Chromatography Forum

I am working on transitioning to a 20m 0.18ID 0.36um film Rxi-5SilMS column for semivolatiles analysis. When I use 1m of Intermediate Polarity Deactivated guard column I have terrible tailing of the DCM solvent. it tails out until after the elution of the third internal standard half way through the run. If injecting without the guard column I see almost no DCM in the background.

Would it be the polarity of the guard column causing the problem? When doing EPA 525.3 I had to add the guard column to remove this problem with the Ethyl Acetate solvent. I am going to try some non-polar guard column and see if that helps once it arrives. Just wondering if anyone else has seen such problems before.

I do 525.2 using a 0.25 DB 5MS column and don't see tailing with EA, DCM however!!!
What is your starting oven temp, we use 80C.

Normally we start at 50c to be able to separate Pyridine from the solvent front. On the instrument that I use solvent venting LVI I start at 50c then ramp quickly to 90c with short hold then proceed. With either of these I see the solvent in the background at high levels until the oven reaches 150c or higher. Remove the guard column and the solvent peak drops straight down and you can see the first analytes in the total chromatogram. With the tail I can barely see the Internal standard at 20ppm, though in the EIC it is easy to see..

I think I found the culprit, though it really doesn't make sense.

I have been using the Restek MXT metal unions with the 1/32" graphite/vespel ferrules in them to connect the guard to the analytical column. I replace that with a fused silica press tight fitting and the solvent background has dropped dramatically. It is doing it on several instruments, most run drinking water so it doesn't really sound like contamination, but I can see why the silcosteel treated union would hold on to methylene chloride or ethyl acetate.