical community is acutely aware of the importance of transitioning to sustainable and climate-safe energy and chemical processes. Energy storage and conversion through green hydrogen is a promising strategy that requires scientific advancement to thrive. Thankfully, researchers from across many disciplines are bringing their skills and creativity to this topic while the electrochemical community continues to drive collaborative efforts and share their core knowledge.

To find out more about associated topics, download these free Application Notes from Metrohm Autolab.

References

[1] Green Hydrogen Cost Reduction: Scaling up Electrolysers to Meet the 1.5°C Climate Goal; International Renewable Energy Agency: Abu Dhabi, 2020.

[2] McCrory, C. C. L.; Jung, S.; Peters, J. C.; et al. Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction. J. Am. Chem. Soc. 2013, 135 (45), 16977–16987. doi:10.1021/ja407115p

[3] Niu, S.; Li, S.; Du, Y.; et al. How to Reliably Report the Overpotential of an Electrocatalyst. ACS Energy Lett. 2020, 5 (4), 1083–1087. doi:10.1021/acsenergylett.0c00321

[4] Jerkiewicz, G. Standard and Reversible Hydrogen Electrodes: Theory, Design, Operation, and Applications. ACS Catal. 2020, 10 (15), 8409–8417. doi:10.1021/acscatal.0c02046

[5] Nutting, J. E.; Gerken, J. B.; Stamoulis, A. G.; et al. “How Should I Think about Voltage? What Is Overpotential?”: Establishing an Organic Chemistry Intuition for Electrochemistry. J. Org. Chem. 2021. doi:10.1021/acs.joc.1c01520

[6] Lamy, C.; Millet, P. A Critical Review on the Definitions Used to Calculate the Energy Efficiency Coefficients of Water Electrolysis Cells Working under near Ambient Temperature Conditions. J. Power Sources 2020, 447, 227350. doi:10.1016/j.jpowsour.2019.227350

Post written by members of the Metrohm Autolab group, Utrecht, The Netherlands.