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. B 2r Detector 3. Only ions of a particular mass reach the exit slit and continue to the detector. FIGURE P24.67
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 is shown. 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.
A mass spectrometer similar to the one shown is designed to analyze biological samples. Molecules in the sample are singly ionized, then they enter a 0.80 T uniform magnetic field at a speed of 2.3 x 105 m/s. If a molecule has a mass 85 times the mass of the proton, what will be the approximate distance between the points where the ion enters and exits the magnetic field?
A. 25 cm
B. 50 cm
C. 75 cm
D. 100 cm
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