What is an impurity reference standard?

An impurity reference standard is a characterised material — usually a single known impurity of a drug substance or product — used to identify that impurity and to measure how much of it is present. Everything else in this article follows from two words in that sentence: characterised and known.

Three terms that are not interchangeable

The fourth term worth keeping separate is certified reference material. That phrase has a specific meaning tied to ISO 17034: it describes a producer's competence and the metrological traceability of the assigned value. A material can be an excellent impurity standard without being a certified reference material — and a supplier who uses the two phrases as synonyms is telling you something about how carefully the rest of the documentation was written.

What makes a material usable as an impurity standard

Three properties, in this order:

1. Identity, established by more than one technique. A structure drawing generated from a name is not an identity. The claim is normally supported by mass spectrometry and by NMR, with the spectrum attached or available. For an isomer or a diastereomer, this is the whole game — the correct molecular formula proves nothing about which isomer you have.
2. Purity, with its basis stated. "98%" is not a measurement until it says 98% of what, measured how: by HPLC area percent, by mass balance including residual solvents and water, or by quantitative NMR against a primary standard. The basis determines what the number can be used for.
3. Fitness for the method it will be used in. A standard can be 99% pure and still unusable — if it is hygroscopic, if it is light-sensitive, if it does not dissolve in the method's diluent, or if the counter-ion interferes with the detection.

Where impurity standards come from

There are three practical routes, and they leave different fingerprints on the final documentation:

• Isolation from a production batch — the impurity is already being made; it has to be separated from closely related species, which is where co-eluting isomers and residual parent material appear.
• Targeted synthesis — the route is designed for the impurity. This is the normal route for impurities that never accumulate: the yields are small, the purification is the hard part, and the counter-ion depends on the last step.
• Purification of a purchased material — appropriate for simple degradants and for reference materials that are commercially available at technical grade.

An API standard and an impurity standard are different problems even when the molecules are related. An API standard is a large, stable, well-behaved batch whose content can be assigned with high confidence. An impurity standard is often a few milligrams of a substance nobody has made at scale, and the assignment has to survive a much smaller sample.

What the documentation has to state

At minimum: catalogue number, batch number, identity (name, CAS where one exists, molecular formula and weight), the purity or content value with its basis, the storage condition, the retest date and the release date. If any of those is missing, ask — a certificate that omits the basis of the purity figure cannot be compared with a competitor's certificate. We walk through the fields one by one in How to read a certificate of analysis, and a specimen of our own layout is available on the certificate of analysis page.

Picking the right form

The same impurity can be supplied as a free base, a hydrochloride, a sulfate, a hydrate, or a solvate — and the label has to match what the method assumes. Three consequences that catch people out:

• Weighing. A salt contains the counter-ion by mass. If the method calculates the impurity as free base, the salt's formula weight has to be carried through the calculation.
• Water and solvent content. A hygroscopic material weighs differently on a humid day. The purity basis (as-is versus anhydrous) is what makes two weighings comparable.
• Chromatographic behaviour. A salt and its free base do not elute the same way, and a counter-ion that absorbs in the detection wavelength will show up in the blank.

How much purity do you actually need

It depends on what the standard is for, and the two cases are different:

A buying checklist

1. Identity supported by more than one technique, with the spectra available on request.
2. A purity or content value, and the basis it was measured on.
3. The form stated on the label: free base or salt, anhydrous or hydrate.
4. Storage condition and retest date, not just a shelf life.
5. The batch number on the certificate matching the batch number on the vial.
6. For anything quantitative, an uncertainty budget or an assigned value with traceability — not only a chromatogram.

If a supplier cannot answer questions on items 1, 2 and 6, the material may still be perfectly usable qualitatively. It is not a basis for a quantitative filing.