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# Mass spectrometry help please watch

1. Hi guys, can someone please help me understand what the molecular ion peak is here and what the 63 and 65 mean? Many thanks
2. (Original post by sienna2266)
Hi guys, can someone please help me understand what the molecular ion peak is here and what the 63 and 65 mean? Many thanks
Look at the atoms in the molecule and think isotopes
3. (Original post by charco)
Look at the atoms in the molecule and think isotopes
Thanks so I get that M+ is positively charged 2-chloropropane with the Cl having a mass of 35 and the M+2 is positively charged 2-chloropropane with the Cl having a mass of 37.

But still don't quite get where the 63 and 65 come from?
4. (Original post by sienna2266)
Thanks so I get that M+ is positively charged 2-chloropropane with the Cl having a mass of 35 and the M+2 is positively charged 2-chloropropane with the Cl having a mass of 37.

But still don't quite get where the 63 and 65 come from?
Add up the mass of the molecule, CH3CHClCH3 and you get 78 and 80 (for the two different isotopes)

Then if each molecular ion loses a methyl group (m/z=15) you are left with 63 and 65
5. (Original post by charco)
Add up the mass of the molecule, CH3CHClCH3 and you get 78 and 80 (for the two different isotopes)

Then if each molecular ion loses a methyl group (m/z=15) you are left with 63 and 65
Thanks so much! Also, is the major peak here caused by CH3CHCH3 and if so, is there a particular reason why?
Usually major peaks occur for ions which have a functional group at the end of it. For example, the major peak for CH3CH2COCH2CH3 is CH3CH2CO (not specific to this question). So 2-chloropropane does not have a functional group?
Sorry I am a little confused with my terminology
6. (Original post by sienna2266)
Thanks so much! Also, is the major peak here caused by CH3CHCH3 and if so, is there a particular reason why?
Usually major peaks occur for ions which have a functional group at the end of it. For example, the major peak for CH3CH2COCH2CH3 is CH3CH2CO (not specific to this question). So 2-chloropropane does not have a functional group?
Sorry I am a little confused with my terminology
The most intense peak corresponds to the most stable fragment in the fragmentation pattern. Peak intensity in general is related to the stability of the fragment ion formed.

The heights of the peaks do not give much information in organic MS, except in certain cases, such as the presence of a bromine atom, in which case there are pairs of equal intensity separated by m/z = 2.

The highest peak could be the molecular ion, but it could equally be any particularly stable configuration of atoms, as in this case.
7. (Original post by charco)
The most intense peak corresponds to the most stable fragment in the fragmentation pattern. Peak intensity in general is related to the stability of the fragment ion formed.

The heights of the peaks do not give much information in organic MS, except in certain cases, such as the presence of a bromine atom, in which case there are pairs of equal intensity separated by m/z = 2.

The highest peak could be the molecular ion, but it could equally be any particularly stable configuration of atoms, as in this case.
Thanks so much- this makes sense. But I came across a question : "Predict the m/z value of a major peak in the mass spectrum of compound Q"

The answer is 43 or 71.
So they've taken the major peak to be at CH3CH2CH2CO or CH3CO . Why are these the ions that cause the major peaks?
8. (Original post by sienna2266)
Thanks so much- this makes sense. But I came across a question : "Predict the m/z value of a major peak in the mass spectrum of compound Q"

The answer is 43 or 71.
So they've taken the major peak to be at CH3CH2CH2CO or CH3CO . Why are these the ions that cause the major peaks?
Cleavage of the C-C bond adjacent to a carbonyl group must be easier than other bonds, and the fragments formed are relatively stable as a positve charge on the carbonyl group is stabilized by the +I effect of the alkyl group.

The initial ionization is most likely to occur at the carbonyl oxygen as it has two lone pairs, giving the molecular ion

From there, fragmentation would involve cleavage of the C-C bond adjacent to the carbonyl.

Remember that lots of other fragments will be seen, but these are the easiest to form and most intense.
9. (Original post by charco)
Cleavage of the C-C bond adjacent to a carbonyl group must be easier than other bonds, and the fragments formed are relatively stable as a positve charge on the carbonyl group is stabilized by the +I effect of the alkyl group.

The initial ionization is most likely to occur at the carbonyl oxygen as it has two lone pairs, giving the molecular ion

From there, fragmentation would involve cleavage of the C-C bond adjacent to the carbonyl.

Remember that lots of other fragments will be seen, but these are the easiest to form and most intense.
Thank you!

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