Despite tremendous advances in our ability to perform trace
element and isotope analyses at the micron scale using high
resolution ion probes, the capabilities of these approaches do not
extend to isotopic analysis for elements such as Sr and Nd. Thus,
in order to obtain adequate precision and satisfy stringent
resolution requirements (e.g., resolving 
Rb from 
Sr),
researchers have turned to in situ microsampling coupled with bulk
analysis using Thermal Ionization Mass Spectrometry
[TIMS---see Faure, 1986 for a review of this technique].
The technique is as its name suggests: a small amount of material is extracted via mechanical microdrilling and prepared using standard dissolution and ion exchange techniques to isolate Sr and Nd [and any other element that can be analyzed for isotope ratio, or for concentration by isotope dilution---e.g., Richard et al., 1976] for TIMS analysis. Some additional care over normal bulk rock chemistry must be observed. The reduction in sample size commonly requires reduction in blanks so that they do not significantly affect the measured ratios. Sample masses typically contain < 10ng of the element of interest. The most important advance in mass spectrometry that has enabled such small sample sizes to be measured with relative ease is the multicollector, which allows a number of masses to be simultaneously measured (i.e., each collector is fixed on a desired mass) rather than with a single collector which scans over the masses of interest. The advance is not so much one of improved precision through collector or amplifier design, but rather one of optimizing data collection, by being able to collect a much larger fraction of ions arriving at the collector than had been possible with a peak switching single collector. Clearly with the limited number of ions available from microsamples, precision can be best achieved by using as much of the ion inventory as possible for analysis. Ionization efficiency can also be improved by using modified loading techniques---arguably an important avenue to pursue as we refine microsampling techniques, since standard ionization efficiencies for elements such as Nd and Sr are only of order 1%.