Comparison of diffuse versus inverse spatially-offset Raman spectroscopy modalities for analyte detection through barriers

Abstract

Abstract Spatially-offset Raman spectroscopy (SORS) is gaining an increasing popularity in detection of analytes through barriers. SORS can be applied in a number of different excitation modes and head-to-head comparison of different SORS modes are limited in the literature. In this study, a custom system that is capable of switching between two spatially offset Raman excitation modes, namely inverse (iSORS, ring) and diffuse (dSORS, enlarged flat-top circle), were compared at the same power for depth-probing Raman spectroscopy applications. Specifically, depth of detection, signal-to-noise ratio, and intensity ratio as a measure of elimination of barrier contribution to the Raman spectra were compared between the two methods. Transparent and opaque polymeric materials as well as skin tissue were used as barriers to characteristic Raman peaks from other polymers, pharmaceutical tablets or joint-borne gout crystals. Results indicated that diffuse SORS mode provides signal collection from depth layers at significantly greater intensities and at higher signal-to-noise ratio when compared to iSORS. On the other hand, iSORS generally provided a higher depth layer to barrier layer intensity ratio, indicating that iSORS is more effective than diffuse SORS in terms of the elimination of barrier layer contribution. We also demonstrate that barrier signal can be eliminated in diffuse-SORS mode as well by reducing collection zone area using a variable pinhole. Overall, the results of this study indicate that diffuse SORS mode to be useful in detecting the analyte particularly when the barrier layer is thick, or, when presence of barrier layer peaks in a spectrum does not confound the analysis.