Problems getting a good chromatogram

[vl94557]

Hi, I am new to GC and I am trying to sample the head space in a reactor cell that contains CO2 (very large amount), H2S, and water vapor. I am sampling directly from the 20 mL headspace by connecting the cell to the sampling valve. The GC has a TCD, helium carrier gas (50 mL/min), Sulfinert tubing/sample loop (5mL) from Restek, and the column is Silcosmooth, with Hayesep Q packing, mesh 80/100, 2m x 2 mm ID. The H2S is present in a very small amount (15mM throughout the 60 DI water) and heated to about 75 C. I can get a good response from the detector, although the water peak elutes at about the same time as the H2S and the water peak is very large and always tailing. The small H2S peak is incredibly broad and blends in slightly with the water peak. I've tried several different temperature programs, and the best I have found is 30C/4min to 80C/5min at 60C/min, injector at 150C and detector at 270C. Does anyone have ideas on how to sharpen these peaks? The only peak that needs to be quanitified is the H2S. Also, when I am doing a calibration curve, because I am sampling directly from the cell, do I need to calibrate that way also? Thanks for any help!!

[chromatographer1]

Your sampling procedure sounds inadequate and may not be able to be calibrated.

Please explain:

How do you know you are injecting the same amount of gas for each injection?

You should review proper techniques in sampling gas samples.

5mL is a LARGE sample of gas to inject, even on a 2mm ID column.

You can sharpen your peaks by increasing the column temperature.

What temperature do you have the lines going to your sampling valve? And what temperature is your valve?

best wishes,

Rod

[vl94557]

On the reactor cell, I have a metering valve with a Vernier handle with tick marks so I can be sure to allow the same flow each time. The 5mL sample loop was recommended by Restek because the sample contains a very small amount of H2S and they said to increase the sensitivity, I needed to have a larger sample volume. The valve is at the same temp as the cell, about 70-75C, but the line is at room temp. I tried heating the lines as well, but it didn't make much of a difference in the output. If I raise the temperature too high, the water peak takes over the H2S peak. Should I start the temp program at 30C then increase it to a higher temp more rapidly than I have been doing?

Thanks for your input!

[chromatographer1]

Your setup does not indicate to me you can get a consistent sample.

To sample a gas consistently, one should flush the sample loop 3-5X its volume, and stop flow before allowing the loop to come to ambient pressure.

What you say you are doing is not indicative of this procedure.

You should have no bare metal in your sample flow path, or within the GC injector-column-detector path.

Try different isothermal temperatures to maximize your sharpness and separations.

good luck,

Rod