Chromatography Forum

I had just done an air and water check and got ~1% O,N,water. I went to repeat it and got the filament current fault.

I've switched my 5971 over to hydrogen as the carrier. with the minimum flow I can achieve (0.83 mL/min) I get a vacuum of 7.2E-5. The built in vacuum gage at the diffusion pump (accessible in MS software) exit says 44 mTorr.

I put in new PFTB fluid and seals. When the tune valve opens the vacuum goes up to 1E-4 Torr. Can't seem to get the seal any better and it does not pump down any lower when tune valve is open. Is this normal or do I perhaps have a leak in the solenoid or the vial seal?

Is this a source pressure problem somehow made worse by hydrogen? Hydrogen flows so easily that the lowest flow achievable with DB624 column 20m x 200um x 1.2 um; is 0.83 mL/min. If there is a frit I can change in the flow controller please suggest.

Finally, could this be a diffusion pump problem. I've not checked but diffusion oil might be low. I am seeing 25 mTorr at the Edwards E2M2 pump via a DV6 gage.

With hydrogen the gauge will always read higher than it does with helium, since they are actually calibrated for nitrogen, even helium pressure is not the actual pressure in the analyzer. When I was trying hydrogen I moved to long 30m x 0.18mm columns to give as much restriction as possible so that I could achieve low flow rates and still maintain decent inlet pressure.

If the air and water check is looking good then I doubt there is a leak, just a higher pressure with the valve open due to the hydrogen. One thing you might check is the EI/CI switch on the front of the analyzer. If it is in EI mode it will use the resistance of the filament to try and judge if there is too high analyzer pressure, if you flip it to CI it will simply disable that check.

Longer column, check.

Buttoned it back up after finding filaments were intact and hardly used. Surprise, I seem to be able to tune and do air/water checks so not top board problem.

I Got slightly better vacuum this time so now its 6.7E-5 Torr, and when tune valve opens it goes up to 2E-4 but falls donw to 1.2 E-4 if left on scan for some time. Is that good enough for the tune vial valve being open or should I be expecting better vacuum? I guess 1.2E-4 Torr with hydrogen would be about 7E-5 Torr with Helium.

Wondering if topping up diffusion pump oil is necessary. Going to order some before I go the the trouble of opening it to check.

Longer column, check.

Buttoned it back up after finding filaments were intact and hardly used. Surprise, I seem to be able to tune and do air/water checks so not top board problem.

I Got slightly better vacuum this time so now its 6.7E-5 Torr, and when tune valve opens it goes up to 2E-4 but falls donw to 1.2 E-4 if left on scan for some time. Is that good enough for the tune vial valve being open or should I be expecting better vacuum? I guess 1.2E-4 Torr with hydrogen would be about 7E-5 Torr with Helium.

Wondering if topping up diffusion pump oil is necessary. Going to order some before I go the the trouble of opening it to check.

If it will scan and autotune then you should be ok even with a higher pressure reading. Even on my 7000C QQQ when I use hydrogen I get almost an order higher pressure reading on the analyzer, so it is normal for it to be quite a bit higher.