Exploring the upper temperature limits for a wax GC column

When I was traveling in Israel a few weeks ago, my new friend and colleague Efraim Graber of Balirom asked me a question I’ve heard often over the years, “When is Restek going to introduce a high-temperature wax GC column?”  Now I’ve probably paraphrased that question a bit, because it might have been “high-temperature polar GC column”, but for many flavor and fragrance chemists like Efraim, a wax (100% polyethylene glycol) is one of the “go to” columns, along with a non-polar column like a “1” (100% dimethyl polysiloxane) or “5” (5% diphenyl – 95% dimethyl polysiloxane), based on using retention indices to identify flavor and fragrance compounds.  Reasons for desiring a high-temperature wax column include: eluting less volatile components in shorter times, better thermal stability, and lower bleed.  Another one is related to more mass spectrometer use in the industry, and that is installing the “1” or “5” column and the wax column in the same GC-MS system.  Unfortunately, that dictates that the GC oven temperature never exceeds the maximum operating temperature of the wax column (260°C, e.g., for the Restek Stabilwax), when an Rxi-1ms or Rxi-5ms can go to 350°C.

Like most users of capillary GC columns, I assume the vendor puts maximum operating temperatures on the boxes for good reasons, and if you want to see a well-developed treatise on that from my colleague, Tom Bloom, go here: “What is the maximum temperature of a GC capillary column and why is it important?”  But I couldn’t help but think after Efraim told me that a 280°C operational temperature for our Stabilwax column would serve his needs, “Try it, Jack, and see what happens!”

I installed a 30m x 0.25mm x 0.25µm Stabilwax in an Agilent 6890 GC-FID.  I used a Fragrance Allergen Standards Kit and analyzed the A, B, and C mixes 13 times each back-to-back using GC oven programs that went to 260°C (the advertised maximum operating temperature of the Stabilwax), 270°C, and 280°C.  Helium carrier was used at a constant flow of 2 mL/min.  The GC oven program was: 60°C (0.1 min), 10°C/min to 260/270/280°C (5 min).  One microliter split injections at 250°C, split ratio 200:1, were made into a Sky 4mm Precision Inlet Liner with wool.

Overlaid chromatograms for the first run from each series of runs are shown in Figure 1 below to demonstrate the GC column bleed.  Restek tests for Stabilwax bleed at 250°C and this particular column showed 2.7 pa on the QC sheet.  As you can see from the figure, bleed for the Stabilwax approximately doubles for every 10°C increase in temperature.  Now if this was all there was to the story, maybe users could live with the increased bleed from 270 and 280°C Stabilwax final temperatures.  However, I eventually made 273 analyses to 280°C and you can see in Figure 2 that the bleed is going up, up, up over that period of time.  What may not be obvious in that figure, but is easily seen for the zoomed-in chromatogram of Figure 3, is that retention times are shifting earlier and earlier as the Stabilwax stationary phase is lost due to repeatedly exceeding the recommended temperature maximum for the column.  Can we tolerate that retention drift?  Probably not, especially when you combine it with selectivity changes that occur as seen in Figure 4 and the increased bleed over time.

The problem in a nutshell is that polyethylene glycol is just not stable at these elevated temperatures.

I showed this data at a recent R&D meeting and asked Restek stationary phase guru extraordinaire, Roy Lautamo, a man of few words, for a declaration on my experiments with Stabilwax at temperatures above the recommended maximum operating temperature.  Quote.  “Don’t do that.”  End-quote.  Enough said…

Figure 1

 

Figure 2

 

 

 

Figure 3b

 

Figure 4

3 Responses to “Exploring the upper temperature limits for a wax GC column”

  1. Javier says:

    High temperature is great, especially because the analysis time is shorter , but, what about with lower limit temperature? sometimes it’s necessary very low temperatures to separate very volatiles compounds or to achieve chiral separations. Why crosslinking wax phases are no used?

  2. Jack Cochran says:

    This is a great comment, Javier, about low temperature, and I completely agree. Wax columns often have lower temperature limits of 40-50 degrees C, and trying to operate below those temperatures results in “gas – solid” chromatography and broad peaks, i.e. loss of efficiency. You have to go to the siloxane columns to extend the temperature limit on the low side, too!

    As regards cross-linking waxes, that removes/reduces their polar functionality and they won’t have the same selectivity.

    Thanks for your comments!

    JC

  3. Jaap de Zeeuw says:

    Nice study Jack.
    I do not think that the selectivity change is caused by a difference in elution temperature. If you calculate, the difference is only 3 C. The problem for wax phases is that, the selectivity is changing when operated at too high temperature. The phase changes. The Stabilwax is already one of the most stable phases as it remains stable up to 250C. Other phases will show already selectivity changes occurring at 250C.

    I am surprised that only 19 seconds are lost in retention over 271 analysis going up to 280C. It means that there is approximate a loss of 15% of phase which is not bad.

    Still, operation at 280 should not be done. You see also that bleed systematically increases, indicating that a different process is in place changing teh PEG irreverisbly. Tmax iso is 250 and programmed one can use the Stabilwax types up to 260C.

    If higher boilers need to be eluted, use a flow program in the last part of the analysis. Increasing the flow a factor 4 will reduce elution temperatures by approximate 30C. As it can be set digitally, its very easy.

    jaap de zeeuw

Leave a Reply