Two reports on the same product from two laboratories, four percentage points apart, and no method stated on either.
The narrow version of the question is why two laboratories report different purity for the same batch, and which number to believe.
Numbers rather than impressions, if you have them.
Answering the narrow version, because the broad one does not have a single answer. 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.
I would rather be corrected than agreed with, if it comes to it.
Dr.NateNeph said:They are not contradictory, they measure different things, and this is the single most common misreading of a certificate.
Agreed, and mass spec answers a question purity does not: identity. High purity tells you one thing dominates the chromatogram, not that it is the molecule you ordered. A mass within about a dalton of expected is the identity check.
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Shop Reference StandardsDr.CardioMD said:Two reports on the same product from two laboratories, four percentage points apart, and no method stated on either.
This matches mine closely enough to be worth saying so. 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.
Clinical perspective, offered as context rather than as advice.
My vendor review relevant to certificates of analysis: ordered 3 times from my current compounder. Consistency has been excellent.
Shipping: Next-day with cold pack
Packaging: Professional, discreet, proper labeling with lot/batch numbers
Purity: 98.0% average across orders (Janoshik verified)
Customer service: Responsive within 2 hours
Price: $144/month for tirzepatide 10mg equivalent