RESUMO
An axially perpendicular offset (APO) scheme based on an axially perpendicular geometrical arrangement of laser illumination and photon detection, enabling spatially offset Raman spectroscopy (SORS), is proposed as a versatile tool for the minimization of the glass background in direct measurements of Raman spectra of samples housed in glass bottles. This strategy is based on the possibility of isolating glass photons from sample photons by properly locating a detector beneath the sample-housing bottle, because glass photons are much more localized near the glass wall while sample photons are widely distributed throughout the bottle. In addition, the curved bottom of the glass vial enabling forming the conical photon-detection volume would be further effective in exclusion of the glass photons in the acquisition of sample spectra. The APO scheme was validated by measuring the Raman spectra of 66% ethanol housed in four glass bottles of different sizes and colors; the measurements were performed by varying the offset distance from 2 mm to 20 mm. The intensity of the glass background decreased rapidly with increasing the offset distance; on the other hand, the variation in the ethanol intensity was relatively insignificant. In all cases, the offset distance of 16 mm minimized the presence of glass background in the spectra, thereby helping to highlight the pure ethanol bands and producing nearly similar sample spectral features regardless of contained bottles. The results of the Monte Carlo simulation were in accordance with the experimental observations, and the suppression of glass photons in the APO scheme was clearly explained and visualized by the simulation.
