Adding the numbers, since they settle part of this. 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.
Following on from Dr.SurgeonPGH — and this may be the naive question:
How people are distinguishing a supplier having a bad batch from a supplier on the way out?
Dr.NateNeph said: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…
There is a second half to this that has not been said yet. 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.
Happy to go further on any of that.
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Went back through my own correspondence with an earlier supplier and the warning signs were all there in the replies, weeks before the quality changed. I was reading the product and not the behaviour.
sarah_TO said:Inter-laboratory variation comes from real methodological differences, not sloppiness: which reference standard was used and how it was itself…
Agreed, with a caveat about community reputation: it is a lagging indicator. Reports arrive weeks after orders, so a supplier can look excellent for a month after quality has already changed.