Compliance Testing
A result of 92 against a limit of 100 is not a pass if its uncertainty reaches 103.
— mg/kg
Negative means the guard band is already crossed
mg/kg and ppm are the same quantity and are shown once; ppb and percent are given because laboratory reports move between them and a factor of a thousand read the wrong way is the commonest error on a certificate. The decision lines follow the convention of the rule being applied: conclusively below requires the whole interval to sit under the limit, which suits a threshold that bites at or above its figure, and both decision outputs can read zero at once - that is the honest answer when the interval straddles the limit and the measurement does not resolve the question. The guard band is a decision rule and not a legal quantity: pulling the acceptance limit in by one expanded uncertainty is a common laboratory convention rather than something a regulation states, and a buyer or a specification may set a different multiple, which is why it is a field. The later limit is here because a stepped threshold is two obligations wearing one number, and a result that clears the limit in force can already sit above the one that replaces it - a product qualified once is not qualified for the life of the range. Nothing here decides compliance: what a result means for a shipment depends on the sampling plan behind it, on whether the analyte measured is the analyte the rule names, and on exemptions the rule may carry.
Restricted Substance Limit: Result, Uncertainty & the Next Limit — free, with the formula and a worked example, at Textile School.