Residual solvents and elemental impurities sit outside the ordinary limit

An impurity profile on a certificate of analysis usually reads as one list: the specified related substances, any unspecified ones, and a total. Two groups of impurity rarely sit in that list, and a buyer who reads only the related substances section can conclude that a batch is clean without having looked at them at all. Residual solvents and elemental impurities are controlled under their own guidelines, tested by their own methods and reported in their own sections, sometimes on a separate document entirely.

The separation is not administrative. These two groups differ from ordinary impurities in where they come from, in how their risk is judged, and in what a passing result can and cannot tell you. Reading them properly starts with those differences.

Why are residual solvents and elemental impurities controlled separately?

Ordinary organic impurities are related to the drug substance itself: starting materials, intermediates, by-products and degradation products of the chemistry that made it. Their acceptance criteria are set with reference to that substance, its dose and the safety evidence for each impurity, and they are usually separated and measured by the same chromatographic method that assays the API.

Residual solvents and elemental impurities are not related to the molecule. Solvents are left over from the process: from reaction steps, from crystallisation, from washing and drying the solid. Elements may be added on purpose, as catalysts, or arrive by accident, from reagents, process water, equipment surfaces and container contact. Their toxicity belongs to the solvent or the element, not to the drug, so the same solvent carries broadly the same concern whichever API it turns up in. That is why each group is assessed on its own toxicological basis, under its own ICH guideline, rather than folded into criteria derived from the drug substance. The major pharmacopoeias have taken the same approach into their general chapters.

How is the risk of a residual solvent judged?

Solvents are grouped by how much harm they can do, from those that should be avoided in manufacture altogether to those regarded as low risk. The acceptable level of each one starts from a permitted daily exposure, which is then translated into a concentration in the material using the daily dose of the medicine it ends up in. Two consequences follow for a buyer. The same result can be acceptable for one product and not for another, because the dose differs. And a solvent in the group that should be avoided has no business appearing in a profile without a written justification.

The groupings and the acceptable concentrations are set in the current guideline and pharmacopoeia, and they are revised from time to time. Check them against the edition that applies to your own market and dosage form rather than relying on a figure quoted on a certificate.

Which solvents should the certificate report?

A residual solvents test finds only what it is set up to look for. The useful question is not whether the material passed, but which solvents were tested, and whether that list matches the solvents actually used in the route of synthesis, including those in the final steps and in purifying the solid. A certificate that reports two solvents when the process uses four has demonstrated nothing about the other two. If the route has changed since the specification was written, the list should have changed with it.

How are elemental impurities assessed?

Elemental impurities are approached by risk assessment rather than by testing a fixed list on every batch. The manufacturer considers each potential source: catalysts deliberately added, elements carried in with reagents and solvents, water, equipment, and the container the material is stored in. For each element that could plausibly be present, it weighs the likely level against the permitted daily exposure for the intended route of administration. The outcome may be routine testing for certain elements, periodic testing, or a documented conclusion that the assessment itself is sufficient control.

What the risk assessment should tell you

A buyer should be able to see a summary of that assessment, or at least its conclusions: which elements were considered, which sources were identified, and why each element is or is not tested. A certificate that says complies, with no indication of what it complies with, gives you nothing to check. The same goes for a line reporting a general heavy metals test. Several major pharmacopoeias have moved away from that older, non-specific test towards element-specific methods, and a certificate still relying on it raises a fair question about which edition of the monograph the material was tested against.

The finished product carries the final responsibility

The elemental impurities guideline is framed around the finished drug product. The API is one contributor among several, alongside excipients, water, manufacturing equipment and packaging. The manufacturer of the finished product therefore needs data from the API supplier to complete its own assessment, and an API supplier that cannot provide it leaves a gap in the buyer's file.

What should you ask a supplier about solvents and elements?

Ask for the list of solvents used in the process, at least in the final steps, and compare it with the solvents reported on the certificate. Ask whether any metal catalysts are used, and how their removal is demonstrated. Ask for the elemental impurities risk assessment summary, or the statement the manufacturer prepares for customers. Ask which edition of which pharmacopoeia, and which guideline, the specification follows. Where a drug master file or a certificate of suitability exists, much of this sits in the documents described in our note on DMF and CEP documentation.

Where the material comes through a trading or distribution company, the answers still concern the site that made it. They should be passed through intact, not summarised into a general assurance that the material meets the pharmacopoeia.

Residual solvents and elemental impurities are read in the same pass as the rest of the profile, as set out in our guide to reading an API impurity profile. We check both on every certificate as part of our API sourcing work, and they read naturally alongside what a pharmacopoeial grade does and does not tell you.

NJMC Medical Supplies Co., Ltd is a trading and consultancy company in Nanjing, China, supplying medical equipment, medical consumables and active pharmaceutical ingredients to healthcare institutions, sourced from manufacturers in China and India.

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