One major limitation of the present state-of-the-art is that LA-ICP-MS cannot rival SIMS or TIMS for the determination of isotopic ratios across the mass range, nor in spatial resolution. Factors such as ablation characteristics, transient signal, peak resolution, and counting statistics using a scanning electron multiplier do not produce isotopic ratios to the degree of accuracy required for petrogenetic interpretation. However, LA-ICP-MS has been used to accurately determine the heavier isotopes [e.g., U/Pb--- Feng et al., 1993; Walder et al., 1993; Ludden, 1995] providing Pb is present in sufficient quantities (i.e., > 3 ppm) and the unstable ablation of U can be reconciled [e.g., Hirata and Nesbitt, 1995]. Furthermore, LA-ICP-MS does offer a cheaper alternative for trace element analyses without as many interference problems.
By optimizing the laser operating parameters, the amount of
material ablated can be controlled such that depth profiling
through a sample is possible [e.g., Denoyer et al., 1991]
with comparison of surface and bulk compositions possible.
Generally, depth penetration can be controlled to 1-10 
m per
laser pulse. This is of particular significance to the
semi-conductor industry where impurities in high purity quartz
have been identified [e.g., Denoyer and Wallace, 1989],
although it is recognized that for most purposes, the scale of
1 
m depth resolution is still too coarse.
Perhaps one of the most advantageous applications of LA-ICP-MS is semi-quantitative analysis which allows a compositional fingerprint of unknown materials to be obtained, as well as fingerprinting compositional variations across a sample, although sample throughput is enhanced at the expense of accuracy. Critical in semi-quantitative LA-ICP-MS analyses is determining RSFs. By determining RSFs across the mass range, a calibration or response curve is generated using a multi-element standard which can be applied to all elements (from atomic mass 4 to 240) by reference to a single internal standard. Van de Weijer et al. [1992] demonstrated that for each matrix, a different set of RSF values need to be calculated, depending upon where the elements are located in a given matrix [c.f., Jarvis and Williams, 1993]. The accuracy of the procedure depends upon the RSF values used, updating as necessary, and also upon the number of internal standards.
Applications of LA-ICP-MS to a number of different areas have
been reported primarily from laboratories outside the United
States. For example, several studies have been reported where
LA-ICP-MS has been used to analyze sea shells for trace-element
fluctuations, recording major pollution events [ Fuge et al.,
1993]. Marshall et al. [1991] used LA-ICP-MS to determine
the trace elements in solid plastic materials. A defocused beam
was used to analyze polyester, polyethylene, nylon, polyvinyl
chloride, and polypropylene which contained a variety of fillers
and other additives. Semi-quantitative and quantitative analyses
were undertaken using 
C as an internal standard. Results
demonstrated that LA-ICP-MS can be used to explore the spatial
distribution of trace elements in polymeric materials.
The application of LA-ICP-MS to the analysis of whole-rock geological materials (i.e., pressed powder pellets, lithium metaborate fusions) has been described by several authors [e.g., Perkins et al., 1991, 1993]. A recent development has been in the direct fusion of whole-rock samples using a tungsten strip heater cell under an Ar atmosphere to suppress volatile loss and minimize oxidation [ Fedorowich et al., 1993]. By producing glass matrices in this way, dilution due to flux addition (and a potential source of contamination) is eliminated and matrix-matched glass standards are available (e.g., NIST 610, 612 glasses) for quantitative analyses.
Mineral analyses have been reported either analyzing grains [e.g., calcite, aragonite, zircon, olivine, plagioclase, and K-feldspar--- Pearce et al., 1992a] or in thin section [e.g., titanite, zircon, uraninite, apatite, and garnet--- Jackson et al., 1992; garnet and clinopyroxene--- Neal, 1993]. The potential of LA-ICP-MS to the analysis of thin sections is seen in these studies, although spatial resolution, detection limits, and reproducibility need to be improved.
The application of LA-ICP-MS to the analysis of small samples is continually being developed. Furthermore, the area required for a fully quantitative analysis is constantly being reduced. Optimization of ablation and transportation of material will further enhance the sensitivity of this technique and expansion of the LA-ICP-MS analytical technique to geological samples and beyond is anticipated over the next quadrennium.
Acknowledgments. This manuscript has greatly benefited from the comments of three anonymous reviewers and Ken Verosub. This work was partially supported by NSF grant ECS92-14596 to CRN.