Chromatography Forum

this is EI GCMS for non polar compounds with a non polar column with a split injection. I can't be too specific...

In general for the method:
Injection port @295C, Column from sightly below ambient to 320C

I have a samples that have a lot of peaks early (well often a lot to about 3/4ths of the way through the temperature programed run) ... and we care about all the peaks

Right now I am using a low boiling solvent but that is likely obscuring some early eluting peaks and a solvent that would be better for retaining some of the low boilers would help with their recovery ...

Someone suggested using a high boiling solvent that would elute AFTER the peaks towards the end of the run.

I have never seen that done...and intuitively that does not seem right... Has anyone tried something like that and/or is that likely to cause issues?

Thanks,
-Karen

Are you trying to measure very low boiling compounds, like C5-6 stuff?

Typically you want to start measuring after your solvent has passed through the column and is pumped away by your MS (= solvent delay). If you start measuring before that, you can damage the filament.

As far as I know, the solvent doesn't 'retain' compounds in GC. You can condense the solvent on your column after the injection if you start the oven run below the temperature of the boiling point, this is done to focus the analytes.

Using a high boiling point solvent sounds like a bad plan for GCMS.

The filament is not an issue. I can use a timed event to turn it off anytime during the run.

What would it do to the chromatography?

- Karen

Hi Karen

In answer to your query "Someone suggested using a high boiling solvent that would elute AFTER the peaks towards the end of the run. Has anyone tried something like that" - Yes I have!

You really need to tell us about your sample, your target analytes, your column film thickness etc and your solvent. Only in this way can the forum properly help you.

In this way you may well find that there is another approach to your problem.

Kind regards

Ralph

I am not in a position where I can share more than I did (non polar phase mostly non polar compounds - but a lot of them)... But I would like to hear what might be the issues, if any, with doing this.

That would be enough for me to figure out if it is worth trying

Thanks,
- karen

Since all solvent/analyte/column combinations are unique, the best way to find out if it will affect the chromatography is to try it.

Inject a blank with only the solvent to see where it elutes and what the peak shape looks like. Inject it as a dilute solution in a early eluting solvent first to know where the retention time is, that way you will know when to turn off the filament and see how the solvent reacts to the system(long tail, misshapen peak ect,) to better understand how it might affect the chromatography. Then try injecting a standard with the target analytes and see how their peak shapes compare to the current chromatography, this will tell you how the new solvent would interact with them.

If all that looks good, then try a sample and see what happens and how the results compare to historical results. If everything looks good, you have a new method to work with.

I remember using a higher boiling solvent for GC for one assay because a lower boiling solvent would've coeluted with the analytes.

I just can't remember the specifics, and I'm retired now. I'll chime in if I remember.

Peaks eluting before the solvent will be very broad as they will not benefit from
solvent focussing http://www.sge.com/support/training/inj ... ent-effect.

It's probably good enough if you just want to identify something, but quantitation would be iffy at best.

Only the peaks that elute just before the solvent will be significantly wider - you can see a similar effect if you have trace components eluting just before something at percent levels.

The picture on the link is a very idealized situation, in reality the solvent film gets blown down the column by the carrier gas, smearing analyte over a considerable length of column, and then braking into droplets that evaporate in situ leaving little blobs of analyte in the stationary phase. If you are lucky thermal focussing brings all these together as you programme the temperature, if not you get all kinds of exotic peak shapes with leading or trailing steps, spikes and splitting.

If you want a reliable solvent effect you need to anchor the solvent film in place in a porous bed:

APPS, P.J., PRETORIUS, V., ROHWER, E.R, and LAWSON, K.H. 1984. The dynamic film concentrator a new approach to large and dilute samples in GLC. Journal of High Resolution Chromatography and Chromatography Communications 7: 212 214.

APPS, P.J., PRETORIUS, V., LAWSON, K.H., ROHWER, E.R., CENTNER, M.R., VILJOEN, H.W. and HULSE, G. 1987. Trace analysis of complex organic mixtures using capillary gas liquid chromatography and the dynamic solvent effect. Journal of High Resolution Chromatography and Chromatography Communications 10: 122 127

LAWSON, K.H., PRETORIUS, V. and APPS, P.J. 1987. High precision trace analysis in capillary gas liquid chromatography using an inlet based on the static solvent effect. Journal of High Resolution Chromatography and Chromatography Communications 10: 235 239.

APPS, P.J. 1990. High precision sampling of trace gas borne volatiles by the dynamic solvent effect with a comparative review of alternative techniques. Journal of Chromatography 504: 21 43.

APPS, P.J. 1990. High precision sampling of sub nanogram, low parts per billion solutes from liquids using the dynamic solvent effect. Journal of Chromatography 511: 271 279.

I think that I am right in saying that the repeatabilty in the two 1990 papers has never been bettered.

Peter