Feasibility of the quantification of respirable crystalline silica by mass on aerosol sampling filters using Raman microscopy

Abstract

Airborne respirable crystalline silica (RCS) is a hazard that can affect the health of workers, and more sensitive measurements are needed for the assessment of worker exposure. To assess the use of Raman microscopy for the analysis of RCS particulate collected on filters, aliquots of quartz or cristobalite suspended in isopropanol were pipetted onto silver filters. Samples were measured by arbitrarily selecting positions along the filter and collecting spectra at 50 discrete points. The calculated limits of quantification on test samples were between 0.066-0.161 and 0.106-0.218 μg for quartz and cristobalite, respectively. Three respirable quartz calibration dusts (A9950, NIST 1878 and Quin 1B) with different mass median aerodynamic particle sizes obtained similar Raman response relationships per unit mass. The difference between NIST 1878 and Quin 1B was not significant (p = 0.22). The intermediate measurement precision of replicate samples was 10-25% over the measured range for quartz (0.25-10 μg) and could potentially be improved. Results from mixtures of quartz and cristobalite were mostly within 10% of their theoretical values. Results from samples of 6% quartz in calcite were close to the theoretical quartz mass. The upper measurement limit for a mixture of 20% RCS in the light absorbing mineral hematite (Fe 2 O 3 ) was 5 μg. These data show that Raman spectroscopy is a viable option for the quantification of the mass of respirable crystalline silica on filters with a limit of detection approaching 1/10th of that obtained with other techniques. The improvement in sensitivity may enable the measurement of particulate in samples from low concentration environments (e.g. inside a mask) or from miniature samplers operating at low flow rates.

Measurement of Crystalline Silica Aerosol Using Quantum Cascade Laser–Based Infrared Spectroscopy

“…This method also offers the ability to pretreat filter samples to minimize matrix interferences (for example, following protocols outlined in ASTM D7948); however such pretratment may increase detection limits beyond 0.3 μg . While recent studies 9 , 10 have shown that new FTIR and Raman instrumentation can provide similar LODs reported here, QCL-based spectrometers offer unique advantages with respect to development of compact, hand-portable, near real-time aerosol sensors.…”

“…Estimated method detection limits (MDLs) for RCS by XRD and IR range from 1 to >10 μg per sample 7 , 8 . Recent studies using new FTIR instrumentation 9 and Raman microscopy 10 have demonstrated LODs below 1 μg . In consideration of lower occupational exposure limits (OELs), better MDLs are desired.…”

An indirect Raman spectroscopy method for the quantitative measurement of respirable crystalline silica collected on filters inside respiratory equipment

“…Measurement conditions were based on those described in a feasibility study using 5 mm diameter deposit area. 11 An In-Via Raman microscope (Renishaw Ltd, Gloucester, UK) with near infrared laser (785 nm) excitation, CCD camera, and a grating with 1200 l mm À1 was used to collect Raman spectra between 314 and 906 cm À1 . This range was set to avoid the broad Raman band below 300 cm À1 , which was attributed to a trace of silver oxide.…”

Recent advances in linear and nonlinear Raman spectroscopy. Part XII

“…Airborne RCS is a hazard that can affect the health of workers, and more sensitive measurements are needed for the assessment of worker exposure. Their data show that Raman spectroscopy is a viable option for the quantification of the mass of RSC on filters with a limit of detection approaching one‐tenth of that obtained with other techniques . Taddei, Pavoni, and Affatato carried out a comparative micro‐Raman study on standard, cross‐linked, and vitamin E‐blended polyethylene acetabular cups after long‐term in vitro testing and ageing.…”

“…Their data show that Raman spectroscopy is a viable option for the quantification of the mass of RSC on filters with a limit of detection approaching one-tenth of that obtained with other techniques. [73] Taddei, Pavoni, and Affatato carried out a comparative micro-Raman study on standard, cross-linked, and vitamin E-blended polyethylene acetabular cups after long-term in vitro testing and ageing. Micro-Raman spectroscopy allowed them to analyze the wear degradation of the cups on a molecular scale, and as a result, differences in the wear mechanisms were revealed.…”