Page 97 - Inorganic Mass Spectrometry : Fundamentals and Applications

~~~uctively Coupled Plasma Mass ~pectromet~ 87

10-nglg range in geological materials have been reported recently [62]. Precision
of 5% to 10% is typical, although this is also strongly dependent on experimental
parameters as well as sample homogeneity [64]. If a simultaneous detection sys-
tem is used, such as a dual-qua~pole [65] or multicollector sector MS [57,58,
661, isotope ratio precision of 0.24% to 0.004% can be obtained, respectively.
The major 1inGtation of LA-ICP-MS is the need for standards that closely
it
match the properties of the samples. In some cases is possible to use NIST glass
standard reference materials for calibration in the analysis of geological materials
[67,68]. Internal standardization employing MS signals from elements at known
concentrations has been used to improve precision and accuracy. Other tech-
niques, such as acoustic [69] and light scattering [70] measurements, have been
used in an attempt to monitor the relative amount of material ablated. These
approaches seem to work we11 for variations in the amount of material sampled for
similar sample matrices but not for very different types of solids. Dual-sample
introduction systems with either wet [71] or dry [72] aerosol introduction in
addition to laser ablation have also been reported.
By positioning the sample just below the ICP, sample transport losses can
be
dramatically reduced [73]. This “in situ” sampling approach suggests some
interesting possibilities for laser ablation sarnpling. A narrow-signal (0.7-msec)
pulse was produced after each laser pulse. The peak signals were approximately
cell
1000 times the steady-state signal observed when a conventional ablation. and
transfer tubing were used. However, fast, simultaneous detection (such as is
provided by time-of-flight mass spectrometers) is required to take full advantage
of this approach in ICP-MS.
Electrot~e~al ~a~orizatio~. Samples may be introduced into the ICP as
vapor, including atoms, molecules, small clusters, and small particles [74] by
electrothermal heating [75]. A small liquid (~10 pL) or solid (-10 mg) sample is
placed on a graphite furnace or tantalum filament, which is heated by passing a
high dc current through it. The electrothermal source, unlike in elec~othermal
vaporization (ETV)-ato~c absorption spectrometry, does not have to atonGze the
sample, only to transfer it into a form that can be efficiently carried into the ICP.
The vaporizer is part of a closed system through which a carrier gas flows to take
the vaporized sample through a length of tubing into the ICP. The advantages of
this approach are direct sampling without requiring dissolution and ability to
vaporize the water before vaporizing the sample so that molecular ions that
involve 0, such as ArO+, are much less common.
The ETV parameters, including vaporizer temperature, carrier gas flow rate,
drying temperature and duration, pyrolysis temperature, and duration and vapor-
ization temperature, all affect the transient signal produced. Different elements are
carried into the plasma at different times, depending on their volatility.
The transport of sample into the ICP appears to be more efficient when the