Two suppliers, two certificates, two numbers. One says 98%. The other says 99%. The question is whether that single percentage point is worth anything. A framing note first: every compound discussed here is sold for research use only, never for human or animal consumption, and the discussion below is about analytical chemistry on the bench — not laboratory protocols for working with the material.
The short answer is that those percentages describe how much of the analyzed sample shows up as the intended peptide peak on a chromatogram. They do not describe an absolute mass fraction of peptide in the powder. They do not confirm that the cleanest peak is the sequence you actually ordered. And they are bounded by the specific analytical method that produced them. The figure is useful — but it's one line on a longer document, and reading only that line is how laboratories end up surprised by their results.
This piece walks through what the percentage is actually measuring, what the missing 1-5% typically contains, why two reports that both say 99% can describe quite different samples, and how to read the rest of a peptide certificate of analysis so the headline reads in context.
How a purity percentage is actually measured
The plain-English answer: the percentage on a peptide label is almost always an HPLC area percent — the fraction of total detected signal that elutes as the target peptide peak.
The instrument: reversed-phase HPLC with UV detection
High-performance liquid chromatography pumps a liquid sample through a stainless-steel column packed with a solid stationary phase. As the sample moves through the column, its components interact with the packing material to different degrees and end up moving at different speeds. A detector watches the column outlet and records when each component emerges. The output is a chromatogram — a plot of detector signal against time — with one peak per separable component.
For peptides, the dominant flavor of this technique is reversed-phase HPLC, in which the stationary phase is a hydrophobic C18-modified silica and the mobile phase is a water/acetonitrile gradient with about 0.1% trifluoroacetic acid (TFA). The TFA acts as an ion-pairing agent that sharpens peptide peaks and makes related impurities easier to resolve.
What the detector actually sees
Most peptide analytical chromatograms are recorded with a UV detector watching at 214 or 220 nm. That choice isn't arbitrary. The peptide bond itself absorbs strongly in the low-UV region, so every peptide-containing species in the sample produces a signal regardless of which aromatic side chains it carries. Each compound separating from the column outlet shows up as a Gaussian-shaped peak rising above the baseline.
From peaks to percentage
The data system integrates the area under every peak. The purity figure is then a simple ratio: area of the target peak divided by the sum of every integrated peak area, expressed as a percentage. A label that says 99% is reporting that, on the specific method used, the target peak accounts for 99% of the total UV-detected signal — the rest of the chromatogram contains 1% of the area, distributed across other peaks.
That language matters. The figure is a chromatographic relative measurement on a particular instrument run, not an absolute mass fraction of peptide in the dry powder. The broader question of how a research-grade specification sits next to a pharmaceutical specification is its own topic, covered in our piece on research-grade vs. pharmacy-grade peptides.
What the missing 1-5% actually is
The remainder on a high-purity peptide certificate is almost never one contaminant. It's a distribution of synthesis-related species, and knowing what they typically are makes the percentage easier to interpret.
Deletion and truncated sequences
Modern synthetic peptides are built one amino acid at a time on a polymer resin, in a process called solid-phase peptide synthesis (SPPS). Each round removes a protecting group, couples the next amino acid, and washes the resin clean. The chemistry is well-behaved but never perfect. Every coupling step has a small yield gap, and when a step misses, the chain continues without that residue. The result is a deletion sequence missing one amino acid relative to the target.