HELP!!! SPME

[xeim]

Hello,
Please help me!!!

I am using CAR/DVB/PDMS 50/30um- SPME fiber. I am analysing the beer samples and to obtain calibration curves I applied this device in the headspace of standards mixture (ethanol, acetaldehyde, ethyl acetate, propanol, isobutanol, isoamyl alcohol, isoamyl acetate). Unfortunately, I could not get expected chromatogram with SPME fiber. The component were not well separated. Before this procedure I directly injected the mixture of the same compounds and got normally separated peaks in my column. I know that this fiber (CAR/DVB/PDMS 50/30um) is not appropriate for my aroma extraction but at least it is suitable for ethanol extraction. Afterwards, I prepared the ethanol solution just to observe the result and unfortunaly, I got a very long tailing peak. Could you tell me please what is the problem and how I can eliminate it? The GC conditions were: injector-250C, splitless mode, Detector-210C, Column-VF5MS (30x0.2x0.25), Oven- 40 to 120C with 10C/min, . The SPME conditions: 10min extraction at ambient temperature with stirring, 5 min desorption.
Could you tell me please how I can get normally separated peaks in chromatogram?

Best regards,
Ksenia

[rb6banjo]

I do this sort of thing all the time, with the fiber you describe. It's my fiber of choice actually.

Would you append an example of your poor chromatography? Here's mine. I don't need to see ethanol and I do acetaldehyde another way if needed so I only scan to m/z = 50. That's why there's no EtOH in the chromatogram. (Column is Rtx-5, 60 m x 0.32 mm x 0.25 µm from Restek).

[MSCHemist]

That is also the fiber I use all the time. My typical procedure is ~3ml of diluted sample saturated with NaCl with stirbar in 22ml vial, heat to 50 deg C in water bath for 15 min, introduce fiber, incubate 30 min stirring, remove from sample, hit prep run [2:1 split 260 deg Restek 20973 liner] insert into injector and incubate 1min, remove and hit start. I prefer the wax column but I see no reason a db-5 should be a problem.

[rb6banjo]

Wax is better but this just happened to be a 5. The point is that the SPME fiber should be ok for the application.

[xeim]

Thank you guys for your posts. Below I presented the picture of my chromatogram. Could you see it please and tell what is the problem?

I do this sort of thing all the time, with the fiber you describe. It's my fiber of choice actually.

Would you append an example of your poor chromatography?

Here is the chromatogram of standards mix I got. the peaks are veery broad.

[Bigbear]

Do you have a cryo trap?
You maybe also overloading your column. Could you try going split after a min. or so?

[rb6banjo]

One step further. Try to set a low split ratio (2:1 or 3:1) and see what happens if you go with a split injection. The beer chromatogram I show was injected with a 2:1 split.

[xeim]

Do you have a cryo trap?
You maybe also overloading your column. Could you try going split after a min. or so?

no unfortunately I don't have cryo trap.
I thought overloading the column will present peak on the chromatogram with "fronting". And SPME doesn't extract that much component to overload the column. I did with split after a 3 min 1:200 and got the same result.

[Peter Apps]

What is the carrier gas flow rate ?, and what kind of liner do you have in the inlet ?

Peter

[xeim]

What is the carrier gas flow rate ?, and what kind of liner do you have in the inlet ?

Peter

Thank you for the post
He flow rate 1 ml/min. Liner with 2mm ID. And I don't have possibility to use SPME 0.75mm ID liner. Can I obtain normal peaks without the special liner?

[rb6banjo]

If you don't use the low-volume inlet liner, you're going to have problems. I use the low-volume inlet liner. Without it, I fear that you're going to need a cryotrap. The liner is the inexpensive option.

[xeim]

If you don't use the low-volume inlet liner, you're going to have problems. I use the low-volume inlet liner. Without it, I fear that you're going to need a cryotrap. The liner is the inexpensive option.

The problem is that as I can not buy the liner personally from the company they require organization to buy it. Since I have to do everything fast I can not wait until my organization give me the authorization for the liner acquiry. My GC is bruker 430GC with injector 1177. Do you know where or how I can buy the appropriate liner personally without my organization participation?

[Peter Apps]

The problem with the 2 mm i.d. liner is that its internal volume is large compared to the flow rate through the liner under splitless conditions (1 ml/min). You can overcome the problems caused by by the larger internal volume by increasing the volume flow rate, and that is easily done by running the SPME desorption with splitting (as advised in earlier posts). Your detection limit will actually improve with splitting since the peak heights will stay about the same but all the peaks will be narrower and easier to integrate.

Try with 10:1 split and please tell us the results.

Peter

[MSCHemist]

At my last position we did a lot of research on optimizing SPME for Orange juice analysis and ethanol was one of our analytes (it ran ~200ppm). We found that the .75 liner put increased wear on the fiber. We found the optimal liner was 1.0 mm and we use the Restek 20973 liner with a 2:1 split. It is a very cheap liner as well so I'd recommend you get a pack. I use it here at my current position with excelent results.

We also found the best column was the Stabilwax.

[xeim]

The problem with the 2 mm i.d. liner is that its internal volume is large compared to the flow rate through the liner under splitless conditions (1 ml/min). You can overcome the problems caused by by the larger internal volume by increasing the volume flow rate, and that is easily done by running the SPME desorption with splitting (as advised in earlier posts). Your detection limit will actually improve with splitting since the peak heights will stay about the same but all the peaks will be narrower and easier to integrate.

Try with 10:1 split and please tell us the results.

Peter

Thank you very much I tried split ratio 10:1. You can see the result presented below. It helped a lot but still some compounds are not shown in the chromatogram such as acetaldehyde and Propanol.

хостинг фото

Thanks for all of the posts they were very helpful
Could you tell me please How many times I need to dilute the sample with Ethanol concentration about 35% v/v for car/dvb/pdms fiber. I found some articles where the ethanol concentration is adjusted before the trials until 13% v/v. However, in Sigma they told me the concentration shouldn't be high to eliminate the swelling of the fiber. But they didn't determine the exact limit for my ethanol.