Half the Column, Same Chromatogram. Trimming your GC Column and Maintaining Resolution.

Oftentimes, when dealing with complex matrices that have a lot of non-volatile material, you may need to trim the front end of your column to prolong the column lifetime.  I was recently working on a project where I wanted to see how short of a column I could use and still maintain resolution between some isobaric pairs of brominated diphenyl ethers (BDE 49 and BDE 71).

I started with a 15m x 0.25mm x 0.10µm Rtx-1614 column and GC instrument conditions that were optimized by Pro ezGC.  I began trimming loops off of the front of the column and accurately measured the new column length by injecting air into my GC-MS to determine the holdup time.  Then, I used the Agilent GC pressure/flow calculator to decrease the column length until the calculated holdup time matched my experimental holdup time.  This new column length was used in the Agilent GC Method Translator software to calculate my new method.  By doing this the analytes are eluting from the column at the same temperature, no matter what column length we use.  This provides essentially identical chromatograms and maintains resolution (See figure below).  I think I could have continued trimming my column, but I ran into two issues.  First, my head pressure was reaching 0psi because I was operating under vacuum outlet conditions.  Second, the oven ramp rate was getting so fast that my instrument couldn’t properly keep up.  Really this just goes to show that you can trim your column in half (and the run time!), while still maintaining resolution, by properly translating the method.

In order to translate your methods remember the following things:

  1. Measure new column length using holdup time
  2. Input new column length and all original conditions into method translator (select translate only)
  3. Make sure to change ALL parameters necessary of new translated method (including your new column length) into GC software
  4. Check your instrument maximum heating rate to make sure your instrument can keep up to new method conditions.
The resolution between BDE 49 and BDE 71 is maintained while trimming the column almost in half.  The analysis time is also cut in half!

The resolution between BDE 49 and BDE 71 is maintained while trimming the column almost in half. The analysis time is also cut in half!

8 Responses to “Half the Column, Same Chromatogram. Trimming your GC Column and Maintaining Resolution.”

  1. jaap says:

    Very nice example michelle: “the better the chromatographer, the shorter the column gets”

    One can speed up a lot. There is only one thing that we also need to realize. as peaks are eluting closer, they will also run faster in each other when the column ages. That means that one needs to be prepared to do more frequent column maintenance, and also the “lifetime” (nr of injections) of such a column will be reduced.

    jaap

  2. Jack Cochran says:

    Excellent post, Michelle! I’m amazed at how well the resolution between isobaric BDEs 49 and 71 held up even after you got down to about half the length of the original column.

    I’m assuming you never changed the flow rate at all for the different column lengths. Is that correct? What was the flow rate? Constant pressure? Constant flow?

    This looks like a great way for customers to save money and get more sample analyses per column, or in another words, reduce the cost of analysis per sample.

    JC

  3. Jason Hoisington says:

    Are you using a TOF MS? I ask because for some applications I’ve found that as I approach 10 minute runs with single quad full scan MS the scan rate starts to become a limiting factor since I can’t get a decent number of data points across each peak.

    There are several products available for people in a similar boat with their run time being limited by their GC oven rate. Agilent and VICI both make resistively heated column modules, and Zip Scientific makes auxiliary heating coils that can be added to the GC oven. Also, Zip Scientific makes a cooling fan that can bring your oven back to ready state faster, allowing you to reduce cycle times without method modifications.

    Jason Hoisington

  4. Michelle Misselwitz says:

    Thank you Jaap, Jack and Jason for your comments. I used helium as my carrier gas and kept a constant flow rate of 1.6mL/min each time I trimmed the column. This means that the average velocity was increasing as I trimmed the column. The average velocity for the 14.4m column is 67cm/s and 90.6cm/s for the 7.9m column length. Although we are operating faster than what the Van Deemter equation would tell us is optimal, we have the resolution to spare.

    The data was collected using an Agilent 7890/5975 GC-MS operated in selected ion monitoring mode. I did a few scan runs just to figure out where to set my SIM windows but I lost a lot of sensitivity and the peaks were not pretty because, as Jason mentioned, I could not acquire fast enough. I know there are some products that could have helped with the oven heating, but unfortunately I didn’t have any in the lab. Is there one that you use in your lab?

    Thanks again for your comments!
    Michelle

  5. Jason Hoisington says:

    We currently use the GC Racer/Chaser from Zip Scientific ( http://www.zipsci.com/), and we’ve had good luck with it. It can’t quite match the heating rates of LTM columns and the like, but they are easy to install and can be used with off the shelf columns rather than expensive wrapped columns. In addition, the since the Chaser simply aids in GC cooling it can reduce cycle times with no method modifications. In general you’ll get more use out of the Chaser if you have cooler initial temperatures since the difference in cooling rates seems more pronounced at lower temperatures.

    Jason Hoisington

  6. [...] colleague Michelle did some excellent work demonstrating how to maintain the separation of critical pairs while trimming up to half the [...]

  7. [...] pressure control (EPC) can accurately deliver your desired carrier gas flow.   When you need to trim your GC column for maintenance, the new column length needs to be determined to properly translate the current method and maintain [...]

  8. [...] Analysis 2013 in Toronto, Canada with Michelle. She presented a poster highlighting her BFR column trimming work, and gave an oral presentation on some results from a Spring Creek sediment study (a local [...]

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