how do you know which peak in the mass spectrum is the Molecular ion peak

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jonjoshelvey21
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how do u know
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ChemistryWebsite
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Assuming you are studying at A level:

The peak with the largest m/z value is the molecular peak. This is the largest / heaviest possible fragment from the sample molecule as only an electron has been lost to form this ion fragment.

All other fragments created by the MS will have lower mass, therefore lower m/z value.




(Note, it is possible to get a very tiny peak just higher than the molecular ion peak due to isotopes of elements present in the molecule. The genuine molecular ion peak will not be very tiny, but will be easy to see on the mass spectrum).
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jonjoshelvey21
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(Original post by TutorsChemistry)
Assuming you are studying at A level:

The peak with the largest m/z value is the molecular peak. This is the largest / heaviest possible fragment from the sample molecule as only an electron has been lost to form this ion fragment.

All other fragments created by the MS will have lower mass, therefore lower m/z value.




(Note, it is possible to get a very tiny peak just higher than the molecular ion peak due to isotopes of elements present in the molecule. The genuine molecular ion peak will not be very tiny, but will be easy to see on the mass spectrum).
right thanks and am i right in thinking that the mz of this is equal to the molecules Mr?
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ChemistryWebsite
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Yes, the m/z value of the molecular ion peak is the compound's Mr
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jonjoshelvey21
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(Original post by TutorsChemistry)
Yes, the m/z value of the molecular ion peak is the compound's Mr
is the Mr equal to the RFM?
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HateOCR
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If theres two peaks which have high abundance you choose the one thats furthest to the right btw.
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jonjoshelvey21
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(Original post by TutorsChemistry)
Assuming you are studying at A level:

The peak with the largest m/z value is the molecular peak. This is the largest / heaviest possible fragment from the sample molecule as only an electron has been lost to form this ion fragment.

All other fragments created by the MS will have lower mass, therefore lower m/z value.




(Note, it is possible to get a very tiny peak just higher than the molecular ion peak due to isotopes of elements present in the molecule. The genuine molecular ion peak will not be very tiny, but will be easy to see on the mass spectrum).
also whats the point of the Molecular ion peak or the point of timesing the mz by the abundance and dividing by all the abundance added up? do thry not bring the same answer andnwhats the difference? Isnthis only for atoms isotopes
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(Original post by jonjoshelvey21)
also whats the point of the Molecular ion peak or the point of timesing the mz by the abundance and dividing by all the abundance added up? do thry not bring the same answer andnwhats the difference? Isnthis only for atoms isotopes
Finding the molecular ion peak allows you to find the Mr of the molecule (mass) which may be different depending on the isotope, and multiplying isotopic masses by the percentage abundance and dividing by the total abundane gives you the RAM of an element which is an average of all its detected isotopes.
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(Original post by jonjoshelvey21)
also whats the point of the Molecular ion peak or the point of timesing the mz by the abundance and dividing by all the abundance added up? do thry not bring the same answer andnwhats the difference? Isnthis only for atoms isotopes
That is 2 processes for 2 different sample types. You do one or the other but not both.

The molecular peak is for analysis of compounds to find the Mr.

The relative abundance calculations relate to analysis of individual elements as samples, not compounds, and tells you the relative abundance of that element's isotopes.
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