Anal. Chem. 2004, 76, 7396-7399
The Stability of 100 ppb Hydrogen Sulfide Standards Robert Benesch, Malik Haouchine, and Tracey Jacksier*
Air Liquide, Chicago Research Center, 5230 S. East Ave., Countryside, Illinois 60525
An experimental study has been performed to determine the factors affecting the stability of hydrogen sulfide calibration mixtures at concentrations below 1 ppm. Quantitative data was obtained that illustrates that material selection and surface treatment are major factors affecting the shelf life of low concentration reactive gas mixtures. With the proper material selection and surface treatment, hydrogen sulfide and carbonyl sulfide standard mixtures on the order of 50-100 ppb can be maintained for at least 18 months. The impetus for the measurement of low levels of sulfur compounds has come from the hydrocarbon process industry, where the requirement for lower sulfur measurements is steadily increasing. Low sulfur content in simple olefin feedstock for highpurity linear polymers, such as polyethylene and polypropylene, is quite important. Sulfur compounds adsorb onto the surface of the catalysts and prevent the desired reactants from reaching the catalytic surface, thereby deactivating the catalyst. Given the quantity of these products produced annually,1,2 the economic impact can be quite significant. Therefore, common sulfur impurities in the feedstock, such as H2S (hydrogen sulfide) and COS (carbonyl sulfide) must be carefully monitored. To monitor, control, and regulate these compounds, calibration gas mixtures containing these compounds must be used. For these mixtures to be utilized successfully, they must be reliable with respect to stability (shelf life) and concentration. Significant challenges arise as the concentrations of reactive gas mixtures required to calibrate equipment are decreased. Interactions on the cylinder and tubing surfaces, such as chemical reactions and adsorption, are often insignificant, with reactive gas concentrations in the range of 10 ppm and above. These interactions become quite important with reactive gases at concentrations at or below 1 ppm. Since accurate and reliable calibration mixtures are important to process evaluations and also regulatory monitoring, an experimental study was designed to assess the effect of various parameters on the stability of hydrogen sulfide mixtures at concentrations
