You can use the peaks at m/z = 15, 29, 71, and 85 to distinguish between the two compounds.
The major fragmentation peaks come from cleavage of the C-C bonds adjacent to the carbonyl group.
4-Methyl-2-pentanone
The structure is
So (CH₃)₂CHCH₂COCH₃ should fragment as
(CH₃)₂CHCH₂-COCH₃ → C₄H₉ + C₂H₃O. This gives m/z = 57 and 43.
(CH₃)₂CHCH₂CO-CH₃ → C₅H₉O + CH₃. This gives m/z = 85 and 15.
The major peaks should be at m/z = 15, 43, 57, and 85.
2-Methyl-3-pentanone
The structure is
So (CH₃)₂CHCOCH₂CH₃ should fragment as
(CH₃)₂CH-COCH₂CH₃→ C₃H₇ + C₃H₅O. This gives m/z = 43 and 57.
(CH₃)₂CHCO-CH₂CH₃ → C₄H₇O + C₂H₅. This gives m/z = 71 and 29.
The major peaks should be at m/z = 29, 43, 57, and 71.
Differences
The peaks at 43 and 57 are common to both.
The first compound should have a major peak at 85 and a small one at 15.
The second compound should have a major peak at 71 and a small one at 29.
Note: This ignores other fragmentation and rearrangement patterns.
For example, the first compound can generate a major peak at 58 by the McLafferty rearrangement.
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