Chromatography Forum

Hi all,
We are running a C14 & C15 analysis (roughly ppm levels) to demonstrate the use of internal standards in GC-MS analysis. The solvent is hexane and we are using a 20:1 split at 180C.
We have noticed recently that the hexane solvent tails badly into the two analytes peaks which come off around 2 and 3 minutes on our 30 m HP5MS column. The liner is a split liner and the injection port temperature is 250C. This method used to work with no problems. The chromatography of the C14 and C15 peaks is normal other than they lie on the hexane peak of course.

Today whilst running another split analysis of C7-C12 hydrocarbons in hexane with the same column and liner I noticed that the hexane was tailing for over 5 minutes. It was possible to identify hexane as the major contributor to the baseline even when the higher hydrocarbons such as C10, C11 and C12 were being eluted. Again this was never previously the case. Again the peak shapes of the analyte peaks are okay

Any suggestions as what to try. My first instinct is obviously to change the liner but the current liner was clean on inspection perhaps indicating something else. Am I on the right track if I think that the gold seal and/or the split vent may be the next areas to look at.

Thanks in advance
Kevin

I'll lay good odds that you have a fragment of septum in the inlet, or some other crud sitting on the gold seal.

Check also that the column is inserted to the right depth into the inlet, and inspect the first meter or so of the column for particles,blobs of muck, or disrupted stationary phase.

Peter

Are your peaks the same size as they used to be, or have they gotten larger? If they are larger you may have a blockage in your split vent line. Some GCs have a chamical trap in the split vent line, if that gets blocked you are running a lower split, and if it gets bad enough a splitless injection.

I'll lay good odds that you have a fragment of septum in the inlet, or some other crud sitting on the gold seal.

Check also that the column is inserted to the right depth into the inlet, and inspect the first meter or so of the column for particles,blobs of muck, or disrupted stationary phase.

Peter

I'm placing my bet on this, that you have an issue at the GC inlet. You have a GC problem, not a GCMS problem.

You may try to decrease the initial temp to about 60 degrees or increase the split ratio, say about 50:1 or 100:1.
You also try to clean the injector chamber as mentioned above.

I also recommend decreasing the initial temperature and try increasing the split flow 1 or 2 minutes after the injection.

soccerdad007,

What is your column length, what liner are you using, and what GC are you using?

I would interested to hear what a much higher split does.

Best regards.

Thanks for all the great advice to all of you...

I am almost embarrassed to tell the resolution of this problem but for the sake of preventing the same thing happening to someone else, here goes

On close inspection of the liner, I found pieces of a SPME fibre that must have broken off in the liner during a manual injection. Things work great now!!! My only remaining puzzle is why the spme fibre seemed to capture/delay the hexane, thus causing the problem but didn't seem to affect the analyte peaks at all.

Anyway I am happy now that it is working

It happens in the best of families

The reason that you see a tail on the hexane and not on the analytes is that it is present in much higher quantities - blow up the analyte peaks to the same size as the hexane and you will see a tail on them too.

Peter