I would like to monitor the disappearance of headspace oxygen concentrations. I would like to sample the headspace gas through a septum and inject it into the GC.The oxygen will be consumed by microbes degrading an organic substrate.
However, I'd like to incubate the sample at higher than room temperature to simulate composting conditions. When I draw the hot gas into the gas-tight syringe,I'll get fewer molecules of oxygen even if no oxygen has been consumed -- the hot gas will be expanded. Should I try to cool the sample immediately before taking a gas sample? Is there another clever way to handle this situation?
By Anonymous on Thursday, May 8, 2003 - 08:29 am:
At time = 0, the GC detector response represents 21 mole% (for dry air)whatever the temperature. But there is a humidity correction in a sealed headspace which is obviously temperature dependent. Let's assume ~80% relative humidity at 45 deg C. Water vapour accounts for 7.6 mol% (0.8 x 72 mm vap press/760)and will reduce the initial oxygen to 21 x (100-7.6)/100) = 19.4 mol%. That is only significant if you vary the temperature in your experiment, or if you must know the absolute partial pressure of oxygen. In that case you must estimate or measure the humidity and calculate mol% water from vapour pressure tables. If you just want to monitor relative change at constant temperature - ignore the above correction. Just take the sample as normal and monitor relative change in the oxygen peak area.
By ralph on Thursday, May 8, 2003 - 08:36 am:
Its a good question -there are 4 thoughts that spring to mind:
a)yes, cool before sampling although I am not sure how much effect that will have if you are only heating to 37 ish. The gas won't be expanded in the septum sealed vial, only under slightly increased pressure so you should actually get more into your syringe - I'll have to think this through more!
b)if sampling over a time course only take small samples or for larger samples use several vials
c)use a vial containing substrate only as a control which you treat and sample in exactly the same way - relate your results to that. That way you have a comparison and also eliminate your volume concern.
d)to measure oxygen you are probably using a thermal conductivity detector and a suitable column. That column will probably separate nitrogen so you could measure the ratio of oxygen to nitrogen to follow the depletion.