Aerosol Analysis Using Handheld Raman Spectrometer: On-site Quantification of Trace Crystalline Silica in Workplace Atmospheres

A method for aerosol chemical analysis using handheld Raman spectrometer has been developed and its application to measurement of crystalline silica concentration in workplace atmosphere is described. The approach involves collecting aerosol as a spot sample using a wearable optical aerosol monitor, followed by direct-on-filter quantitative analysis of the spot sample for crystalline silica using handheld Raman spectrometer. The filter cassette of a commercially available optical aerosol monitor (designed to collect aerosol for post-shift analysis) was modified to collect 1.5-mm-diameter spot sample, which provided adequate detection limits for short-term measurements over a few tens of minutes or hours. The method was calibrated using aerosolized alpha-quartz standard reference material in the laboratory. Two Raman spectrometers were evaluated, one a handheld unit (weighing less than 410 g) and the other a larger probe-based field-portable unit (weighing about 5 kg). The lowest limit of quantification for alpha-quartz of 16.6 mu g m(-3) was obtained using the handheld Raman unit at a sample collection time of 1 h at 0.4 l min(-1). Short-term measurement capability and sensitivity of the Raman method were demonstrated using a transient simulated workplace aerosol. Workplace air and personal breathing zone concentrations of crystalline silica of workers at a hydraulic fracturing worksite were measured using the Raman method. The measurements showed good agreement with the co-located samples analyzed using the standard X-ray powder diffraction (XRD) method, agreeing within 0.15-23.2% of each other. This magnitude of difference was comparable to the inter- and intra-laboratory analytical precision of established XRD and infrared methods. The pilot study shows that for silica-containing materials studied in this work it is possible to obtain quantitative measurements with good analytical figures of merit using handheld or portable Raman spectrometers. Further studies will be needed to assess matrix interferences and measurement uncertainty for several other types of particle matrices to assess the broader applicability of the method.