The WDXRF Analysis of Industrial Minerals and Geological Materials

There are three main analysis methods:

• Qualitative measurement: This method identifies the elemental composition solely based on the peak positions in the X-ray spectrum. It tells you which elements are present.
• Semi-quantitative analysis: Building on qualitative analysis, this method translates elemental composition into approximate concentrations using a fundamental parameter approach. It relies on pre-measured "line libraries" from pure materials, but for good results, the sample's composition must be known to be close to 100%.
• Quantitative analysis: This is the most precise method, calculating concentrations based on measured intensities. It requires setting up calibration lines using certified reference materials (CRMs) and analyzing their intensities. This approach aims for high accuracy and precision and is preferred for critical applications.

Potential sources of error include sample taking, sample preparation, instrument precision, chemical data of the standards used for calibration, and inaccuracies during regression analysis for calibration lines.

It's crucial to understand the difference between accuracy and precision:

• Accuracy: Refers to how close a measured value is to the true or reference value. Bad accuracy means the results are consistently off the target.
• Precision: Refers to the repeatability or reproducibility of measurements. Good precision means repeated measurements yield very similar results, even if they are consistently far from the true value (bad accuracy). Ideally, both precision and accuracy should be good to obtain reliable and exact concentration results. Errors cannot simply be "deleted" from calibration curves; they indicate a systematic issue that needs investigation.

These are physical effects that can significantly impact WDXRF results, particularly with powder samples:

• Grain size effects: The intensity of an element's signal depends on the number and distribution of particles in the analyzed volume. Different grain sizes (e.g., large vs. fine powder) or preparation methods (loose powder vs. pressed pellet) can lead to vastly different intensities for the same concentration. This is resolved by grinding samples to a consistent, fine powder and preparing them identically, often via pressed pellets or, more effectively, fused beads.
• Mineralogical effects: Elements can exist in different mineral phases within a sample (e.g., calcium in various forms in cement clinker). Since WDXRF measures elements, not their oxide forms, the intensity of an element can vary depending on its mineralogical association. This effect can only be effectively resolved by the "fused bead" method, which transforms the sample into a uniform, non-crystalline material.
• Matrix effects: Occur when elements in a sample influence the X-ray fluorescence of other elements through absorption or enhancement (excitation). For instance, silicon can enhance aluminum's signal. These effects are complex but can be accounted for during calibration, especially when using fundamental parameter approaches or preparing samples as fused beads to create a consistent matrix.

Rigaku offers two pre-calibrated packages to simplify WDXRF analysis for industrial minerals and geological samples:

1. Oxide-FB PAK: Designed for analyzing a wide range of oxides (21-23) in materials like sand, limestone, clays, slags, and cement. It relies on the "fused bead" sample preparation method, which eliminates grain size and mineralogical effects, offering superior accuracy and precision for oxide analysis.
2. GEO-TRACE-PAK: Intended for analyzing trace elements (26) and some main oxides (9) in geological matrices. This package uses the "pressed pellet" sample preparation method. It is not suitable for highly concentrated mineral ores or mine tailings.

Two separate packages are necessary due to fundamental physical effects within powder samples (grain size, mineralogical, and matrix effects) that require dedicated sample preparation methods. Fused beads are ideal for oxides because they homogenize the sample and resolve mineralogical effects, while pressed pellets are suitable for trace elements and certain geological applications where fusion might not be necessary or preferred, but they are more susceptible to grain size and matrix variations. Combining them into one calibration is not physically possible for optimal accuracy across all elements and matrices.

Selecting the appropriate WDXRF spectrometer is crucial for achieving analytical objectives and is a long-term investment (10-15 years lifetime). Key considerations include:

• Analytical tasks and objectives: Define the materials to analyze, specific elements, concentration ranges, desired accuracies, and detection limits.
• Software user-friendliness: Ensure the software is intuitive and aids calibration setup and analysis.
• Sample preparation needs: Account for necessary tools and methods, as good preparation is key to accuracy and precision.
• Budget: Evaluate the initial purchase price and long-term cost of ownership (maintenance, consumables).
• Service and application support: Ensure assistance is available in your region after purchase.
• Future needs: Consider potential analytical needs beyond current requirements, as the spectrometer will be in use for many years. Different models offer varying capabilities (e.g., tube-above vs. tube-below, power, number of crystals, sample capacity, mobility), making it important to consult application scientists who can provide expert advice and run tests.