gc pressure faults (Agilent)

[lmh]

I'm a newbie in GC (liquid chromatographer struggling in a new world). Our GC-MS, an Agilent 6890-based system, has had some methods running for ages, that have suddenly stopped working. They produce a pressure fault during the run.

So far as I can see, these are constant flow methods with gas-saver, and during the temperature gradient as the pressure increases to keep the flow constant, there comes a point where it can't rise further, the flow falls, the target and actual pressures differ, and the error happens

Our system is probably short on gas-pressure generally (see below!).

I've got a couple of questions:
(1) If we switch off gas-saver, the problem doesn't happen, and the pressure rises much higher than it does with the gas-saver on. I don't understand: why should decreasing the gas flow (by using the gas-saver) make it harder for the instrument to maintain the necessary pressure at the start of the column? I'd be very grateful if anyone can point me at an explanation.
(2) Our system is on a gas line that has a 4-bar maximum pressure. Agilent have said we should have 5-8 bar. Clearly our system has been working for years at the substandard pressure. What pressures do people generally use/get-away-with in their labs?

Obviously something has changed in our system that means the methods aren't working any more. Perhaps the filter on the gas line is getting old, I don't know?? I'm wondering if these were methods that were living on a knife-edge and have just fallen over. Any advice gratefully received.

[dblux_]

...
So far as I can see, these are constant flow methods with gas-saver, and during the temperature gradient as the pressure increases to keep the flow constant, there comes a point where it can't rise further, the flow falls, the target and actual pressures differ, and the error happens

Carrier gas supply pressure must be greater than max. column pressure during run.

Besides leaks in the inlet possible -check septum and nuts.

[James_Ball]

5-8bar (70-112 psi) seems a little high to me. We always run ours at 60psi (4.3bar) if I am converting that correctly.

Check the split flow at injection and then in the gas saver mode. I think minimum flow in gas saver mode is 15ml/min, but if you are running a column flow of 1ml/min and a split below 15:1 then you have less total flow at injection than after gas saver starts and you lose head pressure because gas saver is actually increasing your total flow instead of saving gas.

What column size are you using? Also what carrier gas? (I assume Helium but maybe you are using Hydrogen). For helium needing more than 4bar at a flow rate of 1ml/min you would be using a very small diameter column even at above 250C oven temp.

If the gas filter has become clogged then you may not get the needed pressure at the flow rate, but it would still have to be the situation where the gas saver is increasing your total flow instead of reducing it.

[Peter Apps]

When was the last time that you changed the septum ?

Also run a leak check (with a leak seeker not the onboard diagnostics) on all the inlet and gas control plumbing.

Peter

[lmh]

Thanks for such fast replies. We're using helium, the column is 30m * 250um. The method goes up to a fairly drastic temperature (380 degrees; column is rated to 400/430). We're doing splitless injections with 0.9mL/min constant flow to column, 91.9mL/min purge flow, 96.1 mL/min total flow (I'm trying to get my head round the arithmetic of where flows go...; these are just the values from the acqmeth.txt file, since the instrument is running a different method this morning so I can't open the method file itself). The purge time is zero, which I don't really understand, as I thought the idea in splitless injections was to have (step 1) a small flow through the septum purge, but otherwise let all gas go to column, then after time for sample to enter column (step 2) open the split valve with a big flow to purge the inlet, then (step 3; is this the gas-saver in action?) reduce the split-line to a lower value as by now there should be nothing left in the inlet to purge (why don't we close it altogether??).

I'm going to try poking around with the leak-detector. When the runs are finished, I will check the state of the inlet. The septum was last changed only a few injections ago, after the problem happened, because my coworker took out the inlet liner to look for obvious difficulties, and felt she'd damaged the septum in the process, so she changed it. Since we're both beginners, she's asked me to take an independent look (4 eyes better than 2...).

Thanks again

[dblux_]

Your column head pressure reaches 27 psig at 380 degC. So your gas supply pressure 4 bar is sufficient.

Leaks I think.

[Peter Apps]

Hi lmh

Did the problem persist with the new septum ?, if not the trouble has been shot.

There should indeed be a non- zero purge time, with it set at zero you get a split injection with valves opening and closing as the analytes transfer to the column, which is not good for repeatability. If you get big enough peaks with the current methods I would try a proper split method with the same flow parameters rather than going to proper splitless, which might crud up the column.

Peter

[Yama001]

I know quite a few people who like to set the valve time at 0 seconds these days - I have never been sure if they realize they are relying on lucky timing.

[lmh]

Thanks again. Yes, I wasn't sure what a purge-time of zero actually meant (never purge, or purge right from the start) and I fully see that given the almost instantaneous release of material in the inlet and a nominally-simultaneous almost instantaneous turning on of a purge flow, it's pretty much random what actually happens. Ooops.

I'm going to get me and co-worker to give ourselves a couple of clear days where we can just inject a boring standard on a short method while varying all these different parameters to see what they do, how they affect reproducibility and peak area, and just play around so we understand the system.

We still can't find a leak, but that may be our gross incompetence at GC!

[GasMan]

The Gas-saver mode controls the total amount of gas coming into the inlet. Think of the inlet as a paper cub with a small hole in the bottom. If you fill the cup with a fast flow, it will overflow( this is your split vent flow). If you reduce the flow into the cup, such that the flow is less than the leak, you will have no overflow and the level (equivalent to pressure) will slowly drop.This would explain why you see the problem with Gas Saver on, the incoming flow is less than the outgoing flow, where outgoing flow is column flow, septum purge, split flow plus leak!!

One of the things that you find in GC, is that you can have itermittent leaks which are temperature dependent. You can have a system that is leak tight at room temperature, you start a temperature program and because of differences in expansion rates, a leak will occurr which can reseal at a higher temperature. I would check or replace ALL seals in the oven area.

Gasman

[Peter Apps]

Thanks again. Yes, I wasn't sure what a purge-time of zero actually meant from Yama's past it seems that you are not alone ! (never purge, or purge right from the start) and I fully see that given the almost instantaneous release of material in the inlet and a nominally-simultaneous almost instantaneous turning on of a purge flow, it's pretty much random what actually happens. Ooops.

I'm going to get me and co-worker to give ourselves a couple of clear days where we can just inject a boring standard on a short method while varying all these different parameters to see what they do, how they affect reproducibility and peak area, and just play around so we understand the system. Excellent plan, have fun, if you get a chance try some different kinds of inlet liners as well

We still can't find a leak, but that may be our gross incompetence at GC! Do you have a leak seeker ?, if not it can be very difficult to find exactly where gas is being lost, but in this case the consensus is certainly that there has to be a leak somewhere

Good luck.

Peter