Trace Element Determination in Geological Samples Using XRF

By AZoMining Editors

Table of Contents

Introduction
Wavelength Dispersive X-Ray Fluorescence (WDXRF) Spectrometer S8 TIGER
Standard Sample Preparation
Measurement and Calibrations
Results
Conclusions
About Bruker AXS

Introduction

There is a growing requirement for elemental analysis in geology with global issues such as the sustainable development of natural resources, protection of people and environment, information about erosion processes and the geochemical characterization of regions. Today, powerful and economic chemical profiling of numerous samples in a short time has become very important. Actual environmental and geological studies in research require the most precise and reliable information. The industrial exploitation and exploration of minerals requires the least detection limits and highest sample throughput to achieve cost- savings from effective process control.

The determination of traces in geological samples has been previously performed with high performance wavelength dispersive X-ray fluorescence (WDXRF) spectrometry. In order to meet present day requirements for quality data, it is essential that modern instruments provide high elemental sensitivities to achieve low detection limits and high spectral resolution in order to minimize line overlays. To cover the complete range of trace elements in a large variety of different geological materials it is required that any analytical solution is based on certified reference materials and an effective handling of all matrix effects. This report displays the performance of the wavelength dispersive X-ray fluorescence (WDXRF) spectrometer S8 TIGER for determining traces in geological samples.

Wavelength Dispersive X-Ray Fluorescence (WDXRF) Spectrometer S8 TIGER

The design of S8 TIGER is such that needs of all geological applications are served. Key features of the instrument are described below:

  • The optimum level with respect to resolution, detection limits, and reliability are provided for each element.
  • The instrument has a high intensity X-ray tube, and is combined with a compact beam path. It exhibits excellent excitation and high intensity over the whole elemental range.
  • The selection of the analyzer crystal LiF 220 and the 0.23° collimator produces a choice combination for the analysis of traces from scandium to uranium.
  • SampleCareTM of the S8 TIGER offers distinct benefits to users. Since dust particles are a problem for instruments analyzing prepared samples as pressed pellets, sensitive spectrometer components are protected against being affected by sample particles with the 4x protection using a contamination shield.
  • The dust resistant design and sealed spectrometer cabinet dedicates the S8 TIGER for use in heavy industries such as ore processing and mining.
  • The instrument can be conveniently operated with TouchControlTM enabling unskilled operators to learn and use the S8 TIGER within short training times.

Standard Sample Preparation

XRF is especially used for geochemical characterization because of the simple, quick and cost effective sample preparation.

About 10 to 12 g of the dried sample is ground finely and mixed with grinding aid tablets in a ratio 5:1. The material is then pressed with a pressure of 20 tons within 20s to form a stable pellet. This sample preparation method is extremely simple to incorporate into any laboratory environment and is robust enough to avoid contamination. The method is based on cost-saving equipment, safe non-hazardous chemicals and materials. The method can be automated to achieve highest sample throughput.

Measurement and Calibrations

The measurement and calibration factors and conditions are listed below:

  • The analysis of standards and samples were done with optimum excitation with the use of the Rhodium tube with a current of 67 mA and a voltage of 60 kV.
  • The analyzer crystal LiF220 in combination with the 0.23° collimator makes sure that the best separation is attained for adjacent elements.
  • The calibration conducted was based on numerous international certified reference materials symbolizing a huge variety of the most common geological materials.
  • The standards were selected carefully to cover the typical concentration ranges for trace elements including 22 elements overall in the measurement method for quantitative analysis along with additional positions for background modelling.
  • A number of major matrix elements were also measured to correct their influences.
  • For nearly all the elements the Compton method was used based on the Rh Ka1 Compton line as a global matrix correction. The total measurement time with the S8 TIGER was less than 24 minutes.

Results

The calibration curves have been calculated for every element targeting accurate and precise trace analysis. The calibration ranges, measurement times and achieved detection limits are shown in Table 1.

Table 1. Calibration range and measurement times of the geological trace calibration

Element Compound LLD (100s, 3ó) Upper Calibration Range Analysis Time
  [ppm] [ppm] [s]
Sc 0.9 100 30
TiO2 0.001 % 2.6 % 10
V 1.2 500 50
Cr 1.0 300 50
MnO 0.001 % 1 % 6
Fe2O3 0.001 % 20 % 4
Ni 0.7 2500 30
Cu 0.8 1000 30
Zn 0.5 3000 30
As 2.1 350 80
Rb 0.3 3500 20
Sr 0.3 1500 20
Y 0.4 150 20
Zr 0.3 1000 20
Nb 0.3 1000 20
Mo 0.2 150 30
Ba 3.8 2500 50
La 3.4 350 50
Ce 3.8 2500 50
Pb 0.9 2500 80
Th 0.8 1000 80
U 0.8 600 80

The detection limits were calculated according to the Equation 1

where m is sensitivity of the analyte in kcps/mass%,
         Ib is the background intensity for analyte in kcps and
          Tb is the counting time in seconds at the background angle

The analytical accuracy of the trace element application was tested with two international certified reference materials (CRM) Mintek NIM-G and USGS W2A which were independent of the master calibration. The results are shown in Table 2. The data shows the impressive analytical performance of the S8 TIGER with regards to accuracy and reliability. More than 22 elements are precisely analyzed in less than 24 minutes including the process of sample preparation. The achieved accuracy shows the robustness of the trace element application.

Table 2. Accuracy of the International Reference Sample Granite MINTEK NIM-G

Element / Compound Certified Conc. XRF Conc. Mean Value 20 repetitions Abs. Std. Dev. 20 repetitions
  [ppm] [ppm] [ppm]
Sc 36 25 1
TiO2 1.06 % 1.05 % 0.001 %
V 260 235 1.5
Cr 92 86 1
MnO 0.16 % 0.16 % 0.005 %
Fe2O3 10.83 % 11.39 % 0.05 %
Ni 70 70 1.3
Cu 110 106 1.1
Zn 80 80 0.9
As T < 3 -
Rb 21 22 0.6
Sr 190 198 1.2
Y 23 22 0.6
Zr 100 96 0.8
Nb 8 6 0.6
Mo T < 1 -
Ba 170 156 2.8
La 10 11 2.4
Ce 23 18 2
Pb 9 6 0.9
Th 2 1.5 0.6
U T < 1 -

Conclusions

The wavelength dispersive X-ray fluorescence (WDXRF) spectrometer S8 TIGER ensures high performance trace analysis in geological material. The data is obtained with a high level of precision and accuracy showing that the instrumental setup ensures quick and dependable trace analysis. The convenient and cost effective sample preparation in combination with the high analytical speed of the S8 TIGER provides shortest time-to-result. The integration of the S8 TIGER into process and quality control for minerals and mining companies knows no boundaries. SampleCareTM of the S8 TIGER and the dust resistant cabinet ensures the excellent instrument up-time and the TouchControlTM provides easy operation and high safety of basic data. Along with analytical performance, ease and consistency are the most important demands for successful routine operation.

About Bruker AXS

Bruker AXS, an operating company of Bruker Corporation (NASDAQ:BRKR) is a global market and technology leader in materials research and quality control instrumentation for elemental and crystalline structure investigations. We develop and manufacture high-quality analytical X-ray systems and complete solutions for material analysis. For the mining & exploration activities, Bruker offer handheld XRF analyzers and XRD based equipment.

This information has been sourced, reviewed and adapted from materials provided by Bruker AXS.

For more information on this source, please visit Bruker AXS

Date Added: Jun 16, 2011 | Updated: Aug 15, 2013
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