Is 98.2% a realistic purity ceiling for semaglutide given its chain length?

The specifics, since they change the answer: 98.2% · semaglutide.

I would rather understand the derivation than memorise the outcome.

Two people I asked gave two answers that differ by a factor of ten, which is suggestive.

Can someone show the working rather than just the answer?

To be exact about it, understanding purity requires separating the chemistry from the method from the reporting convention, and the three are not independent.

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.

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

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.

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

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

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

To be exact about it, 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.

The Arrhenius relationship for peptide degradation is the basis of accelerated stability testing and also governs how quickly methods drift with temperature.

I would be careful about over-reading a single measurement — treat it as a data point, not as ground truth.

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

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

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.

The limitation is that single-digit micro-impurities become invisible at typical reporting thresholds, so "no impurities detected" means "none above one in two thousand."

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

The honest answer is that the achievable range of plausible purity figures for a given vial is wider than most people expect.

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

The ICH Q3A impurity thresholds and the relevant pharmacopoeial chapters all specify method validation requirements that almost no research-grade certificate claims to meet.

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

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