Page 82 - Inorganic Mass Spectrometry : Fundamentals and Applications

72 Olesik

Second, the sample is isolated, to a large degree, from the doughnut region of the
plasma where most of the energy is coupled. As a result, the energy coupling is not
strongly dependent on the composition of the sample (in contrast to a discharge
between two electrodes, for example). Gas velocities in the center of the plasma
are typically 15 to 20 dsec [8]. The plasma may be about 20 to 30 mm long before
ions are sampled into the mass spectrometer, so the sample spends l to 2 msec in
the plasma.
for
The radio frequency plasma power supply must be designed specifically
ICP-MS. Radio frequencies between 27 and 40 MHz are typically used for ICP
generation. During ignition, the impedance of the plasma varies dramatically from
nearly infinite to nearly zero. The plasma impedance is also a function of the
applied power, gas flow rates, solvent loading (the amount solvent aerosol and
of
vapor entering the plasma per second), and plasma gas composition. ICP-MS
signals are very sensitive to small changes (more than about 1%) in power so
power fluctuations and drift must be minimal. Finally, the plasma potential,
dependent on the power supply and load coil configuration, must be properly
controlled. If the plasma potential too high relative to the sampling plate of the
is
mass spectrometer, a secondary rf discharge or arc forms. The electric field can
also propagate along the ion beam into the interface region between the sampling
orifice and skimmer, so that a discharge forms in this region.
The presence of a strong secondary discharge has several deleterious ef-
fects. The sampling orifice can be slowly vaporized by the arc. This leads to a
higher background for the elements that make up the sampling cone and a reduced
lifetime of the sampling cone. Formation of doubly charged ions (]Sa2+, for
example, because barium has a relatively low second ionization energy, 10 eV,
compared to other elements) is more likely, although molecular oxides might be
more effectively atomized. The ion kinetic energy and spread of ion kinetic
energies are larger, thus reducing the resolution and abundant sensitivity (ratio of
signal at the mass of an ion to that one mass unit away, produced by ions of the
same mass) provided by quadrupole mass spectrometers.
Several different approaches have been used to minimize formation of a
secondary discharge, which results from parasitic capacitance between plasma
the
and the load coil. A balanced load coil can be used where the two ends of a single
load coil are driven by rf signals of opposite phase but nearly equal amplitude [g]
(as is done on Perkin ElmerlSciex ins~ments, called PlasmaLok). Then the
center of the load coil is at 0 V. In some cases the center of the load coil can be
directly connected to ground or to the sampling plate of the mass spectrometer.
~lte~atively~ two separate load coils can be interlaced to form a balanced rf drive
system (as is done on Varian instruments).
A grounded, electrical shield can be placed between the load coil and torch
to reduce the capacitive coupling between the load coil and the plasma in order to
reduce the plasma potential [lo]. A thin metal cylinder, split along its length (to