Analytical Methods:Multi Element

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Multi-element analysis
Human activities can result in materials being deliberately or accidentally incorporated in the soil. This creates characteristic patterns of element concentration. Multi-element analysis simultaneously determines the elemental composition of a soil/sediment, or artefact.

Questions
Multi-element analysis has principally been used in site prospection studies using topsoil samples. It is also used to identify patterns of space use and different functional areas from floor samples. However, it is not always possible to identify the exact cause of these concentrations.

Multi-element analysis may also be useful in studies of erosion within river catchments. It is obviously useful in studies of historic and prehistoric pollution and where specific pollution events are known, element concentrations can provide date markers in alluvial or peat sequences.

Examples of situations where multi-element analysis has been successfully used inlcude:
 * Multi-element soil analysis: an assessment of its potential as an aid to archaeological interpretation.
 * Guildhall (Roman amphitheatre) excavation, London – complementary use of multi-element analysis, soil micromorphology and palynology.

Samples and storage
This technique uses bulk samples which may be frozen, refrigerated or dried. Because of the high variability of soil chemistry it may be necessary to collect large numbers of samples as well as reference samples (off-site minimally disturbed soils or samples of local natural parent materials). A specialist can advise on sampling.

Sample preparation depends on the analytical method being used. Samples are usually dried and ground, before an extraction procedure (often digestion in acid) is performed. X-ray Flourescence (XRF) offers a non-destructive alternative to acid digestion for major elements and some trace elements. A specialist will advise on the most appropriate preparation methods.

Extraction Methodologies
As with phosphate analysis the choice of the extraction method and an understanding of the nature of the fraction that is recovered using this method is vital to the interpretation. A range of techniques can be used from weak acid extractions that essentially recover the plant available fraction, to strong acid (HF) and multi-acid extractions (aqua-regia) that also break down the more resistent soil minerals and effectively recover the total (or near total) element concentrations in that soil.

There has been much debate in recent years over the most effective extraction method. Many researchers (e.g. Well, 2004; Middleton, 2004) have advocated a weak acid method arguing that harsher methods recover the geological fraction that can mask the archaeological signals. Other researchers have used stronger acid extractions to explore the relationship between patterns of element enhancement and historic rural settlements in the UK (e.g. Entwistle et al. 2000; Wilson et al. 2005). Sequential extraction of soils on such sites has supported the use of stronger acid extraction methods by demonstrating that a significant proportion of the anthropogenic signal is in fact retained in the more resistant mineral bound phases (Wilson, et al. 2006).

Analysis
The main techniques to simultaneously analyse multi-elements are:
 * ICP-MS – Trace element concentrations down to parts per billion detection limits and can provide isotopic data, but slower and more costly than other techniques.
 * ICP-AES – Large suite of major and trace elements down to parts per million concentrations, but requires sample digestion.
 * XRF - Non-destructive determination of a large suite of major and trace elements, but sensitive to a number of interference effects.

The method used will often be determined by the availability of the specialist equipment and by the detection limits required. A specialist can advise further.

Data and interpretation
Multi-element analysis generates a large amount of data that needs summarising using graphical techniques and multivariate statistics such as Analysis of variance (ANOVA) prinicple component analysis (PCA) and discriminant analysis. Specialist data analysis and interpretation is highly recommended. It can be difficult to differentiate between local background variation in element concentrations, patterns of element concentration linked with changes in soil type, geology and water-regime, and those patterns of element enhancement attributable to former anthropogenic activity. Local variation in geology, soil type and hydrology (water flow) conditions need to be taken into account in the interpretation of all element concentration data.

Related Techniques

 * Phosphate analysis

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