Two reports on the same product from two laboratories, four percentage points apart, and no method stated on either.
What would genuinely help is knowing 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.
Short answer first, then the reasoning. 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.
RetaRick_CA said:Inter-laboratory variation comes from real methodological differences, not sloppiness: which reference standard was used and how it was itself…
RetaRick_CA has the substance of this right. The condition it depends on is worth stating. 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.
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Shop Reference StandardsSarahChen_PharmD said:Two reports on the same product from two laboratories, four percentage points apart, and no method stated on either.
Can confirm the pattern SarahChen_PharmD describes. 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.
Adding the clinical framing, because it changes how the question reads.
HPLC results interpretation for certificates of analysis: when you get your Janoshik report back, here's what the numbers mean:
- Purity %: Should be >97%. Below 95% is concerning.
- Retention time: Should match reference standard for the peptide
- Related substances: Degradation products should be <2% total
- Single impurity: No individual impurity should exceed 1%
My last batch: 99.4% purity, 0.2% total related substances. Clean chromatogram with single sharp peak at expected retention time. Exactly what you want to see.