Analytical Methods:Element Analysis


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[edit] Elemental Analysis

Summary table
QuestionsIndication of anthropogenic activity, identification of certain soil forming processes, provenance of artefacts and soil materials
ScaleLaboratory analysis
Samples and storageBulk samples, for storage samples should be air or oven dried and may be sieved.
Time and costAnalysis usually takes only a few days, if a number of elements are of interest it may be more cost efficient to use multi-element techniques
General commentsColorimeter, atomic absorption (AA) spectrophotometer or optical emission spectrophotometer (OES) are required for analysis.

[edit] Questions

The measurement of concentrations of different elements in the soil

Case studies illustrating the use of element analysis in archaeology include:

[edit] Samples and sample preparation

Bulk samples are fine for elemental analyses as the soils will be sieved and / or ground as part of the sample preparation process. Ten grams of soil will usually be sufficient (more if the soils are very stony as it is the fine material that is analysed). However, most analytical techniques are destructive so this is required in addition to that needed for any other analyses.

The sampling scheme used will depend on the questions being asked and the size of the area being investigated. Samples may be taken using a grid pattern where it is the spatial pattern of element concentrations that is of interest, for example in site prospection and the identification of areas of former anthropogenic activity. Sufficient samples should be taken at a small enough sampling interval to identify real patterns of enhancement and to separate these from random variation. In other cases samples should be taken from vertical sections, for example to identify the effects of soil processes or to identify periods of anthropogenic activity in a sequence of deposits, replicate samples may be needed from each unit sampled to ensure that high concentrations are related to an overall increase in element concnetrations in that deposit rather than a localised random "hot spot".

As the analyses are usually based on sample mass the soils should be oven-dried (105oC) to remove moisture. If concentrations are to be related back to a particular volume of soil you will also need to measure bulk density. For soils and sediments it is usually the fine material that is of interest so sample will usually need to be sieved (either to < 2mm or <63 microns). Do not use brass sieves if copper or zinc are to be measured and do not use steel sieves if iron is to be measured.

[edit] Analysis

Analysis of element concentrations usually means extracting the elements from the soil or sediment by some form of wet digestion method. There are a huge numbers of different methods available for element extraction and the range of possibilities can be very confusing even for specialists. Each method differes in its "aggressiveness" and hence will extract a slightly different amount of each element. The best method to use will depend on the question you are asking, the nature of the soil you are looking at (e.g. organic matter content, clay content etc.) and the particular elements you are interested. It is recommended that you consult with a specialist first if you plan to undertake the extractions yourself.

Common analytical techniques:

  • Colorimetry
  • Atomic Absorption
  • Optical Emission

Certain analytical techniques such as X-ray Flourescence (XRF) do not require wet digestion.

[edit] Data and interpretation

It is important with this sort of data that you understand the units, usually mg kg-1 or mg l-1 and that you know whether this refers to the concentration in the extract or the concentration in the soil sample. The units mg / kg-1 correspond to parts per million (ppm). However, depending on the concentrations present other units may be used souch as g kg-1 corresponding to parts per thousand, or µg kg-1 correspoonding to parts per billion.

[edit] References

[edit] Related techniques

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