HPLC 6-port valve for GC?

[rekuci]

I found a 6-port valve for an HPLC (Rheodyne model 7125) that I salvaged from the garbage months ago. Although a little dirty, it looks like it's in pretty good shape and could be washed out.

I want to inject hydrogen/air mixtures, either by syringe or directly from a small sampling cylinder at slightly elevated pressures (eg 30 psi), without introducing air into the inlet, since I need to analyze for air in the sample, or at least be sure that hydrogen isn't escaping as air is entering (air can then be determined by difference since the total pressure of the sample will be known). I've been told by chromatographer1 and others that the only way to avoid air contamination is to use a sampling valve.

Will this valve be ok for use with GC? I downloaded the operating instructions for the valve from the Rheodyne website, and it looks simple enough if I purchase a 5 or 10 uL sample loop (it has a 100 uL sample loop on it now) and inject 1-2 mL of sample with a syringe to adequately purge the sample through the loop. Valco actually recommended an HPLC valve to me over the phone, but this could have been an error. What's not clear is if this will actually correct the problem of air entering during the injection, not knowing if air is entering at the moment of injection or if it is continuously seeping in while the needle is in place. Any suggestions are appreciated.

Also, to get a more adequate purge, perhaps I could connect the waste line to a vacuum pump to be able to vacuum purge the sample loop?

[Victor]

Someone has done this before. There is a paper in the Journal of Chromatgraphy about it years ago, but the reference escapes me. There seems no reason why you should not do this, because I imagine the HPLC valve is over-engineered for GC purposes in that it will withstand high pressures. This is not required in GC. I would guess that a GC gas valve is much cheaper.

Remember that gases are more dilute than liquids and maybe you will need to inject a greater volume than is usual in HPLC. Also you need to take adequate safety precautions when playing around with hydrogen.

[rekuci]

I went ahead and installed it...things look leaktight so I tried an injection. It's a little tough to do nicely right now because the valve is just hanging with tubing everywhere. Also the excess line is just going straight to atmosphere. My needles don't seem the ideal length for penetrating the valve seal (it just has a syringe injection port built in, unfortunately...it would be better if I could hook up my own apparatus for injection). When I turn the valve from load to inject while leaving the needle in, the extreme end of the tip is getting bent up.

Injecting 20 uL hydrogen, I get a broader peak than injecting directly into the inlet (probably to be expected, and the tubing ID from the valve to the inlet is too small at the moment). Also the peak area is smaller than direct injection, but the air peak seems to also be disproportionaly smaller or not even visible. Hopefully this isn't because it's just really wide. But it seems to be working, perhaps with some modification this can be improved.