Page 153 - Inorganic Mass Spectrometry - Fundamentals and Applications

I~ductively Co~pled Plasma Mass Spectro~et~ 143

ment analysis. Convenient, rapid semiquantitative analysis and isotope measure-
ments are additional capabilities that make ICP-MS popular. As indicated by the
wide range of applications, ICP-MS currently provides solutions to analysis
problems in a wide range of disciplines.
Substantial improvements have been made in ICP-MS instruments over
just
the last few years and further advances are on the horizon. The quadrupole ICP-
MS instruments have become smaller, more sensitive, easier to use, and less
expensive. Sector-based ICP-MS instruments are growing in popularity and per-
formance, although they are still expensive compared to quadrupole instruments.
Second- and third-generation sector-based instruments hold promise for further
performance improvements. Multicollector sector-based instruments that were
previously useful only for isotope ratio measurements may now provide excellent
elemental analysis performance as well, because of new inst~ment designs.
Detection limits and precision provided by time-of-flight instruments have im-
proved, and commercial instruments have recently become available. Further
improvements in sensitivity may be possible. The speed of complete spectral
acquisition provided by time-of-flight ICP-MS could particularly valuable with
be
laser
sample introduction systems that produce transient signals, such as ablation,
electrothermal vaporization, and capillary chromatography. Laser ablation solid
sampling continues to improve with the use of ultraviolet (UV) lasers and flat
beam profiles. Sample introduction systems for solution samples also continue to
improve. Systems with high analyte transport efficiencies and membrane desolva-
tion are particularly promising.
ICP-MS currently has several limitations. Research continues to develop
new ways to overcome these limitations and improve ICP-MS performance
further. At the same time steps to lower the instrument cost continue to progress.
Spectral overlups, particularly those due to polyatomic ions, remain a
problem in many applications. High-resolution mass spectrometers can overcome
many, but not all, of these overlaps. The use of reaction cells to remove particular
molecular ions chemically is an exciting development that could have a major
impact on ICP-MS performance.
C~emicul matrix effects due to space-charge ion transmission loss remain a
problem. Concentrations of heavy ions as low as 100 ppm can affect sensitivity
and therefore produce an analysis error. Perhaps alternative designs will reduce
space-charge effects, but can the space-charge effects be significantly reduced
while m~ntaining or continuing to improve sensitivity?
~epositio~ sample on the sampler, skimmer, ion optics, and parts of
of
other
the interface can lead to elevated blank levels as well as drift. This,
in omb bin at ion
with the space-charge-induced chemical matrix effects, often requires further
dilution of samples than is desirable. This can also limit the range of concentra-
tions that can be measured for a set of samples even though the dynamic range
may in theory be sufficient. An improved understanding of the chemical and
physical characteristics of the deposition process and means to minimize them is
needed.