Varian GC 3800 pressure does not stabilize with split on

[Chris F]

I am running a Varian GC 3800 (1177 Injector, 30m x 0.25mm x 0.25um capillary column, FID, Autosampler 8400, N2 carrier, Type 1 EFC) and have the problem that the injector pressure does not stabilize when the split is on.

It has been suggested in previous posts about similar problems, that this might be due to a blocked split line. How can I actually clean the split line?

Column flow is 2.5 mL/min (constant flow mode), pressure 23 psi and the split is 40, but the problem persists with higher (5.0 mL/min) or lower (1.0 mL/min) flow rates. When I turn the split off, the pressure stabilizes within 10-15 sec and the GC switches into ready mode. If I then turn up the split (changing and reactivating the method) the pressure remains constant up to a split of 30. When I then increase the split to 40 the pressure becomes unstable (fluctuating between 20 and 25 psi).

I measured the flow (rotameter) at the split vent and the flow is lower than I would expect (?). E.g. at split 30 the reading at the instrument sofware is 82 mL/min total flow, the actual flow at the spilt vent is approx 40 mL/min. At split 40 the pressure starts oszillating and the flow at the split valve starts to fluctuate between nil and burst of 100 mL/min.

The GC has been running with this method before and the problem seems to have started after the injector was cleaned. This leads me to believe that there might be are particle in the split line. How can I check that and how can I clean the split line?

Any help will be highly appreciated, as I am getting kind of desperate...

[Peter Apps]

This looks like an inadequate volume flow of gas to the inlet. What is the total flow when the inlet is in splitless mode ?

What is the septum purge flow ?

As an aside - why are you running nitrogen carrier with such a high flow rate ?

Peter

[Chris F]

this morning I started the instrumend up and ran a few tests "cold", i.e. all ovens off. All flows seemed fine, instrument stabilized, switching split on and off repeatedly. Then I did a few test runs "hot" (injector 300°C, oven 100 - 250 °C) and the problem reappeared.

This looks like an inadequate volume flow of gas to the inlet. What is the total flow when the inlet is in splitless mode ?

What is the septum purge flow ?

"cold" all valures stable, reproducible
pressure 22.1 psi
column flow 2.1 mL/min (set point 2.0 mL/min)
septum purge 8.0 mL/min (I readjusted that to 4.6 mL/min)
split vent 10.0 mL/min / approx. 80 mL/min (split off/on)
total flow (display) 20.8 mL/min / 90.8 mL/min (split off/on)

"hot" values stable with split off, fluctuating with split on
pressure 27.2 psi / 24-31 psi (split off/on)
column flow 2.2 mL/min stable (set point 2.0 mL/min)
septum purge 7.0 mL/min stable
split vent 9.6 mL/min / approx.** 90 mL/min (split off/on)
total flow (display) 17.7 mL/min / 81-91 mL/min (split off/on)
**the gas comes out in short bursts every few seconds. I collected the gas for 30 sec and extrapolated

As an aside - why are you running nitrogen carrier with such a high flow rate ?

The method has been optimized quite a bit for the particular analyte mixture. I have some high boiling analytes, so I use a high flow to shorten analysis time, because I need a high sample through-put. I use a high split to have a high total flow through the injector, because there are substantial amounts of non-volatile components in the samples. In the past low split ratios (hence low total flow) oed to rapid contamination of the injector. Therefore, I was also cleaning the injector more frequently, and now I am afraid that I blocked a line during the last cleaning (I tried to remove charred residues with a cotton swab, which might have been a bad idea).

[Peter Apps]

I think that you right be right about the blocked line. As the next diagnostic step, set the total flow in splitless to 90 ml/min (I don't recall what this setting is called - it might just be the split flow after the split opens). This will show whether it is the higher gas flow itself that causes the problem, or the difference in flow route between split and splitless.

On the flow rate - analysis time tradeoff - you will probably do better using a shorter column with a flow rate closer to optimum. Also, you cannot shift non-voaltiles using a high split flow (or all they do is go and sit in the split line). What you need to do is to reduce the amount of crud that goes into the inlet by diluting the samples - I would try a 10 x dilution at a split of 10:1 with a 15 m column.

Peter

[Chris F]

I think that you right be right about the blocked line. As the next diagnostic step, set the total flow in splitless to 90 ml/min (I don't recall what this setting is called - it might just be the split flow after the split opens). This will show whether it is the higher gas flow itself that causes the problem, or the difference in flow route between split and splitless.

Thanks, really appreciate your help. I am not sure if I understood you correctly, though. So I switche d the split off and set the pressure to 100 psi. That gave:
total flow 121 mL/min
column flow 17 mL/min (displayed, not measured)
septum purge 50 mL/min (measured)
split vent 66 mL/min (measured, constant, no bursts)
So the flows seem to add up.
What really puzzles me is that once I switch the split off, the pressure stabilizes within seconds. If I leave it off for a while and turn the split on again, then sometimes it remains stable for a couple of runs. Once it becomes unstable however, it does not stabilze ever, and the pressure keeps going up and down with a fairly large amplitude.

Edit: The problem occurs with high and low gas flow. I tried 1.0 mL/min and 5.0 mL/min. Also, the gas "bursts" out of the split vent and the pressure fluctuations correlate. While the pressure is rising, there is no gas flow, then the pressure seems to cross a threshhold and the gas comes out and the pressure drops below the setpoint. And repeat...
This seems to suggest a blockage or partial blockage to me, but where would I start looking for it? And how do I unblock it?

[Peter Apps]

It is not the flow through the column that you need to change but the total flow through the inlet. It is possible to set a high total flow through the inlet even when it is in splitless mode. How you set this varies from instrument to instrument - and I am not sure how it goes with yours. The simplest way may be to simply increase the split ratio that it switches to after the splitless time is over, but on some instruments it is a specific setting for splitless injections. You will need to check with your operators manual.

The rationale behind comparing the same high flow in split and splitless is that the inlet plumbing simply re-routes the gas when it goes into splitless mode, it does not change the volume flow as long as the total flows are set the same in split and splitless. So if, with a high flow in both modes, you see loss of control only in split mode you know that it is not the flow itself that causes the problem, but an obstruction in the flow line that most of the gas flows through in split mode but not in splitless mode. This is the tubing and filter going from inlet to EPC module - you do not want to be trying to clean that unless you are sure that they are where the problem is.

Peter

[krishna30]

if i set pressure at 10 psi , it shows actual pressure showing 10.2 psi,always difference is 0.2. i have cleaned already injector with tubing line.but faceing same problem.what should i do?

[mariasol]

if i set pressure at 10 psi , it shows actual pressure showing 10.2 psi,always difference is 0.2. i have cleaned already injector with tubing line.but faceing same problem.what should i do?

Hi, I´m having the exact same problem, always a difference of 0.2 psi in the pressure. did you solve the problem?