Rapid and Accurate On-Site Mineral Analysis

The demand for mineral resources is increasing globally and quick and precise elemental analysis is needed at a satellite laboratory in a mining site for quality check and geological resource survey. One of the solution for such requirements is the compact-size light-weight benchtop wavelength dispersive X-ray fluorescence spectrometer (WDX), Supermini as seen in Figure 1.

Figure 1. Benchtop X-ray fluorescence spectrometer, Supermini.

Even though a benchtop energy dispersive X-ray fluorescence spectrometer (EDX) is normally used for an elemental analysis, WDX, with its high spectral resolution and light-element sensitivity, is more suited for analyzing minerals and rocks as they contain a considerable number of heavy and light element oxide components. Supermini is equipped with a newly developed high power air cooled 200-W X-ray tube, which has about 4 to 6 times higher sensitivity than that of a former bench-top spectrometer. This allows an XRF analysis of a sample with high accuracy. This paper reports the results of an XRF analysis of rocks using a Supermini spectrometer.

Samples

Calibration curves for the XRF analysis were obtained with geological reference materials (JA-1, JA-2, JA-3, JB-1a, JB-2, JB-3, JG-1a, JG-2, JG-3, JGb-1, JGb-2, JR-1, JR-2, JR-3) from National Institute of Advanced Industrial Science and Technology (AIST) Geological Survey of Japan Geoinformation Center were used to obtain calibration curves for the XRF analysis. Three of the samples (namely, JA-1, JG-1a, JGb-1) were used for repeatability and accuracy tests.

Sample Preparation

The fusion bead method was used to prepare the samples. This technique provides precise results by the removal of inhomogeneity caused by particle size and mineralogical effects presented in a sample.

The method followed is detailed below:

• 3.5 g of flux, lithium tetra borate and 0.7 g of sample in the dilution ratio 1:5 were thoroughly mixed.
• Heating of the mixed sample was done at 1200°C in a platinum crucible to fuse the sample using a desktop fusion machine as shown in Figure 2. The sample was finally cooled to form a fusion bead.
• The dilution of the samples with flux during the fusion process brings down the influence of the matrix absorption and enhancement effect of X-rays caused by the coexistent elements.
• Since the dilution ratio of 1 : 5 is lesser than the commonly used ratio of 1:10, its dilution effect is relatively small. This results in a large matrix effect. However, high-sensitivity analysis of trace elements is possible using a Supermini spectrometer because of its superior spectral resolution.

Figure 2. Desktop type bead sampler

Experimental Conditions

Totally ten elements namely Si, Ti, Al, Fe, Ca, Mg, Mn, K, Na and P were measured in the samples. The experimental conditions used for quantitative analysis of the 10 elements using a Supermini spectrometer are listed in Table 1.

Data Analysis and Results

In order to calculate the theoretical matrix correction coefficients caused by the absorption and enhancement of X-rays due to the coexistence of oxide components in a sample, a computer program was used in Supermini.

This program makes use of the fundamental parameter (FP) method to calculate theoretical X-ray intensities by automatically changing the concentrations of all elements presented in a sample, and then the theoretically calculated intensities are used to obtain the matrix correction coefficients. Through the FP method, one can obtain the correction coefficients using typical composition of major elements without the need to use reference standard samples, and hence no cumbersome measurements of standard samples are required. Rigaku’s X-ray fluorescence spectrometers use the FP program for over 20 years and are extensively used for quantitative elemental analysis. The theoretical matrix correction coefficients for all components presented in the samples are listed in Table 2.

Table 1. Experimental conditions

Table 2. Theoretical matrix correction coefficients

Calibration Curves

The corrected calibration curves with uncorrected data plot for SiO₂, TiO₂, Al₂O₃ and T-Fe₂O₃ (total iron oxide) plotted in Figs. 3 to 6 show that the corrected concentrations are considerably different from those of the uncorrected concentrations, showing that theoretical matrix corrections are important to obtain XRF results with high accuracies.

The accuracy of a calibration curve can be estimated using the following the root-mean-square equation:

Where Ci is the standard concentration of the sample i, Wi the measured X-ray concentration, n the number of samples, m = 2 for a linear equation or m = 3 for a quadratic equation. The smaller the value of σ_d, the higher the accuracy. The concentration ranges and values of the accuracy for the corrected and uncorrected calibration curves for all ten oxide components determined by regression calculations are listed in Table 3. An improvement in accuracy for the corrected calibration curve over the uncorrected curve was obtained for each of the oxide components, and drastic improvements in accuracy by about 2 to 3 times were obtained for the corrected calibration cures of 4 oxide components, namely SiO₂, TiO₂, Al₂O₃ and T-Fe₂O₃.

Table 3. Results of the regression calculation

Repeat Measurement Results

The measurements on three standard samples (namely, JA-1, JG-1a and JGb-1) were repeated 10 times to determine the accuracy and precision of the analysis.

Table 4. shows the repeat measurement results for sample JA-1. The average analysis results are very close to the standard values, and the results of the repeat measurements are also very good.

Table 4. Repeat measurement results for sample JA-1. (Unit: mass%)

Figure 3 and 4 show the corrected calibration curve and uncorrected calibration curve for TiO₂ respectively

Figure 3. Corrected calibration curve for TiO₂

Figure 4. Uncorrected calibration curve for TiO₂

Conclusion

A Supermini XRF spectrometer was used for analyzing major and minor oxide components in rock samples prepared by the fusion bead method. The XRF results on the analysis of the major oxide components in rock samples were highly precise and accurate. The results on analyzing relatively low-concentration oxide components such as P₂O₅ and MnO were also successful.

Supermini, a compact-size light-weight spectrometer, can be installed easily in a small laboratory with a limited space. The results reported in this note again confirm that Supermini gives high intensities and has high resolution. A Supermin XRF spectrometer is highly suitable to be used for the XRF analysis of rock samples at a satellite laboratory of a geological resource survey or a mining company.

This information has been sourced, reviewed and adapted from materials provided by Rigaku Corporation.

For more information on this source, please visit Rigaku Corporation.