THF in LC-MS

[Chellie]

Does anybody know whether or not it is advisable to use or not to use THF on LC-MS? I think compatibility with PEEK tubing as well as with the system are the main issues.

If THF can be used, would it be better to use a very low concentration in the mobile-phase? Would it be a bad idea to use a high percentage in a THF/Water mobile phase.

The method in question is for the separation of organics. THF is the optimal solvent for dissolving samples in. ACN can dissolve samples but only at very low concentrations.

Any suggestions?

^_^

edit: the MS ionisation technique involved is ESI

[Noser222]

and didn't notice anything out of the ordinary

[MG]

Years ago, I recall using a mobile phase containing 30% THF with LC/MS, and it worked. I also recall trying to spray a compound in 100% THF (with stainless-steel tubing), and that didn't work. The main concerns are compatibility with PEEK, and ionization.

If you can rid your system of PEEK tubing (and any other incompatible polymers), then I don't see why it would harm the system. Unfortunately, the design of some systems makes this difficult. For example, with Sciex electrospray sources, there is an important length of PEEK tubing between the ESI needle and a grounding union. You cannot replace this with stainless steel because that would short-circuit your ESI voltage. Perhaps fused-silica would be a workable alternative (as is installed by default on the Thermo-Finnigan ESI sources). On Agilent ESI sources, this is not a worry because the ESI needle is held at ground.

On the ionization issue, I recall that spraying 100% THF gave me zero electrospray current, and no useful signal. Mixing in a polar solvent such as water, methanol or isopropanol should help, but you'll have to experiment to see how much is needed. I would be interested to hear of your results.

[Roy]

I agree with MG. I have used THF with other orgaic solvents and never had any problem. Depending on the structure of your compound you may use a little bit of acid in the sample prep itself to enhance ionization.

Thanks.

[james little]

if the THF doesn't give you any signal, try adding 25 mmolar ammonium acetate in methanol post column. I usually add 0.1 ml/min when using 1 ml/min through the column.

This doesn't seem to cause any problems with precipitating the analyte and helps for ionization for many compounds.

[james little]

I've probably used PEEK tubing and THF and even methylene chloride in marginal situations where it could fail with no problems. Especially in some GPC separations for LCMS identification.

I found some information on PEEK tubing on the internet at:

http://www.rheodyne.com/support/product ... htip02.asp

which was useful. In most of my applications, I used stainless steel tubing on the HP 1100 before the column and then only used PEEK behind the column. Thus, even though the pressure is high, the majority of the pressure drop is before the column. I used the PEEK post column to do splits, post column additions, etc.

As the internet link indicated, thicker diameters are better. We usually use 0.005 id.

Often a mixture of acetonitrile and THF (50/50) gives a good separation as is a strong solvent system for eluting highly retained compounds in reversed HPLC if one finds pure THF is not acceptable.

To enhance ionization, we usually add 0.1 ml/min of 25 mmolar ammonium acetate to methanol to the 1 to 1.5 ml/min flow from the HPLC. Can go all the way to 100% THF with no problems of precipitation of most samples and the ammonium acetate.

The use of the PEEK tubing could still be used on the Applied Biosystems interface again as long as the pressure drop post column is not a problem. The only problem would be if plugging in the whole system increased the pressure drastically. This would be probalby would still not be a problem since the overall column pressure limit on the HPLC should shut down the system.

Be safe..

[MG]

Thanks James, that's some good information. Have you found that the post-column ammonium acetate is superior to acetic or formic acid, in general?

Have you ever noticed any bleed from PEEK when running THF for methylene choride through it? I worry that if a solvent causes a polymer to swell, then some of the polymer might dissolve in the mobile phase, but I'm no polymer expert so this may be an unjustified fear.

[james little]

could get some polymer signal, but haven't run in a while.

Our interface on the Quattro Micro and the LCT TOF is very robust. So I don't worry much about what I put into it. Even run phosphoric acid and a variety of less than friendly buffers. Often get a lot of chemical noise, but longer term detrimental effects.

Normally most of my compounds have UV response, so use the UV response (software corrects for offset in time of UV vs MS signal) to line up background subtraction in my MS chromatogram.

Could use an acid such as acetic or formic, but my compounds of interest tend to work better with ammonium acetate.

Good luck.

[Chellie]

Thank you all for your help, it is much appreciated.
I will keep all of your suggestions in mind.

[james little]

Also, forgot to mention that the flow post column would be at times 1.5 eluent from column and 0.1 ml/min from post column addition from an old Waters 510 pump.

This is too high for the mass spec in many cases, so I do a split a T and some PEEK tubing. 100-250 ul/min going to MS, the rest to waste.

For some applications in GPC, have added potassium acetate in Methanol (I think it was around 500 micromolar in methanol?). Potassium acetate is reasonably soluble in organic solvents compared to some other salts.

[JMB]

Chellie,

1. I have only once used THF as a partial component of the mobile phase; generated many high intensity, proton bound oligomers of THF in the lower m/z range, under TSP ionization. Chose not to use it again !!!!

2. Have routinely run dichloromethane through PEEK with neither swelling nor, as far as I could tell, any elution of extractables (but then I was not looking for extractables).

JMB