Basic Principles of the Quadrupole Mass Spectrometer

However, we now need to allow motion along the yz plane too. Since the potentials applied to the upper and lower rods are negative, these are attractive to a positive ion and any ion located nearer to one or other of these rods will be attracted to the nearest rod and may be lost by contact with this electrode. Consequently, the conclusion is that a static quadrupolar field can generate stable motion in one plane (xz in this case) but not in the other (yz). 

Suppose, now that instead of a static quadrupolar held, an oscillating electric field component is also added operating at a radiofrequency (RF) such that 

In the above expression is the net applied potential at time t, U is the static (DC) component, V is the maximum amplitude of the oscillating (AC) component, and v is the 

It can therefore be seen that the combination of static and oscillating components to the applied electrical potentials provides a mass filter that allows passage of ions with only a narrow range of masses. This set of transmitted masses can be changed by altering U, V and V, and thus provides the basis for a mass spectrometer. To produce an actual mass spectrometer the quadrupole filter needs to be combined with a vacuum system, an ion source, and an ion detector at the downstream end of the mass filter. 

The explanation above is very simplified. A more detailed account of QMS would consider the equations of ion motion and the permissible values of U, V and v that transmit ions of a particular miz to the detector. A full description of this topic is beyond the scope of this book and the interested reader is referred elsewhere for further details, such as the 

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