High boiling compounds by GC/MS

[DanCooper]

Hi all,

Random pop quiz...

Does anyone know the largest reported compound analysed [i]via[/i] GC/MS?

Cheers.

[Don_Hilton]

Off the top of my head: Decabromodiphenyl ether is a pretty good sized compound.

[Ron]

The decabromo PBDE is the highest molecular weight compound (959 amu) that I know of that is commonly analyzed by GCMS, there is a perfluorinated triazine that I have seen a mass spectrum of, I believe the molecular weight was 1068, but this was to demonstrate high mass capability and is not routinely determined in real world labs.

[DanCooper]

Thanks both, what's the limiting factor when analysing heavy compounds? i.e. is it mobility on the column, transfer line temperature etc...

[Don_Hilton]

Termal stability will be a limiting factor - as it takes increased temperature to partition larger molecules into the vapor phase. And, in some cases, such as decabrodiphenylether, the compund breaks down with high temperature (making the analysis of brominated flame retardants by GC an interesting proposisiton) or you destroy the GC column. In the analysis of heavy petroleum fractions, the compunds can take the temperature but columns used for the work are short lived.

[Tarapan]

How about shortening the GC column to cut down the time the compound has to stay in the column?

[DanCooper]

cheers,

What I'm interested in is long chain alkane type molecules, oil fractions etc. Is a C80 chain out of the question?

[Peter Apps]

It would be quite a challenge and I doubt that it could be done with off the shelf hardware. If you are up for it, try exploiting the vacuum of the mass spec to exploit what tiny little bit of vapour pressure the compounds have - use a short megabore column and change the inlet plumbing so that the MS vacuum goes all the way back to the inlet (obviously you can't use a split injection etc !!) I'd be tempted to use an uncoated tube for the column - molecules that big will stick to anything and that might give you the partitioning that you need to separate them. You will need temperatures higher than are catered for in any bench top instrument that I can think of, and the mass is above the upper limit of "ordinary mass specs" although in principal an ion trap or a TOF might handle it, so you will probably end up building the whole thing from scratch. Probably there are easier ways Also I hate to think how tiny the molecular ion will be on a C80 hydrocarbon !

If you do crack it, be sure to let us know.

Peter

[Ron]

I believe that C80 is doable, but as Peter states above the molecular ion will be extremely small. All straight chain hydrocarbons above a certain length start to look alike, and you are definitely in the range where all will look alike. You might be able to elute the C80 hydrocarbon with a 15 m x 0.25 mm id with a 0.1 um 100% PDMS phase, if your mass spectrometer can handle above 2 mL/min column flow. You will have to use retention time standards to verify identification, mass spectra alone will probably not give enough molecular ion for positive ID.

[bhuvfe]

cheers,

What I'm interested in is long chain alkane type molecules, oil fractions etc. Is a C80 chain out of the question?

It seems to be possible if you can afford a SBM-GC-MS (http://www.avivanalytical.com/Supersonic-GC-MS.aspx).

I've seen presentations\paper from the guy above of hydrocarbons close to your C80 (polywax 850). The molecular ions using this technique are favored (expecially for hydrocarbons) and so seeing a M+. between 60- 100 % intensity is quite common.

[Don_Hilton]

I have seen above C40 on a 10m x 0.2 ID x 0.2 film 5% phenyl methylsilicone column. You can try what I understand some of the petroleum people do - run your chromatography above the stated upper limit of the column, The column will not last long, but you can get some results.

[Peter Apps]

All else being equal, wouldn't it be better to use a wider bore column with a thin film, rather than a narrower one ? The phase ratio is what is important, not the film thickness per se.

Peter

[DanCooper]

You can get polywax 650 through high temperature metal clad columns onto a FID, I was thinking of attaching it to some wide bore fused silica and inserting that into an MS. As far as I can tell it's just a question of whether the mass spec is up to it...

[bhuvfe]

... As far as I can tell it's just a question of whether the mass spec is up to it...

Indeed. But probably would be a challenge even if you have a high performance turbo pump (like the one used in NCI). Or maybe you can use a restriction and do vaccum-GC-MS?

Why do you need a MS for this application?

[Ron]

Peter, you are absolutely right, the phase ratio is the important factor. Ideally I would use a 15 m 0.32 mm id column with a 0.1 um phase, but most mass spec systems can't handle this column. Next best thing would be a 15 m 0.25 mm id with a 0.1 um phase, most modern GCMS systems can handle this column, and you can probably get C80 through. Not really sure if the mass spec will give significant information beyond what you get from an FID though.