Unexpected SPME-GC/MS chromatogram

[rambiochem]

Hi all

We are doing GC-MS analysis of manually SPME-extracted volatiles from fermented milk. There are three issues with this:
1. The peaks of our interest as analyzed by MS have weird shapes as indicated by arrows in Fig. A
2. There is a huge peak at the beginning of all the chromatograms including that with blank fiber as indicated in Fig B. The mass spectra in this area match with those of water, nitrogen, etc. This peak appears only with manual SPME and not with autosampler injection of liquid standards.
3. There are many peaks of the matrix (siloxane) in the SPME chromatograms and not standards’ chromatograms (liquid injections), meaning there is bleeding from SPME matrix. We do condition the SPME fiber for 30 min in the injector before headspace extraction. The fiber has been used only for about 10-20 times till now.

Can anyone recommend any remedies to troubleshoot these issues?

Many thanks


https://postimg.cc/vx1YY8N8

[MSCHemist]

Your peaks, mainly the early ones are not focusing well.
You can reduce the desorption time
you can reduce the GC start temperature as low as 35 deg C
You can use a thicker phase column

Sometimes peaks focus better on a polar wax coln or a nonpolar db-5 column

The air void peak is normal
siloxane peaks are also normal not much you can do but ignore them

[James_Ball]

Also for samples with a lot of water, you could add something like a Restek Hydroguard guard column at the head of the column to help focus the water and give it a sharper peak.

Are you interested in water and nitrogen or any analyte that has a mass below m/z35, if not start the scan range higher and those will be ignored.

[rambiochem]

Thank you MSCHemist and James_Ball for your replies. We were taking the earlier run under splitless mode. Changing to 1:20 split has solved the problem to a large extent.

Thanks for your time!