Inlet pressure fluctuations and retention time shifts

[Trishia]

I have this problem with my Agilent 6850 (GC-FID), where the flow will go down during a run (then back up to setpoint), causing shifts in retention time. It does not happen every run just every now and then during a sequence of them, which is rather odd (also does not happen consistently; that is not at regular intervals or always with the same drop values). This is what I see in the logbook when it happens:

6850A 1 Not ready:Inlet flow 24.5 at runtime 0.00 17:03:12 10/31/14
6850A 1 Not ready:Inlet pres 11.9 psi at runtime ... 17:03:12 10/31/14
6850A 1 ... 2.66 17:03:12 10/31/14
6850A 1 Not ready:Inlet pres 12.0 psi at runtime ... 17:03:13 10/31/14
6850A 1 ... 2.81 17:03:13 10/31/14
6850A 1 Not ready:Inlet pres 13.0 psi at runtime ... 17:03:13 10/31/14
6850A 1 ... 3.78 17:03:13 10/31/14
6850A 1 Not ready:Inlet pres 13.7 psi at runtime ... 17:03:13 10/31/14
6850A 1 ... 4.48 17:03:13 10/31/14
6850A 1 Not ready:Inlet pres 14.4 psi at runtime ... 17:03:13 10/31/14
6850A 1 ... 5.48 17:03:13 10/31/14
6850A 1 Not ready:Inlet pres 16.3 psi at runtime ... 17:03:13 10/31/14
6850A 1 ... 6.55 17:03:13 10/31/14

Started doing runs without injections, same issue. Checked for leaks with electronic leak detector and could not find anything at any point in the system or gas supply connections (regulator also seems to be delivering fine). Re-installed column with new ferrules, changed inlet liner, o-ring and septa. Did runs in splitless mode to see if issue was related to split ratio or split vent, but it still happened. Didn’t see any drastic temperature changes occurring during runs. Have used specific method before without any problems; also tried with different methods (tried modifying flow/pressure, temperature program, inlet/detector temperature, constant flow vs constant pressure, you name it and chances are I tried it). I was using a Restek Rtx-R1 with Integra-guard column and replaced that with just a length of guard column, but same issue.

The method I originally was running was; Inlet at 275 degC, detector at 275 degC, split ratio of 50:1 (gas saver 20 mL/min at 2.00 min), flow of 2.0 mL/min, T-programming – [65 degC held 0.5 min], [ramp (25 degC/min) to 175 hold 0.25], [ramp (30 degC/min) to 265 hold 2.25], but as I mentioned I also tried many variants of this.

Any ideas where I should be looking next?


Thanks,

Trishia

[dblux_]

Check tightness of the column reducing nut.
Does it happen when method is run without gas saver ?

[Trishia]

So I ran it most of the day yesterday and it does not seem to happen if I turn off gas saver. Would this indicate an issue with the valve or EPC?

[dblux_]

So I ran it most of the day yesterday and it does not seem to happen if I turn off gas saver. Would this indicate an issue with the valve or EPC?

You will need to carrefuly analyze plumbing diagram and the action that takes place when gas saver turns on.
BTW - is carrier gas pressure sufficiently high at the inlet to GC ?

[Peter Apps]

This looks like a leak at the inlet - one that only becomes significant as the head pressure increases to keep the flow through the column constant with increasing temperature. The most likely place is the septum. Do the leak check with the oven hot, or with the inlet pressure increased manually.

Peter

[Trishia]

@dblux_ To my understanding, in Agilent 6850's, the Gas Saver simply changes the split ratio. Yes, carrier pressure is set to like 90 psi (I never had this issue before or with its twin GC).

@Peter I've performed inlet maintenance during the troubleshooting, but still went back and tried to find a leak (increased pressure, tried at higher temperature, during run, etc.), but failed to.

Now if the gas saver is set to 20 mL/min, I occasionally get the drop and I go up to 25 mL/min, same thing. But if I go to 30 mL/min then it doesn't seem to happen anymore... Not sure I understand why that would be required (according to my calculations).

[Peter Apps]

Hi Trishia

Check your actual split flow and actual septum purge flow with the inlet pressure set high. There is more gas leaving the inlet than the EPC is allowing for - somewhere between 25 and 30 ml/min in fact.

Peter

[Trishia]

You mean with a flowmeter? I unfortunately do not have one.

What would this be indicative of, still a leak at some point or...? If it was a small leak, I would assume it would happen every run?


Thanks again

[Peter Apps]

Hi Trishia

Yes, you have to use a flow meter - a simple bubble meter is just as good as a fancy electronic one and a lot cheaper.

The pressure in the inlet is dropping because the gas saver flow of 20 ml/min is not enough volume to account for split + septum purge + column flow + leaks. One of these flows is higher than it should be, or the gas saver flow is less than 20 ml/min. Either way you have to measure actual flows, not the flows that the GC thinks are happening.

Peter

[dblux_]

Have you configured your column dimensions properly in the GC ?
Is column reducing nut firm and tight ?

[gcguy]

Could be a blocked split line, as Peter says confirm the flows are matching the set point. You can also check for leaks by using blanking nuts on the split vent, septum purge and column connection point. Simply block all of these, set a pressure and then turn off the gas. If the pressure drops you have a leak.

GCguy

[Trishia]

@dblux_ - Yes to both questions.

@gcguy and @Peter - I started by plugging the split vent, septum purge and column inlet. Set a pressure, turned it off and it maintained it (split and splitless mode). I also reconnected the column and plugged it at the other end, but same thing.

I measure the flows and I get about 2.9 mL/min off the septum purge, which is good but I do seem to get a higher value than expected off the split vent. For example, setting column flow to 2.0 mL/min and split to 10:1 (split flow of 20 mL/min) and measuring at the vent, I get something closer to 23-24 mL/min (went with a low split ratio due to the limits imposed by my makeshift bubblemeter).

So a little confused still overall...

[dblux_]

... It does not happen every run just every now and then during a sequence of them, which is rather odd (also does not happen consistently; that is not at regular intervals or always with the same drop values). ...

This makes it hard to diagnose.

[Peter Apps]

@dblux_ - Yes to both questions.

@gcguy and @Peter - I started by plugging the split vent, septum purge and column inlet. Set a pressure, turned it off and it maintained it (split and splitless mode). I also reconnected the column and plugged it at the other end, but same thing.

I measure the flows and I get about 2.9 mL/min off the septum purge, which is good but I do seem to get a higher value than expected off the split vent. For example, setting column flow to 2.0 mL/min and split to 10:1 (split flow of 20 mL/min) and measuring at the vent, I get something closer to 23-24 mL/min (went with a low split ratio due to the limits imposed by my makeshift bubblemeter).

So a little confused still overall...

If you have the gas saver set to 20 ml/min and the split is bleeding an extra 3 ml/min then this accounts for all the symptoms that you describe. Is there a way to re-zero and re-calibrate the gas controls on your instrument (I have different hardware to you) ?

Peter

[dblux_]

I would check the whole split vent line (especially connections with split vent trap) for leaks.