In a mass spectrometer, ions of the same charge and speed but different masses are separated because

  • A. heavier ions move faster
  • B. heavier ions follow a path of larger radius in the magnetic field
  • C. lighter ions are stopped by the field
  • D. heavier ions lose their charge

Explanation

The radius mv over qB is proportional to mass when charge, speed and field are fixed, so heavier ions curve less sharply and strike the detector further along. Measuring where each beam lands therefore measures the mass. This is how isotopes were discovered and how their relative abundances are determined.

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