Isotopic studies, SIMS

A practical difficulty of SIMS Li isotope measurement is the paucity of appropriate mineral and rock standards with well characterized isotopic compositions. Although the estimated uncertainties are larger than for the more widespread TIMS and MC-ICP-MS techniques, the capacity to measure at very restricted spatial scales makes this a promising area of exploration, particularly for samples of restricted size and those with complex small-scale structure. The complimentary nature of in situ and bulk data in Li isotope studies shows promise for better understanding complex processes, e.g., fluid-rock interaction (Decitre et al. 2002). 

For the future, even smaller-scale sample areas can be considered for analysis as can isotope ratios. It could be envisaged that multiscale prospection analyses could proceed from optical methods to microfocus synchrotron X-ray fluorescence and SAXS and, finally, to destructive isotopic studies with LA-ICP-MS or to finer resolutions with SIMS. 

Convincing evidence of recycling of volatiles to the mantle and their incorporation into diamonds has recently been reported from sulfur isotope studies of sulfide inclusions within diamond (Farquhar et al., 2002). SIMS analysis of sulfide inclusions has revealed anomalous mass-indepen-dently fractionated sulfur isotopic compositions from the Orapa kimberlite, which are explained as a consequence of the recycling of surface sulfur produced through photolytic chemistry in the Archean atmosphere. Such studies need to be extended to other diamond populations and combined with C-N isotope studies on the host diamonds. 

Isotopic Studies. In this analysis mode the mass spectrometer Is used to determine the relative abundance of different Isotopes of the same element. SIMS Is the only common surface analysis technique that can distinguish the Isotopes of both the light and heavy elements. This mode Is advantageously used In mechanistic studies using Isotopically labelled samples. By using different Isotopes of the same element experiments can be performed without concern for the differences In chemical effects between the species. 

The advent of laser ablation MC-ICP-MS technology allows the rapid in situ determination of the stable isotope ratios of heavy metals commonly found in sulfide ore deposits (e.g., Cu, Zn, Fe, Sb, Ag) providing important information on the source, transport, and depositional mechanisms of these metals. Pb, Pb, and Pb are formed as the end product of radioactive decay and the isotopic variability of lead results because the elements from which the isotopes form were not evenly distributed in ore bodies. Hence, the analysis of stable lead isotopes in annually laminated lake-sediments is a useful method to study lead pollution history as the relative contribution of pollution and natural lead in sediment samples can be calculated. The analysis of lead isotopes by SIMS has also been used to identify the geographical origin of bullets. 

Electron microprobe analysis and secondary ion mass spectroscopy (SIMS) combined with sputtering techniques also provide excellent tools for studying penetration and diffusion of foreign ions. As oxygen does not have a radioactive isotope suitable for tracer studies, SIMS is particularly useful for studying oxygen diffusion employing the stable isotope. 

Studies to determine the nature of intermediate species have been made on a variety of transition metals, and especially on Pt, with emphasis on the Pt(lll) surface. Techniques such as TPD (temperature-programmed desorption), SIMS, NEXAFS (see Table VIII-1) and RAIRS (reflection absorption infrared spectroscopy) have been used, as well as all kinds of isotopic labeling (see Refs. 286 and 289). On Pt(III) the surface is covered with C2H3, ethylidyne, tightly bound to a three-fold hollow site, see Fig. XVIII-25, and Ref. 290. A current mechanism is that of the figure, in which ethylidyne acts as a kind of surface catalyst, allowing surface H atoms to add to a second, perhaps physically adsorbed layer of ethylene this is, in effect, a kind of Eley-Rideal mechanism. 

The impact of an ion beam on the electrode surface can result in the transfer of the kinetic energy of the ions to the surface atoms and their release into the vacuum as a wide range of species—atoms, molecules, ions, atomic aggregates (clusters), and molecular fragments. This is the effect of ion sputtering. The SIMS secondary ion mass spectrometry) method deals with the mass spectrometry of sputtered ions. The SIMS method has high analytical sensitivity and, in contrast to other methods of surface analysis, permits a study of isotopes. In materials science, the SIMS method is the third most often used method of surface analysis (after AES and XPS) it has so far been used only rarely in electrochemistry. 

William Schopf studied supercrustal rock samples from Akilia Raman and ion microscopic photographs showed the presence of carbon-containing inclusions in grains of apatite. The carbon isotope ratio was determined by secondary ion mass spectroscopy (SIMS) the 813C value was -29% 4%, in agreement with earlier analyses. This in turn confirmed the values obtained by Mojzsis (1996), which had been questioned by Lepland et al. three years later. The final verdict on the oldest fossils in western Greenland may not be reached for several years yet (McKeegan et al., 2007 Eiler, 2007). 

Secondary ion mass spectra were measured using a Perkin-Elmer+PHI 3500 instrument. Experiments were carried out with 4 kV Ar ions at beam currents of 3 and 300 nanoamps. Spectra were measured to at least 500 daltons (d). Samples were prepared in the manner used for the XPS studies. For measurements on the pure complexes, sample charging occurred, as evidenced by the inability to record secondary ion mass spectra. To reduce charging, a low energy electron beam (50-400 eV) was rastered across the sample during SIMS analysis. Positive and negative ion SIMS spectra were recorded however, only positive ion spectra are of interest for this discussion. In the spectra only unipositive ions were detected, so that the mass numbers detected correspond to combinations of the various isotopes of the elements in the ion. Thus an ion at m/z 17 d is assigned to... 

Milke R., Wiedenbeck M., and Heinrich W. (2001) Crain boundary diffusion of Si, Mg, and O in enstatite reaction rims a SIMS study using isotopically doped reactants. Contrib. Mineral. Petrol. 142, 15-26. 

The Mn- Cr system can be studied by TIMS, ICPMS, and SIMS techniques. For TIMS and ICPMS work, bulk samples or mineral separates are dissolved and the solutions are passed through ion-exchange columns to produce clean solutions of manganese and chromium. For minerals with high Mn/Cr ratios SIMS can obtain isotopic data while retaining the petrographic context of the measurements. The chromium isotopic compositions may have to be corrected for small additions of chromium from spallation reactions induced by cosmic rays. This is particularly important in iron-rich meteorites. 

Of all the different mass spectrometric techniques for isotope analysis (such as ICP-MS, LA-ICP-MS, TIMS, GDMS, AMS, SIMS, RIMS and isotope ratio mass spectrometry of gases), the greatest proportion of pubhshed papers today concern ICP-MS with single and multiple ion collection.19 Due to its benefits, ICP-MS has now become a widely accepted method for isotope analysis and allows isotope ratios to be measured in a short time with good accuracy and precision.9,19,75 78 As discussed above, as a powerful and universal tool, ICP-MS has opened up new applications for isotope ratio measurements of elements with a high first ionization potential, which are difficult to analyze with TIMS (such as Mo, Hf, Fe). Of all the heavy metals studied, uranium was favoured by ICP-MS and LA-ICP-MS. 

Figure 9.24 b compares the isotope ion distribution of natural 40Ca+ and doped 44Ca+ in a cross section of a root of a Norway spruce. These measurements were carried out by SIMS using 69Ga+ primary ions (lOkeV, 0.1 nA, Ionoptika) with a lateral resolution < 0.1 pm.80 The investigations can be used to study the mechanism of mineral element uptake and kinetics of transport processes in plants as a function of time. About 20 min after the start of the tracer experiment a barrier was observed for the transport of enriched isotope 44Ca+ in the middle of the root.80... 

---