Example:
The mass spectrum of a compound holds only carbon, hydrogen and oxygen gave the following intensity pattern in the region of molecular ion.
|
m z
|
Intensity
(% of molecular ion peak)
|
|
100 (M)
|
100
|
|
101 (M+1)
|
6.67
|
|
102 (M+2)
|
0.41
|
Deduce its molecular formula.
Solution
Since the molecular ion peak is at m/ z of 100 we need to check for the possible molecular formulae from the tables referred above. The following table gives the possible molecular formulae holds C, H or O and having a molar mass of 100.
|
M =100
|
M+1
|
M+2
|
Formula mass
|
|
C4H4O3
|
4.50
|
0.68
|
100.0160
|
|
C5H8O2
|
5.61
|
0.53
|
100.0524
|
|
C6H13O
|
6.71
|
0.39
|
100.0888
|
It is quite obvious that the given mass spectrum is for the compound with a molecular formula as, C6H13O; the compound is cyclohexanol; as the (M+1) is 6.71 and (M+2) = 0.39 which matches along with 6.67 and 0.41 given within the data.
If you look at the table given in the solution to the example 1 you can appreciate that these tables can be used in a yet another way. Suppose we take a high resolution mass spectrum of the compound and find that the molar mass obtained from molecular ion peak is 100.0890 we are quite sure that the compound is C6H13O.
Thus, the molecular formula of an analyte can be obtained either from the intensity pattern of the isotopic peaks or by accurate molar mass determination from high resolution mass spectrum. Now let us learn how to determine the structure of the analyte molecule once the molecular formula is known?