SRM 2958 will contain 5 blank filters and 5 filters at a target concentration of 1000 µg quartz/filter. For cristobalite, NIST SRM 1879a (cristobalite powder) deposited onto filters will be available in a "calibration set" of 6 concentration levels. NIST SRM 2960 cristobalite calibration set will contain 5 blank filters and 5 filters at each of the following target concentrations (µg cristobalite/filter): 5, 10, 20, 50, 100 and 250. In addition, NIST SRM 2967 will contain 5 blank filters and 5 filters at a target concentration of 500 µg cristobalite/filter. These filters can facilitate preparation of routine working standards at known concentration levels. These filters may also be useful in preparation of quality assurance samples.
Industrial hygiene laboratories use a variety of analytical techniques for the quantitative determination of quartz, cristobalite and tridymite [41,48]. The three most commonly used are X-ray diffraction spectrometry (XRD), infrared absorption spectrometry (IR) and colorimetric spectrophotometry. The sensitivities of these three techniques are influenced by the sample particle size. X-ray diffraction intensity varies considerably with particle size, smaller particles (0.2 to 2.0 µm) showing lower intensities [49]. Peak broadening and extinction effects are related to particle size; there is a decrease in intensity at larger particle diameters (>25 µm) due to extinction [50-52]. In addition, the X-ray diffraction intensity is affected by the fact that smaller particles contain a lower proportion of crystalline material because the amorphous surface layer makes up a higher proportion of the total. The infrared absorption response of both quartz and cristobalite is particle size dependent [53,54]. The response increases as particle size decreases to about 1.5 µm; below about 1.5, the response decreases due the presence of an amorphous surface layer. If a sample contains an appreciable portion of amorphous silica, neither XRD nor IR is entirely adequate, and both methods should be used for accurate quantification [55]. Colorimetric methods typically include a precisely timed heating step in which phosphoric acid is used to dissolve amorphous silica. The acid digestion may result in the loss of some of the smaller crystalline silica particles [40]. Since particle size and amorphous layer affect all three techniques, matching the particle size and phase purity of calibration standards with field samples is very important to minimize analytical bias. Because NIST SRM 1878 series (the current being SRM 1878a) consists of quartz with a distribution of particle sizes that is intended to be representative of respirable particles sampled, use a of respirable dust sampler (such as the Dorr-Oliver 10-mm nylon cyclone) should provide a collected dust sample which matches the calibration curve of the SRM [56].
