Reusing nuts (total newbie question)

[lmh]

I'm still new to GC and GC-MS, using an Agilent 6890 GC + 5973 MSD.

When I change columns, is it OK to reuse nuts at the inlet end? At the moment I tend to keep the MS-end column nut on the column when I remove it, and reuse this fitting without any adjustment if I refit the column later (we're having to use several different columns for different analyses on one instrument, so there's more changing of columns than I'd like). At the inlet end I cut the nut off, re-fit a new one, and trim properly each time I fit the column, to ensure a clean inlet-end. I store the column with its ends stuck in the rubber bung thing with which it was delivered, and on installation I hope that I'm not getting lumps of the rubber storage bung-thing stuck in the outlet end (if I were, presumably there would be no gas flow and not much in the way of peaks...).

Very often, my inlet ferule is stuck so tight in the inlet-end nut that I can't get it out, but once in a while I get a ferule that can be pushed out with a bit of wire, and then it hurts to throw it away... are they reusable, or am I just making trouble for myself for a trivial saving? Is it bad practice?

Thanks!

[GasMan]

As long as you have all of the old ferrule material removed and the threads are not damaged, we reuse the column nut.

Gasman

[Peter Apps]

Nuts can be reused indefinitely - just scrape out any old bits of ferrule. I re-use vespel-graphite ferrules as well, and I have a home-made reamer to open the holes up a fraction to get them back onto the column. Using a spetum to seal ends that will not subsequently be cut makes me very nervous, and I use old presstight connectors to join the detector and inlet ends instead. One of the column manufacturers used to supply columns with presstight glass caps - and excellent solution.

Peter

[lmh]

oooh, thanks both. Peter, how do you use the press-tight connectors? I thought they were basically bits of glass tubing with a taper so the column gets stuck in the end, tapering over about an inch. At the inlet end, I have only about 3mm of column protruding from the ferule, which is usually pretty firmly stuck on the column (or am I over-tightening??) so I'm not sure how I'd get enough sticking out the end to make a seal (or do I just cut this end off, use the loose end to seal to the MS end, which is ooodles long before its nut, and then re-fit the inlet-end nut with new ferule (or old if I'm brave enough and can develop my own reamer!) next time I put the column in? We're still very new here about press-tight, only one person has used them to fit a guard to their column (it wasn't me, but I watched with interest), and it was very much read-the-instructions-and-try-this-with-fingers-crossed... we did all the heating-up to make it stick, but didn't use any funny resins, and it seemed to work.

Sorry to harass you with follow-up questions.

Thanks again!

[James_Ball]

For simple storage just pressing them into the press tight should seal them enough to keep out air and moisture.

I normally use septa to cap mine but the press tight seems to be a better idea, as you don't need to trim a tiny bit off each time you use it again, provided you didn't seal it so tight it makes a rough end once removed.

I use the graphite/vespel ferrules on both ends and while it is almost impossible to remove the MS end, the inlet end I can usually pull loose. Try holding the column firmly just below the nut and twisting the nut like you are unscrewing it from the column, sometimes that will make it slip lose, or it will just snap the column which will leave you just where you would have been anyway. I also purchase the deeper inlet nuts from Restek that allow you to use the same ferrules as you do on the MS end. When installing I tighten until I can't move the column up and down any more then about a 1/8 turn more. A trick I also use to get the depth right is I place the column in the inlet with the liner removed, flatten the end of a cotton swab by tapping it on the bench then drop it down the inlet to the top of the column. Then you slide the column down until the swab stops moving, then up again until the swab moves up about 1-2mm, then tighten the nut.

[Peter Apps]

oooh, thanks both. Peter, how do you use the press-tight connectors? I thought they were basically bits of glass tubing with a taper so the column gets stuck in the end, tapering over about an inch. At the inlet end, I have only about 3mm of column protruding from the ferule, which is usually pretty firmly stuck on the column (or am I over-tightening??) so I'm not sure how I'd get enough sticking out the end to make a seal (or do I just cut this end off, use the loose end to seal to the MS end, which is ooodles long before its nut, and then re-fit the inlet-end nut with new ferule (or old if I'm brave enough and can develop my own reamer!) next time I put the column in? We're still very new here about press-tight, only one person has used them to fit a guard to their column (it wasn't me, but I watched with interest), and it was very much read-the-instructions-and-try-this-with-fingers-crossed... we did all the heating-up to make it stick, but didn't use any funny resins, and it seemed to work.

Sorry to harass you with follow-up questions.

Thanks again!

I forgot that you are working with an Agilent - on the Varian/Brukers that I use there is 75 mm of loose column at the inlet end. You can just cut the nut and ferrule off - call it column trimming to maintain performance. Resetting the proper column position at the inlet end is much quicker and easier than at the MS end anyway. When I worked with Agilents I put the column tip about 10 mm above the ferrule so that they column tip was not in the narrow part of the inlet liner where little particles of crud are funnelled down on top of it.
If you are really struggling to get the ferrules off you might be tightening a bit too much. James Ball's guidance is on the money.

Peter

[lmh]

Thanks, it sounds from both of you as though I am tightening a bit too much. I'll try less. I suppose provided the nitrogen peak in the MS isn't bigger than normal, the system must be leak-free. I think I'm paranoid about leaks (or actually, perhaps I should be using my helium leak tester; at the inlet end, it must be helium coming out rather than air being sucked in...)

Sorry I'm so daft: of course James is right: since I'm going to be removing the inlet nut one way or another, I might just as well remove it when I take the column out, and use the glass push-tight thing to keep the column safe, rather than leaving the nut on until I next fit the column, and then cutting it off...

Thanks also for the useful comments about setting how far the column sticks out at the inlet end. It's really interesting to hear how other people do this. The first proper training I got on this suggested fitting the column with a bit too much sticking out, tightening the fitting until the ferule grips the column enough that it won't slip unless you pull, but not so much that it won't slip at all, then carefully pulling the column out to the desired point, and doing up firmly; this was easier if we did the first stage in that little metal gadget that allows you to see what you're doing, rather than in the GC oven. The two of us who tried that seriously here both struggled horrendously (we found it hard to get the ferule gripping the column with just that tightness we needed). Finally someone else taught me a different approach: cut an old septum in half, push the column through its round edge and out through the flat cut side, thread the nut and ferule on top, trim the end of the column to remove any cored septum, rough column bits, gunk, etc., pull the column back through the septum until the right amount sticks out the end, and fit it.

Peter, I'm particularly interested in your idea of leaving a whole cm sticking out at the inlet end. Again, I'd been led to believe that it had to be exactly 3mm or the world would stop turning and no injection would work, but I've found that different people have a different idea of 3mm, and none of us seem to be getting particularly bad effects; it's the same at the MS end, where slight variation doesn't seem to cause the catastrophes we expected.

[Peter Apps]

Peter, I'm particularly interested in your idea of leaving a whole cm sticking out at the inlet end. Again, I'd been led to believe that it had to be exactly 3mm or the world would stop turning and no injection would work, but I've found that different people have a different idea of 3mm, and none of us seem to be getting particularly bad effects; it's the same at the MS end, where slight variation doesn't seem to cause the catastrophes we expected.

With the Varian 320MS that I have the position of the column tip in the MS source is critical to within 1 mm, but the inlet is pretty forgiving. I think that the rationale behind the exact 3 mm in an Agilent inlet is to put the column tip in a zone of high gas velocity in the narrow part of the liner, which supposedly gives better splitting (although I can't see why it would). It also puts the column tip perilously close to the metal surfaces and accumulated gunk at the bottom of the inlet, with a correspondingly higher chance that muck will back-diffuse and get into the column.

Peter

[lmh]

Thanks, really good to know. I'm always inclined to follow instructions until things are working really well, but it's so hard to differentiate the genuinely critical from the crow's-foot-waving: the things I always do in exactly a particular way, because I've always done it exactly that way, and it's always worked, so I'm loath to change, even though actually, it doesn't matter.

Nice example is HPLC column fitting. I know people who insist that you absolutely must have a small flow running when you fit the column because otherwise you might get a bubble in the column. I also know people who are equally insistent that you must Never have even the smallest flow running when you fit the column, because as you tighten the fitting, that flow might force the tube out just slightly before it's truly tight, and that will make a dead volume. I belong to camp 2 (no flow), but objectively I know this doesn't matter in the faintest, because my camp 1 colleagues have no more trouble with dead volumes than I do. I smile secretly when I overhear them explaining why you Must do things their way, and they probably try not to giggle when they overhear me explaining why My Way is Best.

But in GC world I am helpless, I don't know what works, what doesn't, what's essential, and what's not so vital, so I'm really grateful for the tips and advice.

[Peter Apps]

Hi lmh

GC is really much much simpler than HPLC - you can't change mobile phase and there are almost no moving parts to wear out. All the models of gas flow, temperature etc etc are really accurate, and the retention and separation mechanisms are very well understood. EI - MS and capillary GC are a match made in heaven. Inlets could still use some work.

While you find your way into it just follow the manufacturer's instructions. You will very soon get a feel for what actually makes any difference.

Peter

[James_Ball]

For Agilent, as long as the column is near the bottom of the split/splitless inlet you are normally good to go.

I did have a former co-worker install a column into a direct inject inlet on our volatiles instrument and was getting double peaks for every compound. I looked at his column installation and he had pushed the column up to near the septa when it should have been about 5cm lower. The flow from the purge and trap was flowing down to the bottom of the inlet then back up to the top of the column and giving him split peaks.

I have used the half septa trick mentioned above and it can work very well, I have also done that while using a column nut with a used gold seal sitting on top to get an idea of just where the column is sitting in the inlet, works the same as using the silver tool for that purpose. I do mostly volatiles where the inlet is getting sample from the purge and trap transfer line, and we are using straight through 1mm liners, so I try to get the column just a few mm from the bottom of the inlet. On one instrument we have the swagelok adaptor to use a standard 1/16" swagelok nut instead of the column nut, that is where I learned the cotton swab trick since it was much more difficult to get the right height when measuring with the column outside of the inlet.

http://www.restek.com/catalog/view/8572

[MSCHemist]

I actaully started out HPLC and thought GC was harder. All these parameters flow rate, inlet temp, column dimmesions, split ratio and all the different methods of introduction SPME, purge and trap, static headspace...and don't get me started on column nomenclature. C8 C18 simple db-1 db-5 ok db 624; 1701 wtf HP vs DB MS?

I reuse nut/ferrules all the time. I switch column more often then I used to on HPLC. I have a crappy db-5 I use for raw materials and oils based flavors, a better db-5 for quant, a wax column for SPME of flavors with lots of acids and other polars, a db-1 for 3-mcpd, one that is set up for the ODP. I never have to many problems unhooking and rehooking columns at the inlet and I use siltite nuts/ferrules for the quickswap that are designed to be reused over and over I only replace them when they tarnish or I have to hook the column up to my solvent rinse setup.