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Quantitative Analysis of Crystalline Silica by XRD

Kaho Hamada, Miki Kasari, and Keigo Nagao

Summer 2025 Volume 41, No. 2 , 16-24

Regulations on crystalline silica have been becoming increasingly strict worldwide and, in Japan, it has been designated as a material subject to risk assessment. Therefore, it is essential to confirm the applicability of the regulatory cut-off value (0.1%) and to appropriately control the amount of crystalline silica in, for example, glass, ceramics, or bricks based on this threshold. As a result, the demand for quantitative analysis is increasing. However, current regulations do not specify an analytical method for determining silica content in powders.

In this technical note, we propose the use of a calibration curve method employing an X-ray diffractometer for the quantification of crystalline silica. We also compare different diffractometers and slit optics configurations, and consider the optimal approach for accurate quantification.

The limits of detection for three crystalline silica samples were all below 0.1%, the regulatory cut-off value. The quantitative results obtained using both a 0–100% calibration curve (based on a standard sample) and a 0–1% calibration curve (based on a matrix-matching method) showed good agreement with the prepared concentrations, demonstrating the effectiveness of the calibration curve method.

Highlights

  • XRD calibration curve methods can reliably quantify quartz, cristobalite, and tridymite at or below the 0.1% crystalline silica regulatory threshold.
  • Both full-range (0–100%) and matrix-matched (0–1%) calibration approaches provide accurate quantitative results when matrix absorption effects are properly addressed.
  • High-performance divergent beam (DB) optics offer the best detection sensitivity, while benchtop XRD systems are also capable of detecting trace crystalline silica concentrations.

Summary

X-ray diffraction provides a practical, non-destructive approach for quantifying trace crystalline silica in materials such as glass, ceramics, bricks, and mineral powders where regulatory compliance requires reliable determination of concentrations around 0.1 wt.%. A calibration curve method based on peak integration enables accurate measurement while distinguishing among the three common crystalline silica polymorphs: quartz, cristobalite, and tridymite.

Two quantitative strategies are effective. A simple 0–100% calibration curve requires only pure crystalline silica and matrix standards, but quantitative results should be corrected using a spike recovery ratio (SRR) to compensate for differences in X-ray absorption between the analyte and matrix. Alternatively, a 0–1% matrix-matched calibration curve eliminates the need for absorption correction when an appropriate silica-free matrix is available.

Performance was evaluated using multiple XRD configurations, including SmartLab SE systems with Bragg-Brentano (BB) and divergent beam (DB) optics, as well as a MiniFlex benchtop diffractometer. All systems successfully detected crystalline silica near the regulatory threshold, with DB optics providing the highest sensitivity because incident-beam monochromatization reduces background and fluorescence. Detection limits for quartz and cristobalite were well below 0.1%, while tridymite required somewhat greater sensitivity due to weaker peak characteristics.

The study also demonstrates that longer counting times can further improve detection limits and that the calibration curve method produces quantitative values in close agreement with prepared concentrations, making it a robust solution for routine crystalline silica analysis.

Frequently asked questions

XRD directly measures crystalline phases and distinguishes among quartz, cristobalite, and tridymite, unlike bulk chemical techniques that determine only total silica content. Because it is non-destructive and phase-specific, XRD is well suited for evaluating materials that must meet crystalline silica regulatory requirements.

Yes. The evaluated calibration methods achieved limits of detection at or below approximately 0.1 wt.% for all three crystalline silica polymorphs under suitable measurement conditions. Quartz and cristobalite generally exhibited detection limits well below the threshold, while tridymite approached the limit depending on the optical configuration.

The 0–100% calibration uses pure analyte and pure matrix standards, making preparation straightforward. However, differences in X-ray absorption between the matrix and analyte require correction using a spike recovery ratio. The 0–1% matrix-matched calibration uses standards with compositions similar to the sample, eliminating the need for absorption correction but requiring preparation of low-level reference materials with a matching matrix.

The spike recovery ratio compensates for matrix absorption effects that can either suppress or enhance diffraction intensity. Applying the SRR correction to the 0–100% calibration method improves quantitative accuracy and allows results comparable to those obtained using matrix-matched calibration standards.

Among the evaluated systems, the SmartLab SE equipped with divergent beam (DB) optics produced the lowest limits of detection. Monochromatizing the incident beam reduces fluorescence and background intensity, improving the signal-to-noise ratio for trace crystalline silica measurements.

Yes. Although a high-performance laboratory system provides the greatest sensitivity, the MiniFlex benchtop diffractometer successfully detected crystalline silica at low concentrations suitable for many routine analytical applications. Detection performance can be further improved by increasing measurement time.

The strongest tridymite diffraction peak overlaps with a calcium carbonate peak in the test matrix, requiring use of a weaker secondary reflection for quantification. This results in lower signal intensity, making longer counting times beneficial for improving detection and measurement precision.

Increasing the measurement time reduces counting statistics uncertainty and improves both the limit of detection (LOD) and the limit of quantification (LOQ). Appropriate sample preparation, optimized optics, and careful background subtraction also contribute to improved analytical performance.

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