How does orforglipron behave on a C18 column compared to cagrilintide?

Stated plainly: orforglipron · a C18 column · cagrilintide.

Both of these get recommended confidently by different people, which suggests neither is obviously right.

My constraints are cost, measurement resolution and how much handling I am prepared to do — in roughly that order.

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

Specifically, peak shape carries as much information as peak area does, and a badly tailing peak or a shouldered peak is telling you something about the sample or the column that matters.

Detector linearity is why a heavily loaded peak can give false purity by compressing the main peak height while leaving impurity shoulders unchanged.

The part that matters: system suitability checks on replicate injections of a known standard establish whether the method was in control — if the peak area varies by more than two per cent between replicates, something is wrong.

The limitation is that co-elution is invisible — if two species happen to have the same retention time, they will report as a single peak at their combined area.

The practical summary: specify the method, run the same method on every sample you compare, and use orthogonal techniques to confirm the result.

edited 20 Aug 2026 by w_okoye — expanded the table to cover the lower concentration

Stated carefully, the chemistry of peptides at low pH and high organic-solvent concentration is not something most users have intuition for, which is why published methods exist.

Reverse-phase stationary phases use C18 or C8 chains bonded to silica, and the pore size of the silica matters more for a peptide of this chain length than the ligand length does.

Specifically, acetonitrile is the organic modifier of choice because it has a good UV cutoff, a reasonable viscosity and a refractive index that minimises baseline noise.

Inter-laboratory studies using identical methods on identical material show precision well within half a per cent when the method is locked down, pointing to method variability as the primary source of disagreement.

Ask for the chromatogram and the system suitability data, not just the number.

It helps to be literal here: coupling the chromatography to mass spectrometry adds identity information but trades chromatographic quality for ionisation efficiency.

Wider-pore phases — 300 Angstrom rather than 100 Angstrom — have faster mass transfer and narrower peaks for peptides above three kilodaltons, which is almost every peptide you will see.

If two labs give different numbers, the method difference is the first hypothesis, not lab quality.

In practice, reverse-phase HPLC is the workhorse for peptide purity work, but it is almost universally run under conditions that are not optimal for a peptide of this chain length.

Formic acid is the compromise when you need the mass spectrometer on the same run, but the peak shape penalty is real and easily a tenth of a per cent on purity.

The selectivity of a reverse-phase separation depends on the hydrophobicity of the side-chain profile, which is why two peptides can co-elute even if they differ by a residue.

The practical summary: specify the method, run the same method on every sample you compare, and use orthogonal techniques to confirm the result.