Light Element Analysis With WDXRF - Reliably Quantify Boron and Fluorine

There are two primary sample preparation methods for WDXRF:

• Pressed powder:
  • Preparation: Involves drying the representative sample, reducing its particle size (ideally to less than 45 µm) through milling, and then pressing the powder into a compact specimen, often with grinding aids like "Spectra Blend" to ensure homogeneity and a smooth, flat analytical surface.
  • Advantages: Aims for the highest intensities by effectively reducing particle size and achieving homogeneity. It's crucial for light elements (like fluorine and boron) because their signal is read from the top micron layer of the sample.
  • Disadvantages: Requires "matrix matching" and "mineralogical matching" of calibration standards and unknowns. If the same element exists in different mineral forms (e.g., calcium as hydroxide, carbonate, or fluoride), it can lead to different calibration curves, making accurate quantification challenging without proper matching.
• Fusion bead:
  • Preparation: Involves dissolving the sample in a flux at very high temperatures to create a homogeneous glass bead. For volatile elements like fluorine, enclosed fusion systems with a lid can be used to prevent loss. The sample-to-flux ratio can also be adjusted (e.g., 1:2 instead of 1:10) to increase sensitivity for less concentrated elements.
  • Advantages: Eliminates the "matrix problem" and mineralogical effects seen in pressed powders, as all minerals and elements are homogenized into a single, neutral glass matrix. This results in all forms of an element falling on one calibration curve.
  • Disadvantages: Involves significant dilution, which can reduce sensitivity for trace elements. There's also a risk of losing volatile elements during the high-temperature fusion process if not performed in an enclosed system. For boron, common fusion fluxes often contain boron, which can complicate analysis unless accounted for. The recommendation for boron is typically pressed powder due to potential loss during fusion.

Light elements like fluorine and boron are challenging to quantify with XRF because they have a very low fluorescent yield, meaning they produce a weak X-ray signal. Additionally, their signals are read from only the top micron layers of the sample, making particle size and surface integrity critical. Overlaps with other elements (e.g., fluorine with iron or manganese) can also pose problems.

Rigaku's ZSX Primus IV spectrometer addresses these challenges through several unique features:

• Thinner Be window (30 µm): This significantly increases the intensity of X-rays for low fluorescing elements like boron and fluorine by up to 40%, leading to better sensitivity and lower limits of detection.
• Specialized analyzing crystals: Rigaku offers specific crystals like the RX35 for fluorine and the RX85 for boron. The RX35 can double the sensitivity for fluorine (e.g., from 1 kilocount to 2 kilocounts), while the RX85 can increase boron sensitivity by at least 30%, leading to better detection limits.
• Automatic pressure control (Vacuum stabilization): Light elements are highly sensitive to vacuum level fluctuations. This feature ensures a very stable and consistent vacuum, preventing intensity variations (e.g., a 5 pascal change can result in a 2% intensity difference for boron), leading to more consistent and stable results.
• X-ray tube-above design: This ensures the long-term usage of the X-ray tube, no dust on the thin Be window, loss of intensity is reduced, thus offering enhanced reliability and durability. Accidental contamination of the goniometer or inadvertent damage to the X-ray tube is minimized.
• Software features: The software allows for various mathematical corrections, including theoretical alpha corrections (Lachance, de Jongh, and JIS models) and line overlap corrections (e.g., for sulfur and molybdenum), which can improve accuracy, especially for complex matrices or interfering elements. Compton scattering ratio correction is also available for matrix correction in pressed powders, especially for trace elements.

WDXRF provides several significant advantages over traditional wet chemistry methods (like Ion Selective Electrode for fluorine or UV spectroscopy/titration for boron) for the analysis of fluorine and boron:

• Easier and faster sample preparation: Wet chemistry methods often involve tedious and time-consuming sample preparation, extraction, and calibration setup. WDXRF, especially with techniques like pressed powder or specialized liquid sample filters, offers a much simpler and quicker preparation process.
• Reduced calibration frequency: Once a WDXRF calibration is established, it doesn't require frequent re-calibration. The system can maintain calibration stability through "check samples" and "drift correction" procedures, saving significant time and effort in a laboratory environment, especially in quality control settings. Wet chemistry methods often require daily calibrations.
• Fewer interferences (with corrections): While wet chemistry methods can suffer from interferences (e.g., alumina and iron for fluorine), WDXRF software includes mathematical correction models (e.g., theoretical alpha corrections, line overlap corrections) that can effectively eliminate or minimize these issues, leading to more accurate results.
• Suitable for production environments: The speed, ease of use, and stability of WDXRF make it highly suitable for high-throughput quality control and production environments where rapid and reliable elemental analysis is crucial.

Yes, WDXRF can be used to determine fluorine and boron in liquid samples, particularly by employing a specialized "filter method." Traditional liquid cup methods in XRF are not suitable for these elements because the film used in the cups can absorb fluorine and boron, leading to inaccurate results.

The filter method involves:

• Pre-concentration: A dedicated amount of the liquid standard or unknown sample is placed onto a special filter (e.g., Micro Carry, Ultra Carry, Ultra Carry Lite).
• Drying: The filter is then dried at a temperature that ensures the fluorine or boron is not driven off.
• Measurement: The dried filter, now with the concentrated analytes, is then measured in a vacuum environment within a special sample cup assembly. This assembly includes a back-scattering cup (often made of alumina) to eliminate scattered X-rays from the cup itself and ensure only the portion where the liquid was applied (e.g., 20 mm diameter) is measured.
• Increased sensitivity: This pre-concentration onto the filter combined with vacuum measurement significantly increases the sensitivity for boron and fluorine in liquid samples. Calibration curves can be created using AA or ICP standards for ranges like 0-500 ppm for boron and 0-50 ppm for fluorine, demonstrating its capability for low concentration levels in water.

Matrix effects refer to the influence of the overall composition (matrix) and mineralogical form of a sample on the intensity of the fluorescent X-rays emitted by a particular element. This can lead to inaccurate quantification if not properly addressed.

• In pressed powders: Matrix effects are significant because different mineral forms of the same element (e.g., calcium as calcium hydroxide, carbonate, or fluoride) will yield different X-ray intensities, resulting in distinct calibration curves. To counter this, "matrix matching" is crucial, meaning calibration standards must have a similar chemical and mineralogical composition to the unknown samples. The "Spectra Blend" is used to improve homogeneity and minimize these effects. Mathematical corrections like the Compton scattering ratio method can also be applied to account for matrix variations in pressed powders, especially for trace elements.
• In fusion beads: Fusion fundamentally eliminates matrix effects by dissolving the sample into a homogeneous glass matrix. This eliminates errors related to particle size and minerology. This means that regardless of the original mineralogical form, all elements are present in a uniform chemical environment, allowing for a single calibration curve that covers all forms of an element.
• In software: XRF software incorporates various correction models to compensate for matrix effects and inter-element interferences:
  • Theoretical alpha corrections (Lachance, de Jongh, JIS models): These mathematical models adjust for absorption and enhancement effects caused by the presence of other elements in the matrix.
  • Line overlap corrections: Used when the analytical lines of two different elements overlap, leading to an artificially high signal for one. The software can mathematically subtract the contribution from the interfering element.
  • Compton scattering ratio method: This is a specific matrix correction that measures the scattered X-rays (Compton line) from the sample's matrix and uses this information to correct for matrix variations, significantly improving accuracy, especially for trace elements in pressed powders.
  • Background correction: This involves the ratio of the analyte line and background signal to improve the regression of the line. The software allows for different background fitting models (two, three, or four points) to accurately subtract background noise, which can be significant, especially for light elements with wide peaks and high background signals (e.g., boron in glass). 

Rigaku employs several strategies to ensure the long-term stability and accuracy of its WDXRF instruments and calibrations:

• Robust instrument design: The ZSX Primus IV features an X-ray tube positioned above the sample, which minimizes the risk of contamination from falling dust or sample breakage, ensuring consistent long-term intensity.
• Vacuum stabilization (Automatic pressure control): As detailed before, this feature maintains a highly stable vacuum environment, critical for consistent light element analysis.
  • Calibration maintenance procedures / Check samples: Regular measurement of known "check samples" allows users to monitor the performance of the calibration over time.
  • Drift correction: This procedure systematically adjusts the calibration based on the measurements of check samples, compensating for any minor instrumental drift and maintaining the accuracy of the original calibration without needing to re-calibrate daily.
  • Automated SOPs: The software can automate standard operating procedures for calibration maintenance, including checks and drift corrections, simplifying the process for users.
• Strong service and application support: Rigaku provides comprehensive service contracts and application team support to train customers on best operating practices, interpretation of results for checks and drift corrections, and general maintenance, ensuring optimal instrument performance and long-term stable methods.
• Continuous improvement (Kaizen philosophy): As a Japanese company, Rigaku adheres to the Kaizen philosophy of continuous development and improvement, constantly enhancing their products and analytical performance.

WDXRF is crucial for quantifying fluorine and boron across various industries due to their importance:

• Fluorine applications:
  • Fluorite (Fluorspar) analysis: Quantifying fluorine in fluorspar is essential for its use in producing Teflon and in the electronics industry (particularly semiconductor manufacturing).
  • Consumer products: Used in toothpaste and drinking water to prevent tooth decay, requiring precise quantification.
  • General materials: Analysis in other raw materials and finished products containing fluorine.
• Boron applications:
  • Agriculture: Essential for plant growth, making its quantification in fertilizers important.
  • Glass industry: Used to produce heat-resistant Pyrex glass and borosilicate glasses commonly found in laboratories, requiring analysis in both raw materials and final products.
  • Renewable energy materials: Boron finds applications in various renewable energy technologies.
  • Cleaning products: Also incorporated into certain cleaning formulations.
  • Advanced materials: Its unique properties are valuable in the production of semiconductors and various alloys.
  • Tourmaline analysis: Quantifying boron in tourmaline ore.

Both elements, due to their light nature, highlight the importance of optimal sample preparation (especially pressed powders with fine particle size and homogeneity) and the specialized hardware and software features of the WDXRF instrument for accurate and sensitive determination in diverse matrices, including solids (ores, slags, metals) and liquids (water).