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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.

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