Purity vs Fill Accuracy: Why One Number Is Not Enough

Purity is determined chromatographically. A sample is separated into components, and the target compound is reported as a percentage of the total detected peak area. The result is a ratio — it describes composition, and it is scale-free. A sample of 10 mg and a sample of 1 mg with identical composition return the same purity figure.

Content is determined by a quantitative assay against a reference standard. It returns a mass — an absolute quantity of compound found in the vial. Comparing that mass to the amount printed on the label gives fill accuracy as a percentage.

Because one is a ratio and the other is a quantity, neither constrains the other. A vial can be highly pure and underfilled, correctly filled and impure, or fail on both counts. Reporting only the ratio leaves the quantity entirely unstated, which is why a supplier can publish a completely accurate purity result for a vial that is substantially short.

Consider two lots, each labelled 10 mg, each advertised as 99%+ pure. Both claims are true of both lots. Only one of the two lots gives the buyer what the label promised.

These figures are illustrative. They are constructed to show the arithmetic, and they are not any competitor's published result. The point is not that some named supplier ships short vials — it is that a certificate reporting purity alone would rate these two lots as equivalent, because on the only measurement it reports, they are.

• ·Lot A — advertised 99%+ pure · measured purity 99.7% · expected content 10.00 mg · measured mass 10.98 mg · fill accuracy 109.8%
• ·Lot B — advertised 99%+ pure · measured purity 99.8% · expected content 10.00 mg · measured mass 7.10 mg · fill accuracy 71.0%
• ·Lot B has the higher purity figure and contains roughly a third less compound

There are three straightforward reasons, and none of them are conspiratorial.

The first is cost. Quantitative content analysis is a separate test from a purity screen, priced separately, and running it on every lot rather than periodically is a standing expense that does not show up anywhere the customer can see unless the supplier chooses to show it.

The second is that the result is sometimes unflattering. Fill variance is a normal feature of lyophilization at small scale, and a supplier committed to publishing content on every lot will eventually publish a lot that came in under label. Publishing only purity avoids that conversation permanently.

The third is competitive. Purity above 99% is a claim every supplier in this market makes, which makes it safe: it differentiates nobody and therefore exposes nobody. A supplier who starts publishing content invites the comparison, and the comparison is only favourable if the underlying numbers hold up.

The useful questions are narrow and answerable, and none of them require trusting the answer — each can be checked against published documents.

Note that the last question is the one that separates verification from marketing. Publishing content only when it is favourable is a selection effect, and a set of results filtered for flattery carries no more information than publishing nothing at all.

• ·Is a measured mass published alongside purity for the lot I would receive?
• ·Is the expected content stated, so fill accuracy can actually be computed?
• ·Is the figure tied to a lot code I can match against the vial?
• ·Are lots that came in below 100% of label published on the same terms as the rest?

Every Eppix lot is independently assayed before release, and both figures are published for it: purity, and measured mass against expected content, with fill accuracy computed from the two. They appear on the product page before anything is added to a cart, and on that lot's own batch report, with the issuing laboratory's own record linked from it.

Lots that come in under 100% of label are published on the same terms as the rest. That is the part of this position that costs something, and it is also the only part that makes the rest of it worth anything — a supplier who publishes only favourable numbers is exactly where everyone in this market already is.

If you are holding a vial, the batch lookup takes the lot code printed on it and returns that lot's figures directly.