I have been performing a low ppm Sulfide analysis using a Flame Photometric Detector (FPD). To calibrate my GC, I am using a blended gas containing Hydrogen Sulfide, Methyl Mercaptan, Dimethyl Sulfide, and Dimethyl Disulfide at low ppm levels in Nitrogen. This has been working just fine.

A coworker brought in a tank she purchased from a major gas supplier. This tank was certified to contain a low ppm Hydrogen Sulfide concentration in 20% Methane, and ~80% Nitrogen. When trying to analyze for Hydrogen Sulfide, I am obtaining a value that is an order of magnitude lower than the certified value-this happened on two different lots of the gas. I think that because this gas contains 20% Methane, the Methane is somehow interfering with my FPD. We have never had problems with this gas vendor before, and I doubt they could be an order of magnitude off on two different lots of this gas.

Does anyone have any experience performing a Sulfide analysis using a FPD?

Thanks in advance,

Chris

You are probably seeing a response change because the H2S is coeluting with the methane peak under your analysis conditions. FPDs will have a "quenched" response for sulfur when the peak coelutes with a major hydrocarbon peak. A solution would be to choose a column or conditions with which you could chromatographically resolve H2S from methane. Then you will probably see the "true" concentration of H2S in the standard mixture.