Mass Spectrum Example 4

Example 4: chloroethane, CH₃CH₂ClChlorine has two isotopes, chlorine-35 and chlorine-37, which occur in the ratio of about 3:1. Any ion that contains one chlorine atom therefore appears as a pair of peaks 2 units apart, with heights in the ratio 3:1.Molecular ion peaks• CH₃CH₂³⁵Cl⁺: 24 + 5 + 35 = m/z 64• CH₃CH₂³⁷Cl⁺: 24 + 5 + 37 = m/z 66The peak at 64 is three times the height of the peak at 66. This pattern (called the M and M+2 peaks) shows that the molecule contains one chlorine atom.Fragment peaks• m/z 49 and 51 (ratio 3:1): CH₂Cl⁺, formed by loss of a methyl radical. The pair shows the fragment still contains chlorine.• m/z 29: C₂H₅⁺, formed when the molecular ion loses a chlorine atom. This peak is single, because the ion contains no chlorine.C₂H₅Cl⁺ → C₂H₅⁺ + Cl•Method: look for pairs of peaks 2 units apart. A 3:1 ratio means one Cl atom. Subtract 35 from the lower molecular ion value to find the mass of the rest of the molecule: 64 – 35 = 29, which is C₂H₅.