Cannot Get Foreline Pressure Down

[handlovic]

Hello all,

If I leave the column flow on for my GC-MS the foreline pressure will no longer drop below 114mTorr. If I close the column flow it drops to around 50mTorr. Does anyone know how to fix this?

[Consumer Products Guy]

Troubleshoot your GC inlet issue, and fix that.

[handlovic]

Troubleshoot your GC inlet issue, and fix that.

Do you think this is a leak in the inlet? How would you go about diagnosing that? Sorry, I am an amateur chromatographer.

[Consumer Products Guy]

Troubleshoot your GC inlet issue, and fix that.

Do you think this is a leak in the inlet? How would you go about diagnosing that? Sorry, I am an amateur chromatographer.

First I would measure the flow out the detector end of the column.

[benhutcherson]

Can we get more details? What instrument is this? What rough pump are you running?

~40-50mTorr(with He) is about what I get using an Edwards E2M1.5 on an Agilent 5975. That's consistent with what the user manual says. I've run it on a Varian DS 102 before and could get ~20mTorr foreline pressures(about what I get now on the 5971 using that exact same pump) at .5mL/min.

Personally, I'd try fitting a blank ferrule in the transfer line line nut and then pumping down overnight to see what you get. This will let you isolate the problem to either the vacuum chamber or the transfer line.

If it pumps down okay with a blank ferrule, consider these points:

1. Are you using the appropriate ferrule material(I like graphite/vespal) with the correct ID for your column and OD/profile for the transfer line? Also, if using vespal, be sure you tighten it slightly after heat cycling it.

2. Is the column broken or otherwise damaged somewhere close to transfer line, you can get a pesky vacuum leak. The severity depends on the column ID and distance from the transfer line nut.

3. Make sure you're not using a column outside the specs for what the vac system on your MS can support. As an example(and I only know this off the top of my head because I was in a position of needing to do just this) the 5971 specifies that .530mm columns must be 50m or longer.

4. This one is probably rare, but I got a call a few weeks ago to look at a system at work that I don't normally run/maintain. Someone had gotten heavy handed on installing the column, and the transfer line nut inside threads were stripped(fortunately the line itself was fine-that would have been a $$$ repair). They couldn't get it to get to the vacs they were use to. Fortunately, a new nut got it going again.

If you can't pump down with the blank ferrule, you'll need to look at all the seals in the vacuum chamber.

[handlovic]

It is a Thermo Scientific Trace 1300 GC with an ISQ MS. When I close the column flow it reaches vacuum (around 70mTorr, the user manual says under 90mTorr is acceptable). It is only when the column flow is open that it does not hold vacuum. This leads me to think the MS seals are good.

I just replaced the column. Could this be a leak in the inlet or does it have to originate from the transfer line? I used a graphite Ferrel for the injector end and a graphite vespel Ferrel for the transfer line. The size seemed to be correct for the column. I am not sure if I overtightened them, could this cause a vacuum leak?

Thanks for your input, again I am very new to this.

[handlovic]

I did tighten the nut after heating through one cycle.

[benhutcherson]

Did the size SEEM correct, or was it actually correct?

I'm looking now at a 10-pack of Agilent brand graphite/vespal. They are marked .4mm, and say they're for a .1-.25mm column. The ones I have around for larger columns are marked differently.

Also, this may be a stupid question but it's something I've seen-did you install the ferrule in the correct direction? At least on all the other GC-MSs I've used(primarily HP/Agilent and Varian) the "pointy" end of the ferrule goes toward the inside of the nut, and the flat side goes against the transfer line.

[Consumer Products Guy]

Troubleshoot your GC inlet issue, and fix that.

From what you've posted, sounds like the MS is fine, so you're getting air sucked in or too much carrier getting in.