A technique used to determine relative atomic masses and the relative abundance of isotopes, and for chemical analysis and the study of ion reactions. In a mass spectrometer a sample (usually gaseous) is ionized and the positive ions produced are accelerated into a high-vacuum region containing electric and magnetic fields. These fields deflect and focus the ions onto a detector. The fields can be varied in a controlled way so that ions of different types can impinge on the detector. A mass spectrum is thus obtained consisting of a series of peaks of variable intensity to which mass/charge (m/e) values can be assigned. The original ions are usually produced by electron impact, although ion impact, photoionization, field ionization, electrospray ionization, and MALDI are also used. For organic molecules, the mass spectrum consists of a series of peaks, one corresponding to the parent ion and the others to fragment ions produced by the ionization process. Different molecules can be identified by their characteristic pattern of lines. Analysis of mixtures can be done by gas chromatography–mass spectroscopy (see gas chromatography). Other types of mass spectrometer exist. In a quadrupole mass spectrometer the ions pass along a region surrounded by four parallel rods. Variable voltages applied to the rods produce an oscillating electric field. Varying the frequency of oscillation allows different ions to pass through to a detector. In a time-of-flight mass spectrometer the ions are accelerated by an electric field and then enter a drift tube through which they pass to a detector. Different types of ion are distinguished by their time of flight in the drift tube.

http://www.astbury.leeds.ac.uk/facil/MStut/mstutorial.htm An introduction to mass spectrometry by Alison Ashcroft, University of Leeds