The performance of laboratories analysing alpha-quartz in the Workplace Analysis Scheme for Proficiency (WASP)

The Workplace Analysis Scheme for Proficiency (WASP) is a proficiency testing (PT) scheme for the analysis of occupational hygiene and environmental air samples and is operated in the United Kingdom by the Health and Safety Laboratory (HSL) on behalf of the Health & Safety Executive (HSE). One of the 26 analytes available to laboratories is silica (alpha-quartz) on 25 mm Gelman GLA5000 filters. This paper investigates the performance of laboratories participating in the scheme since the HSL took over the production of the samples in 1998. The average relative standard deviation (RSD) of results obtained by a laboratory is 11.5%. This is reduced to 8.5% when the values from laboratories using indirect analytical methods are excluded. Laboratories using indirect analytical methods accounted for some of the most variable data. For the on-filter analytical methods the data suggested a relationship between relative standard deviation and loading that increased gradually from +/-4% at high analyte levels to +/-10-15% at low levels. The average precision estimate for the on-filter analytical methods was found to be 5.6% RSD for the infrared technique and 6.7% RSD for the X-ray diffraction technique. These figures compare favourably with those reported in the published HSE methods. No significant difference was found between the average result reported by laboratories using on-filter infrared (IR) analysis and the average result reported by laboratories using on-filter X-ray diffraction (XRD) analysis. An ANOVA analysis found the repeatability estimate was just as large as the 'between laboratory' variation for both the XRD and IR on-filter analysis techniques. When a limited number of 'realistic' samples were included in the scheme, XRD analysis was found to perform slightly better than IR analysis. The performance of laboratories in the WASP scheme compares very favourably with other published data from a PT scheme where indirect silica analytical methods are predominately used.