Direct-on-filter Raman spectroscopy for quantifying respirable crystalline silica: Analytical challenges and practical considerations.

This study evaluates the feasibility of Raman spectroscopy for quantitatively analyzing respirable crystalline silica (RCS) in coal dust samples using a direct-on-filter (DoF) approach. Key challenges include analytical sensitivity, spatial and chemical nonuniformity, fluorescence interference, and laser-induced heating effects. Although Raman spectroscopy has been proposed as a sensitive method for RCS measurement, a review of the literature shows that, when expressed as RCS mass per deposit area, the limits of quantification (LOQ) for portable Raman spectrometers (0.21-3.25 µg/mm2) are often higher than those reported for Fourier transform infrared spectroscopy (FTIR) (0.018-0.152 µg/mm2). Consequently, quantifying RCS on conventional 37 mm diameter filters is challenging due to insufficient areal loading. Concentrating samples onto a 9 mm diameter deposit area using a filter holder adapter increases Raman signal intensity by more than ten-fold, significantly enhancing sensitivity. Raman microscopes, which have small integration areas, are susceptible to deposit nonuniformity, whereas portable Raman spectrometers with larger laser beam diameters reduce this effect but remain sensitive to chemical heterogeneity. Laser-induced heating and photodegradation can remove organic compounds directly on the filter, thereby reducing both background signal and fluorescence and improving RCS measurement. These findings provide insights into the feasibility of Raman spectroscopy for RCS quantification, particularly in complex sample matrices such as coal dust.