Ethylene glycol RT shift issue

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
  • Hi Wendy,

    I should have mentioned that if the problem is caused by water, increasing the split ratio should also improve the RT precision, as you are putting less water into the system. As indicated above, at least some of the separation of your EG is happening with injected water acting as a component of the mobile phase above 100C and as part of your stationary phase below 100C - Different amounts of water will change the partition ratio of your analyte/stationary phase - More water acting as a stationary phase will increase the retention time of your EG.

    As you say, you are not obviously overloading the system with EG, but increasing the split to 50:1 will still give you plenty of signal for your low level injection. Headspace injections at low split ratios are notoriously imprecise with differing polarity materials, so you should get better results by decreasing the amount of solvent (water) injected. A 50:1 split ratio is often a good place to start when doing method development - I would avoid <20:1 split with a polar compound and an apolar column (or a polar column and an apolar solvent).

    Regards,

    Tim

Reply
  • Hi Wendy,

    I should have mentioned that if the problem is caused by water, increasing the split ratio should also improve the RT precision, as you are putting less water into the system. As indicated above, at least some of the separation of your EG is happening with injected water acting as a component of the mobile phase above 100C and as part of your stationary phase below 100C - Different amounts of water will change the partition ratio of your analyte/stationary phase - More water acting as a stationary phase will increase the retention time of your EG.

    As you say, you are not obviously overloading the system with EG, but increasing the split to 50:1 will still give you plenty of signal for your low level injection. Headspace injections at low split ratios are notoriously imprecise with differing polarity materials, so you should get better results by decreasing the amount of solvent (water) injected. A 50:1 split ratio is often a good place to start when doing method development - I would avoid <20:1 split with a polar compound and an apolar column (or a polar column and an apolar solvent).

    Regards,

    Tim

Children
Was this helpful?