1. Fragmentation of M+
Draw structure of M+ if possible and then draw possible heterolytic cleavages (curved arrows) or homolytic cleavages (fish hooks)
Alternatively, show homolytic or heterolytic cleavage cleavage of the molecule and then remove an electron from one of the fragments.
Fragmentation to give neutral molecule and a radical cation must involve some kind of rearrangement.

2. General Rules
Probablity of bond cleavage depends on stability of fragments produced. (3º > 2º > 1º > CH3) (oxonium > carbocation) (allylic, benzylic > no resonance)
R on cycloalkanes α cleave. Cyclohexenes do retro Diels-Alder.
R on aromatic rings β-cleave to benzylic
Bonds β to heteroatoms cleave
Loss of small stable molecules, H2O, NH3, C2H4, HCN, CO, H2S,etc often occur
Increase branching  decrease M+
Add carbons  decrease M+
Cleave more likely at higher º C
Unsubst cyclic have strong M+ (especially aromatic)
Alkenes cleave to allylic species

3. Alkanes
Straight chain alkanes – homologous series of ions – smooth curve max at 57 or 71. M-15 absent. “picket fence”
Discontinuity = branching. Absence of homolog  no single cleavage possible
Cycloalkanes. Larger M+. More even mass fragments (M-C2H4)

6. Alkenes
Cleave next to double bond to form allylic fragments. (Double bonds can shift in M+.)
Homologous series is -2 (27, 41, 55, 69..)
Cyclohexenes do retro Diels Alder – give even mass fragment (M-C2H4, M-C3H6 etc.)

9. Aromatics No branching = strong M+. Often see M++.
m/z 77, 51 = phenyl (monosubst benzene)
Most prominent peaks are due to beta cleavage of branches to benzylic (really tropylium.)