qeezur
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Two spectroscopic questions
Hi everyone,
I have two basic questions regarding mass and IR spectroscopy:
1) is the fragmentation pattern of ESI-MS same as that of EI-MS ? The same rules apply like inductive and alpha/beta cleavages etc ? Most books i
have read (Pavia for example) give details of fragmentation patterns which are EI-MS based as they all start with odd electron ions (radical ions).
While, ESI-MS ionization process starts with [M+H], so how does that fragment further ? Its a basic issue I am sure but its confusing me ....
2) I have seen many papers in which authors synthesize some chromophore and then start allotting its UV absorption bands to pi-pi* or n-pi* electronic
transitions. What confuses me ... is that done arbitrarily based upon the knowledge that n-pi* transitions must be of high wavelength (less gap
between HOMO and LUMO) so highest absorption is designated as that ? or is there a methodology behind designating bands to particular transitions ?
I shall be very thankful, you guys are an asset !
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Metacelsus
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1) No, definitely not. The results of the two processes are very different. Source: personal experience.
2) Computational models (like TD-DFT) can predict which transitions have which energies. I assume the authors use something like this. In fact, I am
working on doing something very similar with a certain fluorophore.
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qeezur
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Thanks for replying ...
1) lets say if i have to write few lines about ESI-MS spectra of my molecules in a manuscript for a paper, can I just mention like "fragment at 200
m/z emerged because of loss of CxHyOz from molecular ion". I mean the loss happened for sure, right ...
2) You are right that TDDFT should be used. But, none of the papers i have seen do that, they usually just write down something like " absorption at
590 nm was attributed to the n-pi* transition of the -N=N- azo bond" ... it can be pi-pi* i mean ... who knows without DFT ...
I m sorry for being a bit annoying lol ...
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DJF90
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i have been using ESI-MS for over two years now on a daily basis and routinely only see the parent ion [M+H]+ or [M+2H]2+.
Sometimes I see [M+Na]+, but am confident that I have not seen any fragmentation of analytes. ESI is a fairly gentle ionisation method...
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phlogiston
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That is a very important difference in practice: with EI you typically get a large number of fragments whereas with ESI+ usually the most
intense (and often the only) ion is the proton adduct [M+H]+.
Also, EI is commonly used in combination with GC, whereas ESI is very commonly used to interface an LC to an MS.
In the case of LC-ESI-MS, to gain more structural information that can be used to identify an ion or to get better selectivity a complex matrix, a
tandem-MS with a collision cell (or some other method of inducting fragmentation) in between the first and second MS is used.
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"If a rocket goes up, who cares where it comes down, that's not my concern said Wernher von Braun" - Tom Lehrer |
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Metacelsus
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Yes, fragmentation is very rare with ESI-MS. I've only ever seen loss of an equivalent of water, and that isn't really "fragmentation". You should
make sure that your supposed fragment didn't come from something else in the sample.
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Dr.Bob
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There are some common fragments to lose in ESI MS: N-Boc groups often lose both Boc for M+H-100 as well as M+H-56 (loss of tBu), sometimes you can
see acids from esters (in pos/neg switching systems), loss of water, ammonia, and other small molecules is not rare. Much depends on the setup of
the system, ours may be using higher power which may cause more fragmentation. And I often also see M+Na+(23) in our system, likely due to Na
contamination somewhere, like the glass bottles for solvent.
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