Microbeam techniques

There are two principal sources of reliable partitioning data for any trace element glassy volcanic rocks and high temperature experiments. For the reasons outlined above, both sources rely on analytical techniques with high spatial resolution. Typically these are microbeam techniques, such as electron-microprobe (EMPA), laser ablation ICP-MS, ion-microprobe secondary ion mass spectrometry (SIMS) or proton-induced X-ray emission (PIXE). 

The PIXE microbeam technique has a spot size in the range 1-10 pm, and this enables a study of the spatial distribution of elemental concentrations. The advantage of p-PIXE over EPMA is a very much increased analytical sensitivity due to the much lower Bremsstrahlung background generated by the proton beam. The detection limits are of the order 0.1% for EPMA and 0.001% using the p-PIXE technique. 

Methods were discussed to reveal texture in plant materials in situ by X-ray diffraction. A uniform texture in plant materials is generally confined to areas of microscopic dimensions, and the oriented materials are normally of poor crystallinity. Special methods are therefore needed to obtain fibre diffraction from such areas. They may be distinguished as microbeam techniques, artificial orientation of micro-areas, chemical methods improving crystallinity in oriented but poorly crystalline materials, and combinations of these possibilities. 

The examples of application presented were taken from the authors personal experience. They concerned a microbeam technique in which the collimators and camera are adapted respectively to produce X-ray beams down to about 10 microns diameter and registration of the diffraction on flat film 10 mm diameter at distances down to 1 mm from the specimen (l, 2). This enabled fibre patterns to be obtained from wax coatings on plants (1) and a single starch granule C3) in both instances leading to new insights about the ultrastructure of these objects. 

Sweeney R. J., Prozesky V. M., and Springhorn K. A. (1997) Use of the elastic recoil detection analysis (ERDA) microbeam technique for the quantitative determination of hydrogen in materials and hydrogen partitioning between olivine and melt at high pressures. Geochim. Cosmochim. Acte 61, 101-113. 

Modes of occurrence of the elements in coal can be determined using a variety of procedures. Perhaps the most effective method is the use of scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDX). This method can detect and analyze minerals as small as 1 pm in diameter (Figure 14). The SEM-EDX also provides useful information on the textural relationships of the minerals. Other microbeam techniques, such as the electron microprobe analyzer, ion microprobe, laser mass analyzer, and transmission electron microscopy, have also been used to determine modes of occurrence of elements in coal. 

We have shown above that dissolution rates of multiple oxides can be related to the abundance and speciation of hydrogen and hydroxyl radicals at different metal centers at the surface. Since dissolution of most complex oxides is nonstoichiometric, the identity of these centers varies as a function of time and experimental conditions. The selective removal of some cations from the solid surface creates a reacted layer that is depleted in those elements that dissolve rapidly (i.e, modifying cations during basalt dissolution or sodium, calcium, and aluminum in the case of feldspars). As steady-state dissolution is controlled by the dismantling of these altered layers, it is critical to know their chemical characteristics and to identify the main mechanisms that control their formation. Two important findings obtained via microbeam techniques will be presented here. 

The work of Raeburn et al. (1997a,b) used a 2 mm x 250 4,m collection area, which is admittedly much larger than true microbeam techniques (but much smaller than bulk methods ). However, their results compare extremely well with those of wet chemistry and Mossbauer. Given the relatively wide availability of XPS instrumentation, this technique has great potential for future studies. 

Spatial resolution currently achievable with hard X-ray microbeam techniques, down to the 100 nm range, is quite satisfactory. In many cases, spatial resolution at the beam size cannot be realized because of the properties of the sample, notable thickness (because of the penetrating nature of the X-rays, 100 nm resolution can only be achieved in samples <100 nm thick). Improvements in sensitivity can be achieved in two principal ways, increase in incident flux or improvement in detection efficiency. Major increases in incident flux will be impractical in many cases because of the radiation sensitivity of the earth and environmental samples of interest. Improvements in fluorescence detection efficiency will be a more fruitful avenue because current detection schemes use very small solid angles. Energy dispersive detectors that intercept large solid angles would be a major advance in sensitivity enhancement. 

Shepherd, T. J., C. Ayora, D. I. Cendon, S. R. Chenery A. Moissette, 1998. Quantitative solute analysis of single fluid inclusions in halite by LA-ICP-MS and cryo-SEM-EDS complementary microbeam techniques. European Journal of Mineralogy 10 1097-1108. 

A review appeared on speciation of trace elements using radioanalytical methods . Particle-induced X-ray emission (PKE) combined with X-ray spectrometry affords a sensitive multielement analytical method. An advantage of this radiation method is its capability to combine with microbeam techniques, allowing elemental mapping with 1 pm spatial resolution. Applications of PIXE in biology, medicine, geology, air pollution research, archeology and art have been demonstrated . 

Table 4.14. Synopsis of some features of high-energy microbeam techniques...

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