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 the OC (TC) results to some degree, depending on their relative concentrations and the sampling location. Although a 37-mm cassette is often suitable for general industry, the sampler required must be dictated by the sampling environment. e. Carbonates The presence of carbonate is indicated by a narrow peak during the fourth temperature step in helium (Figure 2). Its presence is verified by exposing a second portion of the filter to hydrogen chloride (HCI) vapor prior to analysis. When the acidified portion is analyzed, a diminished (or absent) peak during the fourth temperature step is indicative of carbonate in the original sample. (Note: Acid treatment may sometimes alter the appearance of the EC profile in the thermogram [output signal of thermal-optical instrument], but the EC result itself should not be affected significantly.) A desiccator containing concentrated HCI (added to the desiccator or a petri dish placed at the bottom of it) can be used to acidify the sample portions. The desiccator, or alternative vessel, should be used in a well-ventilated hood. The filter portions are placed on the desiccator tray, and the tray is placed in the desiccator. A wetted pH indicator stick can be used to check acidity. A wetted indicator stick inserted between the desiccator lid and base should give a pH near 2. Portions should be exposed to the acid vapor for about 1 hour (large particles can require more time). After acidification, the tray is placed on a clean, inert surface inside the hood. The residual acid on the portions should be allowed to volatilize in the hood for at least an hour prior to analysis. Environmental samples typically contain little (if any) carbonate, but levels in some occupational samples can be quite high. For example, respirable dust samples collected in limestone and trona mines can contain high levels of calcium carbonate and sodium sesquicarbonate, respectively. In such cases, acidified samples give a better measure of the diesel-source OC (TC). If the carbonate loading is relatively high (e.g., carbonate carbon > 10% of the TC), the difference between the TC results (before and after acidification) gives an estimate of carbonate carbon (CC). Estimation of CC by difference assumes the carbonate is evenly deposited on the filter, which may not always be the case. It also assumes that OC loss through acidification is negligible. The latter assumption is generally true for workplace samples, but depends on sample composition and loading. Alternatively, a carbonate-subtracted OC (TC) result can be obtained through separate integration of the carbonate peak. This applies only to carbonates (e.g., calcium carbonate) that can be removed as a single peak during the fourth temperature step in helium. The carbonate of interest should be analyzed to ensure this is the case. A minor adjustment (lower) to the third temperature step may be necessary to prevent partial loss during this step. Sodium sesquicarbonate (trona) is removed over multiple temperature steps. Samples containing trona should be acidified if a carbonate-subtracted result is desired. (Note: At elevated temperature, trona and other compounds containing sodium can etch the quartz oven wall and sample holder. An etched surface can reduce laser throughput. Avoid direct contact of these materials with the oven wall and sample holder.)

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