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20.Jun.2007 05:56:07 PM

Vapor pressure restriction

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A couple of years ago I purchased a 500 ppm tetrachloroethylene (PCE) standard from Scott Speciality Gases with the balance being carbon dioxide. The product description indicated that the vapor pressure restriction was ~159 psia @ 32 F. When I ordered the standard I think I was told that this was the highest ppm standard for PCE that could be prepared. Is this correct? I assume as the amount of PCE in the cylinder increases the vapor pressure restriction decreases. The intent is to have a gas standard that stays in the vapor phase without condensing. How is the vapor pressure restriction determined? Our studies required PCE standards that were close to the saturated vapor concentration. At 25C the vapor pressure is 18.4 mm Hg. Using the gas law we calculate a vapor concentration of 169,000 ug/L (24,900 ppm). We created a stream of this vapor by purging PCE with carbon dioxide and sampled the vapor using an on-line mass spectrometer. We diluted the vapor stream with a second carbon dioxide stream to create a serial dilution for calibration. We sampled the PCE stream with thermal desorption tubes and were able to find vapor concentration near the saturated concentration listed above. Our correlation between the on-line MS and the thermal desorption concentration was excellent. In our report I will indicate that we weren't able to obtain a commercially available gas standard above a certain level. Your help will be greatly appreciated.


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