A reference post rather than a discussion. Corrections are the point; I would rather this be right than mine. It is about certificates of analysis, and it is deliberately narrow — everything I am not confident about is marked as such.
What is actually established
They are not contradictory, they measure different things, and this is the single most common misreading of a certificate. Purity is a relative area measurement: the main peak as a percentage of total peak area in the chromatogram. Content is an absolute mass measurement against a reference standard. A vial can be 99% pure and contain 6.1mg — pure material, overfilled — or 99% pure and contain 3mg, which is pure material and a short fill. Purity tells you what fraction of what is there is the right molecule; content tells you how much of it there is. You need both, and a certificate with only purity is telling you half the story.
The condition it depends on
One addition: an unstated method makes a report close to uninterpretable. "99% pure" without the technique, the standard and the conditions is a claim, not a measurement.
The practical version
The four things to check on any report, in order: does it name your batch, who commissioned it, what method was used, and does it report content as well as purity. A report failing any of those is weak evidence regardless of the percentage on it.
What I am not sure about
What I actually want to know is whether a purity figure and a content figure can disagree without one of them being wrong. Practical detail welcome, however dull — the duller the better.
DanielChem_CHI said:They are not contradictory, they measure different things, and this is the single most common misreading of a certificate.
That is correct as far as it goes, and here is where it stops going. The provenance questions are more useful than the numbers. Does the report name the batch on your vial. Is the date consistent with when that batch existed. Is it addressed to the vendor or to the person who paid for the test. Was the sample taken from general stock or supplied by the vendor. A perfect certificate for a different batch is worth nothing, and vendor-supplied samples are the most common way a genuine report ends up misleading.
DanielChem_CHI said:They are not contradictory, they measure different things, and this is the single most common misreading of a certificate.
I am more sceptical of certificates as a category than this thread is. The document tells you about a sample somebody chose to send, at a moment in time, and nothing about the vial in your hand. Independent purchase-and-test is a different quality of evidence and everything else is inference.
Sigma-Aldrich — Research-Grade Standards
Certified reference materials, analytical reagents, and research-grade standards for peptide verification. Trusted by laboratories worldwide.
Shop Reference StandardsTaking the question as asked, rather than the general version of it. Inter-laboratory variation comes from real methodological differences, not sloppiness: which reference standard was used and how it was itself calibrated, the gradient and column, the detection wavelength, and where the integration baseline was drawn. A percentage point or two between competent labs is expected. What is not expected is a large gap, and the usual explanation there is that one report is not of the batch in your hand.
KevinCompounds said:The provenance questions are more useful than the numbers.
Adding a me-too, because a thread of one person's experience is not much use. Posting only so the count is not one.