Question

Positive rays of the same element produce two different traces in Bainbridge mass spectrometer. The positive ions have

• A. Same mass with different velocities
• B. Same mass with same velocity
• C. Different masses with same velocity
• D. Different masses with different velocities

Answer 1

Answer: (C)

Different masses with same velocity

A Bainbridge mass spectrometer can be used to separate ions in a gas mixture according to their charge mass ratio $q/m$. The ions are accelerated from rest across the potential difference $V$. So that only ions with a specific $q/m$ ratio enter the detector, the ions are directed through a selector that consists of a magnetic field $B_1$ and two charged conducting plates (plus and minus sigma). The selected ions pass un-deflected through the selector and enter a second region where there is only a magnetic field $B_2$. The ions then move on a circular path until they hit a detector plate that records the path radius $r$. Hence the charges are separated on the basis of velocity, however they may have different masses.