Ask the NIR Experts - Raw Material ID

Question:  I am trying to do Raw Material Identification and I know that FT-NIR can penetrate through the sample bag. But don't you get a residual signal from the bag in the process? Doesn't that affect your calibration?

Thermo Scientific:   Yes, you definitely see a signal from the plastic. However, this issue is easily solved with how you set up your calibration. There are 3 major strategies of factoring bags out of a NIR calibration:

• If the bags are all the same polymer, typically the variability in the bag thickness doesn’t hurt the performance of a Raw Material ID method. So the answer here is to do nothing and leave the bag resonances in your calibration. This is the easiest strategy is actually the easiest to implement. Difficulty: Easy
• Change the regions in your calibration to exclude the polymer resonances. This will perform as if there were no plastic at all. To do this, go into your TQ Analyst chemometric method and simply click and drag until your analysis regions exclude the polymer. This strategy is still easy but involves going into your method to make the change. Difficulty: Intermediate.
• Do an automated spectral subtraction to remove most of the polymer resonances. The tolerances on commercial plastic bag thicknesses is good enough that whatever resonance are left over from the bag after the subtraction will be minimal. You can also set RESULT to reject spectra where the residual is too high. Difficulty: Advanced.