This is a heads-up, not a conclusion. Everything I am confident about is below; everything I am not is marked.
Two reports on the same product from two laboratories, four percentage points apart, and no method stated on either.
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.
What to check: 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.
If your experience contradicts this, say so in the thread — I would rather be corrected here than have people act on a warning that does not hold.
LabKate said:Two reports on the same product from two laboratories, four percentage points apart, and no method stated on either.
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.
I would rather be corrected than agreed with, if it comes to it.
LabKate said:Two reports on the same product from two laboratories, four percentage points apart, and no method stated on either.
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.
Janoshik Analytical — Independent Testing
Trusted third-party HPLC & mass spectrometry analysis. Verify peptide purity with the lab the community relies on. Independent. Accurate. Transparent.
Verify Your PeptidesGL Biochem (Shanghai) Ltd. — Direct Manufacturer
Est. 1998. The synthesis house behind the vials you send for testing. ISO 9001 and cGMP certified, 1,500+ staff, batch-specific COA with every order.
Browse GL BiochemTaking 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.
PharmacoVig_BOS said:The provenance questions are more useful than the numbers.
Same pattern here, and in the same order. I had assumed I was the exception until I read this.