QUANTITATIVE STUDY OF SELF-ABSORPTION IN NEAR-INFRARED FOURIER-TRANSFORM RAMAN-SPECTROSCOPY

Self-absorption occurs in near-infrared Fourier transform (NIR-FT) Ram an spectroscopy when the absolute frequency of a Raman band coincides with an NIR absorption band of the scattering material. This is manife sted by selective attention of specific Raman bands, the degree of whi ch depends on the sample pathlength, concentration and position in the laser beam. These factors can introduce serious errors in quantitativ e FT Raman applications unless care is taken. This paper describes a s imple analysis which allows quantitative predictions of the effects of varying sample concentration and collection optics geometry. The pred ictions for concentration variation have been validated by experiment. A consideration of the experimental design required to minimize self- adsorption indicates that this will also reduce instrument sensitivity and ease of alignment; therefore, there is a trade-off between optimi zing instruments for quantitative applications whilst maintaining sens itivity and ease of use.