Select Page
Benefits of NIR spectroscopy: Part 3

Benefits of NIR spectroscopy: Part 3

This blog post is part of the series “NIR spectroscopy: helping you save time and money”. 

How to implement NIRS in your laboratory workflow

This is the third installment in our series about NIR spectroscopy. In our previous installments of this series, we explained how this analytical technique works from a sample measurement point of view and outlined the difference between NIR and IR spectroscopy.

Here, we describe how to implement a NIR method in your laboratory, exemplified by a real case. Let’s begin by making a few assumptions:

  • your business produces polymeric material and the laboratory has invested in a NIR analyzer for rapid moisture measurements (as an alternative to Karl Fischer Titration) and rapid intrinsic viscosity measurements (as an alternative to measurements with a viscometer)
  • your new NIRS DS2500 Analyzer has just been received in your laboratory

The workflow is described in Figure 1.

Figure 1. Workflow for NIR spectroscopy method implementation (click to enlarge).

Step 1: Create calibration set

NIR spectroscopy is a secondary method, meaning it requires «training» with a set of spectra corresponding to parameter values sourced from a primary method (such as titration). In the upcoming example for analyzing moisture and intrinsic viscosity, the values from the primary analyses are known. These calibration set samples must cover the complete expected concentration range of the parameters tested for the method to be robust. This reflects other techniques (e.g. HPLC) where the calibration standard curve needs to span the complete expected concentration range. Therefore, if you expect the moisture content of a substance to be between 0.35% and 1.5%, then the training/calibration set must cover this range as well.

After measuring the samples on the NIRS DS2500 Analyzer, you need to link the values obtained from the primary methods (Karl Fischer Titration and viscometry) on the same samples to the NIR spectra. Simply enter the moisture and viscosity values using the Metrohm Vision Air Complete software package (Figure 2). Subsequently, this data set (the calibration set) is used for prediction model development.

Figure 2. Display of 10 NIR measurements linked with intrinsic viscosity and moisture reference values obtained with KF titration and viscometry (click to enlarge).

Step 2: Create and validate prediction models

Now that the calibration set has been measured across the range of expected values, a prediction model must be created. Do not worry – all of the procedures are fully developed and implemented in the Metrohm Vision Air Complete software package.

First, visually inspect the spectra to identify regions that change with varying concentration. Often, applying a mathematical adjustment, such as the first or second derivative, enhances the visibility of the spectral differences (Figure 3).

    Figure 3. Example of the intensifying effect on spectra information by using mathematical calculation: a) without any mathematical optimization and b) with applied second derivative highlighting the spectra difference at 1920 nm and intensifying the peaks near 2010 nm (click to enlarge).
    Univariate vs. Multivariate data analysis

    Once visually identified, the software attempts to correlate these selected spectral regions with values sourced from the primary method. The result is a correlation diagram, including the respective figures of merit, which are the Standard Error of Calibration (SEC, precision) and the correlation coefficient (R2) shown in the moisture example in Figure 4. The same procedure is carried out for the other parameters (in this case, intrinsic viscosity).

    This process is again similar to general working procedures with HPLC. When creating a calibration curve with HPLC, typically the peak height or peak intensity (surface) is linked with a known internal standard concentration. Here, only one variable is used (peak height or surface), therefore this procedure is known as «univariate data analysis».

    On the other hand, NIR spectroscopy is a «multivariate data analysis» technology. NIRS utilizes a spectral range (e.g. 1900–2000 nm for water) and therefore multiple absorbance values are used to create the correlation.

    Figure 4. Correlation plot and Figures of Merit (FOM) for the prediction of water in polymer samples using NIR spectroscopy. The «split set» function in the Metrohm Vision Air Complete software package allows the generation of a validation data set, which is used to validate the prediction model (click to enlarge).
    How many spectra are needed?

    The ideal number of spectra in a calibration set depends on the variation in the sample (particle size, chemical distribution, etc.). In this example, we used 10 polymer samples, which is a good starting point to check the application feasibility. However, to build a robust model which covers all sample variations, more sample spectra are required. As a rule, approximately 40–50 sample spectra will provide a suitable prediction model in most cases.

    This data set including 40–50 spectra is also used to validate the prediction model. This can be done using the Metrohm Vision Air Complete software package, which splits the data set into two groups of samples: 

    1. Calibration set 75%
    2. Validation set 25%

    As before, a prediction model is created using the calibration set, but the predictions will now be validated using the validation set. Results for these polymer samples are shown above in Figure 4.

    Users who are inexperienced with NIR model creation and do not yet feel confident with it can rely on Metrohm support, which is known for its high quality service. They will assist you with the prediction model creation and validation.

    Step 3: Routine Analysis

    The beauty of the NIRS technique comes into focus now that the prediction model has been created and validated.

    Polymer samples with unknown moisture content and unknown intrinsic viscosity can now be analyzed at the push of a button. The NIRS DS2500 Analyzer will display results for those parameters in less than a minute. Typically, the spectrum itself is not shown during this step—just the result—sometimes highlighted by a yellow or red box to indicate results with a warning or error as shown in Figure 5.

    Figure 5. Overview of a selection of NIR predicted results, with clear pass (no box) and fail (red box) indications (click to enlarge).
    Display possibilities

    Of course, the option also exists to display the spectra, but for most users (especially for shift workers), these spectra have no meaning, and they can derive no information from them. In these situations only the numeric values are important along with a clear pass/fail indication.

    Another display possibility is the trend chart, which allows for the proactive adjustment of production processes. Warning and action limits are highlighted here as well (Figure 6).

    Figure 6. Trend chart of NIR moisture content analysis results. The parallel lines indicate defined warning (yellow) and action (red) limits (click to enlarge).

    Summary

    The majority of effort needed to implement NIR in the laboratory is in the beginning of the workflow, during collection and measurement of samples that span the complete concentration range. The prediction model creation and validation, as well as implementation in routine analysis, is done with the help of the Metrohm Vision Air Complete software package and can be completed within a short period. Additionally, our Metrohm NIRS specialists will happily support you with the prediction model creation if you would require assistance.

    At this point, note that there are cases where NIR spectroscopy can be implemented directly without any prediction model development, using Metrohm pre-calibrations. These are robust, ready-to-use operating procedures for certain applications (e.g. viscosity of PET) based on real product spectra.

    We will present and discuss their characteristics and advantages in the next installment of the series. Stay tuned, and don’t forget to subscribe!

    For more information

    If you want to learn more about selected NIR applications in the polymer industry, visit our website!

    We offer NIRS analyzers suitable for laboratory work as well as for harsh industrial process conditions.

    Post written by Dr. Dave van Staveren, Head of Competence Center Spectroscopy at Metrohm International Headquarters, Herisau, Switzerland.

    Benefits of NIR spectroscopy: Part 3

    Benefits of NIR spectroscopy: Part 2

    This blog post is part of the series “NIR spectroscopy: helping you save time and money”. 

    Infrared spectroscopy and near infrared spectroscopy – is there a difference?

    This is the second installment in our series about NIR spectroscopy. In this post, you will learn the background of NIR spectroscopy on a higher level and determine why this technique might be more suitable than infrared spectroscopy for your analytical challenges in the laboratory and in the process.

    Spectroscopy… what is that?

    A short yet accurate definition of spectroscopy is «the interaction of light with matter». We all know that light certainly influences matter, especially after spending a long day outside, unprotected. We experience a sunburn as a result if we are exposed to the sun for too long.

    A characteristic of light is its wavelength, which is inversely correlated to its energy. Therefore, the smaller the wavelength, the more energy there is. The electromagnetic spectrum is shown in Figure 1. Here you can see that the NIR region is nestled in between the visible region (at higher energy) and the infrared region (at lower energy).

    Figure 1. The electromagnetic spectrum. (Click to enlarge.)

    Light from both the infrared (IR) and near-infrared (NIR) region (800–2500nm) of the electromagnetic spectrum induces vibrations in certain parts of molecules (known as functional groups). Thus IR and NIR belong to the group of vibrational spectroscopies. In Figure 2, several functional groups and molecules which are active in the NIR region are shown.

      Figure 2. Major analytical bands and relative peak positions for prominent near-infrared absorptions. Most chemical and biological products exhibit unique absorptions that can be used for qualitative and quantitative analysis. (Click to enlarge.)

      The difference in the vibrations induced by IR or NIR spectroscopy is due to the higher energy of NIR wavelengths compared to those in the IR region.

      Vibrations in the infrared region are classified as fundamental—meaning a transition from the ground state to the first excited state. On the other hand, vibrations in the near infrared region are either combination bands (excitation of two vibrations combined) or overtones. Overtones are considered vibrations from the ground state to a level of excitation above the first state (see Figure 3). These combination bands and overtones have a lower probability of occurring than fundamental vibrations, and consequently the intensity of peaks in the NIR range is lower than peaks in the IR region.

      Figure 3. Schematic representation of the processes occurring with fundamental vibrations and with overtones. (Click to enlarge.)

      This can be better understood with an analogy about climbing stairs. Most people climb one step at a time, but sometimes you see people in a hurry taking two or three stairs at once. This is similar to IR and NIR: one step (IR – fundamental vibrations) is much more common compared to the act of climbing two or more stairs at a time (NIR – overtones). Vibrations in the NIR region are of a lower probability than IR vibrations and therefore have a lower intensity.

      Theory is fine, but what does this mean in practice?

      The advantages of NIR over IR derived from the theoretical outline above are:

      1. Lower intensity of bands with NIR, therefore less detector saturation.

      For solids, pure samples can be used as-is in a vial suitable for NIR analysis. With IR analysis, you either need to create a KBr pellet or carefully administer the solid sample to the Attenuated Total Reflectance (ATR) window, not to mention cleaning everything thoroughly afterwards.

      For liquids, NIR spectra should be measured in disposable 4 mm (or 8 mm) diameter vials, which are easy to fill, even in the case of viscous substances. IR analysis requires utilization of very short pathlengths (<0.5 mm) which require either costly quartz cuvettes or flow cells, neither of which are easy to fill.

      2. Higher energy light with NIR, therefore deeper sample penetration.

      This means NIR provides information about the bulk sample and not just surface characteristics, as with infrared spectroscopy.

      However, these are not the only advantages of NIR over IR. There are even more application related benefits:

      3. NIR can be used for quantification and for identification.

      Infrared spectroscopy is often used for detecting the presence of certain functional groups in a molecule (identification only). In fact, quantification is one of the strong points of utilizing NIR spectroscopy (see below).

      4. NIR is versatile.

      NIR spectroscopy can be used for the quantification of chemical substances (e.g. moisture, API content), determination of chemical parameters (e.g. hydroxyl value, total acid number) or physical parameters (e.g. density, viscosity, relative viscosity and intrinsic viscosity). You can click on these links to download free application notes for each example.

      5. NIR also works with fiber optics.

      This means you can easily transfer a method from the laboratory directly into a process environment using an analyzer with a long, low-dispersion fiber optic cable and a rugged probe. Fiber optic cables are not possible to use with IR due to physical limitations.

      NIR ≠ IR

      In summary, NIR is a different technique than IR, although both are types of vibrational spectroscopy. NIR has many advantages over IR regarding speed (easier handling, no sample preparation needed), providing information about the bulk material as well as its versatility. NIR allows for the quantification of different kinds of chemical and physical parameters and can also be implemented in a process environment.

      In the next installment of this series, we will focus on the process of implementing a NIR spectrometer in your laboratory workflow, using a specific example.

      For more information

      about NIRS solutions provided by Metrohm, visit our website!

      We offer NIRS analyzers suitable for laboratory work as well as for harsh industrial process conditions.

      Post written by Dr. Dave van Staveren, Head of Competence Center Spectroscopy at Metrohm International Headquarters, Herisau, Switzerland.

      Benefits of NIR spectroscopy: Part 3

      Benefits of NIR spectroscopy: Part 1

      This blog post is part of the series “NIR spectroscopy: helping you save time and money”. 

      People who are unfamiliar with near-infrared (NIR) spectroscopy frequently ask the question: “Why should I need to know more about this technique, and how can I benefit from it?”.

      In this first installment of in this series of posts, we focus on the main advantages of NIRS over conventional wet chemical analysis methods and will provide examples of the types of parameters that can be measured with NIR spectroscopy.

      Solid vs. liquid samples

      In order to understand the benefits of NIRS, a good starting point is to understand how the NIR spectrum is measured. NIR spectroscopy can be used to analyze different types of samples. However, different instrumentation is required depending on the sample type. Several measurement methods are available for samples ranging from clear liquids to opaque pastes and powders. Choosing the right measurement method, sampling module, and accessories is the most important step to developing robust NIR methods. Below, the different methods are shown for various sample types (diffuse reflection, diffuse transmission, transflection, and transmission).

      Diffuse reflection: Cream, paste, granulates, coarse & fine powders

      NIR light penetrates into and interacts with the sample, and the unabsorbed NIR energy reflects back to the detector. This method is most suitable to measure solid samples without sample preparation.

      Diffuse transmission: Tablets and capsules

      As with diffuse reflection, the NIR light penetrates into and interacts with the sample. This light is scattered throughout the sample, due to interaction with the particles. The unabsorbed NIR light is transmitted through the sample prior to reaching the detector. This method is most suitable to measure solid dosage forms without sample preparation.

      Transflection: Liquids and gels

      This measurement method is a combination between transmission and reflection. A reflector is placed behind the sample, used to reflect the unabsorbed NIR light back to the detector. This method is most suitable to measure liquid samples.

      Transmission: Liquids

      In this situation, the sample is placed between the NIR light source and the detector. NIR light is transmitted through the sample, and any unabsorbed NIR energy continues to the detector. This method is most suitable to measure clear liquid solutions or suspensions.

      Solid sample measurement

      Solid samples (such as powders) must be placed on the window as shown here, secured within an appropriate container or vial. The instrument lid needs to be closed prior to starting the analysis so external light does not affect the results.

      The NIR radiation comes from below, and is partially reflected by the sample to the detector, which is also located below the sample vessel plane. After 45 seconds, the measurement is completed and a result is displayed. As this reflected light contains all the relevant sample information, this measurement technique is called diffuse reflection.

      Liquid sample measurement

      As the image illustrates, for liquid analyses via NIRS, a vial or cuvette must be placed in the drawer of the instrument. After pressing start, the drawer closes automatically and a result is obtained after 45 seconds.

      In this case, the NIR radiation travels through the solution before reaching the detector. This measurement technique is known as transmission.

      Advantages of NIRS

      The procedure for obtaining the NIR spectrum already indicates two main advantages of NIRS: simplicity regarding sample measurement and speed. These and other advantages of NIR are listed here:

      • Fast technique with results in less than 1 minute.

      • No sample preparation required – solids and liquids can be used in pure form.

      • Low cost per sample – no chemicals or solvents needed.

      • Environmentally-friendly technique – no waste generated.

      • Non-destructive – precious samples can be reused after analysis.

      • Easy to operate – inexperienced users are immediately successful.

      How to quantify with NIRS

      NIRS is a secondary technique, which means a prediction model will need to be created first. You can compare this, for example, to HPLC. If you want to identify or quantify a substance with that technique, you would need to prepare standard solutions of the substance and measure them to create a calibration curve.

      This is similar with NIRS: first you need to measure a number of spectra with known concentrations or known parameter values gathered from a primary method such as titration. A prediction model is then created out of these spectra using chemometric software, e.g. the Metrohm Vision software. We will explain in more detail how prediction models are created in another installment of this series.

        Application versatility in all industries

        NIRS is a versatile technique and can be used for various applications, both for chemical and physical parameters. You can find many different application examples for NIR in the Metrohm Application Finder. Here, we have listed representative examples for some industry segments.

        • Polymers: Density of Polyethylene (PE); Melt Flow Rate; Intrinsic Viscosity

        • Chemical: Hydroxyl number of polyols

        • Petrochemical: Research Octane Number (RON) of gasoline; cetane index for diesel

        • Oils and Lubricants: Total Acid Number (TAN)

        • Pharma: Water content of lyophilized products; content uniformity in tablets

        • Personal care: Moisture content and active ingredients in creams

        Overall, near-infrared spectroscopy is a robust alternative technique for the determination of both chemical and physical parameters in solids and liquids. It is a fast method which can also be successfully implemented for routine analysis by staff without any laboratory education. 

        In the next installment we will answer another frequently asked question: “Is near-infrared the same as infrared spectroscopy?”.

        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 Dr. Dave van Staveren, Head of Competence Center Spectroscopy at Metrohm International Headquarters, Herisau, Switzerland.