I've just purchased 0.2M aqueous (m-trifluoromethylphenyl)trimethylammonium hydroxide for the on column derivatisation of fatty acids. It raises the perennial question - should we inject aqueous solution on GC? If using split injection will deterioration of the phase be significant/rapid (am using a PEG phase). To me it is probably analagous to using buffers in HPLC i.e the column doesn't last as long but if you need those conditions you need them?

Try not to allow the water to CONDENSE to a liquid on the column. This is dependent upon the pressure and temperature you are using. Water in the vapor state has little effect on almost any column phase.

Too bad you cannot do your derivatization prior to your injection.

Thanks for that - I'm less concerned with the derivatisation than the general question of whether or not to inject aqueous solvents in GC. Generally I use 30m 0.25 and 0.53mm columns at a head pressure of 11 and 5 p.s.i. respectively. Does anyone have a feel for minimum temperatures for a ramp having injected water before condensation problems could arise?
Thanks
John

I have gotten away with 60C, and in a few caaes 50C, but if I can use 80C or above as a starting temperature I prefer to do that. One thing that may help is to use a 5 meter length of deactivated fused silica tubing as a guard column. If water condenses in a guard column it tends to spread out and not form droplets the way it does on most stationary phases. If you form drops on the wall of the column they can pass through the column rapidly, and cause problems when they hit the detector. You should have no trouble with water on a PEG phase as long as it is a bonded phase. If your derivitizing reagent is non-volatile the accumulation in the column will affect the column life, which is another reason to sacrifice a cheap guard column rather than the analytical column.

The other thought is to see if your derivatizing reagent is available in a methanol solution like some of the other tetraalkylammonium compounds are.

Regards,
Mark

Generally I do fatty acids analysis on the underivatised materials on a free fatty acid phase (FFAP) column using something like diethyl ether as a solvent. This is generally fine for the applications I do. I only got the (m-trifluoromethylphenyl)trimethylammonium hydroxide reagent to see if I could enhance sensitivity where extra is required. It is expensive and from what everybody has said riskier than underivatised analysis.
John

Methyl esters work very well, although they are more time consuming to prepare. At least they go on in organic phase. However, the ability to analyze underivatized fatty acids is great. My philosophy is less (manipulation) is more.

If you look on the Agilent site you will find some useful articles on water injections.

Regards,

Ralph

Water itself will not harm the stationary phase of a bonded and crosslinked capillary column -- these phases will tolerate many injections of water without any noticable difference in performance. Water presents other problems, though:

(1) The vapor expansion volume for water is very large, so you will often end up in a backflash situation causing poor sensitivity, poor reproducibility and/or contamination if the inlet. You can avoid some of these problems by injecting less (0.5 uL instead of 1.0), decreasing inlet temperature and increasing the pressure/flow in the inlet.

(2) Because water is such a polar solvent it can cause problems with analyte peak shape for splitless injections on nonpolar phases under certain conditions. If the phase is nonpolar and you're using water as the solvent then I'd recommend installing a retention gap to help refocus the sample band at the front of the analytical column -- this can improve peak shape dramatically under these conditions.

(3) Water typically carries significant amounts of nonvolatile material (salts) and/or inorganic acid/base material. This material will contaminate and/or damage the column and cause performance problems over time.

Regarding your TMAH material: I've always frowned on the "derivatize in the inlet" techniques. There's just too much that can go wrong, and typically we see these methods provide more variability than one in which you deriv outside of the inlet under "normal" conditions. There's a lot going on in the inlet during injection, and sometimes you can see inconsistent deriv in the inlet under these conditions. It works for some methods fine and can be a big time saver, but if you want the best in stability, reproducibility and accuracy then I'd lean toward doing the deriv on the benchtop not in the GC inlet.

Best regards,
Jason Ellis
GC Column Tech Support
Agilent Technologies

Thanks to everyone. The water in cap GC debate has been bugging me for years. At last I've got some definitive answers on the subject. Next time I have a 'highly charged discussion' (argument) with my colleague 'Cautious' Jim I'll be armed with some facts (always helpful in a highly charged discussion). Thanks again.
John

I just returned from vacation; Jason is correct, and has always posted real information in his posts. We also use 0.5 ul injections when water is injected on 5890s and 6890s with capillary columns.

What is your source for (m-trifluoromethylphenyl) Trimethylammonium Hydroxide in methanol?