Purity on the principal peak
High-performance liquid chromatography separates a sample by how strongly its components hold to the column; a detector records each one as it elutes. The purity figure is the area of the principal peak as a fraction of the total peak area the detector integrated — a ratio between the things that were seen.
Material the detector does not register sits outside that ratio entirely. A lot can read 99.8% on peak area and still hold less peptide than its label mass, because water, counter-ions and residual salts are on neither side of the fraction.
Establishes
- That the principal species dominates the detected material in this lot, at the stated ratio.
- A ceiling on every detectable impurity: no single one can exceed the remainder.
Does not establish
- How much peptide is in the vial. Purity is a proportion, not an amount.
- What the remainder is. The figure is a number, not an identification of the other peaks.
- Anything co-eluting under the principal peak, which is integrated into it rather than against it.
- The condition of the material after the report date, through shipping, storage or freeze-thaw.
