THERMOFISHER SCIENTIFIC QUANT'X ANALYSIS AND INSTRUMENTATION

The following is the analytical trajectory used in the EDXRF analysis of archaeological and source standard obsidian in the EDXRF lab at Berkeley. 

For instrumental information on the Philips PW 2400 go here... or a print version in .pdf format (Shackley 2005).

The ThermoFisher Scientific Quant'X EDXRF spectrometer methods:                    

All archaeological samples are analyzed whole. The results presented here are quantitative in that they are derived from "filtered" intensity values ratioed to the appropriate x-ray continuum regions through a least squares fitting formula rather than plotting the proportions of the net intensities in a ternary system (McCarthy and Schamber 1981; Schamber 1977). Or more essentially, these data through the analysis of international rock standards, allow for inter-instrument comparison with a predictable degree of certainty (Hampel 1984).            

            The spectrometer is equipped with a peltier air cooled solid-state Si(Li) X-ray detector, with an ultra-high-flux end window bremsstrahlung rhodium (Rh) x-ray target with a 125 micron beryllium (Be) window, an x-ray generator that operates from 4-50 kV/0.02-1.0 mA at 0.02 increments, using an IBM PC based microprocessor and WinTrace^TM 4.1 software.  The spectrometer is equipped with a 2001 min^-1 Edwards vacuum pump for the analysis of elements below titanium (Ti).  Data is acquired with a pulse processor and analog to digital converter.  This is a significant improvement in analytical speed and efficiency beyond the former Spectrace 5000 and QuanX analog systems (see Davis et al. 1998; Shackley 2005).

            For Ti-Nb, Pb, Th elements the mid-Zb condition is used operating the x-ray tube at 30 kV, using a 0.05 mm (medium) Pd primary beam filter in an air path at 200 seconds livetime to generate x-ray intensity Ka₁-line data for elements titanium (Ti), manganese (Mn), iron (as Fe^T), cobalt (Co), nickel (Ni), copper, (Cu), zinc, (Zn), gallium (Ga), rubidium (Rb), strontium (Sr), yttrium (Y), zirconium (Zr), niobium (Nb), lead (Pb), and thorium (Th).  Not all these elements are reported since their values in many volcanic rocks is very low. Trace element intensities were converted to concentration estimates by employing a least-squares calibration line ratioed to the Compton scatter established for each element from the analysis of international rock standards certified by the National Institute of Standards and Technology (NIST), the US. Geological Survey (USGS), Canadian Centre for Mineral and Energy Technology, and the Centre de Recherches Pétrographiques et Géochimiques in France (Govindaraju 1994). Line fitting is linear (XML) for all elements but Fe where a derivative fitting is used to improve the fit for iron and thus for all the other elements.  When barium (Ba) data are acquired, the Rh tube is operated at 50 kV and .25-1.0 mA in an air path at 200 seconds livetime to generate x-ray intensity Ka₁-line data, through a 0.630 mm Cu (thick) filter ratioed to a portion of the bremsstrahlung region (see Davis et al. 1998).  Further details concerning the petrological choice of these elements in Southwest obsidians is available in Shackley (2005; also Mahood and Stimac 1991; and Hughes and Smith 1993). A suite of 17 specific standards used for the best fit regression calibration for elements Ti- Nb, Pb, ,Th, and Ba include G-2 (basalt), AGV-2 (andesite), GSP-2 (granodiorite), SY-2 (syenite), BHVO-2 (hawaiite), STM-1 (syenite), QLO-1 (quartz latite), RGM-1 (obsidian), W-2 (diabase), BIR-1 (basalt), SDC-1 (mica schist), BCR-2 (basalt), TLM-1 (tonalite), SCO-1 (shale), all US Geological Survey standards, BR-1 (basalt) from the Centre de Recherches Pétrographiques et Géochimiques in France, and JR-1 and JR-2 (obsidian) from the Geological Survey of Japan, and NBS(NIST) 278 (obsidian) from the National Institute of Standards and Technology (Govindaraju 1994).

            The data from the WinTrace software are translated directly into Excel for Windows software for manipulation and on into SPSS for Windows for statistical analyses when necessary. In order to evaluate these quantitative determinations, machine data were compared to measurements of known standards during each run (Table 1).    RGM-1 is analyzed during each sample run for obsidian artifacts to check machine calibration.  Other appropriate standards from the above list are used for other volcanic rocks.

Table 1. X-ray fluorescence concentrations for selected trace elements of RGM-1 pressed powder pellet (n=35 runs), and whole rock flake from original USGS boulder (n=12). ± values represent first standard deviation computations for the group of measurements. All values are in parts per million (ppm) as reported in Govindaraju (1994). USGS,  and this study. RGM-1 is a U.S. Geological Survey obsidian standard obtained from Glass Mountain, Medicine Lake Highlands Volcanic Field, northern California. 

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Revised:02 March 2010 07:34 -0800