Performance of methods for measurement of exposure to inorganic acids in workplace air

BGIA has organised round robins for the analysis of samples of inorganic acids in workplace air for a number of years. Test samples of the volatile acids HCl and HNO3 are collected from a standard atmosphere and samples of the non-volatile acids H3PO4 and H2SO4 are prepared by spiking filters with acid solution. The last two round robins have also covered the sampling of volatile acids, with up to 15 active participants able to visit the test facility in Dresden and take samples themselves. For other passive participants, BGIA takes samples from the same atmosphere. The acid concentrations generated lie between 0,1 and 1 times the German limit values for HCl and HNO3. The results for the last round robin showed no significant difference between the performance of the active and passive participants. The participant means were in good agreement with the theoretical concentrations and the quality control measurements. For active participants RSDs were between 7% and 14% and for all participants between 8% and 16%. The round robin for the non-volatile acids showed similar results. The participant means were again in excellent agreement with the quality control measurements and RSDs were between 12% and 15%. The BGIA round robins have demonstrated the proficiency of laboratories measuring exposure to inorganic acids in air. However, concerns remain about the performance of published methods. It has shown that the sampling efficiency of sorbent tubes falls off with increasing particle size and hence silica gel tube methods may give low results for acid mists. Another issue with silica gel tubes is that a substantial proportion of the sample can be collected on the glass wool plugs that retain the sorbent. This can be up to 50% for HCl and 100% for HNO3. Hence, low results may be obtained if the glass wool plugs are discarded. Similarly, methods for volatile inorganic acids that use a prefilter to remove particulates usually overlook the fact that the acids can react with coparticulate matter on the prefilter. Low recoveries in the range 30%-50% have been found when sampling HCl through filters loaded with potential interferents. Finally, particulate salts interfere with filter sampling methods for non-volatile inorganic acids. A two-part International Standard is in preparation for inorganic acids by ion chromatography and the issues discussed above are being taken into consideration during its development.