Depth profiling studies, mineral samples

SIMS has become one of the most important tools for the characterization of experimental products because of its minimal sample requirements, high spatial resolution, excellent sensitivity, and unsurpassed ability for depth-profile measurements. Most of the experimental work can be split into two different areas. The first consists of studies examining diffusion rates of different elements in minerals or melts under a variety of pressure, temperature, and fluid conditions, typically by using an isotopically enriched tracer. These analyses are done either by cutting a surface parallel to the diffusion direction and taking a traverse of spot analyses (for conditions in which profiles in the tens to hundreds of micrometers are expected) or by depth-profiling in from the mineral surface to depths of as much as 5-10 micrometers. In the latter mode, depth resolution on the tens of nanometer scale is possible (see Chapter 4). The second area is focused on determining partition coefficients for trace elements between different minerals and fluids/melts at specific temperatures, pressures, and fluid conditions, to provide the data needed to interpret trace element contents measured in natural minerals. This type of analysis typically involves spot analysis of mineral run products. 

Numerous studies have also observed the humification index (HIXem) decreasing with depth in the soil profile. Bu et al. (2010) found a noticeable decrease in all four soil types. Corvasce et al. (2006) and Hassouna et al. (2010) made similar observations to those of Cannavo et al. (2004) showing a decrease in aromaticity correlating to the decrease in HIXem. This decrease has been hypothesized to indicate that larger, more humified molecules are retained on mineral surfaces and the smaller, more mobile molecules can be transported to deeper soil layers. In a smdy on the affect of acidification on soil DOM, Ohno et al. (2007) also observed a decrease in HIXem with depth. They found that the decrease in HIXem with depth was much more pronounced in soil samples in deciduous than coniferous forests. In addition, in both forest types samples from an acidified watershed had higher HIXem values than samples from the reference watershed. 

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