21.4

The Mass Spectrometer

Physicists use mass spectrometers for determining the relative masses and abundances of isotopes.* Chemists use these instruments to help identify unknown molecules produced in chemical reactions. Mass spectrometers are also used during surgery, where they give the anesthesiologist information on the gases, including the anesthetic, in the patient’s lungs.

The physics of a mass spectrometer.

In the type of mass spectrometer illustrated in Figure 21-15, the atoms or molecules are first vaporized and then ionized by the ion source. The ionization process removes one electron from the particle, leaving it with a net positive charge of +e. The positive ions are then accelerated through the potential difference V, which is applied between the ion source and the metal plate. With a speed v, the ions pass through a hole in the plate and enter a region of constant magnetic field , where they are deflected in semicircular paths. Only those ions following a path with the proper radius r strike the detector, which records the number of ions arriving per second.

Figure 21-15 In this mass spectrometer the dashed lines are the paths traveled by ions of different masses. Ions with mass m follow the path of radius r and enter the detector. Ions with the larger mass m1 follow the outer path and miss the detector.

The mass m of the detected ions can be expressed in terms of r, B, and v by recalling that the radius of the path followed by a particle of charge +e is (Equation 21.2). In addition, the Reasoning section in Example 3 shows that the ion speed v can be expressed in terms of the potential difference V as . This expression for v is the same as that used in Example 3, except that, for convenience, we have replaced the potential difference, , by the symbol V. Eliminating v from these two equations algebraically and solving for the mass gives

This result shows that the mass of each ion reaching the detector is proportional to B2. Experimentally changing the value of B and keeping the term in the parentheses constant will allow ions of different masses to enter the detector. A plot of the detector output as a function of B2 then gives an indication of what masses are present and the abundance of each mass.
Figure 21-16 shows a record obtained by a mass spectrometer for naturally occurring neon gas. The results show that the element neon has three isotopes whose atomic mass numbers are 20, 21, and 22. These isotopes occur because neon atoms exist with different numbers of neutrons in the nucleus. Notice that the isotopes have different abundances, with neon-20 being the most abundant.

Figure 21-16 The mass spectrum (not to scale) of naturally occurring neon, showing three isotopes whose atomic mass numbers are 20, 21, and 22. The larger the peak, the more abundant the isotope.

Test your understanding of the material in Sections 21.1, 21.2, 21.3 and 21.4 :

· | Magnetic Fields
· | The Force That a Magnetic Field Exerts on a Moving Charge
· | The Motion of a Charged Particle in a Magnetic Field
· | The Mass Spectrometer

Copyright © 2007 John Wiley & Sons, Inc. All rights reserved.