Analytical Methods:Fractionation

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Questions
Once added to the soil artefacts such as bone, ash, pottery etc. may be degraded and the elements that are released may either be leached down through the profile, be taken up and cycled in the soil by organisms and plants, or become bound to other soil components (either relatively loosely bound to clays and organic matter (exchangeable fraction) or more securely bound with iron and manganese compounds or other mineral fractions. Fractionation techniques are used to identify how an element is bound in the soil.

This information is usually used to augment standard element analyses. Fractionation can help to differentiate between anthropogenic element loadings and the geological background. It can also be used to identify the effects of post-depositional soil processes such as podzolisation and to assess the likely effect of post-depositional processes on soil element concentrations.

Examples case studies in which element fractionation has been used include:
 * Extraction methodologies for multi-element analysis of archaeological soils
 * Early Christian burial site at Plas Gogerddan, Ceredigion, Wales – use of phosphate in the investigation of possible graves.

Sampling
Bulk samples are sufficient for fractionation, although if the total loadings need to be calculated bulk density measurements would also be needed. Usually fractionation is carried out alongside either total phosphate analysis or other elemental analysis so the same samples would be used for both analyses.

Sample storage can significantly affect the partitioning of elements between the different soil fractions. The most usual method of storage is to air or oven dry samples, and it is important to minimize the period of time samples are kept wet in sealed bags. Samples will usually need to be sieved and / or ground before analysis.

Analysis
Fractionation analyses typically consist of a sequence of different extractions on the same sample, starting with the weakest extraction and working through to the strongest (often a total or pseudo-total). The extractions will then be analysed in exactly the same way as for the standard element analysis, though in some cases dilution may be necessary to minimize the effect of the different extraction solutions (matrices).

There is no agreed set of fractionation methods. The BCR (ref) and Tessier (ref) extraction techniques are well established for metal and multi-element fractionation but over time researchers have adapted each of these schemes to there own requirements creating countless different methodologies.

More information on BCR and iron fractionation analyses can be found by clicking on the following links:
 * Fe (iron) fractionation
 * BCR fractionation

Physical fractionation methods can also be carried out to assess the distribution of elements according to particle or aggregate size and density.

Data interpretation
Fractions are operationally defined rather than strictly defined mineralogical or pedological functional fractions, consistency in methodology is, therefore, paramount and it is very difficult to compare results of different methods. The results should be interpreted alongside general soil properties such as pH, clay content and soil organic matter data.

Related techniques

 * Laboratory phosphate analysis
 * Single element analysis
 * Multi-element analysis

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