Toward rapid silica analysis of CPDM samples: Deposition of recovered dust and analysis by FTIR

The ongoing resurgence of severe Coal Workers' Pneumoconiosis in the US has been linked to overexposure to respirable crystalline silica (RCS, which is predominantly present as quartz and regulated as such). Capabilities that enable more frequent RCS monitoring are highly sought. Recent developments include field-based quartz analysis of traditional filter samples-collected on polyvinyl chloride (PVC) filters-using portable Fourier Transform Infrared spectroscopy (FTIR). However, most respirable dust samples in US coal mines are collected with a continuous personal dust monitor (CPDM) that enables real-time tracking of total respirable dust mass concentration. FTIR cannot directly analyze the collected dust sample due to the materials and construction of the sampling substrate. To address this issue, a simple three-step method was envisioned wherein the dust could be recovered into a suspension, redeposited onto a PVC filter using a syringe filter apparatus, and then analyzed by FTIR. The current study was conducted to develop the redeposition and analysis steps. It specifically considers the issues of the PVC filter size and deposition pattern yielded by typical filtration apparatuses and the FTIR scanning locations to establish a model that predicts quartz mass from the spectral data. Of the options tested here, the following combination was found to be optimal: 25-mm PVC filter with dust deposition using an inline syringe filter holder (which yields a "wheel and spoke" pattern), and FTIR analysis at four center-offset locations (90° apart, 8-mm from the center) from which the spectral data were averaged. Under these conditions, the predicted quartz mass on filters with respirable dust deposited from one of two geologic source materials (i.e., representing real coal mine silica sources) was observed to have a standard error of 0.011 mg (11 µg) for samples with an expected quartz mass of less than 0.150 mg (which equated to a total sample mass of less than about 1.5 mg). For samples with higher expected quartz masses, standard error increased, suggesting that dust deposition becomes less uniform with increasing total sample mass.