Scan_Doc0085 - 3.2 Atomic Masses 79 atoms or molecules are...

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Most elements occur in nature as mixtures of isotopes; thus atomic masses are usually average values. It is much easier to weigh out 600 hex nuts than count them one by one. 3.2 Atomic Masses 79 atoms or molecules are passed into a beam of high-speed electrons, which knock elec- trons off the atoms or molecules being analyzed and change them into positive ions. An applied electric field then accelerates these ions into a magnetic field. Because an accel- erating ion produces its own magnetic field, an interaction with the applied magnetic field occurs, which tends to change the path of the ion. The amount of path deflection for each ion depends on its mass-the most massive ions are deflected the smallest amount-which causes the ions to separate, as shown in Fig. 3.1. A comparison of the positions where the ions hit the detector plate gives very accurate values of their relative masses. For exam- ple, when 12Cand l3C are analyzed in a mass spectrometer, the ratio of their masses is found to be
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