X-ray diffraction (XRD) is the gold standard for quantifying crystalline silica content in surface materials. If you’re buying or specifying “silica-free” or “low-silica” products, you need to understand what an XRD report actually tells you — and, just as importantly, what it doesn’t.

How XRD Works for Silica Analysis

XRD works by directing X-rays at a powdered sample and measuring the diffraction pattern. Each crystalline mineral has a unique diffraction “fingerprint.” For silica, the lab looks for specific peaks corresponding to quartz, cristobalite, and tridymite — the three polymorphs of crystalline silica. The intensity of the peaks indicates concentration.

Nine Things to Check on Every XRD Report

1. Accreditation

Is the testing laboratory accredited to ISO 17025? If not, the results may not be accepted by regulators, insurers, or specifiers. This is the first filter. No ISO 17025 = the report is not legally defensible in most jurisdictions.

2. Detection Limit

The detection limit (also called LOD — limit of detection) is the minimum concentration the method can reliably detect. Common detection limits for quartz by XRD are 0.1% to 1%. A result of “< LOD" or "not detected" means the silica content is below that threshold — not necessarily zero. For a product marketed as "silica-free," look for a detection limit of 0.1% or lower.

3. Quantification Limit

The LOQ (limit of quantification) is the minimum concentration that can be reliably quantified (not just detected). LOQ is typically 3x the LOD. Results between LOD and LOQ should be treated as estimates, not precise measurements.

4. Which Polymorphs Were Tested?

The report should specify testing for quartz, cristobalite, and tridymite. Some labs only test for quartz. Cristobalite is a high-temperature polymorph that can form during manufacturing processes — it’s more hazardous than quartz and must be specifically tested for.

5. Sample Preparation Method

XRD results can vary depending on how the sample was prepared (grinding method, particle size, pressing method). The report should describe the preparation method. If particle size isn’t controlled to < 10 µm, results may be unreliable.

6. Reference Standard

What standard was used for calibration? Common reference standards include NIST SRM 1878b (respirable quartz) and NIST SRM 2958 (quartz in a matrix). The reference standard should be appropriate for surface material analysis.

7. Batch Traceability

Does the report include a sample ID that links back to your specific product batch? A “generic” XRD report that doesn’t reference a specific production batch or lot number tells you nothing about the product you’re actually buying.

8. Report Date

Product formulations change. An XRD report from 2023 is not valid for a product purchased in 2026 — unless the manufacturer can demonstrate the formulation hasn’t changed and provides a certificate of conformity linking the old report to the current batch.

9. Signature and Review

The report should be signed by a qualified analyst and reviewed by a senior chemist or laboratory manager. An unsigned, unreviewed report is not a valid test result.

Common XRD Report Red Flags

• “Not detected” with no detection limit stated — meaningless
• Report only shows quartz, not cristobalite or tridymite — incomplete analysis
• No ISO 17025 accreditation — likely not accepted by regulators
• Report provided by the manufacturer’s own lab — potential conflict of interest; independent third-party testing is preferred
• Report dated more than 12 months ago — formulations change, and lab equipment drifts
• No sample ID linking to your batch — the report could be for any product

For a silica-free verification checklist that includes XRD interpretation guidance, see our Verify Silica-Free Claims guide.