Chromatography Forum

Hi,

I am trying to set up a method for analysis of H2, C2H4, C3H6, C3H8, and Hexene.

I'm trying to analyze H2 from ppm to 30%
And hydrocarbons from 0.1% to 100%

This is what i've got so far.
Using back inlet and back detector for H2 analysis
Using front inlet and front detector for hydrocarbon analysis

GC = Agilent 6890
Oven temperature = 120C

For H2
Back inlet = split/splitless
Back inlet gas = N2
Sample loop volume = 2mL
Backflushing with N2 after the H2 comes out for about 1min
Back column = 10m 5A Molesieve 320umID 25um film
Back detector = TCD
Back detector gas = N2

For Hydrocarbon
Front inlet = split/splitless
Front inlet gas = He
Sample loop volume = 250uL
Front column = Plot Al2O3 10m 320um ID 8um film
Front detector = FID

I get really good calibration (mol% vs. area) with a N2/H2 mixture, or a N2/C3H6, or a N2/C2H4, basically any gas with N2.
When running with a high concentration of hydrocarbon and measuring H2; the retention time of the hydrogen shifts down and the calibration curve is different from the calibration curve for the N2/H2 mixture.

I need the large sample loop for the H2 analysis because i'm trying to get down to ppm level detection. I'm thinking the large sample loop for the H2 analysis is injecting too much hydrocarbon onto the molesieve and the backflush is unable to get rid of the hydrocarbon. Also, the hydrocarbon that is flooding the column is messing up the hydrogen analysis. I was thinking about putting a packed Haysep column in front of the molesieve column to take care of the hydrocarbons.

Any suggestions or comments?

Thanks,

Michael

You just need to increase the backflush time/flow. H2 always elutes first so it will not influeced by HCs in the same injection.