Concept

Blank thresholds

When does a blank "fail"? The common conventions, in rough order of use:

  • 10× detection limit as the single failure line (the most common);
  • 5× DL warning, 10× DL failure as a two-tier compromise;
  • ≥90% of blanks below 5× DL as an aggregate form;
  • CIM sets no numeric threshold at all.

Two important refinements beat any DL multiple:

  • A certified blank is not a zero. CDN-BL-10S certifies Au <0.01 g/t — a bound, with a method. OREAS 21e is a "blank" with real certified values for Cu, Fe and Li; OREAS 24c carries 138 ppm Ni. For a certified blank, use the supplier's prescribed limit, not a DL multiple.
  • Economic significance can override. For a 0.5% Cu deposit, failing a blank at 10 ppm Cu is pointless — 10 ppm is economically meaningless there; ~200 ppm would be a sensible limit (Bloom). Mass ratio matters too: carry-over into a 500 g blank reads ~10× stronger than the same contamination into a 5 kg sample.

In geoDB

geoDB resolves a blank's threshold through a ladder, stopping at the first rung that answers:

  1. The certified blank's supplier limit — if the QC sample is linked to a certified blank whose certificate states a < bound for that element, that bound is used. This is the industry default above, applied automatically.
  2. A warning limit you configured on the project for that element (optionally scoped to one method). Element casing no longer matters — an au limit row covers an Au reading.
  3. A detection-limit multiple, if you have set one on the project's QAQC protocol (blank_dl_multiple). This is empty by default and staying empty is a legitimate choice.
  4. Nothing — the reading is reported as "unknown": shown on the page, excluded from the pass rate, never counted as a pass.

The page tells you which rung produced the number in the Limit source column, because the three rungs are different claims — a supplier bound is the CRM vendor's, a warning limit is yours, and a DL multiple is a convention.

Set a DL multiple deliberately, not reflexively. It is a floor, not a recommendation. On a multi-element ICP scan it is meaningless for the rock-forming elements a barren blank legitimately contains — a silica or carbonate blank really does carry percent-level Na, Ca, Fe and Mg, which sit thousands of times above their detection limits without any contamination at all. Configure limits for the elements your blanks are actually there to police.

If your blanks page shows a lot of "unknown", that is the honest reading: no limit exists for those elements yet. Warning limits are managed from the QC pages.