Infrared spectroscopy uses the infrared region (with a longer wavelength and lower frequency than visible light) of the electromagnetic spectrum to induce vibration of bonds in substances. The emission response will give information about the substances’ composition.
For the analysis of organic materials, near-infrared reflectance spectroscopy or NIRS is one of the most used analytical techniques. One reason is that near-infrared radiation (700 to 2500 nm) is mainly absorbed by N-O, C-H, and O-H bonds. These bonds are mostly present in organic matter.
Aside from the analysis of organic bonds, NIRS can be also used in the analysis of minerals in plants. This is due to the occurrence of these minerals in some organic compounds. For example, there are similarities between the wavelengths of detecting magnesium and the peaks of chlorophyll spectrum. Similarities are also present in wavelengths for detecting calcium and peaks of calcium pectate (present in cell walls of plants). It’s also possible to analyze other elements such as potassium, phosphorus, aluminium, sulphur, and silicon.
NIRS is a low-cost technique which gives quick results and requires minimal sample preparation. Samples are prepared by drying and grounding prior measurement. Analysts consider particle size (including consistency) because it affects reflectance values. One reason is reflectance increases as the particle size increases. After sample preparation, analysts pack them into quartz cuvettes. Analysts are then able to collect reflectance readings.
There are still challenges when it comes to using NIRS. One of those is in the mineral analysis, especially during calibration. Applying calibration of one specimen to an analysis of another can be difficult. The reason is there are variations in the formation of organic compounds, the particular season when samples were collected, and the age of plants.
Another challenge is about the presence of substantial amounts of moisture in plant samples. High moisture levels can affect reflectance readings. It also leads to more heterogeneity which causes more noise on the spectrum.
To solve the above challenges, scientists are now developing robust portable NIR meters. These instruments use a variety of fiber-optic probes. These can help analysts obtain quick results during field analysis.
William J. Foley et al. (1998), Ecological applications of near infrared refl̄ectance spectroscopy - a tool for rapid, cost-effective prediction of the composition of plant and animal tissues and aspects of animal performance