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Benefits of NIR spectroscopy: Part 2

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 2

    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.