Chromatography Forum

Hi,

Recently I did some measurement with GC6890-FID system to detect vinyl chloride. The stock solution I purchased was vinyl chloride dissovled in methanol. Then I use DI water to dilute stock solution. But i got a problem that I couldn't separate the peak between vinyl chloride and methanol. I tried a lot oven temperature programs but it didn't work. With these programs, sometimes I just saw one peak, sometimes two peaks were pretty close. I tried DB-5(30m), DB-5ms(30m) and DB-VRX(60m) columns but all didn't work. Could anyone give me some advice? Thanks very much![/img]

Almost certainly a porous polymer will separate the two, first water then methanol, then VC.

But since your non-polar columns wouldn't separate them, the obvious choice would be to use a higher polarity column.

50% Phenyl, or CWax, or OV-225 type phases come to mind, even a OV-200 or 210.

Good luck,

Rodney George

Thanks Rodney. I tried it with GC-FID in another lab this morning and it can separate Vinyl Chloride and methanol well. Both columns are same as DB-5ms(30m*0.25mm*0.25um) . Also I use exact same GC setting in both GC-FID. Is there any other possible reasons can cause this problem? I'm confused why I got quite different results with same GC-FID system. Thanks.

One does not get different results with the same GC-FID system. You have to agree that "Something was different".

Which system is the NORMAL result? Or, I have to ask, is neither NORMAL?

It is up to you to determine the reliability of your results.

The easiest answer is that the columns were not in the same condition and did not have the same selectivity.

But that is not the only possible answer.

Good luck, and I think you may need it if you are not careful.

Rodney George

If you put two chemicals into a solution and got three peaks I would begin to question the purity of the solvents involved, the condition of the liner/septa/gold seal, and possible contamination from rinse/waste vials.

When I do any sort of solvent work on GCs, I always purchase the highest purity solvents that I am able to find.

And Rodney is absolutely correct, you should not get different results with the FID system. Again, I would question the condition of the columns, liners, seals, septa, and contamination from the wash/waste vials.

Hi
Try a DB-624 column. I separate them with this column.

Hi
Try a DB-624 column. I separate them with this column.

Thanks all. I have replaced inlet septum, liner, gold seal with new parts but it seems not working. The peaks of Vinyl Chloride and methanol are still together. I also tried with a DB-VRX column(60m*0.25mm*1.4um) but didn't work.

To fsistere: Could you tell me the dimension of your DB-624 column? Because the DB-VRX I have is also designed for volatile coumpounds but it gave me bad separation. And what retention time for Vinyl Chloride and methanol in your DB-624 column? Thanks very much!

catbear32,

This will sound obnoxious, but what makes you think you are detecting the vinyl chloride at all? First, it is dissolved in methanol so on an FID you will get huge methanol peak with it's impurities and a small VC peak (if lucky and if VC is high enough concentration.) Second, a 0.25, 0.25 -5MS will not have any retention for methanol or VC so I would be very suspicious of that separation as well. Third, I would suspect the VC is squarely under the methanol which is why it is such an ugly shaped peak in all the VOA chromatograms you see posted.

Porous polymer as Rodney suggests, thicker film/longer columns perhaps, or switch to PID or ELCD where you get some selectivity. A gas standard with VC will identify true retention time without the methanol interference.

Best regards.

Hi
I use a DB-624 30m, 0.32mm, 1.8um.
Column at 35ºC. I do head space, but methanol is not the solvent.
CV.- 2.8min
methanol.- 3.6min
If you have the column you can try

Hi
I use a DB-624 30m, 0.32mm, 1.8um.
Column at 35ºC. I do head space, but methanol is not the solvent.
CV.- 2.8min
methanol.- 3.6min
If you have the column you can try

Thanks. Actually in my experiment, I will directly purge vinyl chloride into water and do the degradation. But first I need get the calibration curve for vinyl chloride so I purchased the VC standard from VWR. Because all the commercial available vinyl chloride standard solutions use methanol as solvent, I couldn't avoid the interference from methanol. Does anyone know vinyl chloride standard solution dissoved in other solvents such as propanol that I can purchase? Thanks a lot!