Page 127 - Inorganic Mass Spectrometry - Fundamentals and Applications

Induct~v~ly Co~pled Plasma Mass Spectro~et~ 117

efficiency sample introduction system such as an ultrasonic nebulizer with a
desolvation system [ 164,1651. The ICP is an efficient ionization source. Elements
with ionization potentials less than about 8 eV are over 90% ionized in the ICP
under normal operating conditions, Elements with higher ionization energies,
such as As and Se, are typically 20% to 50% ionized in the ICP. Elements such
as C, Cl, Br, F, S exist mainly as atoms rather than ions in the ICP so their ICP-MS
sensitivities are much lower than those of more efficiently ionized elements. For
lo5 ions produced in
quadrupole ICP mass spectrometers typically only 1 in about
the ICP reaches the MS detector. For sector-based ins~ments, used in a low-
resolution mode, 1 in lo3 ions produced in the ICP may reach the MS detector.
The transport of ions from the ICP to the MS detector depends on travel
through three different regions: from the ICP through the sampler and skimmer,
from the skimer to the mass spectrometer, and through the mass spectrometer
itself to the detector. As discussed, to a first approximation, the flow through the
sampler and skimmer orifice is dominated by neutral gas flow. The gas flow
through the skimmer depends linearly on area of sampling orifice and the area
the
of the skimmer orifice, and inversely on the square of the distance from the
sampler orifice to the skimmer orifice [92]. As long as the skimmer is upstream of
be
the Mach disk, the gas flow through the skimmer should not dependent on the
interface pumping speed because the expanding gas and shock structures prevent
background gas from entering.
When sector-based mass spectrometers are used in a low-resolution mode
the sensitivity can be higher than attainable with current quadrupole mass spec-
trometers. Random background count rates are also typically lower for sector-
based mass spectrometers. As a result, detection limits are one to two orders of
magnitude better than for typical quadrupole ICP-MS instruments.
Particularly for low-mass analytes, sector-based mass spectrometers used in
a high-resolution mode can reduce spectral overlaps due to polyatomic ions,
thereby improving detection limits compared to those of quadrupole mass spec-
trometers. However, ion trans~ssion efficiency decreases significantly as the
resolution is increased. For example, increasing the resolution provided by the
300 to 3000 reduces the ion transmission efficiency
sector mass spectrometer from
by about a factor of 14 [127]. Further increasing the resolution to 7500 results in
another order of magnitude loss in ion transmission efficiency [ 1271. The use of
reaction cells may provide an alternative or complement^ means to reduce
background due to polyatomic ions.
Generally, dissolved solid concentrations should be kept below 0.2% for
ICP-MS. Higher dissolved solid concentrations can lead to deposition sample
of
and skimmer orifices, partial orifice plugging, or deposition of
material on ion lenses that degrade sensitivity and medium-term to long-term
precision. Furthermore, relatively small concentrations of a heavy element (100
ppm or greater) in a sample can cause a decrease in analyte sensitivity, particularly