GC/MS method for short chain alkene?

[saulitbs]

I want to develop a method for using GC/MS to separate and quantitate 2-methyl-2-butene and 2-methyl-1-butene for a student organic lab. We've used the Gow-MAC GC method in the past...its out there on the web. But no GC/MS method seems to be out there. Isothermal? We have a Thermo Trace GC equipped with a 15 meter TG-SQC(non-polar) column, S/SL injector, ISQ MS detector. Solvent would be methanol. Isothermal? Temp program? Anyone done anything like this? Don't want to damage a brand new instrument.

[tlahren]

Wow. That's some pretty good equipment for a student lab. We only had a few old HP 5890s with FIDs and HP integrator machines (6890s were out for at least 7 years at this time as well as computer data systems). That column is the Thermo diagnostic column that came with the instrument if I'm not mistaken. It should work OK for what you want to do. However, I'm more used to a longer (ca. 30m) column for such early eluting (low BP) compounds.

The biggest concern I have is that you are using MeOH as a solvent which has a higher boiling point than your analytes. Most methods call for purge-and-trap concentration for low BP compounds (in aqueous media instead of MeOH). Since you are using an MS detector the solvent can damage the electron multiplier if you are using a simple S/SL injection. One way to use S/SL and not damage the detector would be to setup a solvent delay (time where MS is turned off during the solvent elution). This can be tricky when the solvent elutes after the analytes but it can be done. Usually you will set up a GC method on an FID detector and then use that method on the MS detector already knowing approximately where the solvent (MeOH) peak elutes at. Another way is to set up a generic GC method (eg., 30°C-2min-5°C/min-150°C-10min this is just a quick idea) and set the solvent delay on the MS to something like 10 or 20 min so you are sure the MS (filament and EM) will not turn on until after the solvent has eluted. While the GC program is running you need to monitor the system vacuum. Watch for a large and noticeable decrease in vacuum (increase in pressure). It will most likely be an order of magnitude or bigger change if injecting 1 µL of solvent. Note the time this pressure spike happens and the time it goes back to its starting value. Subtract about 0.5 - 1.0 min from the start time and add about 0.5 - 1.0 min to the end time. This will be the time range you will have to turn the MS OFF. Most MS method setups allow you to turn the MS off at specified times for reasons like this. The systems "Solvent Delay" you can set to 0.0 min as your solvent will be eluting after your analytes. I'm not sure what data system you are using (Xcalibur?) but I'm not sure where to set the MS "OFF" time during the method setup. I'm sure it will be there and it should be rather easy to find.

I hope this helps a little. I think you should be able to setup some kind of method with what you have. Although, the optimum setup would be to use a purge-and-trap concentrator before the GC.

Maybe someone else has a better idea or can add to what I said.

Ty

[saulitbs]

I appreciate the input. I was very concerned about the methanol hitting that filament... it is beautiful equipment. Unfortunately..no FID detector. ISQ on left, ECD on right. We do have Xcalibur software...all 10,000 pages of it. Thanks for your help.

[chromatographer1]

I think you have the wrong equipment to do the job at hand.

Nothing like a space shuttle to do a quick round trip to Walmart?

C5 alkenes usually require a very long and thick film for capillary separation.

And solvents should elute prior to the analytes of interest on a GC-MS, no?

But all this you know already. I would be very unhappy if someone destroyed my nice MS instrument doing the wrong kind of test on it.

Sometimes less is more. A nice packed column and an FID or TCD sure sound good right now. How much would it cost to fix the MS ?

How much would it cost to tell the instructor to do what he wants might be expensive. WORK CAREFULLY.

I do wish you the best of luck.

Rod

[saulitbs]

You are right. The GOW MAC can do this. Thanks.

[mckrause]

It can be done easily and reproducibly using headspace. Spike the methanol into water, and then inject the gas phase above the water. You will not get the methanol onto the column. BTW, you routinely use methanol in GC-MS analysis because if you start at 35 amu you won't see the solvent - it will essentially be invisible to the system. However, it is a TERRIBLE solvent for GC work - you're much better off using headspace for this experiment, and it will introduce your students to not only MS but to some elegant chromatographic prep methods.