Mass spectrometers are important tools in chemistry and biochemis- try for identifying the molecules in a sample. Every molecule has a mass, often unique to that molecule. A mass spectrometer ionizes the molecules and then uses electric and/or magnetic fields to sort the molecules by mass. A graph showing the number of mol- ecules that have each molecular mass is called a mass spectrum. One type of mass spectrometer, shown in Figure P22.82, is a time-of-flight mass spectrometer. A very short, intense pulse of laser light ionizes a sample of molecules very close to an electrode that is held at a po- tential of +18 kV. All the newly formed ions simultaneously begin to accelerate across a 1 cm gap toward a grid-a loose mesh of metal wire that is at 0 V. Most ions pass through the grid and enter a 1-m- long drift tube in which the electric field is zero. Ions are detected when they reach a detector at the far end of the drift tube, and their time of flight since the laser pulse is recorded. 82. Which ions have the shortest time of flight and arrive at the detector first? 1 m B. 9 x 10 m/s C. 1x 105 m/s D. 3 x 105 m/s OVO 1 cm = 0 Drift tube +18 kvo Ionization electrode Detector FIGURE P22.82 Grid Laser beam A. Ions with the smallest mass. B. Ions with the largest mass. C. Ions of all masses arrive at the same time, 83. A carbon dioxide ion CO has mass 44 u, where 1 u = 1 atomic mass unit = 1.66 10-27 kg. What is the ap- proximate speed with which a carbon dioxide ion travels through the drift tube? A. 9 10 m/s 84. An ion would accelerate through the same potential difference if the ionization electrode were at 0 V and the grid at -18 kV. Why is this never done? A. Because it would detect negative ions instead of positive ions. B. Because placing a high voltage on the grid would be dangerous. C. Because it would create an electric field inside the drift tube. D. Because the ions would have negative potential energy.