Analytical Methods:Multi Element

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1 Multi-element analysis

[edit] Multi-element analysis

Summary table
Scale	A specialist laboratory technique
Questions	Is principally used in site prospection studies, and to identify patterns of space use and different functional areas from floor samples. It may also be used to study past pollution events from point or non-point sources.
Samples and storage	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 off-site reference samples. It is best to consult a specialist for advice on sampling.
Time and Cost	Individual analyses can be relatively cheap ( in the order of a few pounds), however because large numbers of replicate samples may be necessary total costs can be quite high. Sampling is key to maximizing the cost / return ratio.
General comments	Involvement of a specialist micromorphologist early in the project is highly recommended to maximise sampling efficiency and hence costs.

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.

[edit] 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:

[edit] 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.

[edit] 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).

[edit] 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.

[edit] 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.

[edit] References

Entwistle, J.A., Abrahams, P.W., and Dodgshon, R.A. (2000) The geoarchaeological significance and spatial variability of a range of physical and chemical soil properties from a former habitation site, Isle of Skye. Journal of Archaeological Science, 27, 287-303.
Middleton, W.D. (2004) Identifying chemical activity residues on prehistoric floors: a methodology and rationale for multi-elemental characterisation of a mild acid extract of anthropogenic sediments. Archaeometry, 46, 47-65.
Well, E.C. (2004) Investigating activity patterns in prehispanic plazas: weak acid-extraction ICP-AES analysis of anthrosols at Classic period El Coyote, northwestern Honduras. Archaeometry, 46, 67-84.
Wilson, C.A., Cresser, M.S., and Davidson, D.A. (2006) Sequential element extraction of soils from abandoned farms: an investigation of the partitioning of anthropogenic element inputs from historic land use. Journal of Environmental Monitoring, 8, 439-444.
Wilson, C.A., Davidson, D.A., and Cresser, M.S. (2005) An evaluation of multi-element analysis of soil contamination to differentiate space use and former function in and around abandoned farms. The Holocene, 15, 1094-1099.