Page 105 - Inorganic Mass Spectrometry : Fundamentals and Applications

Inductively Coupled Plasma Mass Spectrometry 9s

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Quadrupole mass filter: (a) Diagram of quadrupole and connections to rf/dc
power supply. (b) Side view of ion trajectories in two planes.

power supply with a direct current (dc) offset. The voltage on one pair is opposite
is 180" out of phase with
to the polarity of the other pair. The rf voltage on one pair
the rf voltage on the other pair.
Although the ideal cross section of the rods is hyperbolic, the best approx-
imation using circular rods is obtained when the radius of each rod, r, is equal to
1.148 times the radius from the center of the four rods to the inside edge of the
rods, ro [ 1261.
The operation of the quadrupole can be understood by considering the
movement of the ions in the two planes perpendicular to their flight through the
quadrupole. When the potential on two opposite rods negative, positive ions are
is
attracted radially outward from the center the quadrupole toward the negatively
of
biased rods. Conversely, when the potential on two opposite rods is positive,
positive ions are repelled from the rods, effectively focused toward the center of
the quadrupole. The alternating current (ac) portion of the voltage waveform
oscillates with time. The distance ions travel outward from the center during the
portion of the waveform when the ac potential of two opposite rods is negative
depends on the mass-to-charge ratio of the ion, the time the potential remains
negative (inversely proportional to the ac frequency), the magnitude the applied
of
ac potential, the location
of the ion relative to the center of the quadrupole, and the
velocity of the ion. If the masslcharge ratio of the ion is large and the ac frequency
is high, the ion does not move far during each half-cycle of the ac waveform.
the
Therefore, the heavy ions are most affected by average (dc) voltage applied to
by
the rods. In contrast, lighter ions are more significantly affected the oscillating,
ac voltage.
If the average (dc) voltage on the rods positive (as is true for the rods in the
is
to
x-z plane, as defined in Fig. 3.13a), ions tend be repelled from the rods toward
the center of the quadrupole on average. ions that have a masdcharge ratio less