MALDI-TOF mass spectrometry. (1) The protein sample, embedded in an appropriate matrix, is ionized by the application of a laser beam. (2) An electric field accelerates the ions through the flight tube toward the detector. (3) The lightest ions arrive first. (4) The ionizing laser pulse also triggers a clock that measures the time of flight (TOF) for the ions.

[Information from J. T. Watson, Introduction to Mass Spectrometry, 3d ed. (Lippincott-Raven, 1997), p. 279.]