Gérard Hopfgartner 1.1
Mass spectrometry started about 100 years ago with the work of Sir J.J. Thomson. His interest was the quantitative measurement of the mass and charge of the cathode rays (electrons). For that purpose he constructed the first mass spectrometer (parabola mass spectrograph) and he received in 1906 the Nobel Prize for Physics in recognition of his work . In the next decades the major focus in the development and application of mass spectrometry was dedicated to the studies of isotopes . In 1918 Dempster  developed an instrument in which a strong magnetic field was produced, between two semicircular iron plates, to separate positive ion rays with great resolving power. He also described the bombardment of chemical compounds with electrons forming positive ions. This technique is known today as electron impact ionization and is still widely used in modern mass spectrometry. In the early 1940s the first commercial instruments based on magnetic deflection and electron impact ionization became available. These instruments were mostly applied for the analysis of hydrocarbons in petroleum products. Beyond instrumental development the end of the 1950s saw the application of mass spectrometry for structure elucidation of natural products and the studies of fragmentation patterns. At the same time the concept of several mass analyzers was described, such as time of flight or ion cyclotron resonance.
While the first coupling of gas chromatography and mass spectrometry had been reported in the late fifties  one had to wait for almost another 20 years before the direct interfacing of liquid chromatography with mass spectrometry (LC-MS) was described by Arpino et al. . With the direct liquid interface (DLI) the effluent of the chromatographic column was directly introduced in the electron impact source. Contrarily to gas chromatography coupled to mass spectrom-etry (GC-MS), LC-MS did do not catch on as rapidly. One of the reasons was that the MS interface could only handle LC flow rates of a few microliters per minute. Another limitation was that electron impact or chemical ionization was not suit able for very thermolabile and high molecular weight compounds. It took almost ten years before the LC-MS analysis of larger molecules, using continuous flow fast atom bombardment (FAB), was first reported [6, 7]. For small molecules it was thermospray (TSP)  and particle beam (PB)  which allowed the routine use of LC-MS. Thermospray formed in most cases ammonium adducts, while particle beam yielded electron impact spectra. Within a few years thermospray was rapidly replaced by atmospheric pressure ionization techniques.
Quadrupole mass spectrometers  or quadrupole ion traps are today the most widely used mass spectrometers. The physical bases were described in the early 1950s by Paul and Steinwedel. For his work Paul received the Nobel Prize in 1989 . Triple quadrupole mass spectrometers have become very popular instruments for qualitative and quantitative analysis. Yost et al.  built in 1978 the first instrument and it took four years before this type of instrument was commercialized. The coupling with liquid chromatography or gas chromatogra-phy is well established and benchtop ion traps or quadrupoles are nowadays part of the standard equipment of many analytical laboratories.
For the analysis of macromolecules and in particular for proteins a major milestone was achieved with the development in 1987 of matrix assisted laser desorption ionization by Karas and Hillenkamp  and in 1988 of electrospray ionization by J. Fenn (Nobel Prize in 2002) .
Over the past decade progress in mass spectrometry and its hyphenation with separation techniques has made these tools essential in life sciences. The present chapter will describe current ionization techniques as well as mass analyzers.
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