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