Full evaporation method development HS GC - HELP

Discussions about GC and other "gas phase" separation techniques.

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Hello people,

First - I apologize for the long text. I am hoping that with this long text I will give you enough information about my situation.

I am contacting you now as I am trying to develop an analytical method with whom I will analyse quantitatively the residual monomer and the aldehyde which is forming as a side product during the hydrolysis of the monomer in my latex.

I want to develop a full evaporation analytical method for this compounds - meaning to completely shift the equilibrium to the gas phase. That I am doing by lowering the sample size - 5μL in 21mL vials and by increasing the temperature which should theoretically lower the partial pressure and shift the compound to the gas phase. To improve the extraction of the VOCs that I would like to investigate, I am also diluting my latex sample to a 10% solids content.

Now as I saw, when one wants to develop something like this, he basically varies the time and temperature and calibration till the highest possible concentration is obtained.

The current conditions of the HS and GC are the following:

Gas chromatograph conditions (HP 6890)
Column: Capillary column SGE BP20 (Polar phase)
Length: 25m
Thickness: 1m
Internal diameter: 0.53mm
External diameter: 0.68mm
Detector: FID FID Temperature: 250ºC
Injection: Automatic
Software: ChemStation.
Carrier gas: Helium
Injector temperature: 200ºC
Inlet pressure: 9.51 psi
Column flow: 2.2mL/min (constant).
Split ratio:50:1

Temperature profile: 35ºC (during 6 min); ramp until 80ºC at 10ºC/min (kept constant during 2 min); ramp to 120ºC at 40ºC/min (constant for 2 min).

HP 7694 E auto sampler

Vial temperature: 130ºC
Sample loop temperature: 170ºC
Transfer line temperature: 180ºC
GC cycle time: 28 min
Vial equilibration time: 15 min
Pressurization time: 0.13 min (17.8 psi)
Sample loop filling time: 0.02 min
Loop equilibration time: 0.05 min
Injection time: 1min

Now the problem - I am varying the temperatures and times of the equilibration in the HS (only thing that I vary) and do not see any change. The problem is that the temperature range which I am changing is in between 80 and 150 degrees and without any change of the peak area.

Please have a look beneath. The numbers in the table are the peak areas.All the samples represent the same mixture where I have 1 wt.% of the displayed compounds.

Equilibration temperatures and times
80ºC 130ºC 150ºC
Compounds 5min 30min 5min 30min 5min 30min
Acetaldehyde 707 883 773 778 750 777
Internal Standard 1235 1555 1339 1351 1311 1386
Vinyl Acetate 570 710 640 640 601 620

In the beginning I thought - ok, maybe the extraction is complete and I do not need to investigate anymore. The problem over here is that, when I test the already tested vial the second time, I get slightly lower numbers for the peak areas (approximately 80% of the original values - meaning that I didn't succeed to shift the equilibrium nor extract fully the compounds of interest.

I must say that I looked in the literature and the people who investigated a similar system didn't have a problem - they had an increase in the concentration with different times and temperatures. So, I am sure that I am doing something wrong but do not unfortunately know exactly what.

Can you please help me and tell me what you would recommend in this case?

Thank you again for taking the time in reading the email!
I am used to a Perkin Elmer headspace so I do not know if you have the option of a multiple headspace extraction. (the needle stays in the vial, heats and pressurizes again for another extraction). If you can perform that it is useful. I have run it with up to six extractions and GC runs to see how much of a sample's analyte is staying in the vial. With sample analytes that extract well, the drop is exponential, disappearing after 2 or 3 extractions and with stubborn analytes, you will see the sample linger for all six extractions. With the equilibration temperatures being right at boiling or above boiling you should be already achieving full evaporation. If you want I would cool down the temp and look for an area count drop that way. If you are aiming for the best extraction and highest area count, a small amount of water may help since your analytes are not strongly soluble in water. I have done this in headspace analysis to increase the area count and extraction recoveries from benzene. In my case, I was dealing with 50mg of sample that I was analyzing (not all of the 50mg was volatile and it was softened with about 50ul of n,n-DMA) and I added 50ul of water. I ended up doubling the area count recovery.
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