Help! Ionisation for Mass Spectrometry - AQA Chem

Watch this thread
emrex03
Badges: 12
Rep:
? You'll earn badges for being active around the site. Rep gems come when your posts are rated by other community members.
#1
Report Thread starter 4 weeks ago
#1
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...
0
reply
charco
Badges: 18
Rep:
? You'll earn badges for being active around the site. Rep gems come when your posts are rated by other community members.
#2
Report 4 weeks ago
#2
(Original post by emrex03)
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...
Electron spray is used to give charge to large molecules, like proteins.


Normal MS uses electron impact.
Last edited by charco; 4 weeks ago
1
reply
tony_dolby
Badges: 10
Rep:
? You'll earn badges for being active around the site. Rep gems come when your posts are rated by other community members.
#3
Report 4 weeks ago
#3
(Original post by emrex03)
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 Mr+1. This enables the Mr of the molecule to be determined (you find the highest peak and take off 1unit - the result is the Mr 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 Mr

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/resourc...ECTROMETRY.PDF
Last edited by tony_dolby; 4 weeks ago
1
reply
charco
Badges: 18
Rep:
? You'll earn badges for being active around the site. Rep gems come when your posts are rated by other community members.
#4
Report 4 weeks ago
#4
(Original post by tony_dolby)
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 Mr+1. This enables the Mr of the molecule to be determined (you find the highest peak and take off 1unit - the result is the Mr 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 Mr

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/resourc...ECTROMETRY.PDF
Hi Tony,
very useful link - where does it come from?
0
reply
tony_dolby
Badges: 10
Rep:
? You'll earn badges for being active around the site. Rep gems come when your posts are rated by other community members.
#5
Report 4 weeks ago
#5
It's part of the teacher resources on AQA's website here:

https://www.aqa.org.uk/subjects/scie...eaching+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?
0
reply
charco
Badges: 18
Rep:
? You'll earn badges for being active around the site. Rep gems come when your posts are rated by other community members.
#6
Report 4 weeks ago
#6
(Original post by tony_dolby)
It's part of the teacher resources on AQA's website here:

https://www.aqa.org.uk/subjects/scie...eaching+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?
Thanks
0
reply
emrex03
Badges: 12
Rep:
? You'll earn badges for being active around the site. Rep gems come when your posts are rated by other community members.
#7
Report Thread starter 4 weeks ago
#7
Ah amazing! Thanks so much everyone!!
0
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Back
to top
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

Year 12s - where are you at with making decisions about university?

I’ve chosen my course and my university (14)
28.57%
I’ve chosen my course and shortlisted some universities (20)
40.82%
I’ve chosen my course, but not any universities (2)
4.08%
I’ve chosen my university, but not my course (2)
4.08%
I’ve shortlisted some universities, but not my course (4)
8.16%
I’m starting to consider my university options (6)
12.24%
I haven’t started thinking about university yet (0)
0%
I’m not planning on going to university (1)
2.04%

Watched Threads

View All
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise