Using the Restek EZGC Method Translator and Flow Calculator to Support Shoot-and-Dilute GC Method Development – Going from GC-ECD to GC-MS

Hopefully some of you are following the Shoot-and-Dilute GC work (split injection) we’ve been doing in our lab, as it offers a way to keep your GC systems up longer by reducing the impact of dirty samples on inlet liner and column integrity.  After giving a lecture on the technique at the recent European Pesticide Residue Workshop in Dublin, Ireland, I was challenged by an audience member to tackle Captan and Folpet using Shoot-and-Dilute GC.  Captan and Folpet are two notoriously unstable pesticides that must be determined by GC because they don’t ionize well under ESI conditions for LC-MS/MS.  By unstable, I especially mean under hot splitless GC inlet conditions.  In addition, they fragment heavily by electron ionization GC-MS, so selectivity in complex matrices can be poor enough to mandate their determination using GC-ECD.  I’ve achieved some very promising results already for Captan and Folpet with Shoot-and-Dilute GC-ECD, but that’s not what I’m here to talk about…

I’m here to announce, for the first time on ChromaBLOGraphy, the release of our EZGCTM Method Translator and Flow Calculator (MTFC), a super cool tool for GC method development.  And, I want to show you how I used it in my Shoot-and-Dilute GC work by translating a method from GC-ECD to GC-MS so I could confirm some Captan and Folpet results for strawberry extracts.  In short, I’m using the same nominal length column, a 15m x 0.25mm x 0.25µm Rxi-5ms, for both GC-ECD and GC-MS work.  If you’re thinking, “hey Jack, just use the same carrier gas flow and oven program and the chromatograms will essentially look the same”, nope, sorry.  You need to account for the vacuum-outlet of the MS and adjust the GC oven program accordingly to elute compounds at the same temperatures to avoid elution order flip-flops that could occur otherwise.  This is made exceedingly simple by using the MTFC!  Review the screen capture below, and then download that MTFC and try it out and let me know what you think.

 

MTFC 2

3 Responses to “Using the Restek EZGC Method Translator and Flow Calculator to Support Shoot-and-Dilute GC Method Development – Going from GC-ECD to GC-MS”

  1. Alan Sensue says:

    Nice post Jack. Have you noticed if you change the carrier gas from helium to hydrogen that the required head pressure when using a mass spec becomes negative? What would you suggest to our readers who are using hydrogen as the carrier gas with short GC columns (15 meters or less) with a mass spec? Thanks.

  2. Alan, I’m assuming you are seeing negative head pressures when translating to a shorter column length using hydrogen? The high vacuum pump is less efficient when hydrogen is used, so typically you would want to reduce the carrier gas flow into the mass spec. The easiest way to do this is to reduced the internal diameter of your analytical column when you make your transition from helium to hydrogen.

  3. Jack Cochran says:

    Thanks for the kind words, Alan! Yes indeed, a pure Efficiency method translation from helium to hydrogen when using a 15m x 0.25mm GC column starting at 1.4 mL/min He (1.75 mL/min H2) at lower GC oven temperatures (e.g. 40 degrees C) and with a mass spectrometer as the detector will cause a negative head pressure, something that obviously can’t be tolerated. If you can tolerate it, e.g. if you’re doing split injection, you might get by with starting at a higher oven temperature. In my example above you’d have to get up to about 100 degrees C to get the head pressure on the positive side for hydrogen, keeping in mind that you’re now doing method development work and not just translating though. If you have the MS pumping capacity, you could do a Speed translation, which drives the hydrogen flow up to 2.5 mL/min. You’ll get a significantly faster analysis (factor of 2) with hardly any loss of separating power, and the head pressure goes positive. (<< Check my work on the MTFC!)

    Chris points out correctly that dropping the inner diameter of the column can help (e.g. 0.18 and 0.15mm), and it does so in two ways. You have less hydrogen flow into the MS now (versus a 0.25mm column) and the run time will be even faster. There is a downside though, and that is the loss of sample loading capacity. Users sometimes forget that you need to inject less on these 0.15 and 0.18mm columns if you want to avoid overload and column maintenance (from dirty samples). Inject 2 or even 4 or more times less. You'll make up on sensitivity loss by having narrower peaks on those smaller ID columns.

    Thanks a bunch for your comment and question, Alan.

    JC

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