990 uGC - H2S elution and Backflush in MS5A Column

Hi I am new to GC world.

I have a new 990 uGC with a MS5A column (20m + 3m bf) and a PPU column (10m + 1m BF).

I am trying to obtain calibration curves for a mix which is 4.995% H2S in a CO2 Balance. I know that CO2 is not good for MS5A column. I think the same holds for H2S.

I optimized the method for the MS5A to BF the CO2 in a calibration mixture that has CO2 and CH4. However, I do not know if H2S elutes before or after CO2. How do I ensure that the MS5A does not see any H2S?

0 Lance Nelson over 1 year ago

The MS5A channel uses a Porabond-Q as its backflush column. H2S elutes after CO2 on this column, so if your backflush time is set correctly the CO2 and H2S will be backflushed before reaching the molesieve column.

(Easiest way to set this is to run the calibration standard with CH4. Keep setting the backflush time earlier until the CH4 disappears. Then move it back to the previous value where you saw all of the CH4.)
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0 rpandey over 1 year ago in reply to Lance Nelson

Thank you Lance. What time intervals do I change after I see CH4 peak? Is 1 sec good, or do I have to reduce it further? Thank tou
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0 Lance Nelson over 1 year ago in reply to rpandey

I would decrease the backflush time by 1 sec each run. When you see CH4 disappear, the time is set too early. Since the 990 uGC runs are short it shouldn't take too long to determine the optimal time.
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