Chromatography Forum

I analyze organochlorinated pesticides (EPA 8081B) at an environmental lab, and I've been having a problem with degradation of DDT.

The degradation check I run prior to running samples passes criteria with no problems for both Endrin and DDT. However, after running samples the degradation of DDT is often too high (Endrin is fine). These samples are non-detect for all pesticides, and the only thing suspicious about them is that they are slightly colored.

The degradation problem is fixed after I change out the liner. I'm looking for what could be causing the degradation of DDT so that I can try and fix this issue. Any help/ideas would be greatly appreciated.

Degradation of pesticides like DDT, endosulfan,.. after running samples, is a well-known problem in food & environmental GC analysis. They are very prone to degradation in the inlet due to leftover matrix stuff that is hanging there (from samples), which is why you can restore their performance by changing the liner.

It's not straightforward to give advice how to prevent this issue. Anything that makes your extract cleaner (extra cleanup step, dilution,..) will help, but i guess you don't have the freedom to change the sample preparation.

I have had success with the use of analyte protectants. The idea is that you add a solution of defined matrix (typically ppm amounts of molecules with lots of hydroxyl groups) to both the standards and the samples. This can be added to the vials, or programmed to be injected by the autosampler with each injection. You can google for "analyte protectants GC".

Do you inject in split or splitless mode? If you are doing splitless, try doing a 5:1 split, you won't lose much sensitivity but your liner will stay cleaner and you get less breakdown.

Otherwise it is frequent liner changes to solve the problem.

I once had a batch of ethyl acetate that had something in it that caused those compounds to degrade after 20 or so injections.
When doing a liner change be sure to cool the injector to<70C or risk the chance of activating. Also do not neglect the split vent line (best to install a fresh one, cleaning usually is ineffective) and the bottom seal. When these things do not solve my breakdown, I install a clean injector body.

Thank you all for the advice! I will be doing some modifications taking into account what you have told me, and hopefully that helps with the issue. Fingers crossed.