Carrier Gas

[ltnguy3]

I’m new to GC. Could anyone please let me know what the differences between using hydrogen and helium as a carrier gas?

Thanks a lot.

[chromatographer1]

Hydrogen is much cheaper then helium.

Helium is of limited supply. Hydrogen is in almost boundless supply.

Hydrogen increases the time the analytes remain in the gas phases versus the liquid phase. Thus, Hydrogen will be about twice as fast as helium in performing most separations. The flow rate can be increased without increasing the peak width when one uses hydrogen.

TCDs will last longer with helium gas.

best wiehes,

Rod

[chromatographer1]

Oh, I forgot to mention the obvious.

Hydrogen will burn or explode when combined with air or oxygen, given the proper temperature.

Helium won't do either.

best wishes,

Rod

[ltnguy3]

Thank you so much Rod.

[JI2002]

The main reason for shorter run time by using H2 as carrier gas is because it has much smaller viscosity. Smaller viscosity means lower column head pressure, faster average linear velocity and shorter retention time. If column head pressure with helium as carrier gas can be raised high enough so that same average linear velocity can be achieved as with hydrogen as carrier gas, similar, if not exact, retention time can be obtained.

[dblux_]

The main reason for shorter run time by using H2 as carrier gas is because it has much smaller viscosity. Smaller viscosity means lower column head pressure, faster average linear velocity and shorter retention time. If column head pressure with helium as carrier gas can be raised high enough so that same average linear velocity can be achieved as with hydrogen as carrier gas, similar, if not exact, retention time can be obtained.

IMHO it's not a matter of magnitude of column pressure but diffusion coefficient of mobile phase (mainly) - Van Deemter equation

Wojtek

[Peter Apps]

Also, you can generate hydrogen in the lab, which you cannot do with helium.

Hydrogen is not compatible with MS - or is this another bit of GC-MS folklore - has anyone tried it ?.

Peter

[JI2002]

dblux_,

Diffusion constants of analytes in carrier gas are a few order magtitude bigger than that in stationary phase, mass transfer in carrier gas is a lot faster than mass transfer in stationary phase. The diffusion constants difference between H2 and He for an analyte are negligible in terms of retention time. Here are some numbers:

Diffusion Constants of Dodecane @ 150C

H2 = 0.6 cm2/sec
He = 0.4 cm2/sec
polydimethylsiloxane(stationary phase) = 10 to -7 cm2/sec

I did some tests a few years ago by using H2 and He as carrier gas on GC/ion trap MS. The standard was a 16 compound mixture. When same average linear velocities were employed in the two different carrier gases, almost two identical chromatograms were achieved.

[chromatographer1]

Most of the increase in the speed of analysis comes from the flatter Van Deemter Curve for hydrogen, IE one can use a faster linear flow rate and still have the same number of plates in a given column, 40cm/sec for hydrogen and 20cm/sec for helium will produce a nearly equal number of plates, but the hydrogen analysis will be twice as fast.

best wishes,

Rod

[gcguy]

Also, you can generate hydrogen in the lab, which you cannot do with helium.

Hydrogen is not compatible with MS - or is this another bit of GC-MS folklore - has anyone tried it ?.

Peter

I thought the only issue with hydrogen was a safety one, if vacuum is lost then there is a risk of an explosion in the detector.
I would be interested to hear if anybody does run ms using hydrogen, I am thinking about doing it myself.

GCguy

[GOM]

Hydrogen as a carrier gas for GCMS

I'm glad that Peter raised this one - it's something that I have been considering. One advantage would be fast GC with narrow bore columns.

For conventional GCMS systems the thinking seems to be
a) due to the diffusivity of hydrogen it is very difficult for turbo pumps to compress.
b) high chemical noise relating to the reactivity of hydrogen plasma.
c) it is very easy to get an air/hydrogen mix in the vacuum manifold with potential explosion hazard.

Point C really concerns me, you would have to take great care in ensuring full pump down after changing columns. The problem of hydrogen exiting the vacuum pump probably isn't an issue unless it is in an enclosed space.

Regards,

Ralph

[Peter Apps]

Ralph's point b (reactivity of hydrogen) was the one that was bothering me (because I had never thought of a !). The assumption is that analytes get hydrogenated in the source, and the mass spectra are distorted. This would have no effect on the chromatography of course.

JI2002 - did you see anything unexpected about the spectra of your analytes when you ran with hydrogen ??

In LC-MS, interactions of mobile phase with analyte are taken for granted - maybe we could do the same in GC-MS with hydrogen as carrier ?

If anyone tries this, be sure to let us know.

Peter

[JI2002]

Peter,

I didn't see much differences in the spectra of the target compounds between H2 and helium. Some analytes had identical EI spectra, some had slight differences, such as ion [M+1]+ in H2 could be a little bigger than in He.

I also looked into MS/MS data. Because carrier gas was also used as CID gas in the process of fragmentation of parent ions, the MS/MS spectra were quite different for some parent ions. Some daughter ions were base peak in one carrier gas, but in another carrier gas, they were much smaller.

[Peter Apps]

Very interesting - with the high cost and unreliability of supply for helium it looks as if hydrogen is definitely worth a try.

Peter

[Peter Apps]

Very interesting - with the high cost and unreliability of supply for helium it looks as if hydrogen is definitely worth a try.

Peter