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

Self-absorption occurs in near-infrared Fourier transform (NIR-FT) Raman spectroscopy when the absolute frequency of a Raman band coincides with an NIR absorption band of the scattering material. This is manifested by selective attention of specific Raman bands, the degree of which depends on the sample pathlength, concentration and position in the laser beam. These factors can introduce serious errors in quantitative FT Raman applications unless care is taken. This paper describes a simple analysis which allows quantitative predictions of the effects of varying sample concentration and collection optics geometry. The predictions 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 optimizing instruments for quantitative applications whilst maintaining sensitivity and ease of use.