Illuminating Spectra - Part 5: It is not that simple!
If you have read each part of this column so far, you may think that moisture, oil, starch and protein have very specific NIR peaks associated with them. Well, this is not exactly the case!
NIR bands are not related to any particular molecules (or chemical species). Rather they represent certain molecular bonds (mostly C-H, O-H and N-H). This means for example, any molecule with an N-H bond in its structure can potentially be confused with protein as it overlaps with NIR bands related to protein.
Also for example, protein, oil and starch will all contribute to C-H bands at the same time.
This picture gets more complex when you notice that C-H, N-H and O-H bonds absorb in multiple locations in an NIR spectrum (in the form of overtones and combination bands).
And be ready to be even more confused (sorry) by knowing that C-H, N-H and O-H absorption bonds are pretty close to each other as illustrated in this image.
Image: C-H, N-H and O-H can absorb in multiple overlapping locations in an NIR spectrum.
You only need to zoom in on any part of an NIR spectrum and you can't be totally sure what you are looking at as it can be any organic entity with N-H, O-H and C-H in its structure. This is particularly true when dealing with agricultural samples containing lots of different organic components. No wonder NIR spectra demonstrate broad and overlapping features!
Let's recap: broadly speaking, almost anything can absorb anywhere in the NIR spectra and no single band relates to any single thing! Of course, this doesn't render NIR useless but makes it very hard to interpret. This is the reason for using sophisticated multivariate methods (also called chemometrics) to extract information from the spectra.
There is still a lot to learn from inspecting NIR spectra - more on that next time.
Remember, the Aunir team of chemotricians are here to help with any NIR questions or queries you have. Get in touch if you'd like to speak to a member of the team by emailing firstname.lastname@example.org.