Practical instructions regarding this method of analysis are beyond the scope of this book, however, some details will be given to provide a background for those wishing to make use of commercial services. Some material was originally published in Faithfull (1996).
What exactly is NIRS? The near infrared region of the electromagnetic spectrum is a region having a range of wavelengths slightly longer than visible light but not as long as microwaves or the longer radiowaves. A beam of light from a quartz-iodine lamp is shone on to the sample and the spectrum of the reflected light is analysed by a spectrometer. Some wavelengths of the light beam will have been reduced in intensity because of absorption by certain vibrating molecular bonds (in particular C-H, O-H and N-H). It is not the fundamental vibration that is involved in this region but overtones or combinations of the fundamentals. The spectrum must be sampled at about 700 data points so that the subsequent data processing can unscramble the interacting spectral peaks and relate them to concentration of a substance of interest in the original sample. This process is largely mathematical and statistical and far removed from wet chemistry where, for example, the actual fibre is extracted from the sample and weighed. A computer system is required in order to process the data produced by the spectrometer.
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