Chromatography Forum

Hi all,

I would like to ask for your help. Since a few weeks we've been observing an odd GC baseline (GC-TOF system fitted with PTV and membrane fore pump; 15m/0.1/0.1 column; fast T-ramp from 60º–330ºC): apart from being slightly more noisy than in the past, there's a fluctuating cyclicity in the baseline pattern with a period of a few seconds (see attached screenshot [or here: https://imgur.com/m4Qpz3F]: top trace is TIC 50–550, second is m/z 57, then further below 281, 207 and 85). Now there are three odd/interesting observations— (i) this cyclicity ONLY occurs when solvent is injected (hexane with blanks or samples), air blanks are totally fine; (ii) the cyclicity abruptly disappears once the oven reaches maximum temperature, or once the baseline has risen; (iii) the cyclicity is ONLY exhibited by TIC and m/z 57, not by the column bleed of m/z 281 and 207, or by m/z 85.

The fact that we don't see the fluctuations in all ions suggests to me that this is independent of column pressure variations. No rotary oil pump, thus no issues with contaminating hydrocarbons. Also the occurrence only with solvent injections, and not with air blanks suggests a GC issue, but is awkward. Certainly it's tantalizing to connect the m/z 57 to the injected hexane, but why should it bleed (and from where) for almost the whole runtime of ca. 25 minutes? Has anyone of you had a similar problem before, or has ideas for a possible solution?

Many thanks in advance for any suggestions!!
Chris

The m/z 57 would imply hexane is the key. Have you checked the split vent line? I have had those become contaminated with high boiling garbage before and they adsorb things like hexane or methylene chloride then release them over time, and even though there is a flow out the split vent, some of the analytes migrate counter to the flow and make it to the column.

If it is an Agilent unit, I would remove the copper line to the split vent and either flush it and bake it out or just replace with some clean copper tubing and see what happens, along with cleaning the port where it is connected. I have flushed the line before with solvent and it came out the same color as my standard solutions, so it does capture a lot of stuff.