Abstract
Inhalation exposure to airborne respirable crystalline silica (RCS) poses major health risks in many industrial environments. There is a need for new sensitive instruments and methods for in-field or near real-time measurement of crystalline silica aerosol. The objective of this study was to develop an approach, using quantum cascade laser (QCL)-based infrared spectroscopy (IR), to quantify airborne concentrations of RCS. Three sampling methods were investigated for their potential for effective coupling with QCL-based transmittance measurements: (i) conventional aerosol filter collection, (ii) focused spot sample collection directly from the aerosol phase, and (iii) dried spot obtained from deposition of liquid suspensions. Spectral analysis methods were developed to obtain IR spectra from the collected particulate samples in the range 750–1030 cm−1. The new instrument was calibrated and the results were compared with standardized methods based on Fourier transform infrared (FTIR) spectrometry. Results show that significantly lower detection limits for RCS (≈330 ng), compared to conventional infrared methods, could be achieved with effective microconcentration and careful coupling of the particulate sample with the QCL beam. These results offer promise for further development of sensitive filter-based laboratory methods and portable sensors for near real-time measurement of crystalline silica aerosol.
Highlights
Current standardized methods for measuring airborne respirable crystalline silica (RCS) employ filter-based collection using a size-selective sampling device for collecting the respirable fraction[6] of aerosol, followed by laboratory analysis using either X-ray diffraction (XRD)[7] or infrared (IR) spectrometry[8]
We present a new technique, using quantum cascade laser (QCL)15 -based infrared spectroscopy, with applications to in-field or near real-time measurement of airborne RCS
B, involving direct aerosol collection via a nozzle, leads to a nearly circular deposit in the filter center containing most of the RCS sample mass, with an approximate diameter of 0.5 mm
Summary
Current standardized methods for measuring airborne RCS employ filter-based collection using a size-selective sampling device for collecting the respirable fraction[6] of aerosol, followed by laboratory analysis using either X-ray diffraction (XRD)[7] or infrared (IR) spectrometry[8]. Used standardized methods for sampling and analysis of RCS by IR and XRD suffer from several drawbacks, including inconsistent analytical figures of merit at levels below OELs of interest and large sample collection times[13]. These filter-based methods were mainly developed for regulatory compliance measurement, rather than transient monitoring of worker exposures, and they may not be capable of capturing the short-term but high RCS exposures typical in many construction, mining, and other industrial activities[14]. QCLs offer analytical dynamic ranges over several orders of magnitude Their miniature size and robust performance makes them suitable for field portable instrumentation. Quantitative QCL measurement of RCS is demonstrated for masses exceeding 350 μg per sample, and MDLs
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