What does a 99.4% purity with a list of related substances attached let me conclude?

The specifics, since they change the answer: 99.4% · list of related substances.

I have the document in front of me and I can read the numbers. What I cannot do is interpret them.

I am reasonably comfortable with statistics and completely uncomfortable with chromatography, or vice versa.

What is the correct interpretation, and what is the common misreading?

Start from what the detector sees, because that tells you what the number means.

Temperature affects the dynamics of molecular conformation, and if a peptide has proline residues that interconvert on the chromatographic timescale, the peak will split or shoulder at low temperature and collapse at high temperature.

Mechanically, mass on column affects detector linearity and peak overlap — overloading broadens peaks and hides neighbours, while underloading improves resolution but loses sensitivity.

Published side-by-side method comparisons show that a two-point difference in purity on the same vial is easily explained by method choice alone.

If you are ranking vendors, specify a method and have all samples tested at the same place.

Understanding purity requires separating the chemistry from the method from the reporting convention, and the three are not independent.

The fraction of your main peak that is actually your target versus isomers, fragments or related sequences is invisible without complementary identity testing.

Sample solvent strength affects peak shape — if you inject in strong solvent on a gradient starting in weak solvent, the solvent peak can distort your main peak or create a false shoulder.

Proline conformer interconversion kinetics are well-characterised and the half-life is of the same order as the chromatographic peak width at room temperature.

Compare purity within a single laboratory on the same method, never across laboratories.

Identity and purity are orthogonal, and a high purity says almost nothing about whether the peak is actually what you think it is.

Integration of the shoulder — whether you use perpendicular drop or tangent skim — determines what area gets assigned to the main peak versus the impurity table.

Worth noting that method standardisation is poor in the research-grade space compared to pharmaceutical work, so identical-looking methods can produce different results.

The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.

edited 30 Aug 2024 by vialroom — expanded the table to cover the lower concentration

Concretely, area percentage is not mass percentage, and conflating the two is the most common misreading of a purity figure.

Column pore size affects mass transfer — a 100 Angstrom packing on a 5 kDa peptide restricts diffusion, broadening the peak and potentially hiding small impurities in the shoulders.

One qualification: achieving purity above roughly 98 per cent on a 30-residue peptide is fighting the chemistry of synthesis, not the quality of the purification.

If you are ranking vendors, specify a method and have all samples tested at the same place.