Chromatography Forum

Greetings,
I'm using an HP6890 GC for 8260 Volatiles analysis and the instrument is having a problem holding a constant head pressure. I'm running @ 1.0mL/min, 36cm/sec and have a head pressure of 7.4 on the GC with a total flow of 23.6. The pressure fluctuates from 7.2 to 7.6 with a setpoint of 7.4 with a constant oven temp of 50'C and inlet temp of 230'C and still has the fluctuation with an increase in head pressure as the oven temperature increases. I did notice the the head pressure will remain constant when I decrease the inlet temperature (stayed at a constant pressure with no fluctuation when the inlet was less than 150'C) but started fluctuating again once the inlet temp was raised back to 230'C. Agilent suggested that I clean the split vent (running in splitless) and change the copper tubing and chemical trap but it seems more like a weldment issue now (using a fliptop weldment and merlin septa). Any thoughts? Thanks for your help.

It sure looks like a leak related to the thermal expansion as the temp rises.

Sometimes the lines into the weldment crack. There is a leak check where you cap off the inlet with a no hole ferrule (a 0.8 mm ID ferrule and a paper clip is good as well), cap off the split outlet (leave it slight uncapped at first), set the pressure to ~25 psi and 50 to 100 cc/min total flow in split mode, turn the pressure off and finish capping off the split vent. If the pressure does not drop faster than ~ 0.1 psi/min, you are pretty much leak free.

Hi

First off you have posted this in the GCMS index so I will assume you have a MSD attached, therefore carry out an Air/Water check.
If no MSD then carry out the Agilent GC leak test ( it is in the manual ). Hopefully this will point to a leak.

If no leak then check your line pressure is high enough.

Unfortunately it may be your EPC, these can give the symptom you describe.

GL

Sniff around for leaks using a leak seeker. From the fluctuations the leak is temperature related and intermittent. Try repalcing the Merlin with an ordinary septum and nut.

Peter

Thanks for the info. I sealed off the inlet with a no hole ferrule underneath and a Swagelok fitting on the split vent and the pressure was dropping about 0.1 psi/sec. Most likely a leak in the weldment or the fittings from the purge and trap transfer line? I would've done an Air/Water check but unfortunately I'm the same guy having the network issue so I can't even bring the GC/MS online with ChemStation.

What was the last thing done to the injection port? Could be something simple like a leaking septa ( they will leak if over tightened).

That's a very good question. Other than what I've been doing this morning, the instrument hasn't been touched in about 2 months as far as maintenance is concerned since it's running 8260 Volatiles. @ Peter-I switched out the Merlin seal and replaced it with a regular Restek Thermolite. Still having the same issue. Also used a lead detector and there are no discernable leaks in or around the injection port that I can pick up.

Apparently my total flow is fluctuating as well. Was a solid 23.6 this morning but now its jumping from 0-70+ in a matter of seconds.

This really does look like an EPC problem. Is the split vent flow fluctuating ? - check with an independent flow meter not the onboard pneumatics.

Peter

[quote="Darber99"I switched out the Merlin seal and replaced it with a regular Restek Thermolite. Still having the same issue. Also used a lead detector and there are no discernable leaks in or around the injection port that I can pick up.[/quote]
When you installed Restek septum, did you replace the Merlin septum cap with the original septum cap? The Merlin septum cap is not made to properly seal a regular septum so that could cause your problem.

@Cleh-Yep, made sure that I used a regular septum cap with the thermolite septa. Figured the merlin cap wouldn't seal the same. Actually looking more like a concentrator issue. The pressure is fluctuating with the concentrator on but I needed to cool the inlet down all the way and it wouldn't cool down past 85'C with the transfer line heated. After the concentrator was shut off the pressure stabilized. Even with the concentrator turned on, the pressure will stabilize if we slightly unscrew the transfer line from the weldment. Any idea what would cause this?

THE GC NEEDS TO BE RUN IN SPLIT MODE!!!