How third-party testing actually works

When a material is sent out for third-party testing, the phrase covers a specific chain of events. A sample leaves the party that made or holds the material, arrives at a laboratory that has no stake in the result, is analyzed on that laboratory’s own instruments by its own staff, and comes back as a signed report. Each link in that chain is there for a reason, and each one can be checked.

Receipt and chain of custody

The work starts at the receiving bench, not at the instrument. When the sample arrives, the laboratory logs it: the date and time of receipt, who received it, the condition of the packaging, whether seals were intact, the temperature if the sample was shipped cold, and the identification written on the container. The sample is then given a unique laboratory number, and from that point every aliquot, preparation and data file refers back to it.

This record is called chain of custody. Its job is to show that the material tested is the material that was sent, and that nothing happened to it between the sample bag and the instrument. A report is only as good as this link. If the laboratory cannot show which container it opened, the numbers that follow describe an unknown.

Purity and identity are separate checks

Most requests ask two questions, and they need different techniques. Identity asks whether the material is what the label says. Purity asks how much of what is in the container is that material, and how much is something else.

A common approach to purity is high-performance liquid chromatography, or HPLC. The sample is dissolved, pushed through a packed column, and separated into its components, which pass a detector one after another. The result is a chromatogram: a trace with a peak for each component the method can see. Purity is often reported as the main peak’s share of the total peak area. That figure only covers what the method separates and detects, which is why the method name matters as much as the number.

Identity is usually confirmed another way. Mass spectrometry measures the mass-to-charge ratio of ions formed from the sample, which gives a molecular mass to compare with the expected value. Other laboratories compare the sample against a reference standard run on the same system, or use spectroscopy such as infrared. A purity result without an identity result can describe a very pure sample of the wrong material.

Instruments the lab owns and controls

“On instruments it owns” is not a detail. A laboratory accredited to ISO/IEC 17025, the international standard for the competence of testing and calibration laboratories, has to show that its equipment is calibrated, maintained and fit for the method, and that the people running it are trained and authorized to do so. It has to use methods that are validated or verified for the purpose, evaluate the uncertainty of its measurements, and keep records that let someone else reconstruct what was done.

Accreditation is not a general seal of approval. It is granted for a defined scope: specific tests, on specific types of material, by specific methods. A laboratory can be accredited for one assay and not for another it also offers. The scope is a public document, and checking that the test on a report falls within it is one of the most useful things a reader can do.

Before any result is accepted, the run itself has to pass. Laboratories run system suitability standards and blanks alongside the sample to show that the instrument was separating, detecting and responding as the method requires on that day. If those checks fail, the sample result is not reported; the run is investigated and repeated.

The signed page

The end product is a test report. Under ISO/IEC 17025 a report carries, among other things, the laboratory’s name and address, a unique report number, the client, the sample identification as received, the date of receipt and the date of testing, the method used, the results with their units, and the identity of the person who authorized the report. Many reports also state the measurement uncertainty or the acceptance criteria the result was compared against.

The signature means a named, authorized person reviewed the data and released the result. It does not mean the laboratory vouches for the rest of the lot, for how the material was stored afterward, or for anything outside the tests listed. The report describes one sample, on one date, by the methods shown.

What independence buys and what it doesn’t

A third party is independent of the outcome. It has nothing to gain from a pass and nothing to lose from a fail, and its own accreditation depends on reporting what the instrument showed. That is the value of sending a sample out rather than testing it in house.

Independence does not make the sample representative. If the sample was drawn from one corner of a lot that is not uniform, or chosen because it looked best, the laboratory will report it accurately and the report will still mislead. Good practice is to sample according to a written plan, record who took the sample and how, and keep a retained portion so the test can be repeated. The laboratory controls everything after receipt. Everything before receipt is the sender’s record to keep.