Page 114 - Tandem Techniques
P. 114
Page 95
rods, with a relatively small potential ranging from 10 to 20 volts. Once inside the quadrupole, the ions
oscillate in the (x) and (y) dimensions as a result of the high-frequency electric field.
The stability of the oscillating ions are determined by the magnitude of two parameters (a) and (q)
which are defined by the following equations:
where, (r0) 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) is shown in Figure 2.32
Figure 2.32
Conditions for Stable Oscillation in a
Quadrupole Mass Spectrometer
It is seen that there is a very restricted range of values of (a) and (q) that can permit the mass
spectrometer to operate in a stable mode. The mass range is scanned by changing U and V0 while
keeping the ratio U/Vo constant.
