Conditions: identity confirmation by mass spectrometry · semaglutide.
I am trying to choose between two options that are usually discussed as though only one exists.
I am not optimising for price, but I am not indifferent to it either.
Which axes does this decision turn on?
Specifically, 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.
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.
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.
The practical summary: if you are ordering from a new supplier, budget for content assay on the first lot.
edited 13 Apr 2026 by plate_count_9k — reworded for clarity after a comment
HPLC purity, identity confirmation and quantified content on the vial you actually hold. Reports arrive with the chromatogram attached, not just a number.
Submit a sampleFounded 1998. ISO 9001 and cGMP certified, 1,500+ staff and 200+ patents. The synthesis house behind a great many of the vials that get sent out for testing - batch-specific documentation with every order.
Visit GL BiochemIf a supplier quotes a content figure without stating the purity of their reference standard, they have not told you the uncertainty in the number.
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.
Specifically, 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.
Pharmacopoeial guidance on quantitative methods specifies validation steps for linearity, range, accuracy and precision that most research-grade work does not claim to meet.
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.
If a supplier gives you content without the standard's purity, ask them to provide it.
Put another way, reading a content result requires knowing the purity of the standard against which the sample was quantified, because your result inherits that uncertainty.
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.
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.
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.
Ask for both the purity and the content, and do not accept purity alone.
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 single most common reason for disagreement between a supplier content figure and an independent assay is using different standards.
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 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 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.
The relative standard deviation on replicate quantitations of a homogeneous sample should be below two per cent when the method is under control.
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.