Low level PAH carryover

[LADude]

Has anyone tried to analyze low level PAH in SIM mode with an Agilent 6850 GC/5975 MS? Whenever I run a standard (say 1ppm, 2ppm, etc), I have this background of around 0.1ppm in blank after blank for the late eluting compounds. It does decrease some after a few blanks but not nearly as much as it should. Baking it out doesn't help so I'm not sure what to do now. Oh, after the instrument sits idle overnight, sometimes there are no peaks in a blank run. Then if I run a standard, the whole carryover thing starts again. If anyone has had this experience, I would really appreciate your advice. Thanks

[AICMM]

LAdude,

Something to try; inject 10X concentration standard split 10:1 and see if you get the same carryover.

Are you using pressure-pulse?

Best regards.

[Schmitty]

1/2ppm PAH is not low level on GC/MS. What is your signal to noise at this level?

[LADude]

Oh sorry for the mistake. I am looking down to 0.05 ppm - this is low level to us.

AICMM - Yes, using pressure pulse and I'll try your suggestion as soon as I can get some time on the instrument.

Schmitty - Signal to noise for the late eluters at 0.05ppm is anywhere from 20:1 to 100:1

thanks for the replies and help

[jag]

I believe this carryover is from the injector. How frequently you change/clean the liners? Are you using glass wool plug liners?(This also sometimes causes for carryover especially when the injector temperature is low. Use split flow and maintain the injector temperature around 250°C. Which injection solvent are you using?

[AICMM]

LADude,

My experiences with pressure pulse have been very poor. You might try not using pressure pulse at all. You might be able to compensate for that by injecting 2 uL instead of 1. I am assuming you are using dichloromethane so the expansion volume should fit in the inlet, especially if you do a slow injection. If you start the oven cool enough, you will re-focus the 2 uL and should still get good chromatography.

Be interested to hear how this pans out.

Best regards.

[tima]

Use Agilent's Pressure and Volume calculator to figure out the expansion of your injection solvent. Maybe your injection parameters (i.e. temperature, solvent type, initial pressure and liner) results in carry over. Usually the analytes make their way up inside the lines because the solvent expands beyond the volume of the injection port. Try injecting a smaller volume.