loss of vacuum mid-run

[LoayJabre]

Hi everyone -

I've been having an interesting problem. Around 13 minutes into my run (when the GC temp is at 94C), the roughage pump makes a weird gargling sound for about 3 seconds, and I lose vacuum (foreline pressure shoots up) and I get a massive peak. Even when I don't turn on the mass spec until after that peak, the roughage pump still makes that weird noise and I lose vacuum for about 10 seconds. None of that happens when I run a blank tube or a standard run. The worst part is that this happening at slightly different times every time I run a sample, so even if I turn the mass spec off up to 13 minutes, sometimes it happens at 13.5 or 14 minutes. Has anyone experienced this ? I am using a Thermo TRACE1300-ISQ system with a MARKES Unity 2 Thermal Desorber.

I talked to tech support and a few engineers and they have never heard of the problem. Could it be that the peak is so large that it is overloading the system to a degree that even the roughage pump can't handle it ? My sample is on thermal desorption tubes (so not liquid injection). Could there be moisture on the tube that is expanding when the column reaches a certain temperature ?


Thanks
Loay

[James_Ball]

What is your temperature program for the run?

You may want to check the nuts and ferrules on the column to be sure they aren't cracked and the expansion from changing oven temperatures causing the temporary leak.

[gapeev]

I wander if this gets it nailed:

Contemporary Massspecs and GCs have rather robust power supplies capable of working at least 20 V below nominal voltage.

A rough pump motor is not robust at all and WILL produce a kind of gargling sound when voltage drops enough. RPM may slow down to a point when protection circuitry or starting winding kick in.

If a single circuit branch feeds all three devices (MS, GC and the pump) here is what may be happening: GC draws enough current to cause voltage drop at the pump to a point when it just chokes.