Why is it important that the ions have a known speed?

A. The radius of the orbit depends on the mass, the charge, and the speed. If the charge and the speed are the same, the orbit depends on only the mass.

B. The orbit must be circular, and this is the case for only a certain range of speeds.

C. If the ions are moving too fast, the magnetic field will not be able to bend their path to the detector.

D. The ions are all accelerated by the same electric field, and so will all have the same speed anyway.

If you have a sample of unknown composition, a first step at analysis might be a determination of the masses of the atoms and molecules in the sample. A mass spectrometer to make such an analysis can take various forms, but for many years the best technique was to determine the masses of ionized atoms and molecules in a sample by observing their circular paths in a uniform magnetic field, as illustrated in Figure P24.67. A sample to be analyzed is vaporized, then singly ionized. The ions are accelerated through an electric field, and ions of a known speed selected. These ions travel into a region of uniform magnetic field, where they follow circular paths. An exit slit allows ions that have followed a particular path to be counted by a detector, producing a record of the masses of the particles in the sample.

1. Atoms are ionized and accelerated. Ions of a particular velocity are selected to enter the spectrometer. 2. Ions of different masses follow paths of different radii. Edation Detector 3. Only ions of a particular mass reach the FIGURE P24.67 exit slit and continue to the detector.