3
$\begingroup$

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?

$\endgroup$
6
$\begingroup$

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.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.