A proficiency testing scheme for aromatic hydrocarbons in air by the manual thermal desorption-GC method: a comparison of laboratory performance withthe uncertainty requirements of the European Union Ambient Air Directive

The Workplace Analysis Scheme for Proficiency (WASP) is a proficiency testi ng scheme for the analysis of occupational hygiene and environmental air sa mples and is operated in the UK by the Health and Safety Laboratory. Since 1997, WASP has offered samples of benzene, toluene and m-xylene, at environ mental levels on Tenax, and has about 35 laboratories participating, mostly from industry, local government and consultancy organisations in the UK. T he results reported cover the first 10 rounds of the environmental analytes (1997-1999) and demonstrate the important role of proficiency testing in a ssessing the quality of laboratory performance. Estimates are obtained for within-laboratory precision and the total variability at each analyte level . The estimates of within-laboratory precision suggest that laboratories ha ve more difficulty analysing toluene and m-xylene than benzene. Linear rela tionships for the reproducibility relative standard deviation (RSDT) with l oading level are evident for the analytes at occupational levels. At enviro nmental levels, the relationship between loading level and reproducibility is much less well defined. The standard deviation for the proficiency testi ng assessment for all three analytes at the environmental level is 14%, as derived from the benzene data. Expanded uncertainty estimates (k = 1.96), f or the analysis of samples since the scheme started, are obtained from the average total variance, and are 27% for benzene, 39% for toluene and 36% fo r m-xylene. Although the linear trend of performance against round number w as not significant at the 95% level of confidence (p = 0.23 for benzene, p = 0.3 for toluene and p = 0.32 for m-xylene), there was a general improveme nt in RSDT from 26-34% to about 8-13% 10 rounds later. Currently, for a lab oratory to meet one of the data quality objectives in the Ambient Air Direc tive (indicative measurement of benzene, expanded uncertainty +/- 30% or le ss), it would have to achieve a level of analytical performance to satisfy the category 1 (best performance) limit of better than +/-8.8%. In the last proficiency testing round, discussed in this paper, only 58% of laboratori es obtained performance scores that indicated that they were able to consis tently achieve this level of performance.