### Exact Masses & Isotope Abundance Ratios

Element Symbol Nominal Mass Exact Mass Abundance X+1
Factor
*
X+2
Factor
*
Hydrogen H
D or 2H
1
2
1.00783
2.01410
99.99
0.01

Carbon 12C
13C
12
13
12.0000
13.0034
98.91
1.09

1.1nC

0.006nC2
Nitrogen 14N
15N
14
15
14.0031
15.0001
99.6
0.37

0.37nN

Oxygen 16O
17O
18O
16
17
18
15.9949
16.9991
17.9992
99.76
0.037
0.20

0.04nO

0.2nO
Fluorine F 19 18.9984 100

Silicon 28Si
29Si
30Si
28
29
30
27.9769
28.9765
29.9738
92.28
4.70
3.02

5.1nSi

3.3nSi
Phosphorus P 31 30.9738 100
Sulphur 32S
33S
34S
32
33
34
31.9721
32.9715
33.9679
95.02
0.74
4.22

0.78nS

4.4nS
Chlorine 35Cl
37Cl
35
37
34.9689
36.9659
75.77
24.23

32.5nCl
Bromine 79Br
81Br
79
81
78.9183
80.9163
50.5
49.5

98.0nBr
Iodine I 127 126.9045 100

 * X represents the relative intensity of the lowest mass ion in an isotopic ion cluster. It is set at 100% for this calculation. The factor is multiplied by the number of atoms (n) of the designated element to calculate the intensity contribution from higher mass isotopes. Organic Chemistry Michigan State University

This table is adapted from Introduction to Mass Spectrometry, by J.T. Watson

### Molecular Mass Calculator

C H N O P S F Cl Br I

Molecular Mass

The mass calculator on the right may be used to calculate the exact mass of a molecule based on its elemental composition. Simply enter an appropriate subscript number to the right of each symbol, leaving those elements not present blank, and press the "Calculate" button. Only the mass of the most abundant isotope, relative to C (12.0000), is used for these calculations. For compounds of chlorine and bromine, increments of 1.997 and 1.998 respectively must be added for each halogen to arrive at the higher mass isotope values.

### Isotopic Abundance Calculator

C H N O SSi
Molecular Ion
100%
M + 1
M + 2

The calculator on the left may be used to calculate the isotope contributions to ion abundances 1 and 2 amu greater than the molecular ion (M). Simply enter an appropriate subscript number to the right of each symbol, leaving those elements not present blank, and press the "Calculate" button. The numbers displayed in the M+1 and M+2 boxes are relative to M being set at 100%. Of course, compounds of chlorine and bromine have very large isotope abundances.
A more elegant and complete isotope pattern calculator has been created by Junhau Yan. To use this resource Click Here.
A useful resource, written by Jef Rozenski, calculates all possible combinations of H, C, N & O that give a specific nominal mass. To use this calculator Click Here.