The particulars: 97.1% · a wide-pore C18 column · cagrilintide.
I have the document in front of me and I can read the numbers. What I cannot do is interpret them.
I am reasonably comfortable with statistics and completely uncomfortable with chromatography, or vice versa.
What does this actually establish, and what does it not?
It tells you 97.1 per cent of the integrated area came off a wide-pore C18 column where cagrilintide 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 wide-pore 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.
Put another way, 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.
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 relevant detail is that 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.
Peptide separation by reverse-phase high-performance liquid chromatography is described in the general chapters of the United States Pharmacopeia, European Pharmacopeia and Japanese Pharmacopeia, and the methods converge on essentially the same principles.
The caveat is that HPLC is a purity technique and says almost nothing about whether the main peak is actually your target compound — that is why identity confirmation from mass spectrometry or peptide mapping matters.
The practical summary: specify the method, run the same method on every sample you compare, and use orthogonal techniques to confirm the result.
HPLC purity, identity confirmation and quantified content on the vial you actually hold. Reports arrive with the chromatogram attached, not just a number.
Submit a sampleFounded 1998. ISO 9001 and cGMP certified, 1,500+ staff and 200+ patents. The synthesis house behind a great many of the vials that get sent out for testing - batch-specific documentation with every order.
Visit GL BiochemStart by understanding what the detector is measuring and what that means about how the molecule needs to be prepared and handled before injection.
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.
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.
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.
I would caution against over-interpreting small differences between runs, because the noise floor of the method is larger than most people assume.
If two labs give different numbers, the method difference is the first hypothesis, not lab quality.
edited 8 Feb 2026 by claudia_ferrante — corrected a unit error in the worked example
It helps to be literal here: 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.
Proline-rich sequences are particularly problematic because the isomerisation kinetics are in the same timescale as the separation, leading to split or broadened peaks at low temperature.
The part that matters: temperature affects both the viscosity of the mobile phase and the dynamics of molecular interactions, and a method developed at 25 degrees and run at 40 degrees will not behave identically.
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.
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.
Peptide impurities from solid-phase synthesis include deletion sequences, truncations from premature cleavage, racemised residues from epimerisation and oxidised variants, each of which may have different chromatographic behaviour.
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.
Ask PeptideStack is a static archive. Posting is closed, but the norms are worth stating: answer the question that was asked, show your working, cite the trial or the certificate, and say plainly where the evidence runs out.