COA Interpretation Guide (HPLC & Mass Spec)
How to read and verify Certificate of Analysis reports properly.
The Certificate of Analysis (COA) is the primary document verifying compound identity, purity, and quality. Being able to read and critically evaluate a COA is essential for ensuring research integrity — and for identifying when a supplier's documentation raises concerns.
What a COA Must Contain
A legitimate COA from a serious research supplier should include: compound name and CAS/sequence, molecular weight, lot number, synthesis date, HPLC purity (%), mass spectrometry confirmation, appearance (typically white to off-white lyophilized powder), and storage recommendations.
Missing any of these is a red flag. A "purity" claim without an attached chromatogram means nothing — you cannot verify it independently.
Reading HPLC Data
High-Performance Liquid Chromatography (HPLC) separates compounds by their interaction with a stationary phase. For peptide purity, reversed-phase HPLC (RP-HPLC) is standard. What to look for in a COA chromatogram:
- Single dominant peak: indicates high purity; main peak should represent ≥95% of total area
- Peak symmetry: asymmetric or tailing peaks suggest column contamination or degradation products
- Baseline flatness: significant noise or secondary peaks indicate impurities
- Retention time: should be consistent with the peptide's expected hydrophobicity
Purity is calculated as the area of the target peak divided by total peak area × 100. Reputable suppliers report this value alongside the raw chromatogram, not just as an isolated number.
Reading Mass Spectrometry Data
Mass spectrometry (MS) confirms molecular identity by measuring the mass-to-charge ratio (m/z) of the compound. For peptide verification, electrospray ionization (ESI-MS) is most common. The COA should show:
- Observed molecular weight: should match theoretical MW within ±0.5 Da (or ±0.1% for larger peptides)
- Multiple charge states: [M+H]+, [M+2H]2+, [M+3H]3+ — all should calculate to the same neutral mass
- Isotope pattern: should match theoretical isotope distribution for the formula
- No significant adduct peaks: sodium and potassium adducts (+22 or +38 Da) are common but should not dominate
Interpreting Purity Thresholds
Research purity standards vary by application. General guidance:
- ≥95% purity: standard for most biological research; appropriate for in vitro studies
- ≥98% purity: recommended for receptor binding assays and pharmacological studies
- <90% purity: acceptable only for exploratory screening; not for mechanistic studies
- GMP-grade (≥99%): required for clinical translation and regulatory-relevant research
Verifying Third-Party COAs
Some suppliers send peptides for third-party testing and include those results. When evaluating these, confirm the testing lab is named and verifiable. Be cautious of COAs where the testing date precedes the listed synthesis date, or where lot numbers do not match between COA header and chromatogram footer.
If you have access to analytical instrumentation, consider running your own HPLC or MS verification on receipt — particularly for high-stakes experiments. Retaining a reference standard from first-receipt analysis allows you to verify lot-to-lot consistency over time.
Building a COA Archive
Maintain a digital archive of COAs linked to lot numbers used in each experiment. If questions arise about data reproducibility during peer review or regulatory inspection, the ability to produce COA documentation for every compound used is invaluable.