Two GC columns into on MS detector

[tlahren]

I was hoping someone would have some insight into this application. I have a Thermo Trace GC connected to a Thermo DFS HRMS. I would like to run two very different methods that require two very different columns without switching columns all the time (each column would be connected to a different injection port). I am familiar with MS No-Vent style connectors but they still require you to remove and change columns frequently. I don't think using an on-off valve would work well as flow through the unused column would be stopped and could result in column damage during the oven cycle. Is there any type of valve that will allow the other column to flow freely (using He no H2 or N2) while not being used or can a micro-channel valve be used like a Dean's Switch to divert the analytical flow to the MS?

Any ideas?

Thanks,
Ty

[Peter Apps]

A Deans switch is uneccessarily complicated for this application, and it will have to vent some sample, which will decrease signal:noise. A long as you have clean gas coming out of the end of the unused column you could Y connect both columns to a single transfer line going to the MS. The drawback is that this increases the load on the vacuum pumps, and you get a double dose of column bleed.

Peter

[tlahren]

I guess I thought of that as well but as you said I am concerned about the pumping ability on the increased carrier flow. This might be my only option it sounds. One of my methods runs a Rtx5MS Sil 10m × 0.18mm × 0.1µm @ 1.8 mL/min on a PTV injector while the other one would be a DB-1 30m × 0.25mm × 0.25µm @ 1.2 mL/min. Do you think it would be negligible if for each GC program I lower the unused carrier flow to say 0.5 mL/min or lower just to maintain flow and reduce the volume going to the MS?

Thanks Peter.

[Peter Apps]

Hello Ty

The simple answer is try it and see ! all you need to have is enough flow to stop back dissusion up the wrong leg of the Y-connector. You are presumably doing some fancy fast chromatography on the narrow bore column - can you afford to have the band broadening that will be an inevitable consequence of the Y-connector and the transfer line ?

Peter

[tlahren]

Ah, more things to consider. Thanks for the wisdom Peter. I probably can't afford much band broadening. Trying to reduce it as it is. Actually I'm not doing fast chromatography but faster/cooler elution of very heavy thermally unstable compounds like decabromodiphenylether and other heavy halogen buddies. As you can imagine peak widths for these compounds are still very wide even on the short column. We used to have a 6m 0.25mm column but couldn't get reliable back pressure in the inlet for the system to function properly. Moved to more narrow and longer column and inevitably the peaks got a little wider. It's a tough game to play with these compounds.

I will probably just use a no-vent or give up trying to run both methods on this MS.

thanks again Peter.

[Peter Apps]

Ty

If you want your heavy compounds to come out more quickly and at lower temperatures then you need a column with more gas and less stationary phase - in other words a thin coating in a wide bore column - your narrow bore column has a phase ratio about the same as a 0.25 film in a 0.32 mm id column. You can put a restrictor at the inlet end of a megabore column and let the vacuum from the MS run the column at reduced pressure - this also gives earlier, cooler elution than using conventional pressurized carrier gas but with volume flows that the MS pumps can handle. Jack Cochran from Restek has worked on this - have a look on the Restek website.

Actually, the peak width per se is not that critical as long as the peaks are resolved - and often desperate efforts to get peaks as narrow as possible actually compromise resolution.

Peter

[tlahren]

I have actually seen this effect with the narrower column as you stated. I was trading off many factors in the process; peak width (mostly reducing sensitivity), elution temp, and also analysis time. I never thought to try a restrictor before a mega bore column. I suppose then a 0.32mm × 0.1µm column would probably work great with a restrictor on it? How does the restrictor work exactly? Is it just a more narrow retention gap type setup connected with a union or is it something more complex?

Thanks again,

[Peter Apps]

Ty

The restrictor is very simple; enough narrow bore (50 micron) deactivated fused silica to stop the MS vacuum from sucking all the way back into the inlet (lengths around 100 mm are about right I think) installed at the inlet end as if it was a column, and then connected to the column with a deactivated presstight connector. You can use a very short piece of very narrow bore silica press fitted into the taper of a uniliner but that is a bit more fiddly to put together. Becuase the column is at reduced pressure optimum flow velocities are very high, but you get them with a low volume flow (corrected to atmospheric pressure). A drawback is that if you really have to run at exactly constant flow then you need a pretty fancy programmable EFC, but some GCs come with these as standard anyway, and a little fiddling with settings will get you close enough if you have more basic gas control.

Peter

[AICMM]

Ty,

If I recall correctly, Jaap de Zeeuw was looking at something very similar to this (presented at Pittcon a couple of years ago.) You might ping Restek and see if they have an app note on this idea.

Best regards,

AICMM

[tlahren]

Thanks AICMM. I'll give them a call.