saturated CO2 and CH4 measurement with GC

[jojo]

HELLO THERE,
My experiment goes like this. A bottle is filled with 20 ml of distilled water and is rubber tight. the head space consists of mixture of carbondioxide and methane. The mixture of gas is 50% methane and 50% carbondioxide. I put the bottles first at 70 degree centrigrade water bath. The GC TCD results shows very much increase increrase in the concentration of both methane and carbondioxide. It shows the increase in methane concentration from 50% to 70% and carbondioxide from 50% to 100 %. Can anyone please explain why this happens.

thank you very much

[Don_Hilton]

You start with a mixture that is 50% methane and 50% CO2 - that is out of every 100 parts of the gas you have 50 of each methane and CO2. So far so good.

Now you analyze the gas and find both to increase in concentration to 70 parts out of 100 one gas and 100 parts out of 100 the other gas?

One of us is missing something here. You can not have more than 100 parts out of 100 parts.

To answer your question, we would need some clarification as to how you got the numbers, how you filled the headspace, and how you sampled the headspace gas.

[jojo]

hi,
thank you for the question. The gas used for the experiment is mixture of 50% CO2 and 50% CH4. A 260 ml glass bottle is filled with 20 ml water and tight with butyl robber stoppers and sealed with aluminium cover. Two needles are inserted in the stoppers and are connected by robber pipes. one for the gas in and one for out. The out going pipe is water sealed so that no atmospheric air can get chance of getting mixed. In this way the bottle is flushed with gas for several minutes. finally both needles are carefully removed.
thank u

[krickos]

Hi

well not 100% I follow your experiment and saturation of volatile gases in wateris a bit outside my area of working.

But assuming your water is free from disturbing amounts of carbondioxide, the constant flow of gases might saturate the water (the amount of methane and carbondioxide that saturate will be different). So when you stop the gasflow, immediately the water will release different amounts of methane and carbondioxide to the headspace (according to their individual partions coefficients). Considering the properties of the gases in theory the larger increase of carbondioxide seems reasonble.

But then again I do not even know if its possible to saturate water with those gases at 70°C in a closed vessle. However one trick to get rid of carbondioxide in water is to boil/destill it.

[AICMM]

jojo,

If I understand your post correctly, your area counts for CO2 and CH4 go up in your analysis? However, your area percent should basically still report 50/50, no? Only the two peaks in the chromatogram?

Best regards.