Articles | Volume 9, issue 6
Research article
24 Jun 2016
Research article |  | 24 Jun 2016

Water vapor inhibits hydrogen sulfide detection in pulsed fluorescence sulfur monitors

Anders B. Bluhme, Jonas L. Ingemar, Carl Meusinger, and Matthew S. Johnson

Abstract. The Thermo Scientific 450 Hydrogen Sulfide–Sulfur Dioxide Analyzer measures both hydrogen sulfide (H2S) and sulfur dioxide (SO2). Sulfur dioxide is measured by pulsed fluorescence, while H2S is converted to SO2 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 H2S concentration was generated and the output of the analyzer recorded as a function of relative humidity (RH). The analyzer underreported H2S as soon as the relative humidity was increased. The fraction of undetected H2S increased from 8.3 at 5.3 % RH (294 K) to over 34 % at RH  >  80 %. Hydrogen sulfide 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 H2S concentration.

Short summary
Hydrogen sulfide (H2S) is a malodorous, very poisonous, and flammable gas. It can be detected as SO2 using fluorescence after conversion using a hot catalyst. This technique is well established and as such also recommended by authorities such as the EPA. Our paper describes how at a relative humidity as low as 5 %, significant amounts of H2S pass the instrument undetected. At ambient levels of relative humidity, up to 1/3 of all H2S passes the instrument unnoticed.