For base metals, most CRMs perform reliably. For Hg, As, Se, and Sb at ultra-trace levels, a “passing” QC result may simply mean the CRM cannot detect digestion-related losses.
Key Limitations of Volatile-Element CRMs
1. Matrix mismatch – Many commercial CRMs are oxide or sulfate-based. Real samples are often sulfides, carbonaceous, or organic. A digest that dissolves the CRM may not dissolve the sample, masking bias.
2. Uncertified or high-uncertainty values – Hg and Se are frequently reported as “informational” with wide tolerances at <1 ppm. A 60% recovery can still fall within range, allowing systemic bias to go unnoticed.
3. Stability concerns – Hg adsorbs to glass containers; B can volatilize during CRM manufacture. The certified value may not reflect the actual content at the time of use.
4. Preparation method bias – A CRM certified by 4-acid digest is not directly comparable to results from aqua regia. Method alignment is critical.
How High-Performing Laboratories Manage CRMs for Volatiles.
1. Use matrix-matched CRMs– For arsenopyrite ores, select a sulfide CRM with certified As, not a generic soil standard.
2. Monitor recovery vs. concentration – Plot % recovery for As/Se across the calibration range. Losses typically increase below 4 ppm.
3. Employ paired CRMs– Include one high-level and one near the detection limit. Acceptable recovery at high levels does not guarantee accuracy at ore-grade cutoffs.
4. Review CRM certification basis – If certified by Total Fusion, it will not identify aqua regia losses. Confirm values are reported for “4-acid digestible” or “aqua regia extractable.”
5. Implement in-house reference materials – Maintain a homogenised site sample with established Hg/As values. CRMs validate accuracy; site materials validate ongoing consistency.
Red Flags Indicating CRM Programs Miss Volatile Loss
– Base metals recover 98–102%, while Hg/As/Se consistently recover 70–85% without investigation.
– Oxide CRMs are used exclusively for sulfide projects.
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