Would you accept 96.8% on liraglutide from QSC without a content assay?

The case in front of me: 96.8% · liraglutide · QSC.

I want to decide this in advance so that I am not deciding it under pressure later.

Assume I will follow the plan I write down, so I would like it to be a good one.

What would you do, and what would make you change course?

96.8 per cent purity with no content figure leaves the milligram number unmeasured. Purity says 96.8 of every 100 units of detected area is liraglutide and 3.2 is something else. Content says how many milligrams are in the glass. The two do not constrain each other: a vial can be 96.8 per cent pure and still be under label, because water and counter-ion are part of the gross mass and neither shows up as an impurity peak. If you buy one test, buy the one that changes your arithmetic.

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

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.

Reconciling gross mass to label claim

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.

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.

The method matters more than the vial, which is why specifying a method buys you far more than changing suppliers does.

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.

To be exact about it, 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.

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

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.

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.

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

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.

Worth being precise here: reporting threshold is convention and not chemistry, which is why two certificates with different thresholds disagree by a tenth of a point or more.

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.

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

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.

edited 4 May 2025 by loss_on_drying — added a caveat about sampling

To be exact about it, area percentage is not mass percentage, and conflating the two is the most common misreading of a purity figure.

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

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.