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Mass spec graphs

http://filestore.aqa.org.uk/resources/chemistry/AQA-7404-7405-SG-TOFMS.PDF
page 6

why isn't the one with the greatest height the molecular mass of the element?
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
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 ...
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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
(edited 7 years ago)
Original post by dskinner
http://filestore.aqa.org.uk/resources/chemistry/AQA-7404-7405-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.
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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?
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.
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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?
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!
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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?
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.
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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

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