Isomers identificaion by mass

[Chandresh K. Soni]

hello,
somene tell me about isomer identifiation ( o-,m-,p-)by mass fregmentation pattern. is it possible or not.

[Don_Hilton]

In theory, yes. In practice, I would not expect to be able to use it. I won't say never - because the next person posting will have seen the odd case, but for the positional isomers around an aromatic systems that I routinely encounter, a set of isomers will typically look very similar to each other, if they are not totally indistinguishable.

The molecule is ionized by ejection of an electron - a fairly fast event. The charge may be redistributed to form some intermediate radical ion this is also a relatively fast event. As long as this process is faster than the loss of fragments, isomers may rearrange to the same intermediate ion before fragmentation. And, the uniquness provided by the structures of the isomers is lost. It becomes a question of relative rates - and if your instrument is sensitive and stable enough to reproducably measure the differences in signal, you can use it.

The best way to identify specific isomers is often to find a column that will separate them and then use relative retention time to identify the compunds.

[Vlad Orlovsky]

I don't know structures of your compounds, but here are few applications for separation of various isomers by mixed-mode chromatography. You can use ammonium formate, ammonium acetate, formic or acetic acid, mobile phase selection depends on the properties of your compounds:
http://www.sielc.com/compound_215.html (m-, o-, p-toluidines)
http://www.sielc.com/compound_188.html (aminobiphenyls)
http://www.sielc.com/compound_218.html (aminobenzoic acids)
http://www.sielc.com/compound_287.html (aminosalicylic acids)

[Kostas Petritis]

It can be done but not recommended in general. Most times the fragments that differenciate these molecules are of very low abundance so you really need to inject a lot in order to be able to differenciate them, albeit with marginal sensitivity. Things are getting even more complicated if you have major differences in the concentration of these compounds. I have seen different minor fragments for o,m,p Tyrosine and Leu/Ile. For other compounds like alpha, beta, gamma amino butyric acids you can find major fragments that can differenciate them...