Is a 98.8% result from Medutest comparable to 97.1% from PeptideMeter?

Setup, so nobody has to ask: 98.8% · Medutest · 97.1% · PeptideMeter.

I suspect the honest answer is that it depends, in which case I would like to know on what.

Assume I can obtain either option without difficulty, so availability is not the deciding factor.

Is there a defensible reason to prefer one, or is this a coin flip?

Mechanically, gradient slope is the most powerful parameter and almost nobody mentions it, which is why two reports on the same material disagree by a point.

Buffer versus acid in the mobile phase changes the ionisation state of basic and acidic residues, shifting retention and selectivity — same vial, potentially different separation.

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

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

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

Mobile phase additive choice affects ionisation and peak shape — TFA gives sharp peaks but suppresses mass spectrometry signal, formic acid gives worse peaks but preserves signal.

Gradient slope controls resolution, and gentler slopes resolve co-eluting impurities into separate peaks — so the better method reports the worse purity number.

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

The caveat is that purity without identity is only half an answer — a high purity does not mean the peak is actually what you think it is.

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

edited 2 Apr 2025 by grainne_ahearn — added the method parameters

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

Retention time is sequence-specific and method-specific, so comparing your result to a supplier value using a different method is meaningless without method documentation.

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 only pay for one test, pay for quantified content. Purity is the number everyone quotes and content is the number that changes what you do.

The short answer is that two competent laboratories on identical material will disagree, and the disagreement is almost always explainable by method differences.

Detection wavelength matters because 214 nm sees the peptide backbone while 280 nm sees only aromatic side chains — so truncation impurities lacking a tryptophan are invisible at 280 nm.

Inter-laboratory studies on identical peptide material routinely find half-a-per-cent to a full-per-cent spreads in reported purity on the same sample.

In practice: ask for the chromatogram, check the method section, check the lot number against the vial, and set your accept threshold before you see the result rather than after.