b. Instrumentation Of the possible approaches for OC-EC analysis, a thermal-optical technique was investigated because it offers greater selectivity (a pyrolysis correction for char) and flexibility (automated analysis, programmable parameter files) than previously used methods. While thermaloptical methods have not been widely used in the industrial hygiene field prior to its proposed use for diesel exhaust monitoring, they have been routinely applied to environmental monitoring of particulate carbon air pollution. The thermal-optical analyzer has been described previously [34, 59]. Design improvements have since been made, but the operation principle remains unchanged. The analyzer (Figure 1) is equipped with a pulsed diode laser and photodetector that permit continuous monitoring of the sample filter transmittance. This optical feature corrects for the char formed during the analysis of some materials. As in samples where EC is initially present, char strongly absorbs light, particularly in the red/infrared region. Char is formed through pyrolysis, which is a thermal decomposition process. When some organic substances are heated to elevated temperatures in an inert (non-oxidizing) atmosphere, carbonization (conversion to carbon) occurs. Both volatile products and char (decomposition product containing mostly carbon) are formed in the process, which can begin at temperatures as low as 300 o C. In the thermal-optical analysis, a filter portion (punch) of known area (typically 1.5 cm2 ) is placed in the sample oven, and the oven is tightly sealed. Quartz-fiber filters are required because temperatures in excess of 850 o C are employed. The analysis proceeds in inert and oxidizing atmospheres. In both, the evolved carbon is catalytically oxidized to carbon dioxide (CO2 ). The CO2 is then reduced to methane (CH4 ), and CH4 is quantified with a flame ionization detector (FID). OC (and carbonate, if present) is first removed in helium, as the temperature is increased to a preset maximum (about 870 o C, NMAM 5040). If charring occurs, the filter transmittance decreases as the temperature is stepped to the maximum. After OC is removed, an oxygenhelium mix is introduced to effect combustion of the remaining material. As light-absorbing carbon is oxidized from the filter, the filter transmittance increases. The split (Figure 2) between the OC and EC is assigned when the initial (baseline) value of the filter transmittance is reached. All carbon removed before the OC-EC split is considered organic, and that removed after the split is considered elemental. If no charring occurs, the split is assigned prior to removal of light-absorbing carbon. If the sample chars, the split is not assigned until enough light-absorbing carbon is removed to bring the transmittance back up to its initial value. In general, char is more readily oxidized than diesel-particle EC. The delay (i.e., the transit time from sample to FID) between the laser and FID signals is considered in the split assignment. Ordinarily, the split is assigned in the oxidative mode of the analysis. EC and OC results are reported in micrograms per square centimeter (:g/cm2 ) of the sample deposit. The total OC and EC on the filter are calculated by multiplying the reported values by the deposit area. In this approach, a homogeneous deposit is assumed. For triplicate analyses, the precision (relative standard deviation) is normally under 5%, and it is typically
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NIOSH Manuual of Analytical Methods
