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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?

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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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