Articlebronze2002

Final report on key comparison CCQM-K7: benzene, toluene, m-xylene, o-xylene and ethylbenzene in nitrogen

Franklin R. Guenther · George C. Rhoderick · Walter R. Miller · Alain Marschal · A Medem · Kenji Kato +8 more

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Scientific Measurement and Uncertainty Evaluation Analytical chemistry methods development Calibration and Measurement Techniques

Abstract

The aim of this key comparison was to compare capabilities for the preparation and value assignment of gas mixture standards for benzene, toluene, m -xylene, o -xylene and ethylbenzene in nitrogen (subsequently referred to as BTX), maintained at national metrology institutes (NMIs). The range of the nominal amount-of-substance fractions of the comparison standard is 50 nmol/mol to 100 nmol/mol, higher than regulations in most countries for permitted ambient quantities. Another BTX comparison (CCQM-K10) is planned to test the amount-of-substance fraction of 5 nmol/mol to 10 nmol/mol, which is closer to regulatory levels. Eight NMIs participated in CCQM-K7. As pilot laboratory, the National Institute of Standards and Technology (NIST, USA) prepared two 30 L primary standard gas mixtures of BTX in nitrogen using normal gravimetric procedures. Eight 6 L aluminium cylinders were filled from these parent cylinders for the comparison study. Each participating laboratory was shipped one 6 L cylinder to be analysed and then returned to the NIST, which reanalysed the cylinder contents to verify whether the compounds had remained stable throughout the comparison. Although there were indications that the gas mixture was unstable in some of the cylinders, the results indicated that agreement for a majority of the participants was within 5 % relative. The largest deviations were associated with the analysis of ethylbenzene, probably due to lack of experience with this compound. Ethylbenzene is not regulated in most countries, and thus not normally analysed on a routine basis. The relatively high concentration of mixtures used in this comparison may also have caused higher deviations. Normal analytical methods in current use are optimized for the 1 nmol/mol to 10 nmol/mol concentration range, and therefore the higher concentrations were outside the optimum range for analysis.

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