Try to change the pre column

I did already...

ok, have you also cut some cm of the column that was connected to the pre-column? You can also try to condition the column. Use hig temp. for some hours. Howe does the peaks look like ?

Dear Lino,

you must do conditioning of your column via program temperature to 10-20 ºC below maximum temperature of column.

What do you mean by "not looking good"? Are peaks now tailing where they weren't before? Have retention times shifted to longer/shorter times? How long is your retention gap/pre column, what diameter is it, and what volume do you inject? These are all important related questions.

For on-column to work well, the retention gap must have enough surface area for the film of solvent to deposit without hitting the main column (5 m of 0.250 um tubing is usually sufficient for a 1 uL injection). When it gets contaminated enough to cause problems with the chromatography, I would discard the entire retention gap and replace it.

As to your column, trimming 0.5 to 1 m from the front end and then conditioning it should help bring it back to life if the problem is an accumulation of heavy "junk" at the front end. This could be seen from increased retention times and tailing peaks. Note, if you have been doing a fair bit of conditioning without trimming, you might need to trim more, as during this process you are actually moving the heavy junk down the column. Make sure you have good carrier gas before baking for an extended time at a high temperature or you will damage the phase.

The other problem that could be happening with your column is that the phase is just degraded. This will manifest itself by much higher baseline noise, possibly tailing peaks, and shorter retention times. When you bought your column it should have come with a test chromatogram in the box. Run a chromatogram of some of the same analytes under the conditions specified and see what your retention times look like.

If the phase is degraded, then your column is basically done and you need a new one. If trimming your column works, then you're in luck and you can keep on going.

Hope this helps.

Some good advice here. Your column may well need replacing. If the column is cross linked then you could try removing it and rinsing with a suitable solvent. Rinse from detector end to injector end. If you are using MS as a detector you might also monitor for levels of siloxanes as an indication of column degradation. 60m columns are expensive, the whole column might not be affected you could remove the front half of the column and attached a new 30m column (half a new 60m column works out cheaper than a new 30m) with a press fit connector.

Depending on exactly what your crude extracts are and the solvent they are in this can lead to surprisingly short column life. Perhaps you need to examine the options for sample clean-up for your analysis ?

If you do replace your column it would be beneficial to run regular system suitability checks, monitor peak tailing and peak symmetry, resolution etc.. You can use the classic Grob mix for this as suggested or make up a mix tailored for your own specific needs. It is good practice to define performance characteristics for your method and take appropriate action before the degradation becomes problematic and affects the quality of your analysis.

Peaks are asymmetric, tailing and what is worse, the peak of the internal standard (that was not very concentrated so the peak was not very high) is not appearing anymore...We thought it could be a leak but we re-installed everything several times, changed pre-column (as already suggested), cut a small portion in the union of the column and it is not looking better. The retention times are higher then before and even the solvent (hexane) is coming with 2 minutes delaying from the previous runs...

We also run a program with 10 ºC below maximum temperature of the column for an hour which resulted in what seams a "cleaner" baseline but the problem of losing resolution persists. That is why, excluding all other things, I wondered if could be from the column itself that reached its life time...

I will try some very useful suggestions, for which Im very thankful. Just a question to Collin Allchin, what do you mean with "rising with a suitable solvent"? Just inject hexane?

Silvia,

Sorry I meant removing the column from the GC and then rinsing, not simply injecting hexane. This link will explain the procedure.

http://www.restek.com/Restek-Capillary-Column-Installation-Guide-Section-III-Operational-Information

Colin

Silvia,

Have you checked for leaks in the union and/or checked to see if the column flow is what you expect it to be? What type of union are you using? My personal preferences are press-fits (if you know how to do them and are careful with them) or some of the newer all-metal unions with metal ferrules that SGE has been making. I have had bad luck with unions that use polymer or graphite ferrules causing solvent tailing or developing leaks as the oven cycles its temperature.

A leaky union can cause tailing, will always slow down elution, and will also make some peaks disappear. If you have no leaks, your flow is correct, and flushing the column with solvent (also a good idea) do not work, then I would guess that you'd be best to replace the column. The cost of the column will be small compared to the lost time trying to chase down whatever else it could be.

I will take a look at the alternative unions you just suggested, thank you James Harynuk. We have graphite ferrules and actually that was something I didn´t considered yet: change the ferrules. If anything solves, that is just what we are going to do...change column. Again, thank you all for the suggestions. I have been learning a lot!!

1) I assume if your solvents come out that much later, the gas flow is no longer what it should be, meaning something is pretty untight at some place between gas supply and column out.

2) Often good pre columns give good results even if the main column is gone ... for while ;-).

3) With the extraction procedre you describe I would not be too surprised if the column was gone after 250 injections.

4) in my experience baking the column is not successfull in 95% of the cases - that makes only sense if your only trouble is fats.

5) Chemistry is an experimental science. put in a new column, if the problem is still there, it was not the column :-) :-)

good luck

Try to obtain the test chromatogram of the column (included in the factory box) and compare the peaks, If the pattern is different, than you have problems with the column. Change the guard column and condition the main column as well. You can also try to cut a small piece from the main column. At the end , if you have an increasing drift , your stationary phase is degrading. In this case, buy a new column.

If your chromatograph showing peak tailing and loss of sensitivity even after conditiong the column then you should change the column otherwise following the instructions given in the column leaflet.

Have you regenerated the column by passing N2 gas for certain hour? If not then instead of replacing the column you should regenerate(clean) the column. Hope this could help.

For save money, before you buy a new column, you can try these two ways (or both).

First test: invert the column and but the back on the injector and leave the front on the oven of the GC (no connect to the detector) and leave (under carrier) the column close to the maximum temperature that can it work in isothermal.

Second test: take the column and put the front in a vacuum system and start to suck from the back DCM (for around 1 hour) and after for the same time n-hexane. After that suck air for 30 minutes. In function of the contamination you can change solvents.

After the solvents rinse connect the  column to the GC and pass carrier without connect the column to the detector.

After one hour of purge start to increment the temperature over the boiling point of the solvents.

Live for another hour and after that connect the column to the detector and check the performance of it.