10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Near Real-Time Measurement of Crystalline Silica Aerosol Concentration Using Raman Spectroscopy
LINA ZHENG, Pramod Kulkarni, M. Eileen Birch, Kevin Ashley, Shijun Wei, Centers for Disease Control and Prevention, NIOSH
Abstract Number: 410 Working Group: Instrumentation
Abstract Long-term inhalation of aerosol containing crystalline silica can cause silicosis. Accurate measurement of crystalline silica concentrations is of great significance for the prevention of harmful exposures in industrial workplaces. Current methods have poor detection limits compared to newly imposed exposure limits of 50 μg/m3 in the United States. Moreover, these methods cannot satisfactorily capture short-term exposures (over few minutes) that are typical in many workplaces. Developing sensitive, near real-time, field-portable methods are desired for meaningful exposure measurements and rapid implementation of exposure controls.
We present a Raman spectroscopy based method for measurement of trace airborne concentrations of crystalline silica using various aerosol micro-concentration techniques. Three aerosol micro-concentration techniques were investigated for effective coupling of collected particulate samples with micro-Raman spectroscopy: i) direct analysis on a particulate filter after focused aerosol collection using a converging nozzle, ii) analysis of dried particulate deposit on a filter obtained directly from the aerosol phase using the Spot Sampler™, and iii) analysis of a spot (∼1–3 mm diameter) obtained by redepositing the particulate sample, after low-temperature plasma ashing of an air sample collected on a filter. The deposition characteristics (i.e., spot diameter, shape, and deposit uniformity) of each technique were investigated. Calibration curves were constructed, and detection limits were estimated for α-quartz using the A1 Raman Si-O-Si stretching – bending phonon mode at 465 cm-1. The measurement sensitivity could be substantially improved by increasing the signal integration time and by reducing the particle deposition area. Detection limits in the range of 8–55 ng could be achieved by micro-concentrating the aerosol sample over a spot measuring 400–1000 μm in diameter. The low detection limits suggest that near real-time measurements of crystalline silica could be achieved with limit of quantification in the range 2–18.5 μg/m3 at a time resolution of approximately 10 minutes at a sampling flowrate of 1.2 L/min. These detection limits were two to three orders of magnitude lower compared to those attainable using current standardized X-ray diffraction and infrared spectroscopy methods. The method was successfully extended to the measurement of α-quartz concentrations in representative workplace aerosol samples. This study demonstrates potential of portable micro-Raman spectroscopy for near-real time measurement of trace crystalline silica in air.