Two certificates showing 99.5% and 99.0% purity look easy to rank. Often they aren't, because the number depends on more than the sample. It depends on how the lab chose to measure and report it. Reading purity numbers from different sources means knowing what's actually being compared.
What can differ between two purity figures
The method used. UV-based purity and mass-based purity (see what HPLC purity does and doesn't tell you) answer related but different questions, and can produce different numbers for the same physical sample. A certificate that doesn't say which one it used isn't comparable to one that does, or to one using the other method.
The equipment used. Different HPLC setups can separate a sample's ingredients differently. A more sensitive setup may catch and subtract small impurities that a less sensitive one folds into the main peak, producing a lower but more complete purity figure.
How the lab reads its own chromatogram. Labs make judgment calls about small peaks near the edge of detection, background noise, and other borderline cases when calculating a peak's size. Two competent labs testing the same sample can report slightly different numbers just from these calls, with neither one being wrong.
What counts toward "100%." The most common method counts the sum of everything detected as the total. It doesn't separately account for water content, leftover solvent, or other non-peptide material unless the method is built to. So a high reading by that method doesn't necessarily mean a high share of the vial's total weight is the target compound.
What this means in practice
A purity number is only strictly comparable to another purity number made by the same method, on the same kind of equipment, under the same reporting choices. Comparing across labs without knowing these details means comparing numbers that may not mean the same thing, even when the sample quality is genuinely similar.
This doesn't make purity figures meaningless. It means the method matters as much as the number. A certificate that states its method (see reading a certificate of analysis) gives you what you need to judge whether a comparison is fair. One that only states a bare percentage doesn't.
What to do instead of ranking numbers alone
Check whether the method is stated, and whether it's the same method across the listings you're comparing. Where it isn't stated, treat the number as useful but not directly comparable to a number from a different, unstated method. A named lab you can check yourself (see comparing vendors on evidence, not claims) is a stronger basis for confidence than the decimal place on a purity figure.
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