When a laboratory result is disputed, the conversation usually drifts to instruments and method. Yet most objections are aimed at the sample: where it was taken from, how it was stored, whether it really represents that batch.
The representation problem
A sample drawn from the top of a tank may not represent the whole tank. Density differences, settling and incomplete mixing affect the measurement directly. That is why samples are taken to a defined pattern rather than from a single point.
If the sample does not represent the batch, the measurement has measured the sample, not the batch.
What has to be recorded
- Point and method: how many points, at what depth, in what order.
- Conditions: temperature, container type, time between sampling and measurement.
- Identity: which batch the sample belongs to — a recorded link, not a label.
- Chain: who took it, who they handed it to, where it was stored.
The fourth is decisive in legal proceedings. If the sample’s passage from hand to hand is not recorded, the other side can say "this sample is not our product", and rebutting that becomes difficult.
Pre-screening reduces the sampling load
A quick assessment in the field decides which batch gets sampled. Sampling rigour then concentrates on a small number of samples; taking careful samples from a few batches produces a stronger file than careless samples from every batch.
Does the sample represent the batch
Whether a sample counts depends first on whether it represents the batch. Oil drawn from the top of a tank is not the same as oil from the bottom; a carton taken from the front row of a pallet has not sat in the same conditions as one from the middle. An analysis run before the representation problem is settled ties a correct result to the wrong batch.
So the point and method of sampling are as worth recording as the result. Without a note of where, from what depth and from how many points it was taken, the analysis report proves nothing on its own.
Where the chain breaks
The time between taking a sample and analysing it is the basis of most disputes. Was the container changed, was temperature held, whose hands did it pass through — none of the three look like they need recording until a dispute arrives and those are exactly the questions asked.
- The container and seal number are recorded at the moment of sampling.
- Handover is done in writing, from whom to whom.
- Storage conditions and duration travel with the sample.
- A witness sample is set aside; if challenged, the second analysis runs on that.
Why a witness sample is kept
A first analysis is a result that can be contested. If no witness sample was kept, there is no second measurement to settle the contest, and the argument shifts from two reports to the credibility of two parties. A witness sample keeps that argument on technical ground.
What is valid is not the analysis but the sample the analysis rests on.