WROXI - synthetic wide-range oxide standards for fused bead major and minor element analysis

WROXI is a set of synthetic standards, together with an application setup for the Minerals edition of the Zetium XRF spectrometer, which enables calibrations for twenty-one major elements in the analysis of oxide materials based on fused beads.

WROXI can be used either as a primary fused bead calibration or to verify customer in-house standards for pressed powder applications. WROXI is not a precalibration. 

Zetium Minerals edition

Introduction

X-ray fluorescence spectrometry (XRF) is used extensively for the analysis of a very wide range of oxide materials and over a range of concentrations from 0 - 100 wt% oxide. Simple sample preparation, high accuracy and precision, and when required, good to excellent detection limits across large parts of the periodic table are the principal reasons for this choice. Accurate analysis is becoming increasingly important for many reasons: for superior production, process and grade control with associated cost savings; minimizing and controlling contamination and environmental pollution during manufacture; quality control of products; and pure research.

Accurate quantification requires accurate, spectrally clean net peak intensities, accurate corrections for inter-element matrix effects and good standards with a wide range of concentrations.

WROXI is a set of synthetic standards, together with an application setup for the Minerals edition of the Zetium spectrometer, which enables calibrations for twenty-one major elements in the analysis of oxide materials based on fused beads. WROXI can be used either as a primary fused bead calibration or to verify customer in-house standards for pressed powder applications. WROXI is not a pre-calibration.

The Minerals edition of the Zetium spectrometer with WROXI standards and the SuperQ software FP algorithm makes up a unique system that consistently produces very high-quality major and minor element analyses, e.g. Mg, Al, Si, Ti, Fe, Mn, V, Cr, Ni, Cu, Zn, Sr, Zr, Ba and Pb, in a wide variety of oxides, silicates, carbonates, sulfates, phosphates, rocks and soils and similar raw materials used in many different industries, such as cement, gypsum, iron and manganese ores, ceramics, bricks, glass, and some heavy mineral ores.

Minerals edition of the Zetium spectrometer

The Minerals edition of the Zetium spectrometer is a fully integrated wavelength-dispersive XRF spectrometer, complete with X-Y sample handler and state- of-the-art software. Engineered for excellence in terms of both analytical and operational performance, it has been configured specifically to meet the needs of users in the mining and minerals industries.

Easily integrated into automated laboratory systems and with a small footprint, the Minerals edition of the Zetium spectrometer provides consistent high-quality data across the full elemental range, from fluorine to uranium, in concentrations ranging from ppm to 100 wt%. It is ideal for medium- to-high-throughput applications in both production control and R & D environments.

The Minerals edition of the Zetium spectrometer comes complete with a set of 20 synthetic multi-element wide-range oxides (WROXI) standards and an application setup for the handling of  fused bead major element  analyses. Using PANalytical’s unique SuperQ software FP algorithm, the WROXI application can determine concentrations of up to 21 common oxides in a wide range  of  rocks, ores and minerals.

Preparation of standards and samples

For the Minerals editon of the Zetium spectrometer- WROXI configuration to operate correctly, high-purity blanks (the customer’s flux), and single- or multi-element concentration standards are essential. The WROXI standards set comprises 20 synthetic multiple-oxide standards. One advantage of using synthetic standards made from traceable compounds, is that the method is brought significantly closer to being a primary rather than a comparative method. The standards are delivered as powders to be prepared as fused beads by the customer, using the customer’s flux, dilution ratios and methodology. They are therefore suited and applicable to every ‘oxide’ laboratory.

The standards are made from pure chemicals, pre-conditioned, weighed, ground and packaged under nitrogen. Detailed instructions on the use and handling of the WROXI standards are included with the application package.

The WROXI standards and certified reference materials illustrated in the figures and tables below were prepared using 1.0 g of sample fused in 10.0 g flux (66 % lithium tetraborate (LiT), 34 % lithium metaborate (LiM) and  cast into 40 mm diameter fused beads. Pure lithium tetraborate can be used for routine samples, depending on the sample type, although the mixed flux  is preferred for preparing the WROXI standards. Also, depending on the type of sample, 0.5 or 1.0 g NH4NO3 can be added as an oxidizer. The excess NH4NO3 disappears completely during the fusion process and so has no effect on the sample dilution. 15-200 mg of LiBr or LiI can be used as a releasing agent. The fusion temperature was 1150 °C using the LiT/LiM mixture. For the accurate analysis of sulfur, for example in cements and gypsum, it is advisable to lower the fusion temperature to 1050 °C to avoid the loss of sulfur during the fusion process.

Many of the oxide samples contain H2O, carbonates and other volatile compounds, which are lost during the fusion process as loss on ignition (L.O.I.). L.O.I. was used as a balance compound in this application.

Table 1. Total measurement time (peak + backgrounds)

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Measurement conditions

40 mm fused beads were measured in 37 mm sample holders using a 37 mm collimator mask. Single unshared backgrounds were used for all channels except Zn, Cu, Ni, Mg and Na for which two background positions were used. One of the Na background positions was shared with  Mg. Kα lines were measured for all elements exept Pb (Lβ1), Ba, Zr and Sr (Lα1). The SuperQ software for Zetium-Minerals contains an application template for the WROXI application and setup only requires the measurement of the standards. The measurement program takes approximately nine minutes. Measurement times for the individual elements used to obtain the data presented are given in Table 1.

Accuracy

The accuracy of the Minerals edition of the Zetium spectrometer using WROXI standards and the SuperQ (FP) calibration model for major and minor element analyses in oxides and related materials is excellent. This is illustrated in an accuracy overview plot for a number of oxides (Figure 1), in plots for individual oxides; Al2O3, P2O5, K2O, TiO2, Mn3O4, and Fe2O3, (Figures 2 - 7), and in a comparison of certified and measured values for 14 Certified Reference Materials (CRMs) of widely varying composition (Table 2).

The absolute and relative accuracy of the WROXI method is also shown for a range of oxides in Table 3. For all comparisons between certified and measured values, the CRMs were measured as routine samples against the WROXI calibration. The combination of WROXI standards and the PANalytical FP calibration model enables accurate extrapolation of calibrations outside the range in the standards. For example, high Al2O3 and high Fe2O3 CRMs have been analysed successfully with WROXI (Figures 2 and 7).

Figure 1. Accuracy overview: comparison of certified and measured values for 8 oxides in the wide variety of CRMs listed in Table 2 

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Table 2. Analytical accuracy: comparison of certified and measured values for twelve major and minor oxides in fourteen CRMs of various types 

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Table 3. Absolute and relative errors for a selection of oxides calculated from a large number (n) of CRMs. Data are presented for two concentration ranges, > 1 wt% and 0.1 – 1.0 wt%.

*Data for SO3 are from a series of cement and gypsum samples specially prepared at a lower (1050 °C) fusion temperature to prevent the loss of sulfur.

table3.PNG

Figures 2 - 7. Accuracy verification: comparison of certified and measured values for the WROXI standards (   ) and a large number (n) of CRMs (   ). The regression values y, x and R2 are for the CRMs only. 

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Precision and instrument stability

The precision, repeatability and reproducibility of the Minerals edition of the Zetium spectrometer are excellent, not only for short-term measurements (15 consecutive measurements, Table 4), but also for longer-term measurements (measurements carried out over a period of fourteen days).

For comparison, the counting statistical error (CSE) expressed in concentration units is also shown in Table 4. Fifteen consecutive measurements of a single sample (fusion disk of a basalt rock GBW07105) demonstrate relative standard deviations better than 0.1-0.7 % at typical concentration levels greater than 0.2 wt% for many of the elements commonly analysed in wide-range oxide samples, e.g. Na2O, MgO, Al2O3, SiO2, P2O5, TiO2, K2O, CaO and Fe2O3.

More importantly, this level of precision is maintained for measurements carried out over a period of 14 days, illustrating the long-term stability of the system.

Table 4. Analytical precision for WROXI fusions of GBW07105 (basalt) 

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What is the FP algorithm?

The intensity of X-rays measured for a given concentration of an element depends on the bulk composition or matrix of the sample being analysed. For accurate quantification we need to make so-called matrix corrections to account for the differences in bulk composition that occur from sample to sample.

PANalytical’s FP or Fundamental Parameters algorithm used in the SuperQ software calculates matrix corrections from the theoretical laws governing the physics of X-rays. As such, FP models have a significant advantage over more traditional influence coefficient-based matrix corrections (e.g. theoretical alphas).

Unlike theoretical alphas calculations, the FP model calculates matrix corrections that are specific to each sample. This enables accurate analyses over very wide ranges in concentration and in very different types of sample. In addition, accurate analysis can be made outside the range of concentrations bracketed by the standards.

Pro-Trace software and standards

The Minerals edition of the Zetium spectrometer can be supplied with the unique Pro-Trace software and standards. Pro-Trace provides the highest quality trace element analysis (Sc to U) and targets a wide range of materials of geological and environmental significance, including rocks, soils, sediments, ores, minerals, mineral soils and fly ash.

Components typeset in bold were present in the spectrometer used to obtain the data in this note. 

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Conclusions

Elemental analysis with XRF is already the key to the control of quality and production processes in the many industries analysing a wide range of oxide materials. The Minerals edition of the Zetium spectrometer – WROXI – SuperQ (FP) system further extends the advantages of XRF as the best analytical method for oxides analysis. It is capable of measuring  all elements required to control the production and manufacture of materials such as cement, gypsum, iron and   manganese ores, ceramics, bricks, glass and some heavy mineral ores.

It has been demonstrated that analyses are accurate and precise and the method benefits from a simple fusion sample preparation. Furthermore, the stability of the system is such that individual calibrations can be used for months. Time- consuming re-standardizations are unnecessary and the resulting data are highly consistent over time.

Although the Minerals edition of the Zetium spectrometer has been configured to meet industry requirements, it can easily be upgraded with features such as increased power, continuous loading for extra speed of analysis, Omnian for complete standardless analysis and Pro-Trace for high-quality trace element analysis. The use of synthetic WROXI standards made from traceable compounds brings this method close to being a primary analytical method, rather than the strictly comparative method using CRMs as standards, which is commonly used in the XRF analysis of wide-range oxides.

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