Tandem Raman and Elemental Aerosol Spectrometer (TREAS) for Near-Real-Time Aerosol Chemical Speciation

Current methods for quantifying aerosol chemical exposures rely primarily on filter-based collection and off-line laboratory analysis, resulting in significant delays in chemical risk assessment. We describe the development and evaluation of the Tandem Raman and Elemental Aerosol Spectrometer (TREAS), a compact instrument designed for the near-real-time measurement of trace elemental and molecular speciation in workplace atmospheres. TREAS employs an automated, cyclical "collection-analysis-ablation" scheme, in which aerosols are focused onto a dry spot, analyzed by Raman spectroscopy (RS), and subsequently ablated by pulsed spark discharges for elemental quantification via spark emission spectroscopy (SES). Laboratory evaluation using occupationally relevant aerosols in the PM_2.5 size fraction, specifically crystalline silica (α-quartz) and titanium dioxide (TiO₂), demonstrated mass limits of detection (LOD) that were significantly lower than those of standardized X-ray diffraction and infrared methods. The LODs for α-quartz were 420 ng (RS) and 900 ng (SES), while LODs for TiO₂ were 155 ng (RS) and 250 ng (SES). Beyond quantification, the tandem approach provided complementary chemical insights: while SES quantified trace elements, RS can successfully distinguish between the chromium(VI) and chromium(III) oxidation states in welding fumes. The study demonstrates the potential of TREAS to provide high-sensitivity, automated, and analyte-specific monitoring of complex aerosol exposures in near real-time.