Page 101 - Inorganic Mass Spectrometry : Fundamentals and Applications

Indu~tively Coupled Plasma Mass Spe~tro~etry 91

(Fig. 3.10~) that deflected ions off center and then back on center, thus preventing
a straight line of sight for photons for fast neutrals to travel from the skimmer into
the quadrupole mass spectrometer without the need for a stop. In some instru-
ments (Fig. 3.10d, e) the mass spectrometer is placed off axis from the sampler-
skimmer axis in order to minimize background without the use of a stop. The
effective focal length of an ion lens depends on the ion kinetic energy (analogous
to the focal length dependence of an optical lens on the wavelength of light). Ion
optic systems for ICP-MS are therefore no~ally designed from multiple lenses in
order to focus ions with a range of ion kinetic energies with fixed lens voltages. An
A
alternative approach is shown in Fig. 3.10f. stop and a single cylindrical lens are
used to focus ions into the mass spectrometer, but the lens voltage is scanned
synchronously with the quadrupole mass spectrometer for best focusing of ions
of each masslcharge ratio.

is
The transmission of ions from the skimmer to the quadrupole mass spectrometer
generally poor (0.1% to 0.01%) [92,105,1 lo], mainly as a result of space charge
induced loss of ions from the beam, Space charge efYects can be thought of in two
ways. Positive ions, in the absence of electrons, in close proximity repel each
other. This tends to force positive ions away from the beam axis, thus reducing the
number of ions that pass through the next aperture or into the mass spectrometer.
Alternatively, the large number of ions in the beam can be considered to shield
ions from the applied electrostatic field produced by the ion optics, resulting in a
defocusing of the positive ion beam. The radial electric field due to the positive ion
beam can become as large as the electric field produced by the ion lens. As a result,
there are severe losses of ion transmission to the mass spectrometer due to space
charge effects.
The space charge field is due mainly to the predominant positive ion, h+,
when only trace amounts of analyte are present in the plasma. Initially, almost all
at
of the ions are moving the same velocity through the skimmer, independently
of
a
their mass. Because heavy ions have higher kinetic energy than light ions, light
ions are more severely defocused by the space charge field. As
a result, a mass bias
toward heavier ions is induced. If sufficiently high concentrations of heavy
elements are present in the sample, they can contribute significantly to the total
positive ion beam current and produce increasingly severe losses of lighter ions
from the positive ion beam, as will be discussed further.
The eeect of the ion beam current on model predicted ion trajectories [ 1051
is shown in Fig. 3.1 l. Important inputs for the model are the location and distance
over which charge separation occurs to produce a positive ion beam. A charge
separation function was assumed (Fig. 3.1 la) for the simulation. When space
a
charge effects are not included, large fraction of the ions pass from the skimmer