Chromatography Forum

Hello, I am trying to set up a GCMS method for the analysis of hexafluorobutadiene. I have tried using a DB-5 (30m x 0.250 mm x 1.00 df) but the HFBD and the impurities all elute within 1 minute and they are not baseline separated. I then went to a RT-U-Bond column (30m x 0.53mm x 20um df) and this has much better separation but some of the peak shapes do not look very good, more like humps. Can anybody suggest a column for HFBD analysis that will separate the HFBD from its impurities without the need for cryogen? TIA!

Interesting molecule.

This material gives a strong molecular ion. Do the other impurities show strong m/z=93 and m/z=162? If not, then I'd just try to use my mass spec as another mode of separation. Sort the TIC by these masses to isolate the analyte from the other impurities. Stay with the 5-phase if the chromatography is best.

That second column if it is 0.53 ID is too large diameter for MS work. Unless you have a triple quad the pumping capacity will limit you to 1-3 ml/minute which is too slow for the 0.53 to give good peak shapes.

The 5 phase or maybe even a 1 phase should work. That is a very non-polar analyte so needs the non-polar column.

Thanks for the replies, I'll give a more non-polar column a try.

There are some more polar columns that are still pretty nonpolar. They add more benzene rings to the stationary phase to increase the polarity.

Rtx-35 or Rtx-50 from Restek. I've never used a 35 but my experience with the 50 is that it bleeds quite a bit at high temperature. It was years ago so perhaps they've improved the quality of the manufacturing process.

Getting some more interaction between the pi-electron clouds of the analyte and the benzene rings could improve your retention.

Because HFBD has a low boiling point, is it possible to have essentially the opposite of a solvent delay? Basically if the HFBD samples are diluted in a solvent, can I have the mass spec turned on for the first part of the analysis then have it turn off by the time the higher boiling solvent makes it through the column?

You could try a DB-624 or DB-VRX, I think that's what I've used in the past but it was a while ago.

Because HFBD has a low boiling point, is it possible to have essentially the opposite of a solvent delay? Basically if the HFBD samples are diluted in a solvent, can I have the mass spec turned on for the first part of the analysis then have it turn off by the time the higher boiling solvent makes it through the column?

I think I've done this in an Agilent ChemStation before, I think there was a way to switch off the mass spec filaments, etc. for a time segment and then have them back on sort of like an in-run solvent delay. Or maybe I'm imagining it.

I think I've done this in an Agilent ChemStation before, I think there was a way to switch off the mass spec filaments, etc. for a time segment and then have them back on sort of like an in-run solvent delay. Or maybe I'm imagining it.

I seem to remember this function as well, but I was more experienced with the older GCMS Chemstations for the 5971 and 5972. I think we switched the filament off after a certain time once it was not needed to keep the data files smaller (we had 386 computer and 486 computers in those days). I remember when we upgraded the Windows on the 386/5971 unit to Windows 3 !!!

Because HFBD has a low boiling point, is it possible to have essentially the opposite of a solvent delay? Basically if the HFBD samples are diluted in a solvent, can I have the mass spec turned on for the first part of the analysis then have it turn off by the time the higher boiling solvent makes it through the column?

Yep, if you're running an Agilent GC when you're in your oven ramp settings there's an option for a post run bakeout. Adjust your ramp so that the run is over right after all your target analytes are off and it will kick over to post run and not collect data.
Heads up the timer starts right away so if you end at 170 and have a 4 minute 260 degree post run you're really only at 260 for around a minute.

Thanks for the replies everyone!