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?

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