Duplicates near DL
A pair of repeats at 0.01 and 0.02 ppm gold is a 100% relative difference — and if the detection limit is 0.01 ppm, it is also completely meaningless: that is instrument noise, not imprecision. Near the detection limit, relative statistics like RPD lose their meaning, and judging them anyway "can skew our perception of the precision".
The convention
Pick a practical floor — a good rule of thumb is 10× the analytical detection limit, commonly called the "line of significance" — and plot it on your precision charts. Bishi states it as protocol: repeat values proximal to the detection limit are "not considered failures."
The implementable two-regime shape (from a production QC system, EarthSoft EQuIS):
| Regime | Test |
|---|---|
| both results ≥ 5× DL | relative test (RPD/HARD) vs the normal threshold |
| either result < 5× DL | absolute test: flag only if |a−b| > 1–2× DL |
Do not simply discard sub-threshold pairs — that hides real contamination and real sub-sampling failure at low grade. Switch the statistic; keep the data.
(Physical basis: precision degrades asymptotically toward the DL; it is optimised around ~50× DL.)
In geoDB
geoDB applies the two-regime test:
- both results ≥ 5 × the detection limit → RPD against the normal threshold for that duplicate type;
- either result below it → the pair is judged on absolute difference, and fails only if the two results differ by more than one detection limit.
The 5× boundary is per-project (Project settings → QAQC protocol → duplicate detection-limit floor); set it to 0 to judge everything on RPD.
Two things worth knowing:
- The pair is re-judged, never dropped. A near-DL pair that genuinely disagrees still fails — the floor removes noise, not evidence.
- The floor can only ever rescue a pair. Where the absolute test would be harsher than RPD, the RPD verdict stands, so raising the multiple can never manufacture a failure.
Every duplicate row says which test judged it, in the Judged by column and in the chart hover; near-DL points plot as diamonds because the RPD threshold line does not apply to them.
(Pairs where a value is below detection are handled separately: both-below counts as agreement, one-below is not evaluable and is reported rather than judged.)