To ensure the accuracy of silica analysis, the sample deposit should be uniform across the filter. Since field samples are not always evenly distributed across the sampling filter, samples may be redeposited onto a different filter for analysis. The sample filter may be ashed (muffle furnace or low-temperature asher) or dissolved (tetrahydrofuran). The silica sample then is suspended in a solvent and redeposited onto an analytical filter by means of a filtration apparatus. Redeposition of the sample is difficult to perform at low sample loadings and requires the laboratory analyst to demonstrate good intralaboratory reproducibility. NIOSH and OSHA analytical methods use a 2-mL "buffer" of solvent in the filtration chimney prior to addition of the suspended sample in order to provide a cushion for more even deposition of the crystalline silica particles. No statistically significant difference in analytical results has been observed between ashing and chemical dissolution of the sample filter [40].
Direct-on-filter techniques are used by the United Kingdom, the European Union and Australia [41]. The sample collection filter is taken from the cassette and analyzed with no sample preparation. These techniques require less time and labor, avoid loss of analyte during sample preparation steps and are amenable to the predominant analytical methods [42]. Direct-on-filter methods should take into account deposition of the sample across the filter (deposition may be non-uniform) when choosing the area of the filter to measure and when comparing results to other methods. Aerosol deposits near the center of the filter are expected to be higher than deposits near edge of the filter. This potential source of error is generally minimized by using the same sampler and sampler conditions to prepare working calibration standards as was used for field sampling. Sample overloading is possible for a sample collected over a full work shift.
Calibration Standards: Calibration issues have proven to be critical in the accurate measurement of crystalline silica. Comparisons of various standard mineral reference materials used for calibration have shown varying degrees of agreement [43, 44, 45, 46]. In a statistical study of reference materials used by analytical laboratories participating in Round 133 of the PAT program, it was observed that use of multiple calibration standards contributed significantly to variability of the analytical results [40, 47]. To ensure accurate measurement of crystalline silica for analyses in the U.S., it is essential that only standard reference materials (SRMs) from the National Institute of Standards and Technology (NIST) (for which both particle size and phase purity have been established) be used to prepare calibration curves for quartz (1878 series, the current being 1878a) and cristobalite (1879 series, the current being 1879a). Efforts are being made to ensure continued availability of the NIST crystalline silica SRMs. No NIST SRM for tridymite is available, since this silica polymorph rarely exists in the workplace. However, a well-characterized sample of tridymite of the appropriate particle size is available from the U.S. Geological Survey and can be used as a reference standard. Tridymite reference material (Ref No. 210-75-0043) may be obtained from Dr. Stephen A. Wilson, U.S. Geological Survey, Box 25046, MS 973, Denver, CO 80225 (telephone: 303-236-2454; FAX 303-236-3200; e-mail: swilson@usgs.gov).
NIST is currently producing and certifying quartz and cristobalite deposited in known amounts (µg/filter) onto 25-mm PVC filters. For quartz, NIST SRM 1878a (quartz powder) deposited onto filters will be available in a "calibration set" of 6 concentration levels. NIST SRM 2950 quartz calibration set will contain 5 blank filters and 5 filters at each of the following target concentrations (µg quartz/filter): 10, 20, 50, 100, 250 and 500. In addition, NIST SRM 2951 will contain 5 blank filters and 5 filters at a target concentration of 5 µg quartz/filter and NIST
