Chromatography Forum

I've recently had two instances of a laboratory contamination I'm trying to track down. I have been doing some trace analysis, concentrating down solvent extracts using high purity nitrogen. Once using hexane and a second time using acetone I have found a silicone oil (PDMS) in my extracts and blanks.

Since it occurred using two different solvents, I suspect it may be in my sample concentration step. I am using compressed Ultra High Purity (UHP) grade nitrogen for my blowdown. I have heard that it is possible to pick up contamination from compressed gas sources either when taking the tank down to near empty, or that there can be some bleed from components of the regulator.

Does anybody have any ideas, experienced any similar problems?

Thanks,
Mike

Hi Mike

You do not say whether you are doing HPLC or GC, but I am assuming GC, and that you are detecting by MS.

By far the most likely source of "silicones" is column bleed if you are using a silicone column. Second most likely is septum bleed.

Make an injection of solvent that has not been concentrated by evaporation, if the silicones are still there they are coming form the GC, not from the solvent, gas or laboratory.

Peter

Actually I should have specified. I am analyzing the extracts by FTIR - evaporating the extracts down on ZnSe crystals and collecting spectra in transimittance mode.

I do have GCMS and HPLC instrumentation in my lab, but in this case FTIR analysis.

Since it occurred using two different solvents, I suspect it may be in my sample concentration step. I am using compressed Ultra High Purity (UHP) grade nitrogen for my blowdown. I have heard that it is possible to pick up contamination from compressed gas sources either when taking the tank down to near empty, or that there can be some bleed from components of the regulator.

Does anybody have any ideas, experienced any similar problems?

The only impurity that normally might increase when cylinder pressure gets low is moisture. The easiest way to check if the cylinder is the problem is to use a different cylinder.

As for the regulator, if you are using one with a rubber diaphragm, that may cause purity problems. Make sure you use one with a stainless-steel diaphragm. (A brass body is fine.)

Oh, and avoid Tygon and similar polymer tubing, they can off-gas plasticizers and such. Copper tubing is a better option. Just make sure it's rated to an appropriate working pressure.

Also, you can use Zero grade nitrogen instead of UHP, if there's a cost or availability issue with the UHP.

How about grease from glass joints?