Concretely: 2.5 mg · 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?
On a 2.5 mg vial every percentage point of water and counter-ion is 0.025 mg, so the answer is a multiplication once you have the net peptide content off the certificate. Settle first whether the 2.5 mg on the label is gross fill or net peptide, because the two differ by exactly the quantity being asked about. If the certificate quotes net peptide content of 85 per cent, a 2.5 mg gross fill holds 2.13 mg of peptide; at 78 per cent it holds 1.95 mg. That 0.18 mg gap is 7 per cent of the label, larger than any purity difference anybody argues about, and it is invisible to a purity figure because water and acetate are not impurity peaks. Amino acid analysis or a nitrogen determination gives you the number; an HPLC area per cent never will.
The honest answer is that you cannot know for certain what you have without a content assay, and the purity number alone is not enough.
A content assay is always paired with a purity assay because purity tells you what fraction of the measured mass is the target and content tells you the total measured mass.
| 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 |
For peptides at 214 nanometres the response is roughly proportional to the number of peptide bonds, so truncation impurities have lower response factors and overestimate content.
The relative standard deviation on replicate quantitations of a homogeneous sample should be below two per cent when the method is under control.
One qualification: a single result from a single vial is a point estimate, and repeating the assay on a second aliquot is worth doing if the first result is surprising.
If a supplier gives you content without the standard's purity, ask them to provide it.
edited 1 Oct 2025 by tare_weight — corrected a unit error in the worked example
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsConcretely, two measures of the same vial can agree on purity and disagree on content by a few per cent, which usually means the content assay used a different standard.
Water content and counter-ion content are part of the gross mass but not part of the content assay result, which is why the two do not sum to label claim.
The purity of the reference standard is stated on its certificate, and your content figure is only as good as that purity certificate is.
I would not rely on a content figure from a lab that is not willing to state their standard's purity on request.
The practical summary: if you are ordering from a new supplier, budget for content assay on the first lot.
edited 1 Oct 2025 by h_pergande — added a caveat about sampling
Most research-grade certificates report purity without content, which is exactly backwards from what users actually need.
If the standard and sample have different absorption coefficients at the detection wavelength, the response factors differ and the inference fails.
Running multiple independent aliquots of the same sample should give results that agree to within the method precision, which is usually one to three per cent.
Worth noting that the standard certificate carries its own uncertainty, usually on the order of two to three per cent, which the sample result inherits.
Ask for both the purity and the content, and do not accept purity alone.
It helps to be literal here: a high purity does not guarantee high content because it says nothing about how much of anything is in the vial — a vial could be 99 per cent pure but only half full.
Peak area for a standard of known weight produces a response factor — area per unit mass — which is then applied to the sample peak to infer sample mass.
Quantitation against a standard requires that the standard be traceable to a national metrology institute, and certificates for research-grade standards claim that traceability.
The caveat is that content assay costs more than purity, so most people do not do it, which is exactly why it is valuable on the first lot from a new supplier.
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.
The relevant detail is that the label claim is usually the gross mass of the lyophilised solid, which is not the same as the content, because the solid contains water, counter-ion and other non-peptide mass.
Quantitation against a certified reference material assumes the sample and standard are treated identically through the analytical method, which is why the method for calibration matters as much as the method for measurement.
Pharmacopoeial guidance on quantitative methods specifies validation steps for linearity, range, accuracy and precision that most research-grade work does not claim to meet.
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.