Water Vapor Inhibits Hydrogen Sulfide Detection in Pulsed Fluorescence Sulfur Monitors

The Thermo Scientific 450 Hydrogen Sulfide - Sulfur Dioxide Analyzer measures both H.sub.2 S and SO.sub.2 . SO.sub.2 is measured by pulsed fluorescence, while H.sub.2 S is converted to SO.sub.2 with a molybdenum catalyst prior to detection. The 450 is widely used to measure ambient concentrations, e.g. for emissions monitoring and pollution control. An air stream with a constant H.sub.2 S concentration was generated and the output of the analyzer recorded as a function of relative humidity. The analyzer under-reported H.sub.2 S as soon as the relative humidity was increased. The fraction of undetected HS increased from 8.3 % at 5.3 % RH (294 K) to over 34 % at RH > 80 %. HS mole fractions of 573, 1142, and 5145 ppb were tested. The findings indicate that previous results obtained with instruments using similar catalysts should be re-evaluated to correct for interference from water vapor. It is suspected that water decreases the efficiency of the converter unit and thereby reduces the measured H.sub.2 S concentration.