Elemental concentrations artifacts samples

In essence, NAA involves converting some atoms of the elements within a sample into artificial radioactive isotopes by irradiation with neutrons. The radioactive isotopes so formed then decay to form stable isotopes at a rate which depends on their half-life. Measurement of the decay allows the identification of the nature and concentration of the original elements in the sample. If analysis is to be quantitative, a series of standard specimens which resemble the composition of the archaeological artifact as closely as possible are required. NAA differs from other spectroscopic methods considered in earlier chapters because it involves reorganization of the nucleus, and subsequent changes between energy levels within the nucleus, rather than between the electronic energy levels. 

Elemental Concentrations in the Artifact Samples (Data given as ppm... 

Element concentrations from XRF spectra. The pXRF spectrum consists of various peaks due to fluorescence, scattering, and detector artifacts such as escape and pileup peaks. The basic procedure for obtaining quantitative compositional information from XRF spectra is to fit a background spectrum, subtract the background, fit the remaining peaks to obtain net peak areas, and use these net areas to compute concentrations, where the concentration calculations include information on the analysis conditions and physical state and major element composition of the sample. 

The principal advantage of the standard-sample bracketing approach is that its application is very straightforward and only two interference-free isotopes are needed, in principle, for an analysis. The method, however, is also particularly susceptible to the generation of analytical artifacts from variations in instrumental mass bias that are induced by residual matrix elements, which are present in the sample solutions but absent in the pure reference standards (Figure 10.10). Hence this approach is typically used only for elements (i) that are present at high concentrations in samples (e.g., the major elements Mg, Si, and Fe of silicate meteorites or Fe from iron meteorites) or (ii) for which the technique of external normalization is not readily applicable (e.g., Ii, B see below). 

In the Mediterranean Sea and Middle East area, for example, there are obsidian outflows only in Italy, in some islands in the Aegean Sea, and in Turkey. Artifacts made of obsidian, however, are widely distributed over much of this vast area. Chemical analysis of many of these artifacts has shown that most of the obsidian used to make them originated in one or another of the outflows mentioned, but also in far-distant places such as Armenia and Iran. Plotting on a graph the concentration of selected elements in samples from obsidian sources against that in samples from sites where it was used, enables the identification of the source of the samples (see Fig. 22). Moreover, this type of analysis also makes it possible to trace the routes through which obsidian (and most probably other goods) were traded in antiquity (Renfrew and Dixon 1976). 

XRF analysis involves directing a beam of X-rays at a small area ( 2 x 5mm) on an artifact or sample and measuring the wavelength and intensity of the secondary X-rays that are fluoresced by the area hit by the primary X-rays. The wavelengths correspond to the elements present, and their intensity is directly related to concentration. 

The elements of the galvanic cell should not be influenced by the chemistry and/or number of samples measured. In particular, the sample should maintain its initial physical and chemical properties, even after storage. In other words such processes as the evaporation of plasma water, gas escape, denaturation of proteins, the cell lyses, etc. should be minimized to avoid analyte concentration changes and artifacts. 

Analytical Procedure. The cold-trap gas phase mercury detection system was designed and used for both laboratory and shipboard measurements of mercury in seawater. The Coleman Instruments mercury analyzer (MAS-50) was incorporated into the analytical system because of its portable and convenient design. However, the effective use of this simple one-element atomic absorption unit requires scrupulous attention to blank determinations for each seawater sample. For example, the undetected presence of either naturally occurring or sampling induced volatile organics which may absorb at the mercury wavelength in the seawater sample can be a serious error. Such artifacts were observed when acidifled seawater samples were stored in low density polyethylene bottles (21), Therefore, the analytical procedure used to determine the mercury concentration in a seawater sample consists of the following steps ... 

Seven of the artifacts from the La Scola site were analyzed by ICP-MS. Samples were ultrasonically cleaned and dried, and then pulverized at liquid nitrogen temperatures using a SPEX 6700 freezer mill. 100 mg of the powdered sample were mixed with 0.2 mL aqua regia and 2.5 mL hydrofluoric acid and dissolved in a Parr acid digestion bomb at 120 °C for 2 hours. Samples were then diluted to a total volume of250 ml so that elements of interest would be present at concentrations of 500 ppb or less. 

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