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Results for contamination:

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contamination

CATEGORY: term
DEFINITION: Materials that are not part of a natural archaeological deposit or assemblage but which have intruded or altered the deposit or assemblage. The term is often applied to samples taken for radiocarbon dating which have been affected by their environment, for example by humus, which also contains carbon, and may be much younger than the sample, thus resulting in an inaccurate age determination.

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amino acid dating

SYNONYMS OR RELATED TERMS: amino-acid dating; aminostratigraphy; amino-acid racemization, amino acid racemization
CATEGORY: technique
DEFINITION: A method of absolute (chronometric) dating which is hoped to fill the gap between radiocarbon dates and potassium-argon dates. It is used for human and animal bone and other organic material. Specific changes in its amino acid structure (racemization or epimerization) which occur at a slow, relatively uniform rate, are measured after the organism's death. The basis for the technique is the fact that almost all amino acids change from optically active to optically passive compounds (racemize) over a period of time. Aspartic acid is the compound most often used because it has a half-life of 15,000-20,000 years and allows dates from 5,000-100,000 years to be calculated. However, racemization is very much affected by environmental factors such as temperature change. If there has been significant change in the temperature during the time in which the object is buried, the result is flawed. Other problems of contamination have occurred, so the technique is not fully established. It is fairly reliable for deep-sea sediments as the temperature is generally more stable.

atomic absorption spectrometry

SYNONYMS OR RELATED TERMS: AAS
CATEGORY: technique
DEFINITION: A method of analysis used to determine the chemical composition of metal artifacts -- especially copper -- and non-metallic substances such as flint. It measures energy in the form of visible light waves and is capable of measuring up to 40 different elements with an error rate of around 1 percent. It is not a completely nondestructive technique, since a small sample must be removed from the artifact (between 10 mg. and 1 g., depending on the concentration of the elements). The sample is first dissolved and then atomized in a flame. A beam of light, of carefully controlled wavelength, is shone through the flame to a detector on the other side. The light takes a defined wavelength corresponding to the emission wavelength of the chosen element. The atoms of that element in the sample therefore absorbs a proportion of the light, measured with a photomultiplier, and a comparison of the intensity of the light with that which has not gone through the sample shows the extent of the absorption, thus providing an estimate of the amount of the chosen element in the specimen. One of the method's drawbacks is that a separate measurement (and a different hollow cathode lamp) is necessary for each element, so that analysis for a large number of elements is time-consuming. There are also problems of contamination with the high dilutions necessary for elements present in high concentrations, so that the method is used for the analysis of minor elements and trace elements rather than for major elements. The results are generally more accurate than those obtained using optical emission spectrometry and the technique's use will probably increase, especially for the identification of sources of metal ores through the recognition and quantification of the trace elements.

hard water effect

CATEGORY: technique
DEFINITION: A potential source of artifact contamination in radiocarbon dating. When material that is radiocarbon-dated has been buried, groundwater may have percolated into it. Groundwater frequently contains dissolved calcium carbonate, where it has passed through limestones. Such carbonate may crystallize within the sample to be dated. As a result, carbon from a source very much older than the sample may be included. Dates from material that has been contaminated in this way will be too old. Samples such as wood and charcoal may be treated with hydrochloric acid to dissolve away the crystallized carbonate, eliminating the problem. Shell samples, which are themselves made of calcium carbonate, cannot be so treated. If the hard water effect is suspected and corrections are not made, the dates should be reported as maximums only.