You are Here: Home

# Mass spec graphs Watch

1. http://filestore.aqa.org.uk/resource...5-SG-TOFMS.PDF
page 6

why isn't the one with the greatest height the molecular mass of the element?
2. Its because when mass spec is performed there is a bombardment of protons to get the ions resulting in the occurrence of protonation (addition of a proton to an atom) giving it an increased mass number by 1.
Hope this helps
3. (Original post by michaelfish1998)
Its because when mass spec is performed there is a bombardment of protons to get the ions resulting in the occurrence of protonation (addition of a proton to an atom) giving it an increased mass number by 1.
Hope this helps
This is unfortunately incorrect ...
4. (Original post by michaelfish1998)
Its because when mass spec is performed there is a bombardment of protons to get the ions resulting in the occurrence of protonation (addition of a proton to an atom) giving it an increased mass number by 1.
Hope this helps
Thanks for your explanation of why 45 the biggest number isn't the actual molecular mass but why 44 is
(seems that explaination was wrong it's because of a H with a neutron or C-13) just looked it up

however my question was why for example 29 wasn't the molecular mass of the molecule, because it has the biggest peak of them all
5. (Original post by dskinner)
http://filestore.aqa.org.uk/resource...5-SG-TOFMS.PDF
page 6

why isn't the one with the greatest height the molecular mass of the element?
If by greatest height you mean the most intense peak, then this is simply the most likely fragment.

If it's an element then it is the most abundant isotope.
6. (Original post by charco)
If by greatest height you mean the most intense peak, then this is simply the most likely fragment.

If it's an element then it is the most abundant isotope.
Oh i see so i have to always take the highest peak with the highest number?
7. (Original post by dskinner)
Oh i see so i have to always take the highest peak with the highest number?
You have to take the signal with the highest m/z ratio (not forgetting isotopes), not the most intense signal.
8. (Original post by charco)
You have to take the signal with the highest m/z ratio (not forgetting isotopes), not the most intense signal.
so in that example i'd take 45 then as the true value of the molecule?
9. (Original post by dskinner)
so in that example i'd take 45 then as the true value of the molecule?
The example tells you that the highest m/z = 44 and that the peak at 45 is due to the presence of isotopes, which I told you (above) to ignore!
10. (Original post by charco)
The example tells you that the highest m/z = 44 and that the peak at 45 is due to the presence of isotopes, which I told you (above) to ignore!
But how would i know this?
11. (Original post by dskinner)
But how would i know this?
You use common sense.

If you are dealing with a molecule with carbon and hydrogen there is a finite possibility of finding 13-C or 2-H in the structure.

But the intensity of the signal will be proportional to that probability.

with 2-H the isotopic abundance is 1%, so you may have a signal about 1/100 times the height of the molecular ion. For 13-C it is even less.
12. (Original post by charco)
You use common sense.

If you are dealing with a molecule with carbon and hydrogen there is a finite possibility of finding 13-C or 2-H in the structure.

But the intensity of the signal will be proportional to that probability.

with 2-H the isotopic abundance is 1%, so you may have a signal about 1/100 times the height of the molecular ion. For 13-C it is even less.
Ok i see

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: June 23, 2016
Today on TSR

### Is this person a genius?

...with these A Level results?

### I think I'm transgender AMA

Discussions on TSR

• Latest
• ## 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.

• Poll
Discussions on TSR

• Latest
• ## 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.

• The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.