Would you accept 99.4% on liraglutide from BCH without a content assay?

Conditions: 99.4% · liraglutide · BCH.

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.

How would you structure this, and what thresholds would you set in advance?

99.4 per cent purity with no content figure leaves the milligram number unmeasured. Purity says 99.4 of every 100 units of detected area is liraglutide and 0.6 is something else. Content says how many milligrams are in the glass. The two do not constrain each other: a vial can be 99.4 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.

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

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 underlying point is that 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.

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

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

The underlying point is that 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.

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.

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

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

The part that matters: reporting threshold is convention and not chemistry, which is why two certificates with different thresholds disagree by a tenth of a point or more.

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.

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.

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.

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

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.

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 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 29 Oct 2024 by fill_volume — expanded the table to cover the lower concentration

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

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

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.