Nitrosamine and NDSRI standards: what to test for and what to buy

Two families, two different problems

“Nitrosamine” covers two structurally different groups, and the distinction decides what you have to buy.

Small nitrosamines — N-nitrosodimethylamine (NDMA), N-nitrosodiethylamine (NDEA) and their close relatives — are small, volatile molecules. They are not part of the drug substance; they arise from process chemistry, from solvents and reagents, or from packaging and storage. They are typically determined by headspace GC-MS/MS or by LC-MS/MS after extraction.

NDSRIs — nitrosamine drug substance-related impurities — are the secondary-amine-containing drug substance or its close analogues that have themselves been nitrosated. They are large, non-volatile, and structurally specific to one API: the nitrosated form of the same molecule the method is already measuring. They are determined by LC-MS/MS, and they usually have no catalogue entry anywhere until someone makes them.

The reason the distinction matters commercially: a laboratory that already holds NDMA and NDEA has covered the first family and none of the second. The second family is a per-API problem, which is why NDSRI work so often ends in a custom synthesis rather than an order.

What a laboratory actually needs on the shelf

• The small nitrosamines as certified standards, in solution or as neat material, with a stated purity and — for quantitative work — a stated uncertainty. NDMA and NDEA are the two that appear in most method scopes.
• The labelled analogue of each small nitrosamine used for quantitation. Isotope dilution with a deuterated internal standard is the usual way to control for the extraction and ionisation losses these methods suffer from. See the labelled standards held by drug substance.
• The NDSRI itself, for the specific API in scope. Either the nitrosated drug substance or the nitrosated analogue that the risk assessment identified.
• A labelled NDSRI where the method is quantitative and the matrix is difficult. This is the least available item on the list, and the one most often left out of a project plan.

Why “purity by area percent” is not enough here

Nitrosamine methods are usually limit tests or trace assays, and the result is a number with a regulatory consequence. A certificate that states only a chromatographic purity by area normalisation tells you what fraction of the peak area was the main peak. It does not tell you the mass fraction of the analyte, and it does not give you the uncertainty of the assigned value. For a limit test where the acceptance criterion is expressed as an absolute amount, that difference is the whole measurement.

What to look for on the certificate: the basis on which the value was assigned (mass balance and/or quantitative NMR are the two common routes), the expanded uncertainty with its coverage factor, and the traceability statement. If a supplier cannot state the basis, the value cannot be defended in review.

Specifying an NDSRI correctly

An NDSRI enquiry usually fails on description rather than on chemistry. The information that decides whether a quotation is accurate:

• The parent drug substance, by name, and the CAS number of the drug substance.
• The site of nitrosation, if the risk assessment identified one — a secondary amine in a side chain behaves quite differently from one in a ring.
• The salt or free-base form required. A nitrosamine is usually requested as the free nitrosamine, but the counter-ion in the drug substance still affects the route.
• The quantity and whether solution presentation is acceptable. Many laboratories want a pre-made solution at a stated concentration, which removes a weighing step at trace level.
• The intended use: identification, limit test, or quantitative assay with uncertainty statement. The third is a different product from the first two.

A structure drawing or the SMILES string shortens this considerably. Where the impurity has no CAS number — which is common for NDSRIs — a structure is the identifier.

Practical traps

• Assuming one nitrosamine standard covers the class. Each NDSRI is specific to its API; a generic nitrosamine standard cannot substitute for it.
• Leaving the labelled standard out of the plan. It is often the longest lead item, and the method cannot be validated quantitatively without it.
• Using an unlabelled standard for quantitation by external calibration where the method suffers matrix effects, then attributing the recovery problem to the instrument.
• Overlooking formation in the sample. Nitrosamines can form during sample preparation under acidic conditions in the presence of nitrite. A standard cannot correct for an artefact created downstream of it — that is a method question.
• Storing solutions too long. Small nitrosamines are volatile and solutions can change concentration; check the supplier’s stated shelf life and handling for the presentation you buy.

Where to start

Send the parent drug substance and, if you have it, the nitrosation site from the risk assessment. If the compound is not in any catalogue, that is the normal case for NDSRIs rather than an obstacle: non-compendial impurities are quoted from a structure, and a CAS number is not required to quote one. Write to [email protected] with the structure or SMILES.