Near-infrared spectroscopy

What is NIR spectroscopy?

Near-infrared spectroscopy (NIRS) is a method that uses light between the visible and infrared wavelength ranges (approximately 800 – 2500 nm) and measures the absorption of light as a function of wavelength. The near-infrared light that interacts with the sample has specific wavelengths that are absorbed based on certain chemical bonds (e.g. C-H, O-H, S-H, and N-H) being present in the sample.

NIR spectroscopy is suitable for examining both irregular surfaces, such as food and feed, and more carefully prepared samples, such as pharmaceuticals. NIRS is a quick and low-cost method, with the capability of analyzing hundreds of samples in a day with very little running costs. The data from a single NIR spectroscopy run can be used to predict several different results.

How does NIR spectroscopy work?

Near-infrared spectroscopy is based on the use of electromagnetic radiation. A diffraction grating is used to separate light into desired wavelengths. This light is then directed at the sample, and the light scattered off and blasted through the sample is collected by a special diode array detector that measures its transmittance and absorbance. A reflectance spectrum is then created to present the obtained data. The transmitted and absorbed light depends on the composition of the sample.

When electromagnetic radiation is directed at the sample, the carried energy of the radiation causes different interactions, such as overtones in molecular vibrations in certain chemical bonds present in the sample. Different bonds interact at different energy levels. The overtones of different bonds absorb energy at specific wavelengths that are characteristic of their structure.

It is known that several overtones can overlap within one spectrum. Therefore, it can be difficult to assign specific features to specific chemical components. This is where NIR calibration comes in handy. With carefully selected calibration samples and reference analysis methods, NIR spectroscopy can be calibrated to ensure high precision in chemical concentration analysis.

What is NIR spectroscopy used for?

NIR spectroscopy is commonly used for determining the compositional and functional properties of the sample, identifying materials, and as part of process control. The most common fields of study where NIR spectroscopy is used are agriculture, food and feed analysis, pharmaceuticals, and different medical and physiological diagnostics.

In the food and feed sector, NIR spectroscopy is an effective tool for measuring moisture, protein, fat, free fatty acids, ethanol, density, solids, organic acids, carbohydrate profile, and other important constituents. NIRS also has applications in material science, as it can be used as a tool for measuring film thickness and studying the optical characteristics of nanoparticles.

What are the differences between NIR and IR spectroscopy?

NIR and IR spectroscopy both use electromagnetic radiation and measure absorbance and transmittance to gain data from a sample. But when comparing NIR spectroscopy and regular IR spectroscopy, there are a few key differences. Light in the near-infrared region can typically penetrate deeper into the sample when compared to regular infrared radiation. This means that NIR spectroscopy is not a very sensitive method but is very useful when studying bulk materials.

Regular IR spectroscopy is more accurate in identifying functional groups and materials. Near-infrared spectroscopy can be used in the presence of interfering substances, such as glass or plastic containers. These materials can be used as cell windows and focusing lenses. Mid-IR radiation is absorbed by many common materials and therefore more expensive special materials have to be used in lenses and windows to maintain high precision.

Another major difference between the two methods lies in that quantitative calculations with NIR are more complex, calibration is more complicated and calibration data is not very well transferable to other instruments. Calibration data in mid-IR is much more generic and therefore much more easily transferable to other instruments.

Sample requirements and preparation

NIR spectroscopy requires little to no sample preparation. As a technique it is non-destructive, meaning it does not harm the sample. The method is suitable for all kinds of samples that have molecules with C-H, O-H, S-H, and/or N-H bonds. Water is reasonably transparent in the NIR region making it a suitable solvent for different applications.

Need a NIR analysis?

Measurlabs offers NIR spectroscopy analyses for different industries and research purposes. Our experts are also always happy to help with any questions relating to the method or its suitability for your samples. You can contact us through the form below or by emailing us at info@measurlabs.com.