Energy Dispersive X-Ray

Fluorescence Analysis (ED-XRF)

The ED-XRF analyses were carried out by Dr. A. Burkhardt at the IFZAA Laboratory (Institute for Non-Destructive Analysis + Archaeometry), in Basel, Switzerland - analytik@balcab.ch - using a SPECTRACE QuanX Spectrometer.

Methods

For the excitation an X-ray tube with a rhodium target and a 125 micron Beryllium-window was used. The X-ray generator was operated in a series of KV increments between 4 KV to 50 KV with current adjustable in 0.02 mA increments and a maximum power of 50 W, using cellulose, aluminium, palladium and copper filters. The selection of different filters (cellulose, Al, Pd-thin, Pd-med, Pd-thick, Cu-thin, Cu-thick) in combination with the acceleration voltage is required to optimise the background.A thermoelectric cooled Si(Li)- detector with a comfortable Peltier system with a detector window of 12 microns thickness was used. For the light elements (Na to Ti), the system was operated in vacuum which was required to improve the sensitivity. A complex measurement procedure was optimised for the measurement conditions for all elements of the periodic table from atomic number 11 (Na) to 92 (U). A collimator with a diameter of 2 mm was used to condense and focus the X-Ray beam. The average weigh of the samples was 0.5 to approx. 2 ct. For each corundum sample four different energy spectra were collected with a total of 600 seconds lifetime. To avoid strong diffraction peaks, the sample were rotated. The penetration depth of the X-rays in the sample for a particular analysed element and the intensity of the excited signals (M-L-K lines) are correlated with the acceleration voltage and the atomic number. The penetration depth is increasing from microns for the K-lines of the light elements (Na, Mg, Al) to millimetres (Ga-K, Sr-K, Zr-K) as well as for the L-lines of the heavy elements such as Pt-L, Au-L, Pb-L, Bi-L. In a corundum matrix the penetration depths is for example: 3 microns for Na, 10 microns for Cl, 75 microns for Cr, 120 microns for Fe; from Ga to Sr the penetration depth is increasing from 360 microns to 1.3 mm and for the L-lines of Pt to Bi it is increasing from 390 microns to 1.5 mm. The penetration depth of the strong K-lines of silver (Ag) is 4.7 mm in an Al2O3 matrix. After the primary energy spectrum was saved on a Pentium-PC the raw intensities for four energy spectra for each sample were calculated and combined in an intensity file.

The “Fundamental Parameter Programme” was used to quantify the raw data and the results were normalized to 100% (weight-percent). Standards were used to setup a standard-table. The quality of the quantitative ED-XRF data (accuracy, precision and detection limits) were found to be critically related to the correct selection of the analytical parameters (collimator, voltage, current, filter) as well as on the quality of the standards. The standards are used to obtain calibration curves. The ED-XRF method cannot be used to differentiate between thin layers or in-homogeneities in the samples, such as natural inclusions or small scale chemical layering. Therefore, the data are averaged chemical analyses, with the additional inconvenience of the different penetration depth of the M-, L-, and K-lines. In the first survey of our project a database of 1200 ED-XRF spectra have been collected and only 20 elements including Na, Mg, Al, Si, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Ga, As, Ge, Rb, Sr and Au were quantified. Using the advantage of the SPECTRACE QuanX system all spectra can be recalculated and quantified for a selection of other elements (atomic number Z = 11 to 92). The ED-XRF quantitative data are compared with the data of LA-ICP-MS for selected samples. (Tab. 3b). LA-ICP-MS have already compared to another ED-XRF instrument on a larger scale and found to be compatible (Lit. 8) with the exception of Gallium (Ga), therefore confirming that trace element determinations have been checked for accurate consistency with other methods as to the best of our possibilities at this stage.The analytical error on the concentrations for TiO2 is given here as maximal approx. +/-0.01 wt-%, for Cr2O3 approx. +/-0.01 wt-%, for Fe2O3 approx.+/- 0.01 wt-% and for Ga2O3 approx. +/- 0.002 wt-% and for V2O3 approx. +/- 0.01 wt-%. These errors are small enough to allow us to interpret the data (See Fig. 23). Errors on the lighter elements (such as Mg) are considerably higher for this method. Li, B, and Beryllium are not measurable by ED- XRF analyses due to limited detection capabilities.

ED-XRF Results

The trace elements used for this report’s results are given in Table. 5 and shown in Fig.20, 23, 25 and 33.
The data revealed considerable variation mainly in the trace elements Iron (Fe), Chromium (Cr), Vanadium (V), Titanium (Ti), and Gallium (Ga). The elements Na, Mg, Ca, K, Mn, Ni, Cu, Zn, As, Rb, Zr, Tl, Bi, Ge and Sr concentrations are occasionally found. No final conclusion are given here to interpret the presence of these trace elements.

Fig. 20 Representation of ED-XRF data on the dominant trace element concentrations in natural colored sapphires unheated (N) and enhanced by heat with the new E(IM)-method, in oxide wt.-%. These intense saturated colors are found in the sapphires originating from Songea (Tanzania).