Positive artifact sulfate formation from SO2 adsorption in the silica gel sampler used in NIOSH Method 7903.

NIOSH Method 7903, which uses one section of glass fiber filter and two sections of silica gel, has been developed to determine the total concentrations of acid mists in workplace air, although certain gases are suspected to cause interference. In this study, experiments were performed to investigate the roles of sulfur(IV) oxidation and sulfur dioxide (SO2) adsorption in causing artifacts in sulfuric acid measurement. First, sulfur(IV) oxidation, under four combinations of water bath temperature and Na2CO3 solution concentration, was examined to investigate the effect of the extraction process of NIOSH Method 7903. It was shown that sulfur(IV) oxidation to form sulfate could reach 100% within just 2-3 min, following the extraction process of NIOSH Method 7903. The results demonstrate that, using the procedure, SO2 adsorbed by the silica gel and the glass fiber filter easily yields artifact sulfate. Sulfur dioxide adsorption under various flow rates, SO2 concentrations, and sampling times was also investigated. The experimental data were fitted to a deactivation model to determine the adsorption rate constant and the deactivation rate constant. The model can serve as a tool for estimating the artifact sulfate if the SO2 concentration is available.