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Why purity and quantity are not the same thing

In brief

Purity answers "what share of the contents is the declared substance"; quantity answers "how much of it is in the vial". A vial labelled "10 mg" at 99% purity can contain 6 mg of peptide — and both figures are formally honest. They are different measurements by different methods and must be confirmed separately.

Illustration for the research article

Two different questions

Purity is a relative property of composition: the main substance's share of everything the analytical method registered. Quantity is an absolute property: the mass of substance in a specific vial. Neither can be derived from the other.

Example: a vial holds 6 mg of 99%-pure peptide against a declared 10 mg. The purity report is impeccable, while a 40% underfill stays invisible if nobody measured quantity.

Why the declared mass can differ from the actual

A lyophilised peptide is not a pure substance on a scale: vial mass includes water, residual salts, and counter-ions. Some manufacturers also state gross mass before correcting for peptide content. The label and the actual amount of active substance are different values until analysis connects them.

How quantity is measured

Peptide quantity is assessed by dedicated methods: quantitative amino-acid analysis, nitrogen analysis, calibrated UV spectrophotometry, or corrected gravimetry. Serious documentation reports "peptide content" separately from purity — a good sign when you see it.

The practical takeaway

When reading documentation, look for both figures. If only purity is documented, quantity remains unconfirmed — and vice versa. The Moscow Peptides catalogue always treats these as independent fields with their own verification statuses.

References

  1. Electrospray Mass Spectrometry and Tandem Mass Spectrometry of Synthetic Multicomponent Peptide Mixtures: Determination of Composition and Purity — Analytical Biochemistry, 219(2), 261–277 (PMID 8080083; DOI 10.1006/abio.1994.1266), 1994.
  2. Reversed-Phase High-Performance Liquid Chromatography in Peptide and Protein Analysis — Encyclopedia of Analytical Chemistry (DOI 10.1002/9780470027318.a1631), 2000.