ECOC comparison exercise with identical thermal protocols after temperature offset correction : instrument diagnostics by in-depth evaluation of operational parameters

A comparison exercise on thermal-optical elemental carbon/organic carbon (ECOC) analysers was carried out among 17 European laboratories. Contrary to previous comparison exercises, the 17 participants made use of an identical instrument set-up, after correcting for temperature offsets with the application of a recently developed temperature calibration kit (Sunset Laboratory Inc, OR, US). Temperature offsets reported by participants ranged from −93 to +100 ◦C per temperature step. Five filter samples and two sucrose solutions were analysed with both the EUSAAR2 and NIOSH870 thermal protocols. z scores were calculated for total carbon (TC); nine outliers and three stragglers were identified. Three outliers and eight stragglers were found for EC. Overall, the participants provided results between the warning levels with the exception of two laboratories that showed poor performance, the causes of which were identified and corrected through the course of the comparison exercise. The TC repeatability and reproducibility (expressed as relative standard deviations) were 11 and 15 % for EUSAAR2 and 9.2 and 12 % for NIOSH870; the standard deviations for EC were 15 and 20 % for EUSAAR2 and 20 and 26 % for NIOSH870. TC was in good agreement between the two protocols, TCNIOSH870 = 0.98 × TCEUSAAR2 (R 2 = 1.00, robust means). Transmittance (TOT) calculated EC for NIOSH870 was found to be 20 % lower than for EUSAAR2, ECNIOSH870 = 0.80 × ECEUSAAR2 (R 2 = 0.96, robust means). The thermograms and laser signal values were compared and similar peak patterns were observed per sample and protocol for most participants. Notable deviations from the typical patterns indicated either the absence or inaccurate application of the temperature calibration procedure and/or pre-oxidation during the inert phase of the analysis. Low or zero pyrolytic organic carbon (POC), as reported by a few participants, is suggested as an indicator of an instrument-specific pre-oxidation. A sample-specific preoxidation effect was observed for filter G, for all participants and both thermal protocols, indicating the presence of oxygen donors on the suspended particulate matter. POC (TOT) levels were lower for NIOSH870 than for EUSAAR2, which is related to the heating profile differences of the two thermal protocols.