Help! Ionisation for Mass Spectrometry - AQA Chem

Hi! When do you use electron impact ionisation and when do you use electron spray?
I've heard it depends on the substance but I don't know how to choose which method to use...

Hi! When do you use electron impact ionisation and when do you use electron spray?
I've heard it depends on the substance but I don't know how to choose which method to use...

As the previous poster has said, if you have a large molecule like a protein you want to use electrospray. This latter method normally involve the molecule picking up a proton from the solvent. This means that not only that the molecule is now +1 but also that is has a mass of its M_r+1. This enables the M_r of the molecule to be determined (you find the highest peak and take off 1unit - the result is the M_r of the molecule)

The electron gun is a 'hard' technique. You still normally generate a molecular ion, but this ion has +1 charge but the same mass as the molecule you're investigating. The molecule has merely lost one electron. However, in rare cases the molecular ion is absent because the molecule has been broken apart by the high energy electrons. This techinque is not very good for large molecules.

This breaking apart process is called fragmentation. It can actually be quite useful for identifying small molecules. Let's suppose that one molecule has an ethyl group but an isomer doesn't. If there's a peak at 29 m/z then it's a very good indication that an ethyl group is present. The ion that is detected is CH₃CH₂⁺ This knowledge helps to identify between small molecules of identical M_r

The new AQA spec doesn't seem to include identifying fragments, but this electrospray stuff is included instead. Herewith link to a resource from the AQA website: https://filestore.aqa.org.uk/resources/chemistry/AQA-7404-7405-TN-MASS-SPECTROMETRY.PDF

It's part of the teacher resources on AQA's website here:

https://www.aqa.org.uk/subjects/science/as-and-a-level/chemistry-7404-7405/teaching-resources?f.Resource+type%7C6=Teaching+guides

There are only four in total, and AQA only seem to issue these when the spec. is unclear or the approved textbooks are not good enough! I'm not a teacher, so perhaps there are other resources in a secure part of the site?