Ask three chemists what "99 percent" means on a label and you may get three answers. The figure could be chromatographic purity, a titration assay or a biological potency. Each is legitimate, each answers a different question, and mixing them up leads to wrong concentrations, failed specifications and confused supplier conversations.
Purity: what else is in there?
Purity describes how much of the material is the target compound relative to impurities. In practice the term is most often used for chromatographic purity, the area of the main peak as a percentage of all peaks detected by HPLC or GC.
Chromatographic purity is useful, but it has blind spots:
- It only sees what the detector sees. Water, inorganic salts and many solvents are invisible to a UV detector.
- Area percent assumes every compound gives the same response per unit mass, which is rarely true. An impurity with a strong chromophore can look larger than it is, and one with a weak chromophore can be underestimated.
- Impurities that co-elute with the main peak or never leave the column are not counted.
So a material reported as 99.8 percent by HPLC area may contain 3 percent water and still look excellent on the chromatogram.
Assay: how much is actually there?
Assay is a quantitative measurement of the content of the target substance in the material as a whole, usually expressed as percent by weight. It is determined against a reference, for example:
- Titration, such as acid-base, redox, complexometric or non-aqueous titration.
- HPLC or GC against a reference standard of known content.
- Quantitative NMR against a certified internal standard.
Assay captures water, salts and other non-detected material because they dilute the result. That is why a material can show 99.8 percent purity and 96.5 percent assay at the same time.
Why assay can exceed 100 percent
Specifications such as 98.0 to 102.0 percent are common, especially in pharmacopoeial monographs. Titration methods have finite precision, and some are not fully specific, so an impurity that also reacts with the titrant can push the result above 100. A result of 100.6 percent is not impossible, it simply reflects method variability and specificity.
Potency: how active is it?
Potency is used when the property that matters is biological or functional activity rather than mass. It is common for antibiotics, enzymes, hormones and some biologics, and is often expressed in international units, units per milligram or micrograms of activity per milligram.
Two lots of the same antibiotic may have the same chemical purity but different potency, because closely related components contribute different amounts of activity. In such cases the potency value, not the purity, is used to calculate how much material to weigh.
For reference standards, suppliers often state an assigned content or certified value. This is effectively an assay figure, established by mass balance, quantitative NMR or comparison with another standard, and it is the number to use in calculations.
The basis question
Every assay or potency figure has a basis, and it must be stated:
- As is: the result applies to the material exactly as it sits in the bottle.
- Dried basis: corrected for loss on drying.
- Anhydrous basis: corrected for water content, usually by Karl Fischer.
- Solvent-free basis: also corrected for residual solvents.
To convert an anhydrous assay to an as-is value, multiply by (100 minus water percent) divided by 100. An anhydrous assay of 99.5 percent with 4.0 percent water corresponds to about 95.5 percent as is.
Mass balance as a sanity check
A mass balance estimate combines the separate measurements:
Content = (100 minus water minus residual solvents minus inorganic residue) multiplied by chromatographic purity divided by 100
For a reference standard, this approach is often the basis of the assigned value. For a routine material, comparing the mass balance figure with a direct assay is a quick way to spot an inconsistent certificate.
Which number to use
- Preparing a quantitative standard solution: use the assigned content or assay on the as-is basis, or convert it to as is.
- Judging suitability for synthesis: chromatographic purity tells you about by-products, while assay and water content tell you how much reagent you are really adding.
- Dosing biologically active materials: use potency.
- Comparing suppliers: compare like with like. A 99 percent HPLC purity figure from one supplier and a 98 percent titration assay from another are not in conflict.
A worked comparison
Consider one lot of an amine hydrochloride with the following results on its certificate:
- HPLC purity: 99.6 percent area
- Water by Karl Fischer: 1.8 percent
- Residual solvents: 0.3 percent
- Residue on ignition: 0.1 percent
- Assay by non-aqueous titration, anhydrous basis: 99.4 percent
The mass balance estimate is (100 minus 1.8 minus 0.3 minus 0.1) multiplied by 0.996, which gives about 97.4 percent. Converting the anhydrous titration assay to an as-is basis gives 99.4 multiplied by 0.982, about 97.6 percent. The two approaches agree closely, which supports the certificate.
If the analyst had used the 99.6 percent HPLC figure to prepare a standard solution, every result would have been biased by more than 2 percent. That is the practical cost of mixing up the three numbers.
Questions to ask a supplier
- Is the headline figure purity, assay or potency?
- Which method was used, and against what reference?
- On which basis is it reported?
- Are water and residual solvents measured on the same lot?
Zemixal certificates state the method and basis for quantitative results. If a figure is unclear, write to contact@zemixal.com with the lot number. Terms such as loss on drying and residue on ignition are defined in the chemistry terms glossary, and the quality page describes the documentation supplied with each lot.