NMAM 5040 results, these laboratories reported high EC contents for the two OC standards (about 52% for sucrose and 70% for EDTA). Similar findings (i.e., positive bias) for thermal methods having no char correction were obtained recently in another international round robin [72]. In contrast to round robin results obtained previously [35], relatively good agreement was seen in a recent comparison [78] between NMAM 5040 and a thermal method (ZH 1/120.44) used in Germany. The comparison was limited to two laboratories. Method ZH 1/120.44 specifies a 550 °C maximum in nitrogen. The other European laboratories that participated in the previous round robin [35] used variations of this method. In the European laboratories, nitrogen is used as the inert gas, and carbon determination is based on coulometric titration of carbon dioxide. For the comparison [78], samples were obtained in a mine where diesel equipment was being operated. The samples had a much higher EC content (about 50%) than did the round robin samples. No charring was noted in the thermograms, and only a minor amount of carbon was removed above 550 °C. Although differences in the OC-EC results were again seen, they were minor relative to those obtained in the round robin [35]. The mean EC fractions (EC:TC) found with methods ZH 1/120.44 and NMAM 5040 were 0.53 (F = 0.19) and 0.46 (F = 0.15), respectively. The relatively minor difference in the reported fractions was attributed to the different thermal programs employed. IMPROVE method. Another thermal-optical method, called the IMPROVE (Interagency Monitoring of Protected Visual Environments) method [79], also was included in the NMAM 5040 round robin [35]. Relatively good agreement between NMAM 5040 and the IMPROVE method was obtained, although the IMPROVE EC was consistently a bit higher. The carbon analyzers used for the two methods are based on similar measurement principles, but they differ with respect to design and operation. For example, the optical correction in NMAM 5040 is based on filter transmittance, whereas that for the IMPROVE method is reflectance based. Different components are used to monitor these signals. The instrument (Sunset Laboratory, Inc., Forest Grove, OR) used for NMAM 5040 incorporates a pulsed diode laser (670 nm) and photodetector positioned on opposite sides of the filter. The instrument (Desert Research Institute, Reno [DRI], NV) for the IMPROVE method uses a quartz tube and fiber optic to measure helium–neon laser light (632.8 nm, unmodulated) reflected from the filter surface. In addition to instrumental differences, NMAM 5040 specifies a higher maximum temperature (870 °C) in helium than the IMPROVE method (550 °C). As discussed in the preceding section, a higher temperature is used to better remove refractory OC components and carbonates. In the NMAM 5040 analysis, the transmittance of some samples continues to decrease as the temperature is stepped to 870 °C, which indicates charring is not complete at 550 °C. More exhaustive charring results in a lower EC result because the correction for it is larger, and volatile pyrolysis products can evolve to a greater extent. Although environmental samples contain only small amounts (if any) of carbonate, levels in some workplaces (e.g., mines, construction sites) can be relatively high. Collection of carbonate can be prevented (or minimized) through use of an impactor [62]. Two comparisons [73, 74] between the NMAM 5040 and IMPROVE methods were conducted recently. In one [73], the NMAM 5040 EC was typically less than half the IMPROVE EC, but a Sunset Laboratory instrument was not used. Instead, samples were analyzed on a prototype instrument by running two different thermal programs [73]. The results reported for NMAM 5040 (emulated on DRI instrument) may not be representative of those obtained
3/15/03
246
NIOSH Manuual of Analytical Methods
