In order for a compound to be analysed in mass spectrometry it must first be ionised, forming an adduct ion. In many papers (e.g 1 [PDF], 2) these adduct ions are described in the form:

  • [M+H]+
  • [M+Na]+
  • [M-2H]2-

Where M is the analyte, the other content of the square brackets is the ion, and outside the brackets is the resulting charge. The IUPAC Gold Book also mentions this format for protonation without, as far as I can tell, describing it further.

Is there some reference actually describing this convention - ideally encompassing more complex scenarios such as dimerisation or neutral losses as well - around?

  • $\begingroup$ Molecular Ion peak is represented by M+ without bracket. i think this convention is used to describe the adduct is now the part of molecule $\endgroup$ – Khan Sep 12 '16 at 12:41

Your convention is generally correct; as more complicated adducts and ions are formed (clusters, adducts, neutral losses, etc) things start to become a free for all where the clearest terminology wins.

As of now, just go with (where A means adduct):

$$\ce{[M + nA]^{n+}}\text{ or }\ce{[M - nA]^{n-}}$$

Protonated molecules can be denoted as $\ce{[MH]+}$ (common) or $\ce{[M + H]+}$ (technical). The $\ce{[M + H]+}$ convention is more precise because if you write $\ce{[MH]-}$ for a deprotonated ion the interpretation becomes muddled (i.e is this a hydride adduct or is this a deprotonated ion?).

One important think to keep in mind that is Molecular Ions ($\ce{M^{+.}, M^{-.}}$) are not the same as $\ce{[M + H]+}$ or $\ce{[M - H]-}$ and that the terminology is not interchangable.

  • $\ce{M^{+.}}$ or $\ce{M^{-.}}$ retains the elemental composition of the ion and is ionized through the removal or addition of an electron.
  • $\ce{[M + H]+}$ and $\ce{[M - H]-}$ are protonated and deprotonated molecules, respectively. They are not 'pseudomolecular' ions as there are no such thing as pseudo ionized specties.
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  • $\begingroup$ What I was really hoping for was a standard (IUPAC or whatever) which laid something out, but I guess that doesn't seem to exist? $\endgroup$ – Jivlain Sep 13 '16 at 9:17
  • 1
    $\begingroup$ Looking at the requirements from scientific journals in mass spectrometry, only the Journal of the American Society for Mass Spectrometry references to "Helpful information about nomenclature and descriptions specific to mass spectrometry can be found in Mass Spec Desk Reference, 2nd Ed., by O. David Sparkman, Global View Publications, Pittsburgh, PA, 2006." So I guess this book could be considered a standard in the field. The problem is that it seems to be out of print. $\endgroup$ – PLD Sep 14 '16 at 15:05
  • $\begingroup$ And Patrick B.'s answer seems to me as accurate as possible. $\endgroup$ – PLD Sep 14 '16 at 15:07

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