Setup, so nobody has to ask: 60 mg · a GLP-1 receptor agonist.
The units are where I keep going wrong, so please be explicit about them.
I have sanity-checked the order of magnitude and it seems right, which is not the same as being right.
Is my approach right even if my number is wrong?
More usefully, 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.
Comparing content results from different laboratories requires knowing whether they both used certified reference materials or whether one used an in-house standard of unknown provenance.
| Test | Answers | Does NOT answer |
|---|---|---|
| RP-HPLC, area % | What fraction of detected material is the target | How much target is present |
| Quantified content | Milligrams of peptide per vial | What the impurities are |
| ESI-MS identity | Whether the molecular weight matches | Purity, or isomeric substitution |
| Peptide mapping | Sequence, localised to a fragment | Quantity |
| Karl Fischer | Water content of the solid | Solvent content |
| LAL endotoxin | Pyrogen load in EU/mg | Sterility |
| Sterility test | Growth in defined media over 14 days | Endotoxin, or bioburden count |
The underlying point is that the purity of the reference standard is stated on its certificate, and your content figure is only as good as that purity certificate is.
The relative standard deviation on replicate quantitations of a homogeneous sample should be below two per cent when the method is under control.
The limitation is that a quantitative method is only as good as the standard it uses, and a cheap standard is a false economy.
Ask for both the purity and the content, and do not accept purity alone.
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsPut another way, the short answer is that purity says "what fraction of detected material is the target" while content says "how many milligrams of the target are present," and those are two different things.
System suitability for a quantitative method is stricter than for purity because a small systematic error in the standard directly translates into an error in the sample result.
More usefully, the single most common reason for disagreement between a supplier content figure and an independent assay is using different standards.
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.
On the detail: content assay requires a reference standard of known purity and traceability, which is why it costs more than purity testing does.
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.
If a sample shows high purity but low content, the explanation is usually that the standard used for quantitation had a different purity than claimed.
Pharmacopoeial guidance on quantitative methods specifies validation steps for linearity, range, accuracy and precision that most research-grade work does not claim to meet.
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 Nov 2024 by Dr_Elias_Weiss — added the method parameters
Reading a content result requires knowing the purity of the standard against which the sample was quantified, because your result inherits that uncertainty.
If the standard and sample have different absorption coefficients at the detection wavelength, the response factors differ and the inference fails.
Where content data have been published from testing services on common peptides, the spread between services on identical material is typically a few per cent.
Ask for both the purity and the content, and do not accept purity alone.
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.
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 practical summary: if you are ordering from a new supplier, budget for content assay on the first lot.
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.