Custom Synthesis of Impurities: From Structure to Characterised Standard

Custom synthesis of impurities is the deliberate preparation of a specific process impurity, degradation product or mutagenic impurity, followed by purification and full characterisation, so that it can serve as a reference standard for identification, method validation and routine quantification. It is needed when no pharmacopoeial or commercial standard exists, or when the available material is too impure or too poorly documented to support your specification. This article walks through the process from a proposed structure to a characterised standard with a usable certificate of analysis.

When you need a custom-synthesised impurity

ICH Q3A(R2) for new drug substances sets the thresholds that drive most requests. Once an impurity exceeds the identification threshold it must be structurally characterised, and if it exceeds the qualification threshold its biological safety must be established at the specified level.

Source: ICH Q3A(R2), Attachment 1. Drug products follow the equivalent thresholds in ICH Q3B(R2).

Typical triggers for synthesis are:

• Confirming a proposed structure. An unknown peak has been assigned a structure from LC-MS and NMR data on a crude or enriched sample. Synthesising the proposed compound and matching retention time and spectra is the most convincing confirmation.
• Specified impurities in a specification. Quantifying a specified impurity against its own standard, or determining a relative response factor, needs well-characterised material. ICH Q3A notes that where the response factors of the drug substance and the impurity are not close, the drug substance can still be used as a standard only if a correction factor is applied or the impurity is in fact overestimated.
• Method validation. ICH Q2(R2) describes specificity and accuracy through spiking known impurities. Where impurities cannot be obtained, an orthogonal procedure and justification are needed, which is usually more work than making the compound.
• Mutagenic and nitrosamine impurities. Under ICH M7(R2), an impurity without carcinogenicity data but with mutagenic concern is commonly controlled at a threshold of toxicological concern of 1.5 µg/day. Trace methods at these levels need authentic standards, often including N-nitroso derivatives of the API itself.

From structure to synthetic route

There are three ways to obtain an impurity, and the right choice depends on its origin and the quantity needed.

Route design

For targeted synthesis, the chemist starts from the API route. Most process impurities arise from a known step: an unreacted intermediate, a regioisomer from a non-selective alkylation, a dimer, a des-halo or over-oxidised analogue. Starting from the API or a late intermediate often gives the shortest route; a des-methyl impurity might be made by selective demethylation, or an N-oxide by controlled oxidation. When the impurity is structurally remote, a fresh route from commercial building blocks is cleaner, because it avoids carrying traces of the API into the standard.

Points to settle before work starts:

• Stereochemistry. If the API has stereocentres, specify whether you need a single isomer, a diastereomer or a racemate.
• Salt or free form. The form should match how the impurity behaves in your method, or be clearly stated so potency can be corrected.
• Quantity. Identification and spiking may need tens of milligrams; a standard for routine release testing over several years may justify gram quantities.
• Stability. Reactive impurities such as aldehydes, epoxides or some N-nitroso compounds may need inert, cold storage and solution stability data.

Purification to standard grade

Reference-grade material usually needs a chromatographic purity well above what is often accepted for research chemicals, because any impurity in the standard biases the result for the impurity you are measuring. Flash chromatography removes bulk by-products; preparative HPLC resolves close-eluting isomers; crystallisation or salt formation improves purity and gives a more stable solid. Each step is monitored by HPLC using a method close to the one in which the standard will be used, so that critical pairs are known to be resolved. Residual purification solvents and water must be removed as far as practical and then measured, since they reduce the content of the impurity in every milligram weighed.

The characterisation package

ICH Q3A(R2) asks that reference standards used to control impurities be evaluated and characterised according to their intended uses. ICH Q7 (section 11.18) says that where no standard is available from an officially recognised source, an in-house primary standard should be established with appropriate testing to establish fully its identity and purity. In practice the package for an impurity standard looks like this.

For a newly synthesised impurity, identity rests on the spectra agreeing with each other and with the proposed structure: integrals that match the proton count, HMBC correlations that place substituents unambiguously, and a mass consistent with the molecular formula. See how 1D and 2D NMR supports this, and our article on impurity reference standards for what a complete certificate of analysis should contain.

Common characterisation pitfalls

• Relying on mass spectrometry alone: positional isomers share the same mass and often the same fragments, so only NMR (or a matched authentic sample) distinguishes them.
• Reporting purity from a single HPLC method that was never shown to separate the likely isomers or the API from the impurity.
• Ignoring hygroscopicity: a standard that picks up water after opening drifts in potency, so storage and handling instructions belong on the certificate.
• Assuming the salt stoichiometry rather than measuring it, for example by NMR integration of the counter-ion or by elemental or ion analysis.

Assigning purity and potency

Chromatographic purity is not the same as content. A standard that is 99% pure by HPLC area but holds 2% water is over-assigned by roughly 2% if purity is used directly. Two approaches are common.

Mass balance

Potency (%) = HPLC purity (%) × [100 − water (%) − residual solvents (%) − residue on ignition (%)] / 100

Worked example: HPLC purity 98.6%, water 0.8%, residual solvents 0.3%, residue on ignition 0.1%. Potency = 98.6 × (100 − 1.2) / 100 = 98.6 × 0.988 = 97.4%. This assumes all organic impurities respond similarly to the analyte at the detection wavelength, which should be checked where response factors may differ.

Quantitative NMR

qNMR measures the analyte directly against a weighed internal standard of known purity:

P_x = (I_x / I_std) × (N_std / N_x) × (M_x / M_std) × (m_std / m_x) × P_std

where I is the integral, N the number of protons giving the signal, M the molar mass, m the weighed mass and P the purity. It needs a well-resolved signal, adequate relaxation delay and accurate weighing, and it is valuable for scarce material and as a cross-check on mass balance. Agreement within about one percentage point between the two approaches gives good confidence in the assigned value.

Briefing a custom synthesis project

A clear brief saves weeks. Send the supplier:

1. The structure (ideally as a structure file or SMILES), with stereochemistry and salt form defined.
2. How the structure was proposed: LC-MS data, relative retention time, and the API route step thought to generate it.
3. Quantity required and intended use (identification, relative response factor, validation spiking or routine standard).
4. Minimum purity and the characterisation tests you need on the certificate of analysis.
5. Your HPLC method conditions, so purity is reported in a comparable system.
6. Any known safety concerns, especially for suspected mutagenic or highly potent compounds.

Labelled compounds

Stable-isotope-labelled analogues (deuterium or ¹³C) are used as internal standards in LC-MS and GC-MS methods for trace impurities, correcting for matrix effects and recovery. Label position matters: deuterium on exchangeable sites such as OH or NH is lost in protic solvents, and the mass shift must be large enough to avoid overlap with the natural isotope pattern of the unlabelled compound. Check isotopic purity, as unlabelled material in the internal standard adds directly to the analyte signal. For N-nitroso impurities, see our overview of nitrosamine impurities.

How SDS Labs can help

SDS Labs offers custom synthesis of impurities and labelled compounds alongside small-scale synthesis of organic compounds and labelled APIs, with impurity profiling and problem-solving studies. Characterisation is carried out in-house on a 400 MHz FT-NMR (1D and 2D experiments including qNMR), LC-MS, MS, FT-IR, HPLC, GC, TGA and DSC, under ISO/IEC 17025:2017 and ISO 9001:2015 quality systems.

You can also search our impurity catalogue for in-stock standards. To discuss a structure, use our contact page, call +91 90295 20144 or email info@sdslabs.in.

Frequently asked questions

References

• ICH Q3A(R2) Impurities in New Drug Substances (ICH)
• ICH Q7 Good Manufacturing Practice Guide for Active Pharmaceutical Ingredients (ICH)
• ICH Q2(R2) Validation of analytical procedures (EMA)
• M7(R2) Assessment and Control of DNA Reactive (Mutagenic) Impurities in Pharmaceuticals (US FDA, July 2023)
• ICH M7(R2) Addendum: compound-specific acceptable intakes (EMA)