Chromatography Forum

Hi, my co workers and I have been analyzing Organochlorine Pesticides, PCB's, EDB/DBCP, and Herbicides (EPA 8151). for over a decade on 3 old HP 5890's. We have never experienced a long-lasting instability problem. Calibrations for the pesticides, PCB's, and EDB/BDCP, continue. to show what we consider normal stability,(days or even weeks between maintnence and calibrations). However, the herbicides are rarely stable for more than 2 hours after calibration on all 3 instruments. This has been enduring for almost 5 months. We cannot find anythign wrong with any of the 3 systems, and are completely baffled.

Mr. Turcato,

I would look at the derivatization. Considering all three instruments, considering continued stability on the other tough one (EDB) this is where I would look.

I would also question the gas supply but only after thoroughly hammering the derivatization question.

Best regards.

I am unsure how derivatization issues cause instability or drifting of the calibration. Obviousley we are derivatizing our samples. In addition to that we derivatize our standards in house. The herbicide clibrations are linear with average RF's usually under 10%, and ICV's pass within 5%. Thats well within limits. If we inject one of our mid standards that is the same standard that was used in the calibration curve most responses for most of the compounds begin to drift within a few injections. It is noted that when the responses drift they usually drift HIGH. However some of the herbicice compounds responses stay unchanged while others increase sometimes over 100% in 12 hours. We looked into possible gas problems. We use helium with identical batch numbers on all our GC and GC/MS instruments, and can find no issues with the carrier gas. Not sure about the nitrogen for the detectors or a possible scrubber problem. We feel that if it were a gas issue that other clasifications of compounds such as the pesticides and PCB's would also be affected.

Are you saying that when responses for herbicide increse by 100% the responses for PCB remain the same?

Is it common problem in all three GCs. Instability mean, Is it the area changed between injection? Are the area between GC are different.?

I am stating that the area remains constant on all 3 GCs rigged with 3 diffrent phase columns for Organochlorine Pesticides, PCB's, and EDB/DBCP. All three systems are unstable for Herbicides. prior to 5 months ago all three systems were stable for herbicides as well. All three systems perform well with initial calibrations for herbicides but become unstable (sometimes drastically) within hours after calibration.