Part of our Complete Guide to Research Peptides.

Understanding Certificates of Analysis (COAs) and HPLC Purity Testing

A Certificate of Analysis, or COA, is the document that confirms what is actually inside a vial — not just what the label claims. For research peptides, where consistency directly affects reproducibility, understanding how to read a COA is one of the most useful skills a researcher can have when evaluating a supplier or a specific batch.

What HPLC Actually Measures

HPLC (High-Performance Liquid Chromatography) is the standard analytical method for determining peptide purity. In simple terms, it separates a sample into its individual components based on how each one interacts with a column and solvent system, then measures how much of the sample is made up of the target peptide versus other compounds.

The result is a chromatogram — a graph showing a series of peaks. The target peptide should appear as one large, dominant peak; smaller peaks alongside it represent impurities such as truncated sequences or leftover synthesis byproducts. Purity is calculated as the area of the target peak divided by the total area of all peaks, expressed as a percentage.

Why HPLC Alone Isn’t the Full Picture

HPLC confirms how clean a sample is, but it doesn’t confirm what the sample actually is. That’s the role of mass spectrometry (MS), which measures the molecular weight of the compound and compares it to the calculated theoretical weight for the correct sequence. A COA that reports HPLC purity but no MS identity confirmation only tells you half the story — a high purity number for the wrong compound isn’t useful for research.

Method development for peptide purity analysis is itself an active area of analytical chemistry research, since closely related peptide impurities (such as isomers) can be difficult to separate using standard methods. For a detailed technical review of this challenge, see: A strategy for assessing peak purity of pharmaceutical peptides in reversed-phase chromatography methods (ScienceDirect).

What a Complete COA Should Include

• Batch or lot number matching the vial label
• HPLC purity percentage, ideally with the chromatogram image included, not just the number
• Mass spectrometry results showing observed vs. calculated molecular weight
• Test date and manufacture date
• Identifiable testing laboratory, ideally an independent third party rather than in-house-only testing

Purity in the 95–98% range is generally considered standard for RUO-grade material suitable for most discovery-phase research, while >99% purity is typically sought for more sensitive applications such as structural biology or precision binding assays.

Our Approach

Every product in our catalog is supplied with third-party HPLC verification confirming >99% purity. We believe researchers should be able to evaluate the actual data behind a purity claim, not just take a number at face value.

Reading a Chromatogram: What to Look For

When reviewing a COA, the chromatogram image itself often reveals more than the summary purity number alone. A clean, well-resolved main peak with minimal shoulder peaks or baseline drift suggests a well-controlled synthesis and purification process. Researchers evaluating research peptides from a new supplier should request the actual chromatogram image, not just the stated percentage, since visual inspection can reveal issues like co-eluting impurities that a summary number alone would not disclose.

For researchers comparing multiple suppliers of the same research peptides, requesting COAs from the same batch or lot number they would actually receive is important, since purity can vary between production runs even for the same compound from the same manufacturer. A supplier that provides generic, non-batch-specific documentation across all customers is a red flag worth noting when evaluating sourcing options.

This page is provided for general educational reference. All products sold by PNW Peptides LLC are supplied strictly for laboratory research use (RUO) and are not for human or veterinary consumption. See our RUO Disclaimer.

Frequently Asked Questions

For a product-specific example of reported purity and measured quantity, see the analytical summary on our Retatrutide research peptide page. That summary relates to a tested 10mg sample; it does not provide a separate result for the 20mg option.