Evaluation of Direct-on-Filter Methods for the Determination of Respirable α-Quartz

Abstract

Two direct-on-filter methods for the determination of respirable α-quartz in air by X-ray powder diffraction (XRD) were compared with NIOSH Method 7500. The two direct-on-filter methods are direct-on-silver membrane filter and direct-on-mixed cellulose ester (MCE) filter with XRD analysis directly on the collection filter. Both the silver membrane and the mixed cellulose ester methods were subjected to a side-by-side comparison with NIOSH Method 7500 using a laboratory-generated atmosphere of mine dust. The comparison showed with 95 percent confidence that any single measurement by the direct-on-MCE method would produce a result within 25 percent on any single measurement of Method 7500 under the same experimental conditions over the range 200–1200 μg. The comparison also showed the direct-on-silver method would produce a result within 30 percent, with 95 percent confidence over the same range. There is some suggestion of a slight negative bias of the direct-on-silver method with respect to Method 7500 which may be due to difference in the particle size distribution between the mine dust of the collected samples and National Bureau of Standards Standard Reference Material 1878 (NBS SRM 1878) of the calibration samples.

Calibration curves were obtained from two different procedures. One curve was prepared by directly depositing α-quartz (NBS SRM 1878) aerosol directly on the filter, whereas the other curve was prepared by filtration of an isopropanol suspension of the same reference material on the filters. Although there was no difference in particle size distribution, there was a marked difference in the two curves. This dictates that samples must be referred to calibration curves which were prepared in an analogous manner. For the preparation of direct-on-filter calibration samples, a small aerosol generation system is presented.