Gas Chromatography - Tandem Techniques - The Quadrapole Mass Spectrometer > Page 38



where, (ro) is half the distance between opposite rods of the quadrupole system and the other symbols have the meaning previously ascribed to them.


The oscillations of the ions will only remain stable for certain combined values of (a) and (q). Outside these values the oscillations become infinite and will strike the rods and become dissipated. The relationship between (a) and (q) for stable oscillation is shown in figure 26. It is seen that there is a very restricted range of values for (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.