Why would the 2+ molecular ion of butane have a mass spectrum peak at 29?

Question 7 of Edexcel's June 2014 Unit 2 exam paper reads:



> 
> In a mass spectrum of butane, C4H10, where would a peak be seen for
>  the molecular ion if it had a charge of 2+?
> 
> 
> 


The answer is 29.


My question is: why?




The mass spectrometer observes the mass to charge ratio (*m*/*z*). Most of the time, small organic molecules like butane will be singly charged by the ionizer, which means that the mass can be read off directly since *z* =1.


If butane was somehow ionized to the +2 state, then the mass spec would observe 58/2=29, which is the mass of butane (58 g/mol) divided by the charge (+2).


This becomes particularly important for larger compounds with multiple easily ionizable groups, such as peptides. A series of peaks corresponding to different charge states can be used to identify the compound of interest.


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