Peak shape for Grob mix in new VF5ms column

I installed and conditioned a new column (30m X 0.25 mm X 0.25 micron VF5ms + 10m EX guard; part # CP9013) at 320 °C for 30 mins and then ran repeated Grob injections. For the 10 split injections (A_071822_005), all compounds have a symmetrical, gaussian peak shape except for a couple of the compounds such as dicyclohexylamine and 2-ethyl hexanoic acid. For the splitless injection  (A_071822_006), the earlier eluting peaks such as decane, 1-octanol, and undecane have split peaks but the later eluting peaks (C10 to C12 fatty acid methyl esters) all have symmetrical peaks. 

The temperature gradient for this run is 40 ° for 1 min, increase 40°C/min to 300 °C, hold for 4.5 mins. The inlet temperature is 275 °C and for the splitless injection, the purge flow to the split vent is 40 mL/min at 1 min. The injection volume is 1 microL. The Grob mix (Programmed Test Mix, Supelco, catalog # 47304) is diluted 10-fold into dichloromethane.

Why do the earlier eluting compounds have split peaks? Does it have anything to do with the EPC? Any suggestions are appreciated, thanks!  

  • Are the retention times of the peaks pretty stable, even the ugly ones?  Are the peak heights pretty stable?  If so - it's not an EPC problem.   If any peaks are great - it's probably not an instrument problem at all.    These peaks are gigantic - nearly 10,000,000 counts tall. Dilute your diluted standard 100:1 and try again. I bet the peak shape is at least slightly better.   Your column does not have that all that much sample capacity.

    What is your column flow rate?  Inlet pressure?    If you're using Helium carrier gas at 1.2 ml/min column flow and 40°C initial oven, the inlet pressure should be about 9.2 psi.

    Using the Vapor Volume Calculator, 1uL of DCM injected at 275°C and 9.2 psi makes ~643 uL of vapor.  Which liner do you have installed?  Some liners cannot hold that much vapor and that much vapor will nearly exceed most liner's maximum.   Try injecting only 0.25uL or even 0.1uL  first since that's an easy test.

    Next to try would be a pulsed splitless injection.  If your system has a turbo pump, try a pressure pulse of 25 psi for 0.25 minutes and then a purge flow to split vent of 60ml/min at 0.75 minutes.  This temporarily increases the column flow to 3.3 ml/min. The higher inlet pressure means that the 1uL of DCM only expands to 259uL and the higher flow rate in the liner means that it transfers into the column faster, too.  If your system has a diffusion pump, limit the pressure pulse to 16 psi so that you don't exceed the 2.0 ml/min maximum flow into the MS.

    Next is to change solvents to something with a higher boiling point and lower expansion coefficient.  DCM's boiling point is 39.6°C - and your column is at 40°C - so the DCM does not stop and give the solvent effect trapping and peak width reduction that is part of the reason to use splitless mode.

    and let us know how it goes.


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