Validation of ppb/ppt Sulfur Gas Standards by Independent Analytical Methods

Abstract

Reliable measurements of sulfur-containing gases at ultratrace concentrations have two basic prerequisites: 1) analytical methods with sufficient detectability, specificity, and reproducibility; and 2) accurate standards of these sulfur gases which bracket the expected analyte concentration range in actual air samples. Whereas several potential measurement methods for these sulfur gases have been reported, direct calibration of these methods at low ppb to ppt concentrations has been impossible because of difficulties in preparing dependable, verified standards over this concentration range. A system has been developed in our laboratory for the preparation and validation of sulfur gas standards at concentrations down to 0.01 μg S/m3. The calibration procedure uses commercial G-CAL permeation devices whose emission rates (0.1 to 10 ng S/min) are established via a metal foil collection/flash desorption/flame photometric detection (MFC/FD/FPD) technique that can be standardized with aqueous sulfate standards. The validity of this calibration scheme has been determined by intralaboratory comparisons among four independently calibrated analytical methods and by a collaborative interlaboratory study.