We use a Thermo Noran QuanX EDXRF spectrometer with a large sample chamber. The size of the sample chamber allows us to non-destructively analyze large artifacts. The specific conditions that we employ have been slightly modified since we first began operating the spectrometer, and when we make modifications, we give each method a new name based upon when it was developed. Our current method is "11-09" (meaning that it was developed and calibrated in November of 2009). The 11-09 method generates quantitative or partially quantitative data for 20 elements: Na, Mg, Al, Si, K, Ca, Ti, V, Mn, Fe, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Ba, and Pb. The analytical conditions of the 11-09 method are posted below:
Each condition listed above generates a range of x-ray energy by sending the tube (target) through filters, or through no filter at all. Each condition is sensitive to different elements, based roughly on atomic weight. For example, the Low Za condition (6kV with no filter at all) is best for exciting the lightest elements. To run one archaeological sample through the four conditions takes about a half an hour. Elements in the sample produce different characteristic x-rays when they are excited by the initial x-ray beam. A detector in the spectrometer sorts the x-rays generated by the sample into different energy ranges. The information collected by the detector is used to generate a spectrum that shows how many x-ray photons were detected at different energy levels. The spectrum posted below shows the different peak intensities for Rb, Sr, Y, Zr, and Nb for a sample of Mauna Kea adze quarry basalt:
To determine the quantity of each element in a sample (usually measured in parts per million for 'trace' elements, and percentages for 'major' elements), the peak intensities shown above are compared with peaks generated from samples of known composition. This is called 'calibration.' To calibrate the 11-09 method, we have used 27 geological standards. These standards are rock and sediment samples that have been extensively analyzed in different labs. Geochemists publish articles on each standard that describe the concentrations of various elements in the standard. Then the standards are distributed as homogenized rock powders to other labs. To calibrate our method, we make pressed pellets from the powdered standards using a 25-ton press, and then we analyze the pellets. In the graphs below, you can see how the QuanX's calculated value for each standard compares with the published values. Note that we have a very high degree of agreement between calculated values and published values for the 'Mid-z' trace elements Rb, Sr, Y, Zr, and Nb, and poorer agreement for the lighter elements. In a perfect world, all the red dots would be on a straight line, but they are not because of a number of factors ('matrix effects' is one of the biggest problems). The graphs thus demonstrate a little about the level of analytical error that can be expected with different elements.
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