Concretely: 97.1% · liraglutide.
I want the working, not the result — I need to be able to redo it with different numbers.
I care about the precision as well as the value — I want to know how many figures are real.
Is my approach right even if my number is wrong?
97.1 per cent is at the top of what stepwise synthesis delivers on a chain this long, and it is reachable rather than fictional. Every coupling is high-yielding and none is quantitative, so the deletion and truncation sequences that survive purification are what occupies the remaining 2.9 per cent. Above roughly 98 per cent you are fighting the purification rather than the synthesis, which is why a 97.1 per cent figure on liraglutide deserves a method question — column, gradient, wavelength — rather than either belief or dismissal.
More usefully, the single most important distinction is between what purity measures — the fraction of detected material that is your target — and what you actually want to know — how much of the material in the vial is your target.
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.
| Component | Typical share | Counted in purity? | Counted in content? |
|---|---|---|---|
| Target peptide | 88–94 % | Yes, as main peak | Yes |
| Related impurities | 1–3 % | Yes, as other peaks | No |
| Counter-ion (TFA or acetate) | 2–8 % | No | No |
| Residual water | 2–6 % | No | No |
| Bulking agent, if present | 0–40 % | No | No |
Worth being precise here: 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.
The ICH Q3A impurity thresholds and the relevant pharmacopoeial chapters all specify method validation requirements that almost no research-grade certificate claims to meet.
Worth noting that method standardisation is poor in the research-grade space compared to pharmaceutical work, so identical-looking methods can produce different results.
Compare purity within a single laboratory on the same method, never across laboratories.
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsSpecifically, 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.
On the detail: 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.
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.
If you are ranking vendors, specify a method and have all samples tested at the same place.
edited 3 Jun 2024 by plate_count_9k — added a caveat about sampling
More usefully, identity and purity are orthogonal, and a high purity says almost nothing about whether the peak is actually what you think it is.
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.
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.
The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.
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.
The fraction of your main peak that is actually your target versus isomers, fragments or related sequences is invisible without complementary identity testing.
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 17 Jul 2024 by j_wierzbicki — clarified the distinction between purity and content
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.
Published side-by-side method comparisons show that a two-point difference in purity on the same vial is easily explained by method choice alone.
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.
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.
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.