Wild;ly varying responses using the same organochlorine pest standard on the Agilent 7890B

Greeting forum denizens. I’m running organochlorine pesticides on an Agilent 7890B with micro-ECD detectors, and I’m having a problem with the results on my rear column. The rear column is a Restek CLPesticides column and the maximum program temperature on this is 340 C. My oven ramp varies from a low of 210 C to a high of 320 C. My detector temperature is 320 C and the temperature on the injection port is 160 C.

When the instrument has not been used for a day or so I start off with injecting a 2.00 ppm pesticide mix to prevent adsorption on the column, followed by two hexane rinses. For some time I have been having a problem with my resulting peak areas. They vary quite a bit. To use one compound as an example (a-BHC, the first eluting pesticide), my first injection has an area of 11679.2. My second injection has an area of 31087.8, and the third injection has an area of 22964.8. In all cases the peaks look nice and sharp with no tailing. They are noticeably smaller in height.

The next day I switched the front and rear injectors to check for possible problems with them. I figured if the injector was the problem then my issue should follow it and I should then see this problem on the front column and not the rear. But the problem seems to he stopped. Does anybody know why this would happen? Was it just that I didn’t sufficiently prime the column the previous day? Why would I only have this problem with the rear and not the front. This is very confusing to me.

Additional info: The front column is a Restek CLPesticides 2 column run under identical conditions. Again, it is not having this problem. All three injections are consistent.

  • When I ran PCB/Pest my inlet was 250, not sure if that's a problem or not.

  • Thanks for the info. I've considered the injection port temp, but I'm not sure why that would only be a problem on the rear. Both injection ports are at 160 C, but I can certainly give that a shot.  You didn't have a problem with DDT or endrin breakdown? I thought I read endrin breaks down about 230 C. Am I incorrect?

  • Hello  ,

    May I ask what is the expected area for the compound you mentioned? In other words, when you are not seeing this issue, is the area closer to 11679.2 or closer to 22964.8? Since the area seems to be decreasing and not varying up and down, I wonder if there is some sort of contamination/carryover issue that gets washed away with subsequent runs.

    What type of injectors are you using? From what I understand, swapping the injectors between inlets results in neither of the channels showing the issue you are describing?

  •   Thank you for the response. 

    The first question is a good one. I'll make a single compound standard of a-BHC and run it. However, this is happening across all compound. I'm just using that one as an example. I considered carryover, however, in the hexane run after I run the 2.00 ppm primer there are no pesticides, so I discounted that notion. But I'm going to be gone from tomorrow until next Wednesday. Perhaps on returning I'll run a few standards w/o priming the columns.

      Interestingly enough, all my injections are pretty consistent today. The problem arose after not using the GC while I was on vacation last week. Still, why only on the rear column? That is really throwing me off.

      I'm not totally sure what you mean by the "type of injectors". If it's helpful, they are both Agilent G4513A.

  • https://www.restek.com/globalassets/pim-media/instruction-sheets/124-01-004.pdf   

    All the Restek parameters have the injector at 250.  I don't recall much breakdown issues.

  •  ,

    For the first question,  I meant is more as a comparison between what is the expected area vs the area that you see. If the area of the a-BHC on the front injector is closer to 22964.8, it could be an indication that the initial area of 11679.2 there is some extra sample coming from... somewhere.

    If the expected area is indeed closer to 11679.2 and the signal just decreases, that indicates a separate problem.

    As for the injectors, I was just curious if it is a model I am familiar with. 

    I understand now they seem to be working fine, so not much troubleshooting to be done. The reasons you'd see something like this on back and not the front could be because of the age difference between the columns, syringe or inlet consumables, that I can think of. If the problem happens again and it may indicate carryover, try changing the syringe, doing some inlet maintenance and see if it improves the situation.

    If the front and back channels are identical (injector, inlet, column and detector), one way to rule out column problems is to swap columns on the inlet side. If it's a column problem, it would follow the column.

    All this is from a HW perspective, I am entirely unfamiliar with the application. Hopefully you can get more insight on that from other users that use similar setups with same, or similar applications.

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