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 toprevious comparison exercises, the 17 participants made use of an identicalinstrument set-up, after correcting for temperature offsets with theapplication of a recently developed temperature calibration kit (SunsetLaboratory Inc, OR, US). Temperature offsets reported by participants rangedfrom −93 to +100 °C per temperature step.Five filter samples and two sucrose solutions were analysed with both theEUSAAR2 and NIOSH870 thermal protocols.

z scores were calculated for total carbon (TC); nine outliers and threestragglers were identified. Three outliers and eight stragglers were foundfor EC. Overall, the participants provided results between the warninglevels with the exception of two laboratories that showed poor performance,the causes of which were identified and corrected through the course of thecomparison exercise. The TC repeatability and reproducibility (expressed asrelative standard deviations) were 11 and 15% for EUSAAR2 and 9.2and 12% for NIOSH870; the standard deviations for EC were 15 and20% for EUSAAR2 and 20 and 26% for NIOSH870.

TC was in good agreement between the two protocols, TC_NIOSH870 = 0.98 × TC_EUSAAR2 (R² = 1.00, robust means). Transmittance(TOT) calculated EC for NIOSH870 was found to be 20% lower than forEUSAAR2, EC_NIOSH870 = 0.80 × EC_EUSAAR2 (R² = 0.96,robust means). The thermograms and laser signal values were compared andsimilar peak patterns were observed per sample and protocol for mostparticipants. Notable deviations from the typical patterns indicated eitherthe absence or inaccurate application of the temperature calibrationprocedure and/or pre-oxidation during the inert phase of the analysis. Lowor zero pyrolytic organic carbon (POC), as reported by a few participants,is suggested as an indicator of an instrument-specific pre-oxidation. Asample-specific pre-oxidation effect was observed for filter G, for allparticipants and both thermal protocols, indicating the presence of oxygendonors on the suspended particulate matter. POC (TOT) levels were lower forNIOSH870 than for EUSAAR2, which is related to the heating profiledifferences of the two thermal protocols.