peak tailing for C1C6 analysis by GC-FID

[pacerlaser]

hello, I am setting up a C1 to C6 method involving just the individual straight chain alkanes methane through hexane and I'm doing hand injections with a syringe. I seeing some pretty extreme peak tailing occuring on all the peaks but it does seem get worse later in the run. I'm using the same flow rates that I have in the past with a FID and I have a uniliner for the inlet liner. I have the column on the inlet side going through a gold seal and seated directly into the bottom of the uniliner. Any ideas why the peak tailing is happening?

[Peter Apps]

As far as I know Uniliners were not designed for this application - because the column seals to the liner there is no extra purging around the column tip when in the split mode - all the gas in the liner has to go into the column. With diffusive mixing between the sample and the carrier gas going in at the top of the lner this gives an exponential dilution, and tails on the peaks. With larger molecules that need column temperature programming the tails are focussed sharp again at the top of the column, and in my experience Uniliners work very well in this sort of application. If you are using a megabore (530 micron) column the faster flow rate helps to transfer the sample quickly to the column.

If you really need splitless injections I would go for a classical splitless liner and a splitless time of 20s to 1 min. As a quick and dirty experiment just move the column down by 3 mm so that it does not seal to the Uniliner, and open the split after 30 s.

Peter

[nvalentine]

What type of column are you using?

[pacerlaser]

the column is an Agilent GS-ALUMINA 30 meter with a diameter of 0.530 megabore.

[chromatographer1]

If you are using the 4mm ID , not the 1mm ID uniliner then Peter's comments are appropriate to your situation.

How to fix this?

You need to increase the column flow through the inlet so the vortexing of the sample is not allowed to happen, (10-20cc/min?)

AND

you need to inject SLOWLY injecting your sample 5 microliters? 10? 20? 50?

This will of course widen your initial peaks that do not focus well (C1 - C2) but with trial and error you may find your solution.

I did direct injection of 5 microliters of dichloromethane using this liner and was able to increase the peak areas by almost an order of magnitude with this technique.

With this column you should be able to do the analysis you desire but you need a good seal of the column against the liner and the proper flow and injection technique.

Good luck,

Rod

[AICMM]

Pacerlaser,

Tailing or fronting? Also, what concentration are you shooting? This may be a method best run split.

Best regards,

AICMM

[nvalentine]

On solid phase columns, overloading results in tailing rather than the fronting associated with liquid phase columns.

[pacerlaser]

Thanks for all the great responses, I've seem to have gotten better results by dropping the column down about 3 mm on the inlet and increasing the column flow up to 13 mL/min. The peak shape looks a ton better. Thanks again!