Testing GLP-1 and incretin analogues
On this class of compound, the purity figure is the least informative number on the report, and it is the one most people read first.
The incretin analogues are the most-submitted class this laboratory sees, and they are also the class where the mismatch between what people order and what they are actually asking is widest. Almost everyone submitting one wants to know whether the vial holds what the label says. Most of them order a test that structurally cannot answer that.
Currently listed in this family: Cagrilintide, Cagrilintide Tirzepatide Blend, Cagri/Sema Blend, Liraglutide, Retatrutide, Semaglutide, Tirzepatide. What follows applies across the group, because the analytical characteristics that matter are shared.
What makes this class analytically distinctive
They are mid-length, not short
These are peptides in roughly the 1.5 to 5 kDa range rather than the short sequences a great deal of peptide testing deals with. Practically, that means related substances sit closer to the parent peak in retention than they would on a shorter sequence, and it means identity has to be established against reference data for the specific analogue rather than on a general impression of the class.
They are acylated
The fatty-acid side chains that give this class its pharmacokinetic behaviour are also the source of its characteristic impurity profile. Partial acylation, missing side chains, and side-chain variants are all chemically close to the parent, which places them close to it in reversed phase. Resolving them is a gradient and wavelength problem, and a method that does not resolve them integrates them into the principal peak and reports a higher purity than the sample deserves.
They are sold by the milligram
This is the commercial fact that determines what the analytical question should be. A vial is bought as a stated number of milligrams, so the claim being made is quantitative, and only a quantitative measurement tests it.
Why the purity figure misleads here specifically
Area purity is a ratio: principal peak area over total integrated detected response. Two things follow, and both bite hardest on this class.
First, underfilling is invisible. Half-filling a vial removes material from the numerator and the denominator in the same proportion. The ratio does not move. A vial holding 3 mg against a 10 mg claim can report 99% purity with complete honesty.
Second, non-absorbing mass is not in the denominator at all. Counter-ion, residual moisture, and bulking agents are real weight in the vial that the ultraviolet detector never sees. On a class where the gross powder in a vial can be substantially more than the peptide in it, the gap between “the powder weighs what it should” and “the peptide weighs what it should” is exactly where the interesting failures live.
Put together: a purity-only result on an incretin analogue tells you the material present is largely one compound. It does not tell you there is enough of it, and on this class that is usually the whole question.
What a complete answer looks like
- Identity against reference spectra. The sample compared against reference data for the analogue named at scope review, using a selection of FTIR-ATR, Raman spectroscopy, capillary electrophoresis with UV detection, and UV-Vis spectroscopy, chosen to suit the molecule. This is what distinguishes one analogue from another, a distinction that matters commercially, since several members of this family are close in both structure and retention. Where a result looks anomalous, LC-MS confirmation can be arranged at a contract laboratory and is named on the certificate where it was used.
- Purity by HPLC-DAD. Under a gradient chosen to resolve acylation variants rather than to produce a flattering number, with resolved related substances reported individually.
- Content by quantitative LC. Against a calibrated reference standard, on a stated basis, compared explicitly against the nominal fill on the request. This is the measurement that tests the label claim, and it is the one that has to be asked for.
The first two are the standard peptide analysis. The third is an addition, and on this class it is the one worth making.
The reference standard constraint
A content result is only as good as the standard it is calibrated against. For the established analogues in this family, suitable reference material is generally obtainable. For newer research-stage members it is not always, and where it is not, the honest position is that purity and identity can be reported while a quantitative content figure cannot.
That constraint is settled at scope review rather than discovered at reporting time, which is why the request asks which analogue you believe the vial contains before a price is confirmed.
Blends
Combination vials in this family are common and are handled under blend analysis rather than a single-analyte panel. Every expected component and its nominal amount has to be declared, because overall chromatographic purity and per-component findings are different results reported separately, and a blend submitted without a component list is analytically a blind screen.
Where two declared components are close in mass or retention, additional method work may be needed to report them individually. Feasibility on that is assessed before acceptance.
Submission notes specific to this class
- Send a complete unopened vial whenever content is in scope; an opened unit compromises the weight basis
- State the nominal fill in milligrams, and say whether that figure is peptide base or gross powder if you know
- Do not reconstitute before shipping unless the laboratory has approved it in writing
- Name the specific analogue rather than a category, since several in this family are close analytically
- For blends, list every expected component with its nominal amount
What no result on this class establishes
Identity, purity, and content together describe the chemistry of the vial that was received. None of them establishes clinical safety, efficacy, sterility, regulatory status, or consistency across untested units. A measured content figure is a chemical measurement and does not convert to a potency or activity claim.
Further reference
Sources and related guidance for this article:
- ICH Q2(R2): Validation of Analytical Procedures
Framework for evaluating analytical procedure performance; this reference does not certify CPT or an individual result.
- CPT testing methods and reporting scope
Published service scope and technique-specific reporting information.
Common questions
Which panel should I order for a GLP-1 analogue?
The peptide analysis with quantitative content added is the usual recommendation for this class, because the question on an acylated incretin analogue is almost always how many milligrams the vial actually holds. Content is not part of the standard analysis and has to be requested, and a purity-only result on this class routinely looks excellent while leaving the label claim entirely unexamined.
Can a semaglutide or tirzepatide vial be pure and still underdosed?
Yes, and it is the characteristic failure on this class. Area purity is the principal peak’s share of total detected response, so underfilling reduces numerator and denominator together and leaves the percentage unchanged. Only quantitative content measured against a calibrated reference standard compares what is present against what the label claims.
Why do these compounds show a series of peaks in a mass spectrum?
Because at this mass range electrospray ionisation puts several protons on one molecule, so a single compound appears as a series of charge states rather than a single line. The molecular mass is recovered by deconvoluting that series, and a consistent series (every state pointing at the same underlying mass) is what makes the assignment credible. This matters when reading a mass spectrum from any laboratory; here, identity is confirmed against reference spectra rather than by mass spectrometry, and LC-MS is used only as confirmatory work at a contract laboratory.
Can you test a blend of two incretin analogues in one vial?
Yes, under blend analysis. Every expected component has to be declared with its nominal amount, because a blend assessed without a component list is closer to a blind screen than to a targeted panel. Co-eluting or closely related components can require additional method work, which is settled at scope review.