Reference standards and their certificates

Most measurements in a laboratory are comparisons. An instrument response is turned into a concentration by comparing it to the response from a material whose value is known. That material is a reference standard, and the result can be no better than what is known about it. What is known about it is written on its certificate.

Reference materials and certified reference materials

The vocabulary is precise. A reference material is sufficiently homogeneous and stable with respect to one or more properties to be fit for its intended use in measurement. A certified reference material is a reference material whose property values have been assigned by a metrologically valid procedure and come with a stated uncertainty and a statement of metrological traceability. The difference is the certificate: a certified value carries an uncertainty and a chain of comparisons to a stated reference, and an uncertified value does not.

ISO 17034 sets out the general requirements for the competence of reference material producers, including how homogeneity and stability are assessed and how values are assigned. Accreditation to ISO 17034 is the outside assurance that a producer works this way. ISO Guide 31 describes what the certificate, the label and the accompanying documents should contain.

Pharmacopeial reference standards are a related but separate category. They are established for use with the tests in a pharmacopeia, and their suitability is defined by that use. Many carry no stated uncertainty, because within the pharmacopeial test they are treated as exact. Used outside that context, they need the same scrutiny as any other material.

What the certificate should state

• The identity of the material, the producer, and the unique lot or batch designation.
• Each certified property value with its expanded uncertainty and the coverage factor or confidence level used.
• A statement of metrological traceability: what the value is traceable to.
• The method or methods used to assign the value.
• The intended use, and any uses for which the material is not suitable.
• The period of validity of the certificate, and the conditions under which it holds.
• Storage and handling instructions, including any minimum sample mass.
• The date of certification and the name of the person who approved it.

Conditions on the value

A certified value is a statement with conditions attached, and the conditions are where most misuse happens. The value may hold only for the material as supplied in its unopened container. Once opened, a hygroscopic or volatile material can change, and some certificates say so directly: the value applies to first use, or for a stated time after opening under stated conditions. A minimum sample mass appears on many certificates because homogeneity was only demonstrated down to that amount; a smaller portion may not be representative of the certified value.

Drying instructions are another condition. Some values are certified on a dried basis, and the certificate gives the drying procedure. Weighing the material as received gives a different quantity than the one certified. Storage temperature, protection from light and the period of validity complete the list. A standard used outside any of these conditions has an unknown value, whatever the certificate says.

Primary, secondary and working standards

Certified and pharmacopeial standards are expensive and limited in quantity, so laboratories often qualify their own secondary or working standards against them. That is legitimate when it is documented: the working standard is a characterized material, compared against the primary under a written procedure, assigned a value with its own uncertainty, and given its own expiry or retest date. The record should show which primary lot it was compared against, so that a problem found later in the primary can be traced to every working standard made from it.

Keeping the record intact

A reference standard needs its own log: when it was received, when it was opened, each use and the quantity taken, the storage conditions, and the certificate filed with it. Certificates are sometimes revised after issue — a stability finding shortens the validity period, or a value is reassigned — so the current version matters, and the laboratory should know where the producer publishes revisions. When a revision arrives, the laboratory has to be able to find which of its results relied on the affected lot. That is a question the usage log answers and nothing else does.

The same log answers a quieter question at the end of a standard’s life. When the validity period runs out, or the container is down to less than the minimum sample mass, the standard is retired, and the log shows the date. Results produced after that date with that standard are the ones a reviewer will want to look at, and a log that stops without a retirement entry leaves the question open.