A prominent purity percentage is easy to compare and easy to overinterpret. Before deciding what it tells you, identify whether it is a catalogue specification, a result for a particular sample or an unexplained promotional number. This is a first-pass guide to the listing; the certificate and method guides cover the deeper analytical questions.
Identify the kind of claim on display
| Statement | What it establishes as written |
|---|---|
| Purity specification: at least 98% by HPLC | A stated criterion and broad method, not an exact result for every lot. |
| Reported HPLC area purity: 99.2%, lot X | A lot-associated reported result, whose method and report still need checking. |
| 99% pure | A percentage with an unresolved quantity, method and material basis. |
These statements are deliberately hypothetical. Their purpose is to show why two percentages can play different roles even when they look similar on a product card. Preserve words such as minimum, specification and reported rather than copying every number into a single “purity” column.
A listing may legitimately summarise a longer document, but the summary should lead to the detail needed to understand it. If that detail is missing, record the missing field instead of treating a polished presentation as analytical support.
Ask which percentage is being reported
Bachem's peptide guide distinguishes chromatographic purity from net peptide content and notes that water and counterions affect the latter. This is why a displayed chromatographic percentage should not automatically be read as the fraction of the vial's total mass occupied by the target peptide.Bachem — Peptide Guide (opens in a new tab)
Nor should the label be transformed into a claim about every possible contaminant. The method and reporting basis define the measurement's scope. If the listing mentions only a chromatographic result, do not silently add microbiological or unrelated chemical tests to what it claims.
Follow the claim to the relevant report
Check whether the linked document refers to the listed identity, selected size and stated lot. A generic example report can explain the format, but it should not be mistaken for the selected material's result. Keep the distinction visible in your notes.
If a page offers several sizes or variants, inspect which document is associated with the selected option. A result for one preparation is not automatically a result for every other variant sharing the product name. This is especially relevant to blends or complexes, whose descriptions may include more than one amount field.
When the match is unclear, a precise question is efficient: “Which report supports the displayed percentage for this selected item?” That asks for the missing connection without presuming that the number is either valid or invalid.
Do not rank suppliers using decimal places alone
Imagine two hypothetical listings: 99.0% and 99.5%. Without compatible methods, reporting rules and sample context, those numbers do not establish a complete quality ranking. They may be results from different analytical circumstances or one may be a minimum specification.
A first-pass comparison should therefore include columns for claim type, method, material match and accessible supporting report. The percentage belongs beside those fields. If a comparison cannot be completed, say which information is absent rather than using the largest number as the answer.
This is not a reason to ignore purity. It is a way to use the value for the question it can actually answer. A relevant, well-described result is more informative than an isolated number with finer rounding.
Move from the listing to the analytical question
Once the displayed claim and report are matched, read the result against its stated criterion. If the question concerns how the percentage was produced, continue to the HPLC method guide. If it concerns how much material is present, use the separate content result and its basis. Choosing the right next question prevents a purity number from doing the work of an entire specification.
Sources and further detail
- Bachem — Peptide Guide (opens in a new tab)
Net peptide content discussion, page 6. Supplier-specific manufacturing defaults are not assumed to apply to Novum.
Sources checked 19 September 2026. Worked examples are illustrative unless a supplied report is explicitly identified. This article has not undergone independent scientific peer review.