Splitting GC-MS column

[Rich]

Dear Spectroscopists:
I have a HP5972-MS (with a 5890 oven) and am looking to separate out both heavy hydrocarbons and permanent gases for analysis in the MS. Some samples would be predominately composed of permanent gases (CO, CO2, N2) while others would contain heavy liquid hydrocarbons (C5-C20). Right now I am using a 30m RTX column (0.25mm, Restek) and am considering splitting the flow to a 30m Qplot (or equivalent column effective at separating permanent gases, particularly nitrogen and CO). The two columns would join back together at a Y-splitter and then enter the transfer line through a short guard column (about 5m). What are the ramifications of doing this? Obviously, a gas mixture would elute out pretty fast on the RTX column producing unresolved peaks (at less than 1 minute, perhaps). Does it stand to reason that there would be a significant portion going through the Qplot that would allow for sufficient separation? How would this play out?

Rich

[Peter Apps]

Hi Rich

Some drawbacks come to mind.

The pump capacity on the 5972 is limited, so running two columns in parallel will mean either that there is too much gas flow into the MS, or that the flows through the columns are way too slow. Thsi problem will be particularly acute if you use a megabore PLOT (which most PLOTs are).

The heavier hydrocarbons would get onto the PLOT column and take forever to elute, so your baselines and background would continually rise. The PLOT would probably also be poisoned.

How are you going to inject gas samples and hydrocarbons with the same injector - I would think that one needs a valve and loop, and the other a liquid injector ?

Peter

[Rich]

we would just inject a gas sample with a gas tight syringe. Works pretty well. Might there be another column good at separating permanent gases with 0.25mm ID (like the RTX)?

Rich