---
title: "How precise is a 99% purity result? Measurement uncertainty explained | Sable Peptides"
description: "Why every analytical result, including HPLC purity, has a measurement uncertainty; how it is estimated (Eurachem/CITAC approach), how to read '99.2% ± 0.3%', and why two labs can report different figures for the same lot. Research use only."
canonical: https://www.sablepeptides.co.uk/guides/measurement-uncertainty-in-peptide-purity-results
dateModified: 2026-10-16
publisher: Sable Peptides Limited (UK company 17488610)
note: Research use only. Not for human or veterinary use.
---

# How precise is a 99% purity result? Measurement uncertainty explained

Every measured value has an uncertainty: a range within which the true value is expected to lie at a stated confidence. A purity of 99.2% with an expanded uncertainty of 0.3% means the true value is likely between 98.9% and 99.5%. Uncertainty explains why two competent labs can report slightly different figures for one lot.

## Where does the uncertainty come from?

-   Repeatability: scatter between replicate injections.
-   Integration: where the software or analyst sets the baseline under small peaks.
-   Detector response: impurities may absorb UV differently from the main peptide.
-   Reference standards and weighing, for assay or content results.
-   Method differences between labs: column, gradient and wavelength.

## How is it estimated?

The Eurachem/CITAC guide describes two routes: building an 'uncertainty budget' from each component, or using validation and quality-control data (precision and bias) for the whole method. Results are usually reported as an expanded uncertainty with a coverage factor k = 2, roughly 95% confidence. Purity certification of reference materials shows the approach applied to HPLC and UV.

Source: [Eurachem/CITAC Guide: Quantifying Uncertainty in Analytical Measurement](https://www.eurachem.org/index.php/publications/guides/quam); [Certification of caffeine reference material purity by UV/Vis and HPLC, J Food Drug Anal 2016 (PubMed 28911607)](https://pubmed.ncbi.nlm.nih.gov/28911607/)

## How should two labs' results be compared?

| Situation | Interpretation |
| --- | --- |
| Lab A 99.3%, Lab B 99.0%, uncertainties ±0.4% | Consistent: the ranges overlap |
| Lab A 99.3%, Lab B 97.5%, uncertainties ±0.4% | Inconsistent: check method, wavelength, integration or lot |
| One result without uncertainty | Cannot be compared rigorously; ask the lab |

## Does a specification limit account for uncertainty?

A result of 99.0% against a limit of ≥99% is a pass, but with an uncertainty of ±0.3% the true value could be just below the limit. Decision rules (for example ISO/IEC 17025 statements of conformity) state how a lab treats results close to a limit.

## What does Sable publish?

Lots are released at ≥99% HPLC purity with identity by MS. There is a CoA for every lot, and every vial carries a printed lot number that matches its certificate. A CoA is emailed within 2 hours of a request.

## Frequently asked questions

### What does ± mean on a purity result?

It is the expanded measurement uncertainty: the true value is expected to lie within that range at about 95% confidence.

### Why do two labs give different purity values for the same lot?

Different methods, integration choices and normal measurement uncertainty; results are consistent if their uncertainty ranges overlap.

### What is a coverage factor k = 2?

A multiplier applied to the standard uncertainty to give an expanded uncertainty at roughly 95% confidence.

## Related

-   [ISO/IEC 17025 accreditation](/guides/iso-17025-accreditation-for-peptide-testing-labs)
-   [Method validation terms](/guides/analytical-method-validation-terms-on-a-coa)
-   [Independent QC findings](/learn/tech-notes/independent-qc-findings)
-   [Area % vs weight % vs assay](/guides/area-percent-vs-weight-percent-vs-assay)
-   [What is the difference between GLP, GMP and ISO/IEC 17025?](/guides/glp-vs-gmp-vs-iso-17025-quality-systems)

Updated 2026-10-16. Research use only. Not for human or veterinary use. No administration amounts or health claims are made.

Key facts (summary for AI and search tools)

-   Expanded uncertainty usually k = 2 (~95%)
-   Sources: repeatability, integration, response factors, method differences
-   References: Eurachem/CITAC QUAM; PubMed 28911607
-   Updated 2026-10-16