Melgar
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Analysis of methyl-chlorinated toluene
What would be a good chromatography technique for analyzing a mixture of toluene and its chlorinated deriviatives? I would need to know, with a fair
degree of accuracy, the ratio of toluene to benzyl chloride to benzal chloride in a mixture. It would be nice to know the amount of benzotrichloride
too, but this isn't as important.
I already ran a GC/MS analysis, but it couldn't get the numbers for toluene, just the other three.
I suspect this mixture is too volatile for TLC or paper chromatography.
Fluorescence spectroscopy is one that I don't know much about, but seems like it could work. And I know where I could get access to the equipment
too.
Proton NMR is good at analyzing products, but not so much for quantities or ratios, so this is probably out.
HPLC seems like the one I'll have to use if all else fails, but I suspect this is my most expensive option.
So, anyone have an idea of what would be a good technique for me to use?
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matei
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Why GC/MS didn't work? You didn't have a peak for toluene in the chromatogram?
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Melgar
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The retention time was such that it didn't capture toluene. I had someone else run the test for me and she didn't seem to think they could set a
range that would capture all three that I needed.
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tmb
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Do you have a halide-sensitive detector? If you cannot retain toluene - which seems odd - then it would be the only compound not detected. Are all of
these compounds stable at elevated temperatures? I would have thought not.
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Melgar
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Hey, if I understood all the ins and outs of the GC/MS equipment, I wouldn't have had to get someone else to run the test for me. Apparently, if I
want more accurate results, I'm also going to need standards, which would probably cost me an arm and a leg.
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tmb
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Maybe you could hydrolyse to benzyl alcohol, benzaldehyde and benzoic acid; and use column chromatography? If the person can't retain toluene on a GC,
maybe it would be a waste of money to continue with them?
(A DB-17 column will retain toluene, if you wish to continue with that. But if you have to educate the 'experts'...)
[Edited on 22-7-2011 by tmb]
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Paddywhacker
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What you say makes no sense. You can get different separation using different columns .... silicones, carbowax, polyesters, etc.
Optimize the separation by using a thermal profile.
For a crude estimation use either the peak heights or the peak area as a measure of concentration.
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Melgar
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So Paddywhacker - you think column chromatography is the way to go?
tmb - in any case, GC/MS is probably out unless I can find some other lab to do it that can retain at least the three I need.
Sorry - I'm no expert on chromatography/spectroscopy. I've been reading some books on the subject, but that's no substitute for experience.
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tmb
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Thinking about it - If the components are stable, why not just fractionally distill?
(As an aside, if you are interested in a fantastic book on chromatography and all of 'separation-science', I would highly recommend 'Chemical
Separations - Principles, Techniques and Experiments' by Clifton E. Meloan. He's a good old-school analyst who wasn't afraid to make his own column or
detector; or to realize that paper chromatography has something going for it.)
[Edited on 23-7-2011 by tmb]
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Paddywhacker
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I meant GLC columns, not benchtop columns.
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matei
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As Paddywhacker said you should optimize the temperature program. Because toluene is volatile and is not very polar it tends to get pretty early out
of the column, so the temperature program should be something like: 45 deg. C (5 min) and then ramp to 250 deg. C (8 deg./min, hold at 250 for 10
min). And another thing: GC/MS software has a feature called "solvent delay", which means that the MS ionization source is turned on some minutes
after the sample was injected (usually 2 to 5 minutes, this is to protect the filament of the ionization source from the large quantity of solvent
vapour entering the MS). You should override "solvent delay" because toluene might be going out of the column in this interval when no data is
aquired.
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Nicodem
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Quote: Originally posted by Melgar  | | Proton NMR is good at analyzing products, but not so much for quantities or ratios, so this is probably out. |
Why would it not be so good for determining ratios? The only thing that bothers is the singlet of the toluene's methyl, but toluene can be removed
prior to the analysis (chromatographically on small scale or by fractionation on the large scale reactions).
The only thing that is more accurate and at the same time simple enough (not requiring analytic method development) is using GC with a FID detector.
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