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THEN, NOW, AND FUTURE The Mass Spectrometer Thomson used electric and mag- electrons off the compound to pro- to the number of ions. The spectrom- netic fields to determine the duce a positive ion. eter can produce a graph of the charge-to-mass ratio of electrons. Acceleration: High-voltage plates charge-to-mass ratios for a sample by In later experiments, he made sim- then accelerate a beam of these ions moving the detector or by varying ilar measurements for positive into the velocity selector. the electric or magnetic fields. ions. These experiments led to the Mass spectrometers can detect development of the mass spec- Velocity Selection: The velocity compounds in concentrations as trometer, an instrument that can selector has crossed electric and small as a few parts per billion. detect compounds, measure iso- magnetic fields arranged such that These versatile machines have a tope masses, and determine molec- only the ions that have a speed of huge range of applications in sci- ular structures (Figure 15.8). V : E Many mass spectrometers use a B pass straight through. Ions ence, medicine, and industry. four-stage process enclosed in a with slower or faster speeds are Questions vacuum chamber: deflected away from the entrance to 1. Write an expression for the Ionization: If the sample is not the detection chamber. Thus, all the already a gas, the ion source vapor- ions entering the next stage of the radius of the path of ions in a mass spectrometer. izes it, usually by heating. Heating mass spectrometer have the same may also break complex com- known speed 2. How does the mass of an ion pounds into smaller fragments that Detection: A uniform magnetic field affect the radius of its path in are easier to identify. Next, the neu- in the detection chamber makes the the detection chamber? tral compounds in the sample are ions travel in circular paths. The 3. Describe how you could use a ionized so that they will respond to radius of each path depends on the mass spectrometer to detect electric and magnetic fields. Usually, charge and mass of the ion. Ions an athlete's use of a banned the ion source knocks one or two arriving at the detector produce an performance-enhancing drug. electrical current that is proportional ionization velocity chamber selector detection ............t............"... chamber vaporized sample acceleration heater plates vacuum pump detector chart positive ions recorder amplifier Figure 15.8 Mass spectrometer a. A uniform magnetic field in the detection chamber of a mass spectrometer causes ions to travel in a circular path. Write an equation for the radius of the path. ( b. How is the radius of the path an ion makes in a detection chamber affected by the ion's mass? c. The air we breathe is made up of Nitrogen, Oxygen, Carbon Dioxide, Water Vapor and Argon. Explain how a mass spectrometer can be used to determine the percent composition of each gas