Hi all:

I am runing a method containing about 40% THF and 60% ageous on Waters symmetry C18 150x4.6x3.5um column. I ran into two problems:

1. The baseline was good at the begining, but started to devastate after several days of runing.
2. The back pressure was pretty high at about 270 bar (3000psi).

I heard that THF is not the best mobile phase to choose. It may eat up the pump seal and damage the pump. Is this correct? What precautions should one take to prevent this from happening?

And is high back pressure generally associated with THF method?

Thanks in advance!

Chris

We use the column you stated and THF routinely. It's an excellent solvent for polymeric samples. That back pressure is quite high. I'm assuming your flow rate is no more than 1mL/min. We do not see abnormal pressure with THF, but there will be a significant baseline rise with a UV detector. We use HP 1090 and Agilent 1100 LC and do not experience experience pump seal damage. Hell, we run 100% THF GPC analysis 24-7-365 and changes the seals maybe twice a year.

What is your HPLC tubing?

At the THF concentrations you mentioned you shouldn't use any PEEK tubing.

Kostas

The pressure with THF-based mobile phases is not much different from methanol-based mobile phases at equal composition.

Kostas:

I was using stainless steel tubing. Thanks,

Chris

On the baseline: If you are not loading "dirt" on your column with the samples, check whether your THF already contains peroxides, maybe with starch/iodide sticks (has to be interpreted with care as the method is very sensitive).

Do check your THF for peroxides as HWM suggests above. This is also a useful safety precaution.

You didn't tell us your procedure. If you are not already premixing your mobile phase, try doing this. The system I used would not cope with mixing THF and water on-line; I got too many bubbles and heat even with vigorous Helium degassing. Premix, then degas....worth a try. I think THF and water like each other less than methanol or ACN or water.

David

At my lab, we don't premix the THF with anything, and don't have issues. We have Agilent 1050s and 1100s. We purchase THF in 1 liter bottles without preservative so we don't have opened bottles sitting around for long.

Could you be using preserved THF? THF is frequently sold with approx 250 ppm BHT dissolved to avoid peroxides. There is a chance that you are loading the column with BHT and that could be messing with your baseline. Like Anon. above, I buy unpreserved THF in 1L bottles and I buy only as much as I'll need in the near future.

With regard to your high pressure, I think that's just a function of mobile phase viscosity and your small particle size. According to the table in Snyder's tome, THF at 40% in water has a viscosity of 1.38 cP, which isn't far off that of a 40% MeOH in water solution (at 1.62 cP). Also, remember that a 3.5uM particle is going to give a higher back pressure per unit length than a 5.0uM particle.

Hope this helps...

I WOULD LIKE TO KNOW IF I'M ALLOWED TO USE PEEK TUBING USIN A MOBILE PHASE WITH 5% OF THF
THANK YOU

This is totally OK. The main effect of THF on PEEK is to lower the burst pressure but the effect is proportional to the THF concentration. A 5% THF composition has a negligible effect on burst pressure.

Chris,
I'm really not familiar with the term "burst pressure" could you elaborate please.

Once we met similar phenomena: our QC lab used the THF from another supplier ( Riel-de-Haen)then R&D ( J.T.Baker). The column lost the separation capability very rapidly, but when was washed with 95% water 5% ACN solution followed using the THF from J.T.Baker the resolution returned. Just try another THF manufacture.
Good luck!

Dr.Alex Weisman

Anon (Saturday, September 21st)

Burst pressure is simply the pressure at which the tubing is likely to rupture. The burst pressure is a function of the polymer, the tubing dimensions and the mobile phase composition. All polymeric tubing (at least all the tubing that I've ever tested) exhibits lower maximum pressure when solvent is present than when the mobile phase is 100% aqueous. The effect is most pronounced for "good solvents" such as THF and methylene chloride but, of course, which solvent is a "good solvent" depends upon the specific polymer. Anyway, there is a bit of information about this on the Web (http://www.rheodyne.com/tech_tip2.html). Contrary to one of the postings above, PEEK is compatible with up to 100% THF but the maximum operating pressure is quite a bit lower (1500 psi) than observed for 100% aqueous eluents.

Chris,

I guess that you are refering to me when you say that I said that the PEEK is not compatible with the THF. In fact I didn't say that, but as it was interpretating like this please let me explain myself.

Indeed the PEEK is compatible up to 100% THF. However except the lower burst pressure, THF cause physical swelling. When setting the HPLC with PEEK (especilly if it is of small I.D.) you have the tendance to further decrease its I.D. in all the conections when screwing (i.e. with the column, when using as loop etc.). Extented use of THF, will swell these parts and partially or totally plug them. Of course cutting the end of the fittings will solve the problem at least for a while.

Most manufacturers do not recommend the use of 100 % THF with PEEK tubing. The web site with the most of information in PEEK can be found here:

(http://www.netmotion.com/htm_files/property_peek.htm)

I hope I clarified my point of view,

Kostas