Solvent selection for GC/MS

[BrianJR]

Hi all...my first post! My question concerns solvents for GC/MS. Are there any solvents that I should not use due to damage they may cause to the column? Some of the solvents I am interested in using include methanol, acetonitrile and ethyl acetate. The column we are using is a Supelco MDN-5 with COC injection.

[sassman]

Capillary columns can handle most common solvents. Definitely do not use a solution containing strong acid or base as this will hydrolyze the bonded phase. You should probably also avoid TFA, formic, and acetic acids. Some even frown upon ethyl acetate saying that it can degrade to acetic acid, but I think it is not a problem in reality.

Another consideration to take into account is what happens in the injector. Solvents with a high coefficient of expansion such as water expand very quickly in the injection port causing a sort of explosion which is not dangerous but not good for your chromatographic peak shape. HP used to have a little downloadable program called flowcalc to calculate the expansion volume, but I can't find it now.

[BrianJR]

Thanks Sassman! A retired technician once told me not to use methanol, but I can't remember why. A quick google search didn't turn up anything either, so I hope there won't be any problems. I'll try it with an old column just in case though...

[oscarBAL]

Hello Brian; I realy don't know why neither; but i could Think that maybe methanol can carry water and this could decreas your filaments life in your EI source, but do not sounds believable to me...

[Don_Hilton]

Methanol has a trick to it. If you inject methanol into a relatively cool methyl silicon column, it does not adsorb or make a nice film in the column. Because your analytes disolve in the condensed phase, you have material in globs along the column. The methanol goes away and peaks, particuarly early ones come out split and poorly shaped (solven effect gone wrong!)

With methanol on a methyl silicon column, you have to have the column warm enough that the methanol does not condense. And, with COC, this may not be the way you are trying to go...

With a mass spec, I use about any solvent. I've even injected samles with high water content. Just watch out about having filaments on duing the solvent peak or significant tail of the solvent peak. If there is significant gas pressure, you will sputter material away from the hot filaments. Any solvent will do this.

Pick a solvent that will disolve your sample well - and not leave gunk in the syringe or analytes partitioned between undisolved residues in the vial and the solvent. The solvent should be low boiling so that it will clear the column before your analytes begin to elute.

Good luck.

[tima]

You don't use methanol because it doesn't do well with splittless injection. If you go to the Agilent site and down load their pressure/flow calculator, you can determine the expansion volume of different types of solvents, liner types and injection port temperatures.

Methanol and water behave very poorly. Say you have a 1 cc total volume in the injection port and you inject 1 uL of water. Well at 250 C this will expand beyond the volume of the injection port and possibly depositing some of your analytes in the lines. Ghosting of your analytes will occur on the very next injection. So methanol is the next worst solvent for this. I think its otherwise harmless to the column itself. TImothy

[carras]

Hi everyone. I'd like to comment on two issues raised in the thread:

- the problem with the "solvent -effect gone wrong" Don wrote about can be easily solved using a retention gap before the analytical column. As a plus it would protect your column from "semivolatile dubris" found in biological samples (such as mine). That is simply a lentgh of deactivated capillary tubing without stationary phase. Nowadays some vendors offer columns with a retention gap built in (so you don't have to build it yourself). The cheap alternative is buying the retention gap and the connectors and install (or rather, build) it. It is not difficult to do and once done correctly the connetion won't break under normal operation conditions (be sure to follow the instructions provided with the kit). I've been using them for many years.

- solvent expansion in the injector: it is feasible to alliviate it using pulsed pressure injection (available in Agilent instruments) and I suppose in most other GCs); increasing the pressure reduces the volumen of the vaporized solvent.

[Ron]

One other thing to consider is the concentration. For most analyses a split injection can be used instead of splitless. If you do a split injection the expansion volume of the solvent is not as big an issue.