A Higher Initial GC Oven Temperature Can Cause DDT Degradation – Is There Anything That Does Not Cause DDT Degradation, Jack?

In recent blogs I’ve relayed a litany of things that can cause DDT degradation when analyzing samples via GC-ECD as part of EPA Method 8081b for organochlorine pesticides, but have you considered that the GC initial oven temperature might also play a role?  (OK, this guy has finally gone off the deep end, because there is no way that can make a difference.)  Well, yes, I may be Common Loony, but I have the data to prove it below.  It happens when you’re using an Agilent GC and you don’t have wool in the liner and you make that ultra fast injection with the Agilent 7683 autosampler and you have a higher initial oven temperature. 

Notice in the figure below that DDT breakdown percentage goes from about 5 up to about 9 just by increasing the oven temperature start from 40°C to 120°C.  That’s because with the Agilent splitless inlet design, the bottom of the liner, and more importantly, the metal inlet seal, reside inside the GC oven.  A hotter oven means a hotter seal means more DDT breakdown if you are NOT using wool in the liner to stop the injected sample.

I show two photographs below that are significant to the work.  Taken from inside the Agilent 6890 GC oven, one is the split/splitless inlet with the nut warmer cup in its proper place (some of you are thinking that the proper place for that #$%^ thing is in the trash can, I know).  The other photo shows the inlet without the nut warmer cup.  I USED the nut warmer cup for my experiments here.  (If you look really closely you can see that the bottom of the inlet is actually cooler when the nut warmer cup is absent and the initial oven is relatively cool.) 

OK, I was just kidding about looking really close at that photo to get a reading on the inlet temperature.  But wouldn’t it be great if you could do that?  No more burned fingers!

Check out the bad juju that goes down when not using the nut warmer cup.

Keep your nut warm this winter and improve GC pesticide response.

4 Responses to “A Higher Initial GC Oven Temperature Can Cause DDT Degradation – Is There Anything That Does Not Cause DDT Degradation, Jack?”

  1. I would have expected a less linear line with temperature. degradation is usually exponential with temperature. I hope that our customers have red Grob’s recommendations on good injection practice. When doing splitless injection, we should start at 15C below the boiling point of the solvent, so that may help a little on this effect.

    The next steps for reducing DDT /Endrdin challenges are:

    – impact of guard tubing
    – coupling
    – analytical column stationary phase
    – elution temperature ( gas flow, temp program and beta)
    – detector temperature

  2. Jack Cochran says:

    Hi Jaap:

    I was a bit surprised as well that we didn’t see a bigger effect on DDT degradation as we went up in temperature, but I’m not sure that the temperature increase at the bottom seal was linear, since there is an insulator (the infamous nut warmer cup) on the inlet fitting that is in the GC oven.

    As regards starting 15°C below the boiling point of the solvent with the intial GC oven temperature, that is only necessary if you want the “solvent effect” for analyte focusing. Remember that we also have “cold trapping”, where you can start the GC oven temperature as high as ~ 150°C below the boiling point of your analyte. With EPA Method 8081b samples, it’s desired to start the oven temperature as high as possible to increase sample throughput, especially since oven cool-down time can be relatively long. And in this case, with hexane as the solvent, if we try to start the oven around 50°C to accommodate the “solvent effect” (hexane’s boiling point is around 69°C), it takes quite a bit of time to cool-down. Actually, we sometimes see our customers starting at higher GC oven temperatures than I show here even, and many of our applications show that, too (e.g., http://www.restek.com/Technical-Resources/Technical-Library/Environmental/env_A017).

    I think I need to go back and do some additional work!

    Jack

  3. At a Lab in Seattle WA, a predecessor of mine would clean the injection port by flushing with solvent, bobbing off the column end & inserting a new liner. The IP had to be overhauled because of the excessive DDT breakdown, and once reassembled clean would also have unexceptable degradation. What is a Chemist to do? Inject the dirtiest extract you could find!! Breakdown was then acceptable & he would continue on with the analysis.

  4. Jack Cochran says:

    Interesting, and somewhat surprising! I think it all depends on what kind of “dirt” we’re dealing with, eh? At least for active pesticides, things like methamidophos, acephate, omethoate, to name a few, “priming” the inlet can yield a higher response. In fact, the “analyte protectants” articles out there by Poole, Lehotay’s group, and others make a science out of this phenomenon. But I’ve found that a really dirty extract can actually hasten DDT breakdown, especially if it’s an extract with high amounts of chlorophyll.

    Thanks for the comments, Andreas!

    Check this out: The Impact of a Dirty Marijuana Extract on Pesticide Recoveries from a Splitless GC Inlet (http://blog.restek.com/?p=2985)

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