What does 97.1% purity on a C18 column actually tell me about an ecnoglutide vial?

Stated plainly: 97.1% · a C18 column · ecnoglutide.

I would like help reading this properly rather than being told what conclusion to reach.

I have the full report including the method section, so I can quote specifics if that helps.

What can I legitimately conclude from this figure?

It tells you 97.1 per cent of the integrated area came off a C18 column where ecnoglutide comes off, and the remaining 2.9 per cent did not. That is an area statement at one wavelength, not a mass statement about the vial: 2.9 per cent of area is only 2.9 per cent of mass if every impurity absorbs exactly as strongly as the parent, which none of them do. It also says nothing about how many milligrams are in the glass — water, counter-ion and a short fill are all invisible to it. What a C18 column does add is a constraint on what could be hiding: a column that retains by hydrophobicity separates deletion sequences well and separates isomers of identical hydrophobicity not at all.

The limit of this technique for these molecules is reaching a point where small improvements require proportionally large investments in method development.

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.

Reconciling gross mass to label claim

Trifluoroacetic acid at 0.1 per cent is the standard ion-pairing agent because it suppresses tailing by neutralising the basic residues, but it also suppresses mass spectrometry signal.

One qualification: the limit of detection on a reversed-phase HPLC is set by the noise on the baseline, which for these molecules is usually in the range of a tenth of one per cent or less, and anything smaller is not reproducibly detectable.

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

Coupling the chromatography to mass spectrometry adds identity information but trades chromatographic quality for ionisation efficiency.

Mobile phase pH at the point where you inject must match the mobile phase pH at the start of the gradient, or the sample will not be focused at the column head.

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.

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

The method is the measurement in reverse-phase chromatography, more so than in almost any other analytical domain, and two methods that look identical can easily produce different results.

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

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 resolving power of a separation is quantified by the resolution parameter R, defined from the heights and widths of adjacent peaks, and pharmacopoeial methods typically demand R greater than 1.5 for a method to be considered validated.

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

edited 30 Apr 2026 by plate_count_9k — added the method parameters