Hi folks,

As much as I am familiar and competent with most aspects of HPLC, I'm a GC idiot, so please bear with me.

I have an old HP 5890 GC attached to a 5971 MSD and both ran OK the last time I used them several months ago. Upon setting up the instrument this time, however, things were different. I proceded with instrument set-up as usual and when I went to pump down the MSD, I found that it would pump down reasonably well initially (to about 120 mTorr), then the pressure would gradually rise to about 500 mTorr. It would remain there for a while (maybe 5 min on average, but variable), then drop back to an acceptable value (again, about 120 mTorr), remain OK for up to 30 min, then begin the whole cycle again. I initially thought that I had a leak where the (capilary) column goes into the detector, so I replaced the ferule and the brass nut, but to no avail. The system is older, obviously, and about a year ago I was told by our service engineer that my diffusion pump could go at any time, so this could be the case I suppose, but I'd like to hear from annyone that might have ideas or may have seen a similar thing.

Thanks!

Chris

Mo' Info...

If I remove the column and plug the detector inlet, the MSD pumps down just fine - pressure steady @ <100 mTorr! Is it possible for a leaky septum or bad connection at the inlet to cause this problem?

THANKS!

Chris

What's the id of your column?

The column is a 320uM x 30M HP Innowax capilary.

It's possible for a bad connection to cause this problem, but I believe that the issue here is with your column dimensions. A 30M x 320uM column is not usually used for MS work because of the higher volumetric flow rate. For example, at a head pressure of 5psi and an oven temperature of 50C, the flow rate will be above 2ml/min. This is generally over the limit for reliable operation on MSD systems. It's likely that your pump cannot handle these flow rates.

Agree with Brian K, you've isolated the problem to be GC-related, not MSD-related. We've always used 0.2 or 0.25 mm id capillaries.

I also agree; using such column dimensions would require an MS with turbomolecular pump, not a diffusion pump.

Thanks - will try a smaller ID. I have used this column before on this particular GC with reasonable success in the past, though as I said, I'm not the most experienced GC jockey on the planet. LC is my primary expertise.

Will let you all know what happens!

Chris