The ion microprobe

It is possible to measure nearly any type of sample for almost any element with little or no preparation. Only a few mg of sample is required, and the measurements are non-destructive in that the sample is generally undamaged. Measurements take only 1-20 min of beam time. Elemental mapping showing the variations in elemental concentrations can be measured over the surface of a sample using the ion microprobe for an area as large as 5 x 5 mm. 

A unique feature of the ion microprobe is the potential to measure both elemental concentrations and isotopic ratios in the same spot. This capability was particularly valuable in the present study since the possible correlations between the 26Mg/24Mg and Al/Mg ratios are central to the interpretation of Mg isotopic anomalies. The Al/Mg ratio is calculated from the... 

The principal advantage of the ion microprobe (as opposed to the Auger microprobe) is the ability to obtain depth profiles for trace elemental species present in the analytical volume. The characterization of coal fly ash clearly illustrates this point (11-14). Auger detection limits are comparable to BSCA, and thus only elements with bulk concentrations greater than 1% by weight in fly ash (Si, Al, Fe, Ca, S, Na, K) can be... 

The distributions of trace elements between minerals and within a suite of related rocks provide powerful tools for constraining the origin and history of rocks and meteorites. Trace-element abundances for rocks typically are part of the data set collected when determining bulk compositions. Trace element compositions of minerals require more powerful techniques such as the ion microprobe or the laser-ablation inductively coupled plasma mass spectrometer (ICPMS). 

Advances in techniques for handling and analyzing very small particles have allowed detailed examination and characterization of IDPs. Especially useful instruments include the transmission electron microscope ( ), synchrotron facilities, and the ion microprobe. 

Fig. I, Schematic representation of the ion microprobe mass analyzer. (Bausch Lomh/ARL)...

The ion microprobe has also been applied in a preliminary fashion to the rubidium-strontium dating technique. The correlation of the ion mieroprobe results with the independently determined isochron indicates that it may be possible to obtain useful results for samples on a micrometer scale from this dating technique. 

The ion microprobe mass analyzer s unique features permit three dimensional microanalysis of all elements in the periodic table and in... 

Recent studies on iron sulfide minerals in coals, minerals in coals, and in situ investigation of minerals in coal all used the scanning electron microscope (SEM) as the primary analytical tool. The ion microprobe mass analyzer (IMMA) is more sensitive than either the energy-dispersive x-ray spectrometer or the wavelength-dispersive x-ray spectrometer, both of which are used as accessories to an electron microscope. 

In 1967 Liebl reported the development of the first imaging SIMS instrument based on the principle of focused ion beam scanning [24]. This instrument, the ion microprobe mass analyzer, was produced by Applied Research Laboratories (Fig. 4.5). It used an improved hollow cathode duoplasmatron [25] ion source that eliminated filaments used in earlier sources and allowed stable operation with reactive gases. The primary ion beam was mass analyzed for beam purity and focused in a two-lens column to a spot as small as 2 pm. The secondary ions were accelerated from the sample surface into a double focusing mass spectrometer of Mattauch-Herzog geometry. Both positive and negative secondary ions were de-... 

Hitachi announced the development of the third commercial microprobe instrument, the ion microprobe analyzer IMA-2 in 1969 [30]. This instrument placed a scintillator close to the sample for secondary electron imaging. A Wien filter, for primary beam mass selection [31], and an electron spray, for charge compensation on insulating samples [32], were added later. 

The ion microprobe has been widely applied to the analysis of trace elements in a variety of geological materials, as the excellent spatial resolution provides the ability to study either elemental zoning patterns in a large mineral or to analyze phases that are too fine-grained (or are too limited in quantity) to make separation and bulk analysis practical. In comparison to other techniques, the ion probe also of-... 

SHRIMP 238U/206Pb ages as old as 7.2 Ga, much older than the Pb/ Pb Concordia age of 2.68 Ga. Isotope dilution thermal mass spectrometry analysis of the same zircons demonstrated that the ion microprobe U/Pb ratios were too low. This was attributed to the presence of a labile Pb component within amorphous microdomains that had an anomalously low U/Pb ion yield, which invalidated the SHRIMP U-Pb ion yield calibration. 

Figure 1 U-Pb Concordia diagram showing the results of an analysis of a detrital zircon crystal by ion microprobe (SHRIMP) followed by analysis of the same crystal using TIMS. Both error ellipses are plotted at 2a. The best estimate of the age of crystallization of the zircon is identical for both techniques however, the TIMS analysis is an order of magnitude more precise than that obtained using the ion microprobe (source Samson et aL, 2003).

Figure 2 Size of a typical pit produced in an accessory mineral using an ion microprobe during an 18 min analytical run (from Stern, 1997) compared to the size of an ablation crater made from a single pulse of an excimer laser (from Horn et al., 2000). Bottom drawings show generalized cross-sections of the spots made from the two techniques. Note the considerably smaller volume of mineral excavated during the ion microprobe analysis.

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