(a) The energetically unstable molecular ions can break up into smaller pieces.
The simplest case that a molecular ion breaks into 2 parts – one which is another positive ion, and the other is an uncharged free radical. This is known as fragmentation.

The uncharged free radical will not produce a line on the mass spectrum as only charged particles will be accelerated, deflected and detected by the mass spectrometer.
The uncharged particles will be removed by the vacuum pump.

CH₃CH₂CH₂COCH₃⁺• - m/e = 86

CH₃CH₂CH₂COCH₃⁺• --> CH₃CH₂CH₂CO⁺ + CH₃•

CH₃CH₂CH₂CO⁺ - m/e = 71

CH₃CH₂CH₂CO⁺ --> CH₃CH₂CH₂⁺• + CO•

CH₃CH₂CH₂⁺• - m/e = 43

b) Since different substances have different fragmentation pattern, it is possible to identify 2 different substances with the same M_r.

E.g.
Which one of the following gives the complete mass spectrum illustrated on the left?

A CO₂
B C₃H₈
C N₂O
D a mixture of CH₄ and N₂

Answer: Choice A

Remark:
Why do we eleminate Choice B?

Although C₂H₄⁺ has m/e = 28, we will never get this ion as we cannot loose a CH₄• radical from C₃H₈⁺• molecular ion (CH₃CH₂CH_3⁺•).