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is necessary.     The Quadrapole Mass Spectrometer   The most popular mass spectrometer that is used in combined gas chromatography/mass spectrometry systems is the quadrapole mass spectrometer, either as a single quadrapole or as a triple quadrapole. Theses types of mass spectrometer can also easily provide MS/MS spectra (a technique that will be discussed later).   A diagram of a single quadrupole mass spectrometer is shown in figure 25. The operation of the quadrupole mass spectrometer is quite different from that of the sector instrument. The quadrupole spectrometer consists of four rods which must be precisely straight and parallel and so arranged that the beam of ions is directed axially between them. Ideally the rods should be hyperbolic in cross section but in practice less expensive cylindrical rods are nearly as satisfactory. A voltage comprising a DC component (U) and a radio frequency component (Vcoswt) is applied between adjacent rods, opposite

(a) and (q) that can allow the mass spectrometer to operate in a stable mode. The mass range is scanned by changing U and Vo, while keeping the ratio U/Vo constant.     Figure 26. Conditions for Stable Oscillation in a Quadrupole Mass Spectrometer   The quadruple mass spectrometer is compact, rugged and easy to operate and consequently is a popular instrument for use in combined gas chromatography/mass spectroscopy combinations. Unfortunately, the mass range of the quadrapole spectrometer does not extend to very high values but, as will be seen later, when dealing with different interfaces, under certain circumstances multiple charged ions can be generated and identified by the mass spectrometer. It will be seen that the presence of multiple charged ions, in effect, can significantly increases the mass range of the device.