Working of a double focussing mass analyser:

In double focussing mass spectrometer, the molecular and the fragment ions coming out of the ionisation chamber are passed through an electrostatic analyser. This allows the fragment ions with only a limited range of kinetic energy to go over to the magnetic sector. In other words this analyser causes the energy focusing of the ions. In the magnetic sector analyser these ions are then dispersed according to their  m /z ratios and sent to the detector.

McLafferty rearrangement is a mechanism of fragmentation that accounts for prominent characteristic peaks in the mass spectrum of a number of molecules belonging to different classes. The essential requirement for the rearrangement is that the molecules containing a carbonyl group must have abstractable hydrogen at a position γ to the carbonyl group. For example, the mass spectrum of phenylpropylketone gives at a m /z of 120 and its existence can be accounted for by McLafferty  rearrangement as follows: