Chromatography Forum

Hi I am a new bird here. So I have a question regarding to the peak sequence of two columns installed in series.

first column: Hayesep Q 80/100 1m x 1/8"
second column: Molesieve 5A 80/100 1m x 1/8"

Sample Gas: CO 15%; H2: 5%; CO2: 20%; CH4: 10%; N2: 40%; C2H6: 10%

Carrier Gas: Ar

Detector: TCD

If the two columns are installed in sequence, what is the sequence of the gas coming to the TCD?

Depending upon the amount of activation of the MS5A column has..... if it is not fully activated the separations and order may change,

H2 N2 CH4 CO

not eluting: CO2 and C2H6

I assume these are truly in sequence and you are not backflushing or performing any valve operation other than injecting the sample.

best wishes,

Rodney George
consultant

vicnn,

Hydrogen will be good sized, nitrogen (40%) should be pretty good sized, CO will be much smaller and so will CH4.

Best regards.

This may not be so easy.

Normally CO will elute from the HayeSep Q column before methane. However, if the MoleSieve column is well conditioned, CO will elute after methane. As Chromotographer1 mentions, this is dependent on how the Mole Sieve column is conditioned.

Depending on the lengths of the column and temperatures used, it is thus possible for CO to elute before methane, with the methane, or after the methane. I would recommend that you have a sample that only has methane present in Argon, then you can be sure which peak is which.

Gasman

Are you sure these are in series and not being valved? CO2 will not elute from the MS column.

It would be more typical to do the CO2 separation from all the rest on the Q column and separate the methane, CO, N2, etc on the MS column.

In my mind, when I troubleshoot this type of separation (ours is capillary), I see the H2,O2,C1,CO as a single unresolved peak eluting from the Q column before the CO2 and heavier HC's. Then the H2, O2, C1, CO are resolved on the MS, but you've got to bypass the MS column somehow to get the CO2 et al to the detector.

As larkl stated this system probably has switching valves between the two columns to prevent the CO2 reaching the mol sieve column. This means depending on how the instrument has been tuned the elution order can change.

My current elution order is:-

H2 - valve switch - CO2 - Valve switch - O2 N2 CH4 CO

Any valve switching should be very obvious as strangely shaped peaks (positive or negative), probably bracketing CO2.

Rich