Resources
USP vs EP impurity naming, side by side
The same compound can carry an EP impurity letter, a USP related compound letter and a catalogue name. How to map them from structure rather than from the letter.
Published 27 September 2026 · ImpurityLab technical team
Two systems, three labels
The European Pharmacopoeia and the United States Pharmacopeia both identify organic impurities by letter. Both assign those letters independently, against a specific monograph. Neither one has any obligation to agree with the other.
The result is that a single chemical entity can carry a compulsory letter in the European monograph, a completely different letter in the United States Pharmacopeia, and a third name in the catalogue you order it from. None of the three is wrong. They answer different questions.
| Label | Assigned by | Scope |
|---|---|---|
| EP Impurity A | The European Pharmacopoeia monograph for one named substance | Valid only inside that monograph |
| USP Related Compound A | The United States Pharmacopeia monograph for one named substance | Valid only inside that monograph |
| Catalogue name | The supplier | Valid only for that catalogue number |
The letters are not ranks and they are not consistent between monographs. Impurity A of one substance has nothing in common with Impurity A of another, and the letter says nothing about potency, abundance or elution order.
Letters are assigned per monograph, not per molecule
This is the part that causes the most work, and it follows directly from how monographs are written. Impurities are defined inside a monograph for a specific substance. The same molecule appearing in three monographs therefore receives three definitions.
Two examples drawn from the aspirin family show the pattern clearly.
- Salicylic acid. In the acetylsalicylic acid monograph it is designated as an EP impurity. The same molecule also has its own designation in the salicylic acid monograph, and a related-compound designation in the United States Pharmacopeia. Three names, one structure, one catalogue number.
- Salsalate. The compound known as salsalate is at the same time an EP impurity of acetylsalicylic acid and a different EP impurity of carbasalate calcium. Moving from one product to the next changes its identity on paper without changing its structure.
Once you see the pattern, the rule becomes obvious: the letter belongs to the monograph, and the molecule belongs to chemistry. Only the second one travels.
A related trap is edition change. Letters are assigned as new impurities are characterised, so a monograph can gain an impurity letter in a later edition. A method written against edition n can cite letters that edition n+1 defines differently. Record the edition alongside the letter whenever you write the mapping down.
Where the two systems genuinely differ
Beyond the letters, there are real differences in vocabulary, and they surface in the acceptance criteria rather than in the naming.
- Terminology. Both pharmacopoeias use specified and unspecified impurity categories, alongside reporting, identification and qualification thresholds, because both implement the ICH Q3A and Q3B concepts. The wording differs; the structure of the argument does not.
- Impurities with defined and undefined structure. Where a structure is known, the monograph states the chemical name against the letter. Where it is not, the letter is retained with a description of the structure that is known. Both pharmacopoeias do this, and the practical point is the same for you: a letter with no chemical name is not enough to buy against.
- Related compound versus impurity. The wording varies by monograph and by era. Older texts still use letter designations for impurities that newer texts would call related compounds, so age of the monograph is often a better predictor of the wording than the pharmacopoeia itself.
The naming is not the useful comparison. The useful comparison is the acceptance criterion and the qualification threshold, which follow ICH and are therefore broadly aligned across both systems.
Mapping a compound across pharmacopoeias
Four steps, and the order matters, because starting from the letter guarantees a false positive.
- Start from the structure. Chemical name, molecular formula, molecular weight, CAS registry number and structural notation are the identifiers that survive translation. A letter is not an identifier.
- Read the monograph text, not just the acceptance criteria table. The letter is defined in the impurities section, and that section is what tells you which molecule the letter refers to in this edition.
- Check the salt and solvate form. An impurity designated as a hydrochloride in one source may appear as the free base in another. The letter may or may not follow the form.
- Record the mapping in your standard inventory. One line per compound, listing every letter it carries, with the pharmacopoeia and edition. The next person pays the cost of redoing the work if it is not written down.
Where this bites you
Three specific failures, all common, all avoidable.
- Buying two standards for one impurity. A method referencing an EP impurity and a validation report referencing the United States Pharmacopeia related compound, both describing the same structure, ordered separately against two catalogue numbers. The material is financially duplicated and the documentation now has two lots to reconcile.
- Citing an impurity the compendial method does not describe. A filing that names a letter which the current monograph edition assigns to a different molecule. This is a finding, not a typographical error.
- Assuming the elution order. Letters are assigned during characterisation, not by retention time. An impurity method transferred between laboratories with an assumption that A elutes before B will fail specificity in a way that looks like a column problem.
The expensive version of all three is a batch on hold. The cheap version is a column in an inventory spreadsheet: compound identifier, pharmacopoeia, edition, letter, catalogue number.
What to put on the order
If you are specifying a standard for a pharmacopoeial method, give the supplier the structural identity and the acceptance criterion you have to meet, not only the letter. The structure pins the molecule; the letter alone pins a document.
If you are specifying a standard for a developmental method, you already have the structure, and the pharmacopoeial letter is context rather than a requirement. Ask for the certificate to state the chemical name, the formula and the form as supplied, and the nomenclature problem stops being yours to solve.