Further round-robin tests to improve the comparability between laboratories of the measurement of carbon in diesel soot and in environmental samples

Objectives: An informal European coordination group organized two round-rob in tests on filters collected from environmental, workplace and diluted die sel emissions. Previous inter-laboratory comparisons have shown that experi mental samples give reasonably good results in terms of the dispersion arou nd the mean, from all the participating laboratories. However, there were s ignificant differences between the laboratories owing to a narrow distribut ion of the results within a single laboratory. In order to gain a better un derstanding of the differences obtained between the laboratories, it was de cided to carry out more round-robin tests and to investigate further the po ssible factors which may influence the results. Methods: The first round-ro bin (RRT3) was performed on six different samples (eight replicates) analyz ed by ten laboratories. The range of loading was 40 to 138 mug cm(-2) of to tal carbon (TC). Laboratories used their own thermal procedure parameters. The second round-robin test (RRT4) was performed on three different diluted diesel emissions (two replicates) samples analyzed by 13 laboratories. The range of loading was 21 to 37 mug cm(-2) TC. Laboratories analyzed samples using imposed temperatures (500, 650 and 800 degreesC) and imposed duratio n (12 min). Results: Inter-laboratory coefficients of variation for diluted diesel emission samples were 10% for RRT3 and ranged from 6 to 19% for RRT 4. The influence of the desorption temperature was clearly demonstrated and the results tended to show that a desorption temperature of 650 degreesC c ould be an acceptable compromise. The influence of the organic carbon/eleme ntal carbon (OC/EC) ratio was shown to be insignificant with pure diesel so ot samples. Conclusions: It was expected that a significant improvement wou ld be seen in the inter-laboratory dispersion by the use of a common standa rdized thermal desorption program, but the objectives of these RRTs were on ly partly reached. This paper provides new information that will be useful in the elaboration of a standardized procedure for the European Normalisati on Centre (CEN TC 137 WG2 - General requirements for measuring procedures).