GC/MS air water check and inlet mode

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

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Agilent 6890 GC with 5975 MS; helium gas as carrier, 30m x 0.25 mm column. I've noticed that if I run the air/water check when in splitless mode (inlet pressure = 10.6 psi, total inlet flow = 4.8 mL/min, column flow = 1.2 mL/min, constant flow), I get high values for the nitrogen and oxygen (569.1, 161.67) but if I do the same again in split mode (50:1, inlet pressure 10.6 psi, total inlet flow = 64.5 mL/min, column flow = 1.2 mL/min, constant flow) then the values are way more reasonable (9.94, 2.21). Any idea why that would be, because if anything I would have expected the opposite...


Thanks!
Could be a leak at the top of the weldment, either at the o-ring or septa, or at the split vent line port. The higher flow could mask the leak in those places. Possibly also if the leak is in the incoming line, but like you said, I would expect the venturi effect to pull in more air, though to reach the higher flow you will be running a little higher pressure in the incoming lines from the flow controller so it could also mask the leak a little.

I had one acting very similar and it didn't go away until we moved that injection port to the rear position to install a LVI port in the front position. I believe taking apart the flow controller and putting it back together during the swap solved the leak but can't be certain.
The past is there to guide us into the future, not to dwell in.
Yeah, if I set the split ratio to 0.1:1 (didn't even know that was possible) to get a total inlet flow of 4.7 mL/min, inlet pressure at 10.6 psi, I do observe higher values for N2/O2 as well (167.87, 47.57), so it would seem to support that theory. But I did a leak test after inlet maintenance and the pressure didn't even drop by a decimal of a psi over well an hour. I also used cooling spray (1,1,1,2-tetrafluoroethane) and never once detected a thing. PLUS, I have an electronic leak detector and I must have spent at least 45 minutes slowly checking every possible connection along the entire path and still nothing. It's driving me crazy.
Did you open the back and check in the controller itself? It could be between the gas line inlet and the outlet to the weldment, possibly leaking in one of the solenoids.

I also saw one other that was a crack in the line going to the weldment where it had been flexed too much, that one was not easy to find, until it finally broke.
The past is there to guide us into the future, not to dwell in.
Setting split flows below 5 mL/min is not a good idea, or ratios below 2:1. You're not going to get very good results.

I recommend that you cool down your heated zones, consider just venting the MS, and remove column from inlet. Install a column nut with a no hole ferrule, or a column nut, ferrule, and short length of column that is sealed at the end with a septa (this way something leak tight is plugging the inlet. Install an 1/8th inch blanking plug on the septum purge outlet of the EPC (NOT the split vent, the septum purge.)

Replace septum and liner o-ring. Once that is done, set the inlet to 25 PSI and split mode, set the split flow to something reasonable like 50-100 mL/min (you still set a column flow though no actual flow will be coming out. You just dont want to set sometihng like 0.1 or 999 mL/min. 50-100 is good.)

Once it reaches 25 PSI, set the inlet pressure off and watch to see the pressure. drops. You don't want more than a few PSI over several minutes. You can repeat this in splitless mode, though I would do it first in split.

If you are seeing leaks, you'll need to troubleshoot further. If it holds pressure then it isn't a leak.
Just following up to say that I eventually found a (very small) leak at side-plate, which I was able to resolve. Air/water checks now come out good. Thanks!
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