Describe the Principle of Dempster's Mass Spectrometer.
The Dempster mass spectrograph was developed by Arthur Jeffrey Dempster. A slow beam of vapour of the sample is bombarded in high vacuum by high energy electron between two plates each having a slit. Therefore, gaseous atoms form monopositive ions. These ions are strongly attracted to the negative plate when a potential (E) of 500-2000 volts is applied between the plates. The accelerated beam of these ions then comes into the magnetic field at right angles to their path and follows circular path. If e be the charge of the monopositive ion, then electrical energy (E.e.) goes to the particles as K.E.-
If H be the magnetic field, then magnetic force (H.e.v) exactly balances the centripetal force (mv2/r) hence,
The r is constant for a particular spectrometer and if H is kept the same during the experiment, then m ∝ 1/E.
By adjusting, particles of mass m are made to strike the collector. Each ion has a very small current. The ion current is amplified and fed to a recorder which gives the plot of relative abundance against mass number. The plot of Ne isotopes is shown below.
