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Ethylene glycol RT shift issue

wendywen

I am using GC6890 /MS5973 and COmbi PAL headspace to analyze ETHYLENE GLYCOL and other solvents. But always got RT shifts with concentration increase.

anybody could shine any lights for me to find the bug?

 

Here is detail infor:

 

Ethylene Glycol Peak RT shift issue

 

concentration of ethylene glycol = 2.5 ug/ul, pipette the following amount to 10 mL headspace vial for each calibrator

 

Calibrator 1, Ethylene Glycol Peak RT = 8.35 min, Amount = 0.8 uL

   Calibrator 2, Ethylene Glycol Peak RT = 8.38 min, Amount = 1.5 uL

   Calibrator 3, Ethylene Glycol Peak RT = 8.47 min, Amount = 2.5 uL

Calibrator 4, Ethylene Glycol Peak RT = 8.92 min, Amount = 5 uL

    Calibrator 5, Ethylene Glycol Peak RT = 9.22 min, Amount = 10 uL

 

NOTE:

1)other solvents RT are always consistent without any shifts

2)The calibrators were prepared by spiking pure analytes into triacetin.

3) 250 uL headspace gas was injected by CTC Combi PAL.

4) The peak width becomes wider with increasing concentration, peaks is showing tailing for calibrator 4 and 5

5) Peak area is linear for first three calibrators, then suddenly drop for calibrator 4, then back for calibrator 5.

6) RT for same calibrator (repeated injection) are consistent without any shift

7) All above obServed information could be duplicated by running another batch of 5 calibrators. We repeated several times, always got same results like above.

8) Inlet liner is Agilent ultra inert PN 5190-2293

9) GC conditions: Inlet 250 C, split 10:1, flow 4.0 ml/min, oven 35C hold 1.5 min then to 300 C at 30C/min

MS interface 280 C, Scan 15-125 amu.

Parents
  • 0

    Thanks for the info Wendy,

    Depending on the polarity of your column I would expect DMSO to elute near EG, and triacitine (glycerin triacetate?) after EG. I have not used triacitine, but it was an old GC/MS trick to determine volatiles by split injection by dissolving them in DMSO - Generally, even with similar amounts of DMSO injected for each concentration, the retention times could change.

    Cheers, Tim

  • 0 in reply to timstrutt

    Tim

     

    Why RT change with DMSO even same amount?  Thanks !  Wendy

  • 0 in reply to wendywen

    Small variations in concentration, different speeds of cooling/evaporation/sorption/desorption in injector, particularly at low split ratios and lowish injection temperatures...

    Expanding my post: In your case, if you are still using the same concentrations:-

    Calibrator 1, Ethylene Glycol Peak RT = 8.35 min, Amount = 0.8 uL

    Calibrator 5, Ethylene Glycol Peak RT = 9.22 min, Amount = 10 uL...

    If most of the solvent is in the vapour phase you would be injecting roughly .03mg of solvent for Calibrator 5 and .002mg for Calibrator 1 (This is where it gets tricky, and my mental arithmetic could be wrong - It is late at night here and I am definitely old enough to need my beauty sleep) 0.03mg is also very roughly the same amount of stationary phase that you could expect coating ~1 metre of capillary column. If your peak is ~3 seconds wide and the carrier is at ~30cm/sec, your analyte peak also occupies ~1 metre of column - So your analyte will also be retained by very roughly about as much solvent as there is stationary phase material in 1 metre of your column for your Calibrator 5 and less than 1 tenth of that for Calibrator 1. You would expect more solvent to increase the retention time of your analyte as the polarity of your solvent is similar to your analyte...

    Tim

  • 0 in reply to wendywen

    What kind of area counts are you getting EG at the lowest standard?

    Ron

  • 0 in reply to timstrutt

    Great points thanks a lot!!

     

    Wendy Wen

     

    MW Labs LLC

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