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 Six sets of air samples were collected. Two precleaned quartz-fiber filters were loaded into each of eight 37-mm cassettes. In four of the these, a Teflon filter was then placed on top. The bottom quartz filters in all eight cassettes, as well as the four directly under the Teflon filters, provided measures of adsorbed OC. Filters in four additional cassettes served as passive field blanks, which meant that no air was pulled through them. The second (bottom) quartz filter in the cassettes with the Teflon filters were compared to the bottom quartz filters in those having only a quartz pair. If negligible adsorption occurs on Teflon, the bottom quartz filters in the two different sampling configurations should give equivalent results. To avoid possible variability due to lot-to-lot differences [67], filters from the same lot were used to collect a given sample set. Portable dust chamber. A portable dust chamber [69] designed for sampling in mines was used for sample collection. The chamber allowed simultaneous collection of up to eighteen samples. Twelve cassettes were mounted inside the chamber in a symmetrical fashion. Samples were collected on six different days, five days in a loading dock area and a sixth near an outside smokers’ shelter. A diesel truck was operating in the loading dock, over different periods, on three of the days. Personal air pumps were programmed to run at 2 L/min over an 8-hour period. In one case, the pumps were stopped after 23 minutes. The short sampling period provided a low loading of diesel exhaust (a light deposit was visible) and was of interest with respect to the amount of adsorbed carbon collected in the short time frame. Results for the six sample sets are shown in Table 1 and Figure 3. The QQ2 results correspond to the bottom quartz filters in the four cassettes containing quartz pairs only. The TQQ1 results are for the four filters directly beneath Teflon filters; TQQ2 results are for quartz filters beneath the TQQ1 filters (i.e., top Teflon, middle TQQ1, bottom TQQ2). The mean carbon on the bottom quartz filters ranged from 1.35 to 3.44 :g/cm2, and the variability (RSD) ranged from about 4% to 12% on a given day. The loading found on the two sets of bottom filters (QQ2 and TQQ2) was not statistically different, so all bottom filters were pooled in the calculation of the mean. The pooled means for the bottom quartz filters (BQ), the means for the quartz under Teflon (TQQ1), and the corresponding OC and EC loadings are plotted in Figure 3. Results for the TQQ1 filters were only slightly higher than those for the bottom quartz filters, even with the 23-minute sampling period (set 4). Thus, using a separate cassette that has a top Teflon filter appears unnecessary. Although differences between the amount of carbon adsorbed on a quartz filter under another quartz and on one beneath a Teflon filter reportedly can be significant [65, 67], these studies concerned environmental monitoring at much higher flow rates. The negligible differences reported here most likely relate to the lower flow rates and filter face velocities used.

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NIOSH Manuual of Analytical Methods