Illuminating Spectra - Part 3: How dry is my sample?!
Liquid water is the easiest of all to spot in the NIR spectra unless your sample is very dry. Water molecules absorb the NIR light more than any other chemical species. This makes NIR a sensitive tool for measurement of moisture.
Water absorption bands are such broad and dominant features in the NIR spectra that sometimes they can swamp or mask the information from other species.
Two main absorption bands of liquid water are located around 1450nm and 1940nm (with the second one being almost always a head and shoulder taller!).
The graph below shows spectra of different dairy products with different moisture content (2%, 15%, 50% and 80% from bottom to top). Notice the enlargement and broadening of moisture bands as the water content increases (i.e. dryness decreases) in the samples. See if you can draw some conclusions about the relative fat content of these samples just by looking at their spectra (see Illuminating Spectra Part 1 for some hints).
There are other important features of water bands such as their sensitivity to temperature but more on that in a future issue.
Thank you to Bruker for providing the spectra files.
To convert the wavelength units from nm to cm-1 please check out this webpage.
(Just be aware that this is a guideline that we use for solid agricultural samples. From time to time you might encounter samples that don’t react in the same way depending on specific sample composition.)