What I have: an HPLC purity test · dulaglutide.
I have used one of these for a while and I am considering switching, which requires a reason.
What I care about is reproducibility, because a result I cannot repeat is not useful to me.
What does each option buy me, and what does it cost me?
Mechanically, most research-grade certificates report purity without content, which is exactly backwards from what users actually need.
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.
The relevant detail is that 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.
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.
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.
The practical summary: if you are ordering from a new supplier, budget for content assay on the first lot.
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsTo be exact about it, 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.
The single most common reason for disagreement between a supplier content figure and an independent assay is using different standards.
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.
If a supplier gives you content without the standard's purity, ask them to provide it.
edited 14 Dec 2025 by cake_intact — clarified the distinction between purity and content
It helps to be literal here: quantified content is how many milligrams of peptide are actually in the vial, measured against a calibrated reference standard, and it is the only measurement that directly answers that question.
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 relevant detail is that 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.
Quantitation against a standard requires that the standard be traceable to a national metrology institute, and certificates for research-grade standards claim that traceability.
Ask for both the purity and the content, and do not accept purity alone.
On the detail: 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.
If the standard and sample have different absorption coefficients at the detection wavelength, the response factors differ and the inference fails.
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.
The practical summary: if you are ordering from a new supplier, budget for content assay on the first lot.
Content assay requires a reference standard of known purity and traceability, which is why it costs more than purity testing does.
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.
Pharmacopoeial guidance on quantitative methods specifies validation steps for linearity, range, accuracy and precision that most research-grade work does not claim to meet.
The limitation is that a quantitative method is only as good as the standard it uses, and a cheap standard is a false economy.
If a supplier gives you content without the standard's purity, ask them to provide it.
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.