analyzing samples collected at sub atmospheric pressure?
Posted: Thu May 12, 2005 9:20 pm
by beeko
Has anyone ever analyzed gas samples collected at sub atmospheric pressure for trace concentrations of CO2, H2, CH4, O2 and N2?
I did some quick internet research and found that a Discharge Ionization Detector (DID) could detect the gases at low levels but I am unsure as to how to get the sample into the analyzer since it is at sub atmospheric pressure. Does anyone have any ideas.
Thanks in advance for any help and suggestions.
gas analysis
Posted: Fri May 13, 2005 3:30 pm
by chromatographer1
I assume your sample is in a cylinder what was originally a vacuum cell which then 'sucked' in the gas that was sampled. This sample was never brought to ambient pressure.
To transfer this partial pressure gas you will have to 'suck' out a sample through a sampling valve and then inject the sub-athmospheric pressure loop onto a GC column. Depending upon your definition of 'trace':
low ppb, low ppm, high ppm ????????
a TCD or a DID or pHID detector may meet your needs.
Both the DID and the pHID require a very high purity Helium carrier gas. A DID can be very difficult to install leakfree so that it will work well.
A pHID using a Getter (like Valco's) is rather straightforward and most technicians are capable of installing and setting such a detector up. Valco's design can also be installed on many commercial GCs easily. (No, I do not work for Valco but have used their equipment for years)
Be mindful that each time you sample your subambient sample (reducing the already subambient pressure of your cylinder) you will be reducing the amount of gas in your sample loop and change the concentrations of the components relative to ambient pressure.
Without knowing more details this is the best advice I can offer.
Rod