What is the significance of an organic compound's M peak?

**Background**


On the topic of identifying an organic compounds using peaks generated in *Mass Spectrometry*, a rule of thumb expressed in the UK educational textbook, CGP, is that that "the M peak is the one with the second highest mass/charge ratio".


**Question**


What is the significance of this M peak anyway?




"M" stands for **molecular** ion. It is often represented as $\ce{M, M^{+} or M^{+~.}}$. In the mass spec experiment, an electron is knocked out of some of the sample molecules. The peak at M/q (q=1) represents the molecular weight of the entire molecule, less an electron, before it fragments to smaller ions. From precise measurement of the molecular ion (*e.g*. 82.34567890) you can determine the exact molecular formula of your sample compound. This can help greatly with sample identification.


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