Chemistry A level,mass spectrometry questions

Bromine is an element in group 7 it has two isotope 79 Br and 81Br that naturally occur in roughly equally proportion. complete the mass spectrum to give the distribution and location of the peaks for this sample so it's given the graph of relative abundance against mass /charge ratio and you have to find the peaks
how do you do that??? Help pls

Reply 1

BankaiGintoki

Original post by Alhdibensjk

Bromine exists as a diatomic molecule, BR2.
So multiply by 2
M/z at 79 * 2 = 158 for Br 79
M/z at 81 *2 = 162 for Br 81
So draw 2 vertical straight lines at m/z at 158 and 162.
Equal proportion - so is at 50% on the Y axis.

(edited 1 year ago)

Reply 2

UtterlyUseless69

Original post by BankaiGintoki

This is an incomplete solution.

You forget that:

(a) You can also have 79Br-81Br as well as (79Br)2 and (81Br)2. Thus you should also have a peak at m/z = 160.

(b) To say that it is 50% (79Br)2 or 50% (81Br)2 isn’t quite correct. If you use a probability tree diagram, you’ll see why.

(c) Have you considered possible fragmentation of the Br2 molecules and what other ions can form?