Chromatography Forum

Hi all!

I have an Agilent 6890 GC with an FID and use a db-5 fused silica capillary column.

Suddenly all my late eluting peaks started to tail extremly (in an ongoing sequence: one chromatogramm was ok, the next: tailing. The tailing was so extreme that a small peak "vanished". Replacing the liner solved the problem for some peaks, but not for all: Menthol - still tailing; Thymol (eluting later): ok.

Shortening the column on the injector site had no further effect. (I cut off 1 m).

Has anybody an idea what could cause that? (The method I use is old and usually well behaved).

When checking what was beeing injected before, I remembered that a colleague had injected some kind of alcoholic drink the day before - probably undiluted by the look of the chromatogramm. I don't know what kind of beverage it was (and she is on holiday now so I can't ask her); but maybe it contained sugar - could that be the cause of my problems? What happens if a sugary-solution is injected on a GC?
(But note that the tailing started not immediately; it began after about ten injections).

Sugar can make a mess in an inlet liner - but you have changed that out. With menthol tailing and thymol not tailing: What inlet temperture and initial oven temperature are you using? Have these changed since things were working well?

Thanks for the answer.

Inlet temperature is 280 °C, initial oven temperature is 45 °C, hod time is 6 minutes, than quickly to 80 °C, hold time 7 minutes, then the temperature rises with 1,3 °C/min.
Menthol elutes at 29 min, Thymo at 40 min.

The method has not been changed.

Try injecting some trimethylsilyl derivatizing agent a few times (like BSTFA) which will react with anything having hydroxy groups still stuck on the column, and make them more volatile. And raise up the temperature for a while. These modern capillaries can be back-flushed with solvent, and there are little kits to do this - it's a little hassle, but I've done that successfully.

And scrub the stainless steel part of the inlet and rinse.

mikani,

How old and how long is the DB-5. Could it have been toasted? I agree with CPG that it could be rinsed but if it is old it may not be worth it. You could also try reversing the column as a quick check on column health. Not a cure but a troubleshooting tool.

Best regards.

Do you have another collumn you could put into the instrument? If there was a sufficient quantity of sugars and related compounds it is possible that decomposition products made it well past that first meter of the column. And, if there is no improvement, you can always take the new column back out fo the instrument, plug it, and put it back in the box for later.

With the information here, the problem be in the inlet or column. A rather messy sample was run and you have already done the obvious on cleaning the inlet - swapping the column out is fairly quick and easy.

Two other things that you can do; change the gold seal at the bottom of the inlet, and remove the column from the oven and visually inspect it with a bright light behind it to see there are any contaminants, thay will show up as discolourations or irregualrities in the stationary phase.

You do not say whether you or your absent colleague are injecting split or splitless - a splitless inection of an alcoholic beverage could put contaminants a lot further into the column than the 1 m that you have removed.

Peter

This linked document from Agilent is helpful. You would be surprised what can get into your split vent tubing and chemical traps, too. I've had to replace the copper tubing and the chemical traps before to resolve problems with an extremely finicky method I was running frequently for a while. Some pretty revolting stuff can come out the split vent connection depending on what people were running before you. It may seem weird that it would have a significant affect, but it can.

In your shoes, I would do the entire Agilent recommended cleaning procedure and I would consider replacing the copper and the traps, too. Plus of course change the septum and liner, and the gold seal. At least at the end of that you wouldn't be worried about your inlet anymore . Just as a tip, I couldn't find the size of gun brush Agilent recommends, and I went with a .30-.32 calibre 8mm brush I found in the hunting section at Canadian Tire instead. I believe Agilent asks for a wider brush, which I can't imagine forcing down the inlet, the 8mm is hard enough!!

Very good info from agilent. I have a GCMS that is tailing terribly bad for all of the earlier compounds in a V8260 method run. I will try replacing the split tubing.

I have seen many times wierd things happen on these monsters.

As for the original post, I would change the column before I tried anything else. I have seen some semivolatiles diesel range organics GCs need a column replaced after some really nasty samples have been run through. Worst case scenario is that you have a new column that has been opened if this does not fix the problem.

Thanks for the many answers!

Unfortunately, we don't have a second column, but I think we'll buy one because it seems changing the column is the next logical step. (I already had changed the gold seal - had no effect).