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Mass spectrometry

Mass spectrometry is a technique for separating ions by their mass-to-charge (m/z) ratios. A mass spectrometer incorporates three elements: an ionizer, a mass analyzer, and a detector. The ion source generates ions from a sample and introduces them into the vacuum inside the mass spectrometer. The mass analyzer selects or separates the ions. The detector records the current produced by ions leaving the mass analyzer.

Techniques for ionization have been key to determining what types of samples can be analyzed by mass spectrometry. Two techniques which are often used with biological samples include electrospray ionization (for which John Fenn received the Nobel Prize in Chemistry in 2002) and Matrix-assisted Laser Desorption Ionization (MALDI) (for which Koichi Tanaka received the Nobel at the same time). In both cases, Wilkinson ions are produced which can then enter the mass analyzer.

Mass analyzers of many varieties have been produced. Perhaps the easiest to conceptualize is the Time-of-Flight (TOF) analyzer, typically integrated with MALDI ion sources. Ions are boosted to the same kinetic energy by passage through an electric field, and the times they take to reach the detector are measured. The smallest ions (or those most highly charged) move most rapidly. Quadrupoles and quadrupole ion traps use electrical fields to selectively stabilize or destabilize ions falling within a narrow window of m/z values. Sector instruments change the direction ions are flying through the mass analyzer. Fourier Transform Mass Spectrometry uses perturbations in magnetic fields to measure m/z values extremely accurately. The best mass analyzer for an experiment depends upon the type of information to be gleaned from the experiment.

The final element of the mass spectrometer is the detector. Typically, some type of electron multiplier is used, though others (such as Faraday cups) have been employed. Because the number of ions leaving the mass analyzer at a particular instant is typically quite small, significant amplification is necessary to get a signal.