7 Key Metrics in a Certificate of Analysis You Must Understand
A COA is only useful if you can read it. These 7 metrics are the ones that actually matter for evaluating peptide quality — and most researchers only check one or two. For research purposes only.
1. HPLC Purity Percentage — The Headline Number
High-Performance Liquid Chromatography (HPLC) purity is the most commonly reported and most frequently referenced metric on a peptide COA, and for good reason — it tells you what percentage of the material in the vial is actually your target peptide versus impurities. The measurement works by separating compounds in the sample based on their interaction with a chromatographic column, then detecting each compound as it elutes using UV absorbance (typically at 220nm, which detects the peptide bond). The target peptide should appear as a dominant peak, and purity is calculated as the target peak area divided by total peak area. For research-grade peptides, expect 95% minimum and prefer 98%+. Premium products should consistently deliver 99%+. However, the raw percentage only tells part of the story — you also need to consider the HPLC method used, the gradient conditions, and whether the method is adequate to resolve closely related impurities from the main peak. A reported 99% purity on a poorly optimized HPLC method might be masking co-eluting impurities.
2. Mass Spectrometry (MS) — Identity Confirmation
If HPLC tells you how pure the sample is, mass spectrometry tells you what the sample actually is. MS measures the mass-to-charge ratio of ionized molecules, providing the observed molecular weight of the compound. The COA should report the expected molecular weight (calculated from the target amino acid sequence) and the observed molecular weight from the MS measurement. These values should agree within the instrument's measurement precision — typically within 1 Dalton for standard electrospray ionization MS, or within 0.1 Da for high-resolution instruments. The most common way to report this is the [M+H]+ ion (the protonated molecular ion), though you may also see [M+2H]2+ or other multiply charged species for larger peptides. If the observed mass does not match the expected mass, the product is not the target peptide — regardless of what the HPLC purity says. Mass spectrometry is your identity verification, and it is non-negotiable for any serious quality assessment.
3. Amino Acid Sequence — Confirming the Right Peptide
Some COAs include amino acid analysis or sequencing data that confirms not just the molecular weight but the actual amino acid composition or sequence of the peptide. This is particularly important for longer peptides where deletion sequences (peptides missing one amino acid from the target sequence) can have very similar molecular weights that might not be fully resolved by standard MS. Amino acid analysis hydrolyzes the peptide into its component amino acids and quantifies each one, confirming that the correct amino acids are present in the expected ratios. This catches substitution errors where a wrong amino acid was incorporated during synthesis. While not all COAs include this level of characterization, its presence indicates a vendor that goes beyond minimum quality verification. For critical research applications where peptide identity must be absolutely confirmed, amino acid analysis provides an additional layer of confidence beyond MS alone.
4. Appearance and Physical Description — The Sanity Check
This seemingly mundane entry — typically something like 'white to off-white lyophilized powder' — serves as an important physical verification point. When your peptide arrives, compare its actual appearance to what the COA describes. Most lyophilized peptides should appear as a white to slightly off-white powder or cake at the bottom of the vial. Significant deviations — yellow or brown coloration, wet or sticky appearance, powder scattered on the vial walls rather than sitting as a cake at the bottom — may indicate degradation, moisture exposure, or improper lyophilization. Color changes are particularly informative: oxidation of tryptophan or methionine residues can cause yellowing, while more severe degradation or contamination can produce brown discoloration. The appearance description on the COA establishes the expected baseline, and your visual inspection of the actual product is the first and fastest quality check you can perform.
5. Peptide Content (Net Peptide Weight) — The Actual Amount You Got
This is one of the most misunderstood metrics on a COA, and misunderstanding it leads to systematic dosing errors in research. The labeled weight on a peptide vial (e.g., '5mg') refers to the gross weight of material in the vial — but not all of that material is peptide. Lyophilized peptides typically contain counter-ions (acetate or TFA salts from purification), residual moisture, and residual solvents. The net peptide content — the actual mass of the target peptide — may be 60-80% of the gross weight, depending on the peptide and the purification method. For example, a vial labeled '5mg' with 75% peptide content actually contains 3.75mg of peptide. If you calculate your reconstitution concentration based on the labeled weight rather than the net peptide content, your actual concentration will be 25% lower than intended. Good COAs report net peptide content, and you should use this number — not the label weight — for concentration calculations.
6. Batch/Lot Number — Your Chain of Custody
The batch or lot number is the single piece of information that links the COA to the specific material in your vial. Without a matching lot number, a COA is just a generic quality document with no connection to what you actually received. Every time you receive a peptide shipment, verify that the lot number on the vial label matches the lot number on the accompanying COA. If they do not match, the COA is not valid for your specific product. This seems obvious, but it is frequently overlooked — especially when vendors provide a single 'representative' COA for multiple production batches. Each production batch is a separate synthesis run with potentially different purity, yield, and impurity profiles. A COA from batch A provides no quality assurance for material from batch B. If your vendor cannot provide a COA matching the specific lot number on your vial, that is a meaningful gap in quality documentation that should factor into your vendor evaluation.
7. Testing Laboratory Identification — Who Did the Testing?
The credibility of every metric on a COA depends on the credibility of the laboratory that performed the testing. A COA should clearly identify the testing laboratory by name, and ideally include their accreditation credentials. Third-party testing — performed by an independent laboratory with no financial interest in the product passing or failing — is the gold standard. In-house testing, where the vendor tests their own products, introduces an obvious conflict of interest that does not invalidate the results but does reduce the level of assurance they provide. Look for laboratories with ISO 17025 accreditation (the international standard for testing and calibration laboratories), which requires demonstrated competence, impartiality, and consistent operation. You can verify laboratory accreditation through accreditation body databases. A COA from a well-known, independently accredited testing laboratory carries significantly more weight than one from an unidentified or in-house laboratory. When evaluating vendors, the testing laboratory they use is as important as the test results they report.
Research Disclaimer: All information on this page is provided for educational and research purposes only. Products discussed are intended for laboratory research use exclusively. They are not intended for human consumption, therapeutic use, or as dietary supplements. Always follow institutional guidelines and consult published peer-reviewed literature for research protocol development. Not for human consumption.