How Metal Purity Is Verified Before a Shipment Leaves

Three methods, three different questions

Purity is checked by more than one method because each answers a different question. A visual and dimensional inspection asks whether the material is what the description says it is. A handheld XRF reading asks which elements are present at the surface. A laboratory wet assay asks what the material is made of, to a stated precision, in a sample taken in a stated way.

The three are not substitutes for one another. XRF is fast and lets a whole lot be screened, but it reads a surface and is affected by coatings, corrosion and the element being measured. A wet assay is slower and more costly, and it is the result most contracts are settled on.

• Visual and dimensional — form, cleanliness, contamination, wire diameter, whether fittings or attachments are still present.
• XRF — rapid elemental screening across many pieces; useful for sorting and for a first indication.
• Wet assay — laboratory determination of the elements in a prepared sample; the result an invoice usually settles on.

What a certificate of analysis actually states

A certificate of analysis is a statement of results, and its value depends entirely on how the sample behind it was taken. A certificate reporting a purity figure without identifying the lot, the sample or the method tells the buyer very little.

A useful certificate identifies the lot, describes the sampling method, names the analysis method and the laboratory, and reports each element the purchase specification limits — not only the headline purity. It also carries a date and an authorising signature or reference.

Where the buyer nominates the inspection, the certificate comes from the buyer's chosen agency, on samples taken in a way both parties agreed in advance. That arrangement removes the most common source of dispute, which is not the analysis itself but the argument over whether the sample represented the lot.

Sampling is where the argument usually starts

A sample that does not represent the lot produces a certificate that is accurate about the wrong thing. For wire scrap, that means sampling across the bales rather than from the top of one. For ingots, it means taking pieces from across the stack and, where the specification is tight, cutting or drilling a section rather than reading a surface.

The sampling procedure should be agreed at the same time as the specification: how many increments, from where, and how they are combined into the sample that goes to the laboratory. Written down, it takes a paragraph. Left to habit, it becomes the point on which a claim turns.

This is why a buyer's right to nominate the inspection agency is worth exercising. It is not that one laboratory is more careful than another; it is that an agency chosen by the buyer, sampling to an agreed procedure, has no interest in describing the material more favourably than it is.

Acceptance limits: what happens when the result misses

A certificate reporting a figure just below the specification raises an immediate commercial question, and the answer should already be in the contract. The usual options are rejection of the lot, acceptance with a price adjustment, or acceptance with replacement of the shortfall.

Whichever is chosen, the threshold at which it applies should be stated as a number. An agreement that requires the material to meet the specification, without saying what happens when it is marginally outside it, produces a negotiation where a decision was needed.

It is also worth stating who decides in the event of a disagreement — whether a second laboratory, an umpire sample, or the original agency's retained sample is decisive. Keeping a sealed sample until the lot is accepted costs almost nothing and settles most disputes before they become claims.

Putting all of it into a quotation request

All of this can be settled at the quotation stage, and it is considerably cheaper there than later. A request that states the grade, the elements being limited, the sampling procedure, the nominated agency and the consequence of a missed limit gives the supplier everything needed to quote — and gives the buyer a shipment that arrives with its analysis already agreed.

Where a supplier works to a formal quality management system, this is routine rather than exceptional. What it is not is optional: the material's value is a function of its analysis, and the analysis is only as dependable as the procedure behind it.

Frequently Asked Questions