Peak Tailing using Carboxen/PDMS fibers

[Pat Crowley]

Hello,

I have been using a Carboxen/PDMS fiber to analyse both solvents (acetone, ethyl acetate, propanol etc) and BTEX in water. I find that with both analyses I get severe peak tailing using this fiber, but not with other fibers, eg PDMS or PDMS/DVB. I have used injection temps of up up 310 deg C with the Carboxen fiber to desorb these analytes. Has anyone had success in avoiding tailing using this fiber?

Thanks.

[GOM]

Hi Pat,

Would you post your column and conditions?

Thanks,

Ralph

[Pat Crowley]

Hello,

I am using a Supelcowax10 30m, 0.32mm, 0.25df column.
Initial temp 40 deg C (hold 2 mins) to 90deg C @5deg C /min
Injector 180deg C Det 200 deg C

I can desorb BTEX using either PDMS or PDMS/DVB with no difficulty using these conditions, with sharp peaks, but not with CAR/PDMS.

Thanks.

[chromatographer1]

There are several ways to improve the peak shape of volatile components with the Carboxen PDMS fiber. I routinely extract these compounds with the Carboxen-PDMS fiber and with the exception of benzene the peak shape is good with slight tailing. There are several key factors that will help improve peaks shape. Here are some of the key steps:
1. You must use a narrow ID straight liner. I prefer 0.75mm ID. This mimimizes the band broadening in the injection port by increasing linear velocity through the liner.
2. Desorb at 300°C or 310°C which you are already doing
3. The analytical column is very important. For volatiles, I commonly use a 30m x 0.32mm x 4.0µm PDMS column which focuses the analytes well. If you desire to use a 0.25mm ID column, use a column with a 3µm thickness.
4. Open the split vent on the inlet after 0.75min. This will reduce tailing with mimimal loss of analyte.
5. The concentration of the sample is critical. If you overload the fiber you will increase tailing. Small polar analtyes can be in the low ppm range, non polar or moderately polar analtyes such as ethyl acetate should be in the ppb range.
6. Lastly keep your extraction times relatively short. If extracting in the headspace mode for most of these analytes 10-15min is adequate. If you go much longer the analtyes will migrate deeper into the pores which will increase desorption time.

The shape of the analtye and the size play a factor on tailing. Benzene tails much more than toluene or xylenes, even though it has a lower BP. The pi-pi interacitons in the mesopore region makes benzene more difficult to desorb. This can increase peak tailing. For volatile nonpolar anlatyes it is very difficult to competely eliminate tailing, but these steps should improve the peak shape.

Try these steps. You can e-mail me at the address below if you have any additional questions or you would like to see a c-gram.

Robert E. Shirey
Senior Research Scientist
Supelco, Inc.
595 N. Harrison Road
Supelco Park
Bellefonte, PA 16823
(814) 359-5706 Phone
(814) 359-5702 Fax
bshirey@sial.com

[GOM]

Hi Pat,

I use the 50/30 DVB/Carboxen/PDMS fibre with no problems.

Your injection temp seems low to me, although you do say in your initial post that you have been up to 310°C. May I ask few further questions?

What is your purge time on the splitless injection?
Has somebody replaced the 0.75mm liner with a wide bore liner ?
Are you using a narrow bore SPME liner?
Is 40°C the lower listed limit for your Supelcowax
Is the fibre still intact?
Have you tried running the Car/PDMS fibre at 260°C, then changed back to the PDMS fibre (run at 240°C)? I'm trying see if it really is the fibre and not some other change that has taken place.
If PDMS works, why change?

Regards,

Ralph

[Pat Crowley]

Hello,

Thanks to all for replies. In response to all the questions I have listed my parameters as follows;

Extraction Time 15 mins
Desorption Time 1.5 mins
Splitless Time 2.5 mins

I am using a narrow bore (0.75mm id) liner all the time. The lower limit for the Supelcowax10 column I am using is 35 deg C. The CAR/PDMS fiber is intact and when I resample with a different coated fiber the chromatograms are fine. I am working in the low ppb range BTEX (<10 ppb) and the response of the PDMS fiber at this level is quite low (using FID anyway)

My initial desorption temp was 180 deg C as this worked well with both the PDMS and PDMS/DVB fibers. With the CAR/PDMS at this temp the peak shape was very poor. As I tried increasingly higher temps (up to 310 deg C) the peak shapes improved but are still quite bad compared with those obtained with the other fibers.

I will retry the extraction using a thicker film column(maybe 0.25df is too low to focus analytes from the CAR/PDMS fiber)

Will report back with any developments!

Thanks to all.

[GOM]

Hi Pat,

Change your splitless time to 45 seconds first - that may solve your problem.

Regards,

Ralph

[Pat Crowley]

I'll try that first , thanks.

[Pat Crowley]

just thought of something.. do I still leave the desorption time at 1.5 mins?

[GOM]

you can do, to clean the fibre. and change your initial oven time to 45 seconds

Ralph

[Pat Crowley]

Hello,

Reporting back as promised

I first tried reducing the splitless time to 0.75 min using the Supelcowax10 (0.25df) column. I got a slight improvment in peak shape (slightly less tailing) but still not really great. I then tried a ZB-624 1.8 df column and this gave a great improvement in peak shape,(to the point where tailing is hardly noticable). However this column will not seperate the p and m Xylenes. I would like to try a supelcowax10 column with a thicker film, either 0.5 or 1.0 df.
My current column temp conditions are 40 degC (hold 2 min) to 75 degC at 5degC/min. the elution time of the final peak (o-Xylene) is 7.6 min. The linear velocity of the helium gas is approx 25cm/min. Including the cooling down time the run time for each chromatogram is roughly 10 mins. I would like to keep the run time below 15 mins ( the time used for extraction). Would changing to the 1.0 df column increase the run time by too much? I guess I could decrease the run time with this column by raising the initial temp and possibly increasing the temp gradient as well.
Could anyone to give a ball-park guesstimate as to what my retention times might be with a 1.0 df column

Thanks to all.

[chromatographer1]

Would you care for a suggestion?

Try butt connecting a 1 to 5 meter piece of thick film column to ( ahead of ) your thin film capillary column to refocus your analytes.

You may be able to separate your xylene isomers using this two column configuration with a minumum of retention time changes of your peaks.

just a guess from a wild and crazy guy,

Rod

[chromatographer1]

You could also try using a short ( 1 - 5 meter ) thick film Supelcowax column ahead of the 30m 0.25mm 0.25µm column instead of the 624 column. You might see a slight improvement of resolution without the slight changes in selectivity the 624 column will effect.

best wishes,

Rod