Applications of Portable Near-Infrared Spectroscopy

Spectroscopy is the study of matter and the interaction with electromagnetic radiation. Initially, spectroscopy originated from the study of visible light dispersed by a prism into its various wavelengths.

Portable Near-Infrared Spectrometer

Later, the concept expanded to include any interaction of radiation as a function of its wavelength. By using different regions of the electromagnetic spectrum, it allows for the investigation of the physicochemical interactions of various chemical and biological systems, which has important applications in biomedical research.

Infrared spectroscopy is an absorption technique that uses the infrared portion of the electromagnetic spectrum. These regions consist of near-IR (NIR), mid-IR (MIR), and far IR (also known as terahertz). The near-IR region is approximately 14000 – 4000 cm−1 and can excite overtone or combination modes of molecular vibrations.

Typical applications include medical and physiological diagnostics, pharmaceutical, food, and agrochemical quality control, atmospheric chemistry, combustion research, and astronomy.

Basic Theory of Vibrational Spectroscopy

Atoms and molecules change their energy as they transition between distinct energy levels, which cause them to emit or absorb electromagnetic radiation. Different energy transitions result in different frequencies of radiation.

Thus, the principle of spectroscopy is to irradiate molecules in a sample with a select portion of the electromagnetic spectrum to observe changes in molecular behavior. The absorption and emission pattern of radiation for every molecule is unique.

A simple model of a diatomic molecule comprising two atoms linked by a spring is used to approximate the frequency of oscillation by the law of simple harmonic motion. The IR spectrum is a plot of the amount of radiation absorbed (or transmitted) as a function of wavenumber (cm-1).

A simple diatomic molecule can be treated as a harmonic oscillator and is useful to describe a molecular vibration between two atoms, however, it is overly simplistic.

One flaw is that it describes the number of vibrational energy levels as being infinite as the bond length approaches zero. It does not model the effects of Coulombic repulsion between two nuclei, which produces a force in the same direction as the restoring force.

In addition, the harmonic curve cannot predict the interatomic distance at which dissociation of the molecule occurs.

In contrast, the anharmonic model leads to two types of derivations which overcome some of the flaws in the harmonic model. First, the potential energy between discrete vibrational energy levels becomes smaller.

Consequently, transitions that skip two or three energy levels are allowed. Such transitions give rise to overtone bands at approximately two or three times the frequency of the fundamental band. Secondly, an input of energy can simultaneously excite two different fundamental vibrational modes to produce a combination band. These overtones and combination bands can be exploited through the use of NIR spectroscopy.

Vibrations of Molecules

Near-Infrared Spectroscopy

Near-infrared (NIR) spectroscopy is based on molecular overtone and combination vibrations. One advantage of NIR compared to MIR is that it can typically penetrate much further into a sample. Thus, it is useful for the analysis of bulk material and requires little sample preparation.

The molecular overtone and combination bands seen in the NIR are usually very broad, which can be difficult to interpret. Hence, chemometric approaches help to assign specific absorption bands to specific functional groups.

Some common chemometric techniques include Principal Components Analysis (PCA), Partial Least Squares Regression (PLSR), and discriminant analysis.

Portable near-infrared spectroscopy

Instrumentation for near-IR (NIR) spectroscopy is similar to instruments for mid-IR, which generally contains a source, detector, and a dispersive element. Additionally, they can be coupled with a Fourier Transform interferometer, which allows for faster spectral acquisition. Industry requirements for fast, cost-effective, non-destructive analysis in real-time has opened a market for portable NIR spectrometers.

Although portable NIR spectrometers offer several advantages, some considerations may be required such as cost, size, weight, power consumption, robustness, safety, user-friendliness, durability, accuracy of measurement, and high-performance reliability.

Another important characteristic is instrument design and ergonomics to make these instruments truly handheld, which would make them easy to use.

Portable NIR spectroscopy has several applications in the agro-food industry. In particular, the analysis of fruit and vegetables for assessing the internal quality non-destructively in the field under various weather conditions for predicting if the product is ready for harvest.

Portable NIR has also been used for distinguishing between different pig breeds as well as distinguishing between fresh and frozen fish samples. Furthermore, the use of a portable NIR has been used to determine common sources of variability among beer and testing the quality of tea.

The water content of rice crackers has been determined using portable NIR spectroscopy to investigate their hygroscopic behavior.

Additionally, portable NIR has also been used to determine the moisture content of noodles. Furthermore, portable NIR instruments have been used in environmental science to determine contamination levels in a wide variety of soil samples.

References

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Further Reading

Last Updated: Jul 21, 2020

Dr. Grant Webster

Written by

Dr. Grant Webster

Grant is a dedicated senior scientist with a thirst for understanding the unknown. He has a Ph.D. in Chemistry and specializes in analytical and physical chemistry with academic and industry experience in the use of vibrational spectroscopy coupled with chemometrics/multivariate statistics for applications in the life sciences, biomedical diagnostics, and environmental science fields.

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