Assay, Purity and Content: Three Numbers That Are Routinely Confused

Purity, content and assay are three different quantities. Purity is the proportion of the target among the related substances a separation detects, a ratio with no reference to total mass. Content is the mass fraction of a specified component in the whole material, on a stated basis. Assay is a quantitative procedure — usually a comparison against a reference standard of known content — whose result is a content. A lot can be 97.6% pure and 74.2% target by mass, with no contradiction, because the two figures have different denominators.

The companion reference on net peptide content and gross weight works from the bench outward: the vial holds less peptide than its stated mass, and the correction must be carried into a concentration. This page defines the terms themselves, sets out the arithmetic that connects them, and shows why the three figures on a certificate move independently. Worked figures use an invented decapeptide, AGSKLEYFVR, and describe no real lot.

The three definitions

ICH Q6A treats assay as a specific, stability-indicating procedure for determining the content of the drug substance, and lists impurities as a separate test 1. The separation is deliberate. A purity figure does not contain a content, and a content does not contain a purity, although each can be used to estimate the other when the rest of the composition is known.

Purity: a ratio among related substances

Chromatographic purity is the target peak area divided by the total integrated area of all peaks, multiplied by 100. The calculation assumes that every related substance produces the same detector response per unit mass as the target. At 214 nm, where the peptide bond absorbs, that assumption is reasonable for deletion and truncation sequences of similar length, and weaker for species with a different aromatic content. Where a relative response factor has been established for a named impurity, the area is corrected by it before the ratio is taken.

What purity excludes is the whole point. Water, counterion, residual solvent and inorganic salt do not appear as peaks at the detection wavelength. Neither does anything that does not elute or does not absorb. Purity therefore describes the peptide fraction's quality and says nothing about its quantity. The impurity thresholds in ICH Q3A(R2) — reporting, identification and qualification — are all expressed against this kind of figure, not against content 5.

Content: a mass fraction on a stated basis

Content is a mass fraction: the mass of the specified constituent divided by the total mass of the material 2. The constituent must be named — target peptide, all peptide material, water, counterion — and so must the basis, because the same material gives different fractions depending on what the total includes.

As-is content is what a weighing needs, because the balance weighs the material as is. The dried bases are useful for comparing lots, since water uptake changes as-is content from day to day while the dried value stays put. Water must therefore be determined close in time to the assay; a Karl Fischer figure from the certificate date does not describe a vial opened since.

Assay: content by comparison with a standard

The common chromatographic assay compares the sample's target peak with the same peak from a reference standard of assigned content, run under identical conditions. For single-point external standardisation, the content of the sample is the area ratio multiplied by the mass ratio multiplied by the standard's assigned content: w(sample) = (A(sample) ÷ A(standard)) × (m(standard) ÷ m(sample)) × w(standard). A calibration curve generalises the same relation across a range.

The standard's assigned value carries straight into the result. An error of two percentage points in the standard is an error of two percentage points in every assay calibrated against it, and no replication will reveal it. For peptides, assigning that value is itself demanding: metrology institutes use either a mass balance, subtracting every measured impurity class from 100%, or a direct determination such as amino acid analysis traceable to the SI, and compare the two 4. ICH Q2(R2) likewise treats comparison with material of known purity and comparison with an orthogonal procedure as routes to demonstrating accuracy 3.

The two routes also carry their uncertainty differently. An assay result inherits the uncertainty of the standard's assigned value, adds the weighing of standard and sample, and adds the precision of the chromatography, which for a well-run method is a few tenths of a percentage point. A mass balance inherits the uncertainty of every component it subtracts. Water by Karl Fischer, counterion by ion chromatography, solvent by gas chromatography and residue on ignition each contribute, and the counterion term is usually the largest because it is the biggest subtraction. Neither route is automatically better. The assay is only as good as its standard; the mass balance is only as good as its completeness, since any component nobody measured is silently counted as target.

The mass balance that links them

If every non-peptide component has been measured, target content can be estimated from purity: take the fraction of the material that is peptide, then multiply by the fraction of the peptide that is target. That second multiplication is where purity enters, and it is also where the net peptide content figure fits: net peptide content is the first fraction, and target content is net peptide content multiplied by purity.

The two estimates of target content differ by 0.7 percentage point. Whether that matters depends on the combined uncertainty of both routes, which for a lot like this is typically of the same order. Agreement within uncertainty supports both figures. A larger gap in the direction seen here, with mass balance above assay, is the classic sign of something the purity method cannot see: a non-eluting or non-absorbing impurity, or an impurity with a lower response factor than the target. A gap the other way points to the reference standard.

Converting the assay figure to other bases: on an anhydrous basis, 74.2 ÷ (1 − 0.061) = 79.0%. On an anhydrous, counterion-free basis, 74.2 ÷ (1 − 0.061 − 0.164) = 95.7%. All three describe the same lot. The last is close to the purity figure, which is expected, and it is also the figure most easily mistaken for purity when the basis is omitted.

Why the numbers diverge on real samples

How each appears on a certificate

Certificates use the words loosely, so read the method and the basis rather than the label. A line reading assay 97.6% by HPLC area is a purity figure under a different name. A line reading peptide content 76.7% is net peptide content, which includes related impurities. A line reading content 74.2% w/w against a reference standard, as is, is a true assay result.

1. For each figure, identify the denominator: related substances, all peptide material, or the whole material.
2. For every content or assay figure, find the basis. If none is stated, treat it as as is and record the assumption.
3. For an assay, find the reference standard and how its value was assigned. An in-house standard of unstated origin limits the result.
4. Check whether water and counterion were measured, and when. Content figures older than the last opening of the container may not apply.
5. Where water, counterion, solvent and purity are all given, compute the mass balance and compare it with the stated assay.
6. Record the three figures separately in the laboratory's own record, each with its method and basis. Never write one into the field meant for another.

Held apart, the three numbers are complementary: purity describes what the peptide is, content describes how much of it there is, and assay is the measurement that makes the second one traceable 4. Conflated, they produce a single figure that answers none of those questions, and the error goes unnoticed until two lots behave differently at the same nominal concentration.