Sample integrity from collection to instrument

A laboratory result is a statement about the material that went into the instrument. Whether it also says anything about the lot that material came from depends on a chain of steps that mostly happen outside the instrument room: how the sample was taken, what it was put in, how it was labeled and shipped, how it was received, and how it was prepared. A break at any point can make an accurate measurement describe the wrong thing.

Taking a representative sample

Sampling starts with a plan. The plan says how many containers in a lot are opened, which ones, where in each container material is drawn from, and how much is taken. For a uniform liquid, a single well-mixed draw may represent the whole. For a powder that may have separated by particle size in transit, or for a lot filled over several hours, the plan needs several draws from different positions, sometimes kept separate and sometimes combined into a composite.

The equipment matters too. Sampling tools must be clean and made of materials that neither react with nor shed into the sample. Sampling should happen in conditions that do not change the material: a hygroscopic powder sampled in humid air may pick up water before it is ever sealed.

Containers and labels

The sample container should protect the sample at least as well as the original packaging: amber glass for light-sensitive material, a tight closure for anything volatile or hygroscopic, and a container material that does not adsorb the substance being measured. A container much larger than the sample leaves headspace, which can matter for anything that oxidizes.

Every container needs a label that links it, without ambiguity, to its source: the material name, the lot number, the date and time of sampling, the point of sampling and the person who took it. The label must survive the trip, including refrigeration and condensation. A sample whose label has smeared is, for most purposes, no longer a sample of anything in particular.

Chain of custody

When samples change hands, a chain-of-custody record follows them. Each transfer is recorded with the date, the time, the person releasing, the person receiving and the condition of the package. Its purpose is to show that the sample was not substituted, opened or exposed to conditions outside its limits between collection and analysis.

Transit conditions belong in that record. If a sample must stay refrigerated, a temperature indicator or data logger in the package shows whether it did. A sample that arrives with an indicator showing an excursion can still be tested, but the result needs to carry that context.

ISO/IEC 17025 requires a laboratory to have a procedure for handling test items: receiving them, identifying them, protecting them and storing them. In practice, receipt is an inspection. The lab checks that seals are intact, that labels match the paperwork, that the quantity is sufficient and that the temperature on arrival is within limits. Any deviation is recorded and, where it could affect the result, raised with the client before testing continues.

Accepted samples are logged into the lab’s system and given a unique identifier that follows them through every preparation and measurement. From this point forward, a result can be traced to one specific container.

Preparation: the last step before the instrument

Most samples are not tested as received. They are weighed, dissolved, diluted, filtered or extracted. Each step is a chance to change the sample:

• A portion taken from the top of an unmixed container may not represent the whole.
• A hygroscopic sample weighed slowly on an open balance gains water while it is being weighed.
• A solution left on the bench before injection may degrade, especially under light or at room temperature.
• A filter can retain some of the substance being measured, lowering the result.
• Glassware that was not properly cleaned can carry over material from a previous sample.

Validated methods address many of these. Solution stability is tested so the lab knows how long a prepared sample can wait. Filter compatibility is checked. Weighing procedures for sensitive materials are written down. The record of preparation — who weighed what, on which balance, at what time — is part of the raw data and should be retrievable behind any reported result.

Why the record matters as much as the result

When a result is unexpected, the first questions are rarely about the instrument. They are about the sample: where it came from, how it traveled, how long it waited and how it was prepared. A complete sampling and handling record lets those questions be answered from documents. Without it, an unexpected result can only be repeated, not explained.