Choosing the wrong impurity standard can cost weeks: a peak that does not match, a CoA that an auditor questions, or a material that degrades before your validation is complete. This checklist helps QC managers, analysts and procurement teams buy the right impurity reference standard the first time.
1. Confirm exactly which impurity you need
Impurity names are confusing. The same compound may be called "Impurity A" in one pharmacopoeia, "Related Compound B" in another and have a different chemical name in your DMF. Before you ask for a quote, collect:
- The chemical name and structure.
- The CAS number, if one exists (many impurities have none).
- The pharmacopoeial name (IP, USP or EP) if your method follows a monograph.
- The molecular formula and molecular weight.
Matching on structure, not just on name, avoids the most common ordering mistake. You can search our catalogue by name or CAS number.
2. Decide what you will use it for
The intended use decides how much characterisation you need:
| Use | What matters most |
|---|---|
| Peak identification (qualitative) | Confirmed identity (NMR, MS); purity high enough to give one clear peak |
| Quantification and RRF determination | Accurate purity or potency value on the CoA; known water and solvent content |
| Method validation | Full CoA, consistent batch, enough quantity for all validation experiments |
| Trace analysis by LC-MS/MS (for example nitrosamines) | Identity, purity and a matching isotope-labelled internal standard |
3. Check the Certificate of Analysis before you buy
Ask for a sample CoA. A good CoA for an impurity standard should include identity data (1H NMR and mass spectrum at minimum), chromatographic purity, the batch number, storage conditions and a retest date. For quantitative work, look for water content and residual solvents, or an assigned potency. We cover this in detail in What should a CoA for an impurity standard contain?
4. Purity: how high is high enough?
Higher is not always necessary, but the purity must be known. For identification, a clean main peak is enough. For quantification, what matters is an accurate purity or potency value so you can correct your calculations. Be cautious of a CoA that shows only ">95%" with no chromatogram.
5. Salt form and free base
If your API is a salt (for example a hydrochloride), the impurity standard does not have to be the same salt. In reversed-phase HPLC the mobile phase controls how the molecule is ionised, so the salt and free base give the same peak. For quantification, correct for the molecular-weight difference or use the potency given on the CoA.
6. Stereochemistry and isomers
For chiral drugs, make sure you are buying the correct enantiomer or diastereomer. An "(R)" impurity and its "(S)" counterpart may separate on a chiral column but co-elute on a standard C18 column. Check the stereo descriptors in the name and structure on the CoA.
7. Quantity: plan for the whole project
Impurity standards are normally sold in milligram packs. Estimate how much you need for method development, validation (specificity, LOQ, linearity, accuracy, robustness) and future stability time points. Buying one larger batch keeps all your results on the same lot, which makes data easier to defend.
8. Stability and storage
Many impurities, especially degradation products, are themselves unstable. Check the recommended storage temperature, protection from light and moisture, and the retest date. Plan to use the material within that period, and store it exactly as instructed.
9. Isotope-labelled internal standards for LC-MS
For LC-MS/MS methods, a stable isotope-labelled version of the analyte (for example NDMA-d6 for NDMA) corrects for matrix effects and recovery losses. Ask for the labelled and unlabelled standards together. Browse our isotope-labelled compounds.
10. Choose a reliable supplier
- Catalogue depth: one supplier for most of your impurities saves time and keeps documentation consistent.
- Custom synthesis: for impurities that are not available off the shelf.
- Documentation: consistent CoA format with spectra on request.
- Quality system: for example GMP certification and an ISO 9001 quality management system.
- Delivery in India: short transit times and proper packaging (cold chain where needed) protect sensitive materials.
- Responsiveness: quick quotations and technical answers.
Quick checklist
- Structure, name and CAS number confirmed
- Intended use defined (identification, quantification, validation, trace analysis)
- Sample CoA reviewed: identity, purity, batch, storage, retest date
- Salt form and stereochemistry checked
- Quantity planned for the whole project on one lot
- Storage conditions understood
- Labelled internal standard ordered if using LC-MS
- Supplier's quality system and delivery checked
Send us your impurity list and we will match each compound to our catalogue of 5,000+ products or offer custom synthesis. Request a quote.