Details up front: a sterility test · orforglipron.
I want to know what the trade-off actually is rather than which option is fashionable.
I would rather have a defensible reason than a marginal improvement.
So which one, and on what grounds?
Mechanically, reading a content result requires knowing the purity of the standard against which the sample was quantified, because your result inherits that uncertainty.
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.
| 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 |
To be exact about it, 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.
Quantitation against a standard requires that the standard be traceable to a national metrology institute, and certificates for research-grade standards claim that traceability.
If a supplier gives you content without the standard's purity, ask them to provide it.
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsThe relevant detail is that if a supplier quotes a content figure without stating the purity of their reference standard, they have not told you the uncertainty in the number.
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.
Put another way, 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.
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.
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.
The relevant detail is that 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.
The practical summary: if you are ordering from a new supplier, budget for content assay on the first lot.
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.
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.
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.
If a supplier gives you content without the standard's purity, ask them to provide it.
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.
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.
Ask for both the purity and the content, and do not accept purity alone.
edited 8 Sept 2024 by siobhan_deasy — clarified the distinction between purity and content
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.