TCD Issues

[chemviren]

Hi I'm having some trouble with my new TCD, which I replaced an old one with.

The GC is a gas analyzer and in order to detect both Hydrogen and the other permanent gases, I have a valve which switchs carrier & reference gases going to the TCD (N2 & He).

When I have the valve off, N2 is flowing through as the carrier gas & reference gas and the baseline is at -75. I then turn the valve on and He flows through as the carrier gas & reference gas. Instead of staying at -75 or jumping up to a new voltage with a steady baseline, the signal increase greatly with a parabolic slope over about 2 minutes to 33000 and then finally has a steady baseline.

What is causing the parabolic slope increase? Why is it gradual instead of instantaneous like it should be?

Any help would be much appreciated.

[chromatographer1]

What columns are you using?

I would bet money they are not capillary.

best wishes,

ROdney George
consultant

[chemviren]

Its a gas analyzer. I have five packed columns (hayseps & mol sieves) as well as two capillary columns. The TCD is connected to the packed columns.

I think I have narrowed it down to a leak in the valve that switches the carrier gas & reference gas to the TCD from N2 to He (& vice versa). I'm thinking the parabolic slope is a blending of the two gases, hence the gradual increase in signal until it levels out.

Any thoughts?

[chromatographer1]

Are you changing the setting of the gas regulator on the packed column side of the analyzer?

A little more info and I should have a diagnosis.

Rodney George

[AICMM]

chemviren,

AAAAAAAGGGGGGGHHHHHHHHH!!!!!!!!

Best regards.

[chromatographer1]

MM

with half the information, I can only give half an answer and then comes the second question after the first.

But go ahead and tell him the whole story, and save him some time.



Rod

[AICMM]

Rodney,

I realized after I posted that my comment might be mis-construed. I do not wish to upset anyone, it is just that I know that this application can be done much easier. I guess I will have to post the chromatograms to prove it shortly.

Chemviren is in good hands with your advice.

Best regards.

[chromatographer1]

MM

In determing why his NEW TCD is not working like his OLD TCD, I have to assume that something changed, and he was not unhappy with his original application. I think we both suspect that his change of carrier was to do the hydrogen analysis with N2 carrier and his O2, N2, CO, and C1 analysis was with Helium carrier.

I would like to know a bit more about his hardware which he has not as yet been forthcoming and what he changed when he replaced his TCD.

Of course, another question is why did he have to change his TCD as that might be a clue to his problem.

His gradual change in response is due to the slow change of trapped gases in his Mole Sieve column, as N2 does not exchange quickly either in leaving the column or in entering the column packing as it is replacing the He in the interior of the packing. It is something that does not occur in porous polymer packings or in capillary columns.

The resulting upset in baseline is due to the differences in flow in the detector between the two gases due to viscosity. The original TCD may have modified in some way to compensate for this difference.

I would suggest to chemviren that he should contact the vendor who engineered his application and get their help.

Rodney George
consultant

[GasMan]

Perhaps you could tell us what model GC you are working with.

Gasman