Column snaps in transfer line

[GCMS-ISRP]

Hey forum users!

I'm having a little issue with my Thermo DSQII GC-MS. It wasn't tuning initially and it looked like it was due to excessive column bleed so I'm in the middle of installing a new column (TR1-MS 30m, 0.25mm ID, 0.25um) everything was looking rosy until I tried tightening the transfer line using a couple of spanners, the force pretty much snapped the column at the transfer line nut and it came flying out. This has now happened twice, even after heating the oven to 325C to hopefully soften the ferrule a little to put less pressure on the column it happened (I waited for it to cool back down to ~80C before opening) and I'm at a loss, as the vacuum is insufficient to turn on the RF voltage without tightening the transfer line.

Any thoughts? The column itself is exceptionally difficult to cut for some reason, column cutters are inadequate, I resorted to using a scalpel last time because it was too difficult to use the cutter. Maybe I've received a dodgy column and that's the problem?

I'm a bit new to hardware maintenance, help would be appreciated where possible!

Cheers,
Emily

[Peter Apps]

Hi Emily

Is the nut that you are tightening the one that connects the column to the transfer line ? - or is it somewhere else on the transfer line (I do not know your instrument). If it is the column connector nut it sounds as if you are tightening it much too much. As long as the ferrule is the right size for the column you should be able to get it to lightly grip the column by just tightening it with your fingers (I am presuming Vespel graphite ferrules here, pure Vespel is a lot harder, you do get other fancy metal and combination ferrules - if you are using one of those then follow the manufacturers instructions). Once you have the column properly positioned in the source (according to the MS manual) and the ferrule is lightly gripping the column then 1/3 to 3/4 of a turn with a spanner should tighten it leak tight.

NB that you have to put the ferrule on the right way round - check your operators manual.

Peter

[GCMS-ISRP]

Peter,

I think I am officially an idiot! I took apart the nuts of the transfer line and it looks like I was putting the ferrule in the wrong way round! (No diagrams in instructions or with ferrules unfortunately!) Thanks for your help. The system is vaccing much better now but I can still only get it down to around 100mTorr (and getting scared of tightening the transfer line too much to meet the thermo prescribed 60 mTorr) - how much of a difference does that last 40 mTorr make? Would I still be able to do decent mass spec with the system at the pressure it is now?

Emily

[James_Ball]

Peter,

I think I am officially an idiot! I took apart the nuts of the transfer line and it looks like I was putting the ferrule in the wrong way round! (No diagrams in instructions or with ferrules unfortunately!) Thanks for your help. The system is vaccing much better now but I can still only get it down to around 100mTorr (and getting scared of tightening the transfer line too much to meet the thermo prescribed 60 mTorr) - how much of a difference does that last 40 mTorr make? Would I still be able to do decent mass spec with the system at the pressure it is now?

Emily

I am not familiar with the Thermo instrument but on the Agilent ones sometimes you need to cycle the GC oven a few times then go back and tweak the column connection maybe 1/8 turn to complete the seal since a graphite/vespel ferrule will shrink once heated.

A trick I often use to avoid this problem is to keep a small beaker in the oven with a couple of ferrules in it so that they are being heated and cooled with the GC as it cycles, this pre-shrinks the ferrules so they seal up quicker and it won't damage them either.

Depending on the type of high vacuum pump you have, it could take a little while to reach the 60mtorr if there had been air and moisture in the instrument. If the high vacuum pumps are running then 100mtorr should allow you to use the analyzer, just check your high vacuum to be sure you are down to the 5.5x10^-5 torr range or below and you should be good to go. If you can run the analyzer you can then do an air and water check to see if there are any micro leaks left.

[Peter Apps]

Hi Emily

Depending on your instrument having the specified vacuum levels might be important or not. Check your carrier gas flow rate - if it is too high the pumps might be having to work too hard. As long as your air and water are low enough and you can get a tune you can leak check connections by wafting some solvent vapour around them and looking at what the MS detects - if it sees the solvent there is a leak.

Peter

[wraymogg]

Hello,

for Thermo EI 100 mTorr is way too much, this means a massive leak and you can burn your filament. It should be between 30-60 mTorr. However if you are running CI 80-100 mTorr should be fine. Also a high pressure will give you bad sensitivity.

The major leak sources are:
- the o-ring on the transfer line
- the loose nuts and counter-nuts inside GC (usually the one to the injector port)
- cored septa
- bad interlock gaskets or main vacuum room gasket

Also for me it was the pressure of the He gas to the ion trap. I had to measure it and set it at 50 psi.

If you want to be very methodical, you should isolate your MS by capping the transfer line. You should go easy to 30 mTorr. If not, then the problem is on MS side (bad gaskets or o-rings).

If all OK there, connect the column to transfer line and tighten the ferules, but the other end of the column, insert it into a obstructed splitter (might work with a virgin septa too) (don't forget to keep 40 degrees in the oven, higher temperatures will degrade your column if exposed to oxygen). If you still have more than 60 mTorr then is the transfer line connection. Bad ferrules, not tighten well or cracked tube inside transfer line.

If all OK in this point, then is for sure the injector port. It might be the ferrule, the loose counter-nut or there is a cored or sectioned septa. For me it was the counter-nut, I had to tighten it VERY well, this is why I don't like changing liners from bottom.

Regards,
Vlad Popovici

[GCMS-ISRP]

Hi Vlad,

We did have a look for sources of the leak, it was actually the threading of the transfer line nut that was worn and not giving an excellent grip, meaning that the nut needed to be tightened much more than you would expect (even after a decent temperature cycling) - once the nut was replaced the pressure went right down to 29mTorr, and everyone was happy.

Emily