Smaller areas using hydrogen carrier compared to helium.

[pdelt]

I injected the same sample with the same column flow and split ratio using hydrogen as the carrier and helium. My areas were noticeably less using the hydrogen. Has anyone seen this and what would be the cause?

[chromatographer1]

Golly, the answer depends upon which GC detector you are using. Do you know the detector you are using and can you tell us?

best wishes,

Rod

[pdelt]

I,m using a FID detector and helium as a make-up gas in both situations.

[chromatographer1]

You are changing the flame composition when you change the carrier to hydrogen.

This changes the response of the flame.

best wishes,

Rod

[pdelt]

Thanks for the information.

[Peter Apps]

There are two main possibilities; a change in inlet discrimination due to lower pressures or a change in detector response due to more hydrogen going to the flame. Since for some reason you provide only the barest outline of your conditions it is anybody's guess which of these might apply. I would be surprised to find an impact on detector response unless you are using a megabore column.

Peter

[Consumer Products Guy]

I injected the same sample with the same column flow and split ratio using hydrogen as the carrier and .... FID detector and helium as a make-up gas in both situations.

This is curious situation, as pretty much everyone who changes from helium carrier to hydrogen carrier does such to move away from helium cost or availablity. Using helium as the make-up gas only partially removes helium from the whole situation; most use nitrogen as make up gas when using hydrogen carrier.

[pdelt]

Peter,

I was using a 15 meter megabore column.

[pdelt]

Consumer Products Guy'

I understand that nitrogen would be the best choice as a make-up gas. We didn't have anything other than house nitrogen to do our test so I just replumbed the hydrogen to the carrier lines. Thanks.

[Peter Apps]

Peter,

I was using a 15 meter megabore column.

Guessing (again !) that your flow rate is quite high with such a short wide bore column - say 10 ml/min ? - then you have 10 ml/min extra hydrogen gong into the lfmae, and this is reducing its response. I expect that the baseline noise is also worse. What happens if you reduce the hydrogen setting on the FID gas contoller to restore the total hydrogen flow to the flame to what it was before ?

Peter

[chromatographer1]

This was my guess-assumption too, Peter.

While a 0.2mm ID column would not change the flame much with a H2 carrier, the evidence that it had changed the flame suggested a 0.53 or 0.32mm ID column was being used.

Sometimes with so little information given it is difficult to produce a reasonable guess.

But we do try, don't we?

Even a change of H2 from 35cc/min to 37cc/min can make a difference. Certainly, 35 to 40cc/min can make a BIG difference, and doesn't always produce more FID noise, sometimes less, depending upon the makeup and the volume of air flow.

best wishes,

Rod

[Peter Apps]

Hi Rod

Great minds guess alike !!

Changing flows away from the generic "optimum" values can improve signal:noise from an FID, but I supsect that these days anyone who wants minimum lower detection limits would be using a selective detector.

One of the very big advantages of constant flow EPCs is that the flow of hydrogen carrier to an FID stays constant during temperature programming - if you run a megabore column at constant pressure with hydrogen carrier you get some spectacular baseline drift !

Peter

[chromatographer1]

You bring up a good point !

And I can only imagine the possibilities !

I am SO GLAD I did GC in the good ol' days of helium carrier. Something to tell my grandchildren (2 here now, and hopefully, more coming) someday.

best wishes,

Rod

[Peter Apps]

Of course one of the benefits of constant head pressure (were talking the good really old days here) is that during temperature programming the declining volume flow of hydrogen carrier would cancel out some of the baseline drift from stationary phase bleed

Peter

[chromatographer1]

Heavens !

You did not have a dual FID GC?

That is why columns were sold in pairs, so the bleed would be compensated by the parallel detector !

The amps always subtracted the bleed from the non-injected column, and you could switch A-B to B-A when a column got old or contaminated.

Oh, the days of scrubbing out the white powdery silicia deposits with a spatula! It brings tears to my eyes !

Rod