Inconsistent gases analysis with GC

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Top of pagePrevious messageNext messageBottom of pageLink to this message  By steven on Friday, January 10, 2003 - 11:44 pm:


Recently, my company bought a GC sampling valve system with Porapak Q stainless stell column. We intended to analyze acetylene, 1,3-butadiene and methyl chloride gases. The std gases tank contain 5 ppm acetylene, 10 ppm 1,3-butadiene and 100 ppm methyl chloride in nitrogen gases.GC condition

Inlet 100 cel, Oven 70 cel, Det 150 cel. Nitrogen carried flow at 40 ml/min. Sampling loop is 0.5cc.
We started injection after we purge the sampling valve for a while and until the last bubble came out from the outlet tube submerge in a beaker of water.

We get 3 peaks where RT for
Acetylene 0.3 min
1,3-butadiene 0.7 min
Methyl chloride 3.8 min

The 1,3- butadiene recoveries is between 9.9 to 1.1 ppm. While methyl chloride is between 100.1 ppm to 102 ppm. The peak area for 1,3-butadiene is around 60, while methyl chloride is around 490. Peak shape is symetrical.

However, the peak for acetylene is very small (near the detection limit) and the recoveries is around 2 to 5 ppm. Perform leak test on GC. LEak test passed when the valve is on or off.
Can anybody help me or tell me anything which i missed out?? Your kindness is very much appreciated.

Thank you very much in advance.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Pawel CBJ on Saturday, January 11, 2003 - 07:59 am:

What kind of detector do you use? TCD or FID? If TCD you should obtain better recovery when helium is used as a carrier instead of nitrogen. Check detector parameters, probably you can change range to smaller concentration. Also changing sample loop into a bigger one seems to be an easy way to increase peaks.
Hope it helped you,

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