Chromatography Forum

HELLO EXPERTS,
I am very new to gas Chromatography.
I am doing a simple experiment related with synthetic biogas with the use
of GC(TCD). I have used mixture of 50% methane and 50% carbondioxide gas inside a serum bottle containing 10 ml of water and tight with rubber stopper. Then I have Increased the temperature of the system my placing the bottle in a water bath of 70 degeree centrigrade.
I was trying to see the change in th concentration of methane and
carbondixide in saturated case with increase in temperature.I want to
compare the result with the standard (50% methane and 50% carbondioxide.)I use pressure lock syringe for the sample injection.
When I inject couple of sample injections from same sample bottle, There
is not much change in the methane concentration. But in case of
carbondioxide the values are very much fluctuating. I even don't know
which one is the value of carbondioxide. I can see another additional
number(I think water vapor?). Also the graph shows two picks fused with
each other.

For example the printed data is in the form of:

For First Injection
437878.85
282801.83
230473.22

For second Injection
440620.69
148179.70
228457.20

First. What is the column you are using?

Second. Have you injected just methane and CO2 individually without water? Therefore, you can find a retention time, and build a calibration curve for each component. Once this is done you can start looking into saturating in the water.

Third. What is the amount you are injecting? What is the split/split-less ratio? Depending on this the liner, you could be injecting to much. A hint is if you see some pre-tailing you might want to consider injecting less.

Fourth. Since you are manually injecting there are going to some discrepancy in the injections. Also since you are increasing the temperature inside the chamber, the vapor pressure will increase as well; this will also play a role in the you injection consistences.

Hope this helps.

Ideally you'd be using a gas valve injection method. You fill a sample loop and it injects that. Much more consistent.

1. Do you have any leaks in the serum bottles

2. Do you release the pressure in the syringe before injecting? This is important - if not there is where your discrepancy can be.

3. Test the pressure in you serum bottles - heating with gas can increase pressure quite a bit in them (I have done work with methanogens before and have blown up syringe bottles by elevating temperature).

Thank you for your answers. Willnatalies answer actually helped to solve my case. Thanks a lot.