benhutcherson wrote:
I run Nitrogen as a rest gas two instruments(5971, 5975) and have never been able to get a satisfactory tune on nitrogen. It's worth mentioning that these both are diffusion pump instruments, but I've been afraid to even try running them due to the terrible tune.
At least on HP/Agilent stuff, it's worth noting that the vacuum gauge is calibrated for nitrogen. Helium will cause the gauge to read low-in fact the manuals will tell you to multiply the gauge reading by 6 to get the "true" vacuum. The main value in a vacuum gauge for me at least is that even if it's inherently inaccurate under the conditions where it is being used, it is precise, so a change in gauge reading can indicate abnormal conditions in the instrument.
I mention all of that because at first glance the vacuum readings with N2 look terrible on my instruments. If I see 2x10^-5 on He, I might see 4x10^-4 on N2(actual numbers I've observed) with the same column head pressure. If you apply the 6x correction factor, you see that the vacuum is still somewhat worse on N2, but not as bad as the gauge readings taken at face value might lead you to believe. Also, don't forget that, even though the head pressure is the same, the flow rate will be different(don't forget to tell the instrument you're running N2 vs. He so that it can correctly calculate the flow).
My Varian 1200L should be here in a week or two, so once I get it installed and running, I'll be interested to see how the turbo pump in it does with N2. It's getting plumbed separately from the other two GC-MSs, but I'm plumbing in an N2 rest valve for it also.
Hydrogen causes the same problems as far as gauge readings go, so none of the numbers directly relate, but the gauge is a good way to know what "normal" should be once you get things going.
I doubt the 5971 would have enough pumping capacity to handle nitrogen carrier, or you will need to go to the smallest ID column possible to keep flow down and linear velocity up, if staying within the limits of the pumps. The 5975 may handle it but I would definitely be running at least an E2M2 rough pump, those little ones just won't do it. The 7000 has an RV5 so roughing is well sized for nitrogen, I am running E2M2s on the 5973/5975s I have also, it just helps keep things pumped out better even on helium.