Elemental Analysis of Geological Materials Using Energy Dispersive X-Ray Fluorescence Spectroscopy (EDXRF)

Elemental analysis is an important investigative tool in geology and proves useful in extraction process control, mineral exploration and environmental studies.

Energy Dispersive X-Ray Fluorescence (EDXRF) spectrometry is a simple and rapid technique used for the analysis of minerals, ores, rocks, concentrates, exploration samples, etc. This technique requires far less sample preparation compared to other techniques.

EDXRF Analysis of Geological Materials

EDXRF analysis can be performed directly on the powdered material or solid, and thus prevents the costly, time-consuming and potentially dangerous process of dissolving the sample. This benefit, coupled with the portability of the latest instrumentation, makes EDXRF suitable for field analysis.

However, dissolving the sample can effectively remove the analyte elements from the matrix in which they occur. The so-called “matrix effects” are common in geological analysis and can have a significant effect on XRF results.

As a result, the analytical signal from the same concentration of a given element might be different in different matrices – for instance in intermediate, acidic or igneous rocks.

Geological samples often include a wide range of elements at differing concentrations, from trace elements to major components. Besides this, various inter-element interferences can occur and must be taken into consideration.

The analytical precision of an EDXRF system largely depends on its ability to deal with these effects. Moreover, the quality of results can also be affected by the physical nature of the sample.

Sources of Error in EDXRF Analysis

Some of the potential sources of error in EDXRF analysis include poor sample preparation or presentation, sample matrix effects, spectral background, and interferences between closely adjacent spectral lines.

When analyzing geological samples by EDXRF technique, all these effects can be encountered, with the usual manifestations being difficulties with calibration and poor detection limits leading to poor analytical precision.

SPECTRO Analytical Instruments’ EDXRF instruments have sophisticated design features and advanced calibration routines to reduce the effects of matrix and spectral interferences in such a way that instruments that are pre-calibrated from the factory can be relied upon to provide fast and precise results on a broad range of samples having different matrices.

Spectral Interferences

In the basic mechanism of X-ray fluorescence, energetic X-rays from an X-ray tube penetrate the sample and interact with the atoms of the sample to cause energy transitions between the electron shells.

Fluorescent X-rays are then emitted at an energy characteristic of the element from which it arises, and at an intensity relative to the number of atoms of that element in the sample, that is its concentration.

In an EDXRF spectrometer, the detector differentiates between closely adjoining energies to isolate the peaks produced by different elements. Depending on the resolving power of the spectrometer, these spectral lines may lie close together or even overlap in a multi-element sample such as a mineral sample.

In such situations, one element can hamper the measurement of another, by contributing to the analytical signal. High resolution detectors, such as the Silicon Drift Detectors (SDDs) used in the SPECTROSCOUT and SPECTRO XEPOS instruments can help reduce spectral interferences.

Spectral Background

Spectral background originates from scattered primary radiation from the source impinging on the detector. The source radiation comprises both emission lines and continuous (Bremsstrahlung) radiation that are characteristic of the tube anode material.

Therefore, the final observed spectrum includes the desired lines due to elements from the sample as well as the scattered continuum and the scattered tube lines.

The intensity of the scatter depends slightly on the sample’s composition: when samples contain mainly light elements, the proportion of scattered radiation is quite high. The preferred analytical peak will be overlaid on this background.

When background is high with regard to the analytical signal, any errors in calculating the background will be additive to those in calculating the element line.

Spectral background can be reduced by using modified X-rays for sample excitation. This is realized by interposing a secondary target or polarizing between the sample and the X-ray tube in a certain Cartesian geometry. This reduces the amount of scatter similar to the way polarizing sunglasses reduce glare. This approach is utilized in the SPECTRO XEPOS EDXRF spectrometers.

Sample Matrix Effects

Matrix effects

Figure 1. Matrix effects

Even when the spectrometer detection system and sample excitation have been optimized, the direct proportionality between emitted florescence intensity and analyte concentration may still not apply when investigating real samples.

As mentioned above, XRF analysis can be utilized directly on solid samples with little sample preparation. But this would mean that the atoms will be very close to the others in the sample matrix and can lead to interactions between the incident/fluorescent X-rays and the matrix atoms that may intervene with the measured result. These effects are normally called matrix effects (Figure 1).

SPECTRO EDXRF spectrometers use advanced calibration techniques to deal with the difference in sample types that can be encountered. The common approach is based on the Fundamental Parameters (FP) method. Here, besides the intensity of the fluorescence, the effect of the matrix is also considered by comparing the quantified standard against a theoretically computed value.

Sample Preparation

Care must be taken with sampling procedures to achieve an appropriate representative sample. The sample can be first dried and grounded to a powder, usually to less than 100 microns particle size.

Better results can be achieved if the powder is pressed into a pellet, as this helps in removing any voids in the material. Although manual pressing may be adequate, a hydraulic press is often utilized.

In certain samples, variations in hardness may result in inconsistent grinding and thus differences in particle size. When mineralogical effects and particle size are severe, the sample may be prepared as a fused bead, which eliminates these effects.

Analysis of Geological Materials with SPECTRO XEPOS/XEPOS HE

SPECTRO XEPOS/ XEPOS HE

Figure 2. SPECTRO XEPOS/ XEPOS HE

The SPECTRO XEPOS and XEPOS HE (Figure 2) offer excellent precision and sensitivity for laboratory analysis by EDXRF. Both instruments use advanced EDXRF technology. The key components of the measuring system are shown in Figure 3:

  1. X-ray tube
  2. Target changer with up to 8 polarization and secondary targets
  3. SDD detection system
  4. Sample tray with samples

Schematic of SPECTRO XEPOS and XEPOS HE.

Figure 3. Schematic of SPECTRO XEPOS and XEPOS HE.

The SPECTRO XEPOS is suitable for the analysis of geological samples, in particular the major components that are the “light” elements in the range sodium-iron.

In the example below, standard reference materials were examined as fused beads. The beads were prepared using 4.6 g Lithium tetra-borate flux and 0.6g sample material.

In order to optimize the excitation conditions, secondary targets were selected as shown in Table 1.

Table 1. Measuring conditions

Elements kV/mA Target Meas. Time [s]
Na-S 17.5 kV, 2.0 mA HOPG 300
K-Mn, Ba-Nd 35 kV,1.0 mA Co 300
Fe-Y, W-Th 40 kV, 0.88 mA Mo 300

Calibration for trace, minor and major was carried out by determining a range of international geological and mineral reference materials. Tables 2 and 3 show the analytical results on two samples that are also certified reference materials.

Table 2. Analytical results on geological reference material SCo-1 (marine shale)

Oxide Certified [Wt %] Analyzed [Wt %]
Na2O 0.9±0.06 0.94±0.06
MgO 2.72±0.18 2.73±0.04
Al2O3 13.7±0.21 13.50±0.04
SiO2 62.8±0.66 62.42±0.08
P2O5 0.210±0.02 0.205±0.003
K2O 2.77±0.08 2.69±0.02
CaO 2.62±0.20 2.56±0.01
TiO2 0.63±0.06 0.589±0.004
Fe2O3 5.13±0.18 5.17±0.01
Element Certified [µg/g] Analyzed [µg/g]
V 130±13 135±13
Cr 68±5 57±2
Mn 410±30 385±5
Ni 27±4 30±2
Cu 29±2 32±2
Zn 100±8 109±3
Ga (15) 18±2
Rb 110±4 112±1
Sr 170±16 164±2
Y 26±4 23±1
Ba 570±30 588±84
Ce 62±6 64±25
W 1.4 <3
Pb 31±3 34±2
Th 9.7±0.5 11±2

Table 3. Analytical results on geological reference material WS-E (dolerite)

Oxide Certified [Wt %] Analyzed [Wt %]
Na2O 2.47±0.03 2.47±0.08
MgO 5.55±0.04 5.61±0.05
Al2O3 13.78±0.06 13.56±0.04
SiO2 50.7±0.12 50.96±0.08
P2O5 0.30±0.01 0.305±0.003
K2O 1.00±0.01 1.00±0.01
CaO 8.95±0.05 8.94±0.02
TiO2 2.40±0.02 2.43±0.01
MnO 0.170±0.002 0.170±0.001
Fe2O3 13.15±0.07 13.05±0.02
Element Certified [µg/g] Analyzed [µg/g]
S (500) 469±5
V 340±7 305±24
Cr 99±2 102±4
Ni 55±2 63±4
Cu 65±2 58±4
Zn 117±2 112±3
Ga 23±1 21±2
Rb 25±1 25±1
Sr 410±5 394±2
Y 30±1 31±1
Ba 338±6 390±170
Ce 61±1 83±32
W <1 <4
Pb 13.8±0.6 10±1
Th 3.0±0.2 3±1

Correlation plot for rubidium (correlation coefficient: 0.9999).

Figure 4. Correlation plot for rubidium (correlation coefficient: 0.9999).

Correlation plot for thorium (correlation coefficient: 0.9996).

Figure 5. Correlation plot for thorium (correlation coefficient: 0.9996).

These results show how samples with considerably different matrices can be investigated using a pre-calibrated instrument with exceptional recoveries. The calibration procedures offers superior linearity of calibration as shown by the correlation plots achieved for rubidium and thorium elements (Figures 4 and 5).

The SPECTRO XEPOS HE is capable of very low detection limits in geological samples, as illustrated in Table 4. It provides excellent results even when the sample is a loose powder. Although this is a much faster and easier technique of sample preparation, it is usually not preferred for analytical precision.

Table 4. SPECTRO XEPOS HE detection limits in matrix effects, silica matrix and line overlaps may increase the detection limits significantly.

Element/Oxide LOD [µg/g] Element/Oxide LOD [µg/g] Element LOD [µg/g] Element LOD [µg/g]
Na2O 100 Co 2.4 Mo 0.3 Ce 2.5
MgO 30 Ni 1.3 Ru 0.3 Pr 3
Al2O3 40 Cu 0.6 Rh 0.2 Nd 4
SiO2 Zn 0.3 Pd 0.2 Yb 3.1
P2O5 10 Ga 0.2 Ag 0.8 Hf 1.8
S 5 Ge 0.1 Cd 0.3 Ta 1.1
Cl 5 As 0.1 In 0.5 W 0.7
K2O 10 Se 0.1 Sn 1.1 Au 0.5
CaO 10 Br 0.09 Sb 0.7 Hg 0.3
Ti 0.8 Rb 0.09 Te 0.9 Tl 0.3
V 0.6 Sr 0.1 I 1.1 Pb 0.3
Cr 0.5 Y 0.2 Cs 1.4 Bi 0.3
Mn 0.7 Zr 0.4 Ba 1.7 Th 0.2
Fe2O3 3 Nb 0.4 La 2.1 U 0.3

SPECTROSCOUT

When rapid measurements and results are required in the process plant or on site, the SPECTROSCOUT provides a suitable solution. Incorporating many of the sophisticated facilities of the SPECTRO XEPOS instruments, the SPECTROSCOUT is fully portable and can be utilized in remote locations.

The excellent analytical performance of the SPECTROSCOUT can be demonstrated in this multi-element application. Table 5 shows the example analytical results for an international reference material given in comparison to the certified values.

The precision of the measured concentrations for the “light” elements magnesium, phosphorus, silicon, sulfur and aluminum is restricted by particle size effects caused by variations in the applied sample preparation technology of different producers and by mineralogical effects.

The SPECTROSCOUT offers a fast, precise and cost-effective solution for in-situ analysis of major, minor and trace elements in soils and rocks prepared as loose dried powder.

Conclusion

SPECTRO Analytical Instruments’ EDXRF instruments use advanced measurement technology and sophisticated analytical software to handle the various interference effects that originate in complex geological samples. Using simple sampling techniques, precise results can be obtained on different types of samples. The highly sensitive SPECTRO XEPOS and XEPOS HE and the portable SPECTROSCOUT are easy-to-use instruments that provide geochemists with fast and precise elemental analysis of ores, minerals and process samples.

This information has been sourced, reviewed and adapted from materials provided by SPECTRO Analytical Instruments GmbH.

For more information on this source, please visit SPECTRO Analytical Instruments GmbH.

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