The case in front of me: 96.4% · oral semaglutide.
Please show the division. I want to check my own against yours.
I would like the general form as well as the specific number, so I can apply it again.
How many significant figures are actually justified here?
96.4 per cent is at the top of what stepwise synthesis delivers on a chain this long, and it is reachable rather than fictional. Every coupling is high-yielding and none is quantitative, so the deletion and truncation sequences that survive purification are what occupies the remaining 3.6 per cent. Above roughly 98 per cent you are fighting the purification rather than the synthesis, which is why a 96.4 per cent figure on oral semaglutide deserves a method question — column, gradient, wavelength — rather than either belief or dismissal.
The part that matters: understanding purity requires separating the chemistry from the method from the reporting convention, and the three are not independent.
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.
It helps to be literal here: 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.
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.
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsArea percentage is not mass percentage, and conflating the two is the most common misreading of a purity figure.
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.
Tailing factor measures peak shape, and a badly tailing peak spreads into the region where small impurities live, forcing tangent-skim integration that assigns tail area to the main peak.
The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.
In practice, identity and purity are orthogonal, and a high purity says almost nothing about whether the peak is actually what you think it is.
Gradient slope controls resolution, and gentler slopes resolve co-eluting impurities into separate peaks — so the better method reports the worse purity number.
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.
If you are ranking vendors, specify a method and have all samples tested at the same place.
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.
Mass on column affects detector linearity and peak overlap — overloading broadens peaks and hides neighbours, while underloading improves resolution but loses sensitivity.
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 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.
Purity is a method-dependent figure, and that is not a limitation of the measurement, it is a property of what the measurement actually answers.
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.
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.
Worth noting that method standardisation is poor in the research-grade space compared to pharmaceutical work, so identical-looking methods can produce different results.
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.
edited 24 Jun 2024 by Dr_Signe_Baldursdottir — added a caveat about sampling
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