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Selecting and instructing

1. Name the method by its standard. Different standards for the same property give different numbers. State the standard in the enquiry, and check it appears on the laboratory's published accreditation schedule rather than on its website.
2. Get the sampling right first. For microbiological and residue work the sample dominates the result. Sampling plan, container, preservation and time to the laboratory are decided before collection and are frequently the reason a result is unusable.
3. Ask what recovery the method achieves. A limit of detection assumes a recovery from your matrix. Ask for recovery data on a matrix like yours, because the published figure was generated on a different one.
4. Treat failure analysis as an investigation. Failure work delivers a reasoned opinion supported by evidence rather than a measurement. Agree the scope, the techniques and what happens if the evidence is inconclusive, before the work starts.
5. Specify what the report must contain. Method reference, sample description, date and condition on receipt, result with uncertainty, any deviations and a signature. Agree it in advance rather than discovering an omission when the report is needed.

Scope is public and specific

Accreditation bodies publish the exact methods, materials and ranges each laboratory is accredited for. Reading that schedule takes minutes and prevents the discovery that a report cannot be used for its purpose.

Where your method is outside the scope, the work may still be excellent and the report will be labelled differently. Know which you are buying.

Uncertainty belongs in the decision

A result near a specification limit may or may not exceed it, and only the stated uncertainty tells you which. Accredited reports include it; many others do not.

Ask for it, and set internal limits with it in mind. A specification tighter than the measurement uncertainty will fail randomly and waste investigations.

Common questions

Why did two laboratories give different results?

Different standards, different sample preparation or different recovery from your matrix. Naming the standard and asking for matrix recovery data resolves most discrepancies before they occur.

How much does sampling matter?

For microbiological and residue testing, frequently more than the analysis. A perfect method on an unrepresentative or badly preserved sample produces a precise wrong answer.

What does a failure analysis deliver?

An evidenced opinion about the mechanism, supported by the techniques applied. It is not a measurement, and the scope, the techniques and the limits should be agreed in writing at the start.

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