Isotope Abundance. The existence of isotopes was first observed by Aston using a mass spectrometer to study neon ions. When interpreting mass spectra it is important to remember that the atomic weight of an element is a weighted average of the naturally occurring isotopes. Mass spectrometers separate these isotopes and are even used to measure their relative abundance. Although this complicates the mas spectrum, it also provides useful information for identifying the elements in an ion.

Chlorine is an excellent example of how isotope distributions are useful for interpretation. The molecular weight of chlorine is 35.45 u. This is calculated from the natural abundance of ³⁵Cl (75%) and ³⁷Cl (25%). The natural abundance of these two isotopes is observed in the mass spectrum as two peaks separated by m/z 2 with a relative intensity of 3:1. The mass spectrum of CH₃Cl (Figure 21) clearly shows two peaks with the isotope distribution pattern for an ion with a single chlorine atom. CH₃³⁵Cl⁺ (m/z 50) and CH₃³⁷Cl⁺ (m/z 52)⁽¹⁾ are separated by m/z 2 and have the 3:1 abundance ratio characteristic of an ion with a single chlorine atom. Can you identify the other peaks in the mass spectrum of CH₃Cl?

If more than one chlorine atom is present, the isotope abundance is more complex. An ion with two chlorine atoms has three possible isotope combinations. This pattern is apparent in the mass spectrum of CH₂Cl₂ (Figure 22). Ions are observed for CH₂³⁵Cl₂⁺ (m/z 84),

CH₂³⁵Cl³⁷Cl⁺ (m/z 86), and CH₂³⁷Cl₂⁺ (m/z 88). Based upon the probability of each combination of isotopes, the relative intensity of these peaks is 10:6:1. The 3:1 isotope ratio for an ion with a single chlorine atom is observed at m/z 49 and m/z 51. This corresponds to CH₂³⁵Cl⁺ and CH₂³⁷Cl⁺ fragments formed by loss of Cl from the molecular ion. Careful examination of the spectrum also shows ions produced by loss of H and H₂.

The 1.1% of natural abundance of ¹³C is another useful tool for interpreting mass spectra. The abundance of a peak one m/z value higher, where a single ¹²C is replaced by a ¹³C, is determined by the number of carbons in the ion. The rule of thumb for small compounds is that each carbon atom in the ion increases the abundance of the M+1 peak by 1.1%. This effect is seen in all the spectra discussed in this paper. For example, in the n-decane mass spectrum (Figure 16) compare the peak for ¹²C₉¹³C¹H₂₂⁺ at m/z 143 (0.9 % relative abundance) to the peak for ¹²C₁₀¹H₂₂ at m/z 142 (9.0 % relative abundance) . The abundance of the ¹³C peak is 10 % the abundance of the ¹²C peak. Now look at some previous spectra to find more examples of this pattern.

Because all atoms have several naturally occurring isotopes, the patterns discussed here become more complex. Fortunately, most elements common in organic mass spectrometry have one predominant isotope. The high abundance of the two chlorine isotopes is unusual, so they are easy to identify. The relative abundances for isotopes of frequently encountered elements are given in Table 2.

For additional information see: Isotope patterns from Jerry Morine

Abundance of Isotopes for Some Common Elements.^*

Atom	Isotope A mass %**	Isotope A+1 mass %	Isotope A+2 mass %
H	1	100	2	0.015
C	12	100	13	1.1
N	14	100	15	0.37
O	16	100	17	0.04	18	0.20
F	19	100
Si	28	100	29	5.1	30	3.4
P	31	100
S	32	100	33	0.80	34	4.4
Cl	35	100			37	32.5
Br	79	100			81	98.0
I	127	100

^*adapted from McLafferty, F. Interpretation of Mass Spectra (University Science, Mill Valley CA: 1980.

^**By convention mass spectra are normalized so that the most intense peak has an abundance of 100%.

1. To avoid ambiguity the molecular ion is defined as the ion with the most commonly occurring isotopes. For CHCl₃ the molecular ion is ¹²C¹H₃³⁵Cl at m/z 50.