Determination of Crystalline Silica in Dust at Low Concentrations by Low-Temperature Infrared Spectrometry.

The American Conference of Governmental Industrial Hygienists (ACGIH) accepted a lower threshold limit value (TLV) for respirable crystalline silica (RCS) exposure of 25 μg/m3, half of the previous TLV. This change is problematic because the current standard sampling and measurement practices used by NIOSH, OSHA, and MSHA are not sensitive enough to allow an analyst to confidently determine samples acquired near the TLV. In response to this need for a more sensitive method to analyze respirable dust filter samples for crystalline silica, a modification of current NIOSH infrared spectrometric methods is being developed. The additional sensitivity is gained by performing the infrared absorbance measurements at 77 K where absorbance peaks are more intense by virtue of being narrower. A quick-change cryostat has been fabricated such that a sample can be introduced to the spectrometer and cooled to 77 K in 5 min, interrogated for 1 min, and removed and the cryostat readied for another sample in 2 min, for a turnaround time of 8 min per sample, which is brief compared to the time required to prepare and redeposit a sample. Therefore, samples can be acquired and interrogated with legacy samplers, filters, pumps, spectrometers, and sample preparation, the only modification being the addition of a cryostat to the spectrometer. Preliminary experiments demonstrate that the peak-to-background ratio of the quartz signature band near 800 cm-1 increases by approximately 50 % on cooling from room temperature to 77 K. The slopes of the calibration curve derived from standards interrogated at both room temperature and 77 K indicate that the low-temperature method is approximately 25 % more sensitive.