Illuminating Spectra Part 7 - Watch the baseline

NIR spectra not only tell us about the chemical properties but also reveal a great deal about the physical condition of the sample.

One of the features of NIR spectra is that they often exhibit baseline shifts. This means that the spectra show an additive effect (i.e. offset) along the absorption axis. This is usually indicative of variations due to particle size, packing density or porosity and even the presence of air bubbles.

This baseline shift is a very common feature. For example, if you scan sub samples of the same sample multiple times you may see that the resulting spectra are not overlaying each other. Here is why this can happen:

As the particle size or porosity in the sample increases, part of the NIR light that is shone on the sample escapes through the gaps between particles and hence would not find the chance to reach the instrument detector. This translates into a higher absorption baseline. This is also called a pseudo-absorbance to differentiate it from real chemical absorption by the sample.

In the graph below you can see that whole (unground) corn spectra show a relatively high baseline (red line). Now if we grind the sample with a coffee grinder, less light will escape through the sample gaps and more light will reach the detector, hence the baseline shifts downwards (green line). Grinding the sample to a smaller particle size will cause the baseline to shift downwards even further as shown by the blue line. 

Figure: Baseline variation is indicative of particle size or porosity in the corn

Baseline shifts are one of the reasons why pre-processing NIR spectra is important. Pre-processing methods such as normalisation can minimise the physical variability in the spectra so that chemical information becomes more pronounced.

The next time you observe a baseline shift, try to see if you can relate it to the physical properties of your samples.