Peptide Purity Testing: HPLC, Mass Spec and What COAs Show

The Certificate of Analysis (CoA) is the only document that provides you with information about the contents of a peptide vial. This post explains the two main tests – HPLC and mass spectroscopy – what is meant by each entry on the CoA and how to tell whether a test report is credible or worthless.

Why Purity Testing Matters for Research Peptides

Contaminants and Their Impact on Results

Research peptides are synthesized through solid-phase or liquid-phase peptide synthesis, processes that can leave behind truncated sequences, deletion peptides, racemized amino acids, residual solvents and endotoxins. A vial labeled "BPC-157, 10 mg" may contain 10 mg of material, but if only 90% of that material is the correct peptide, the actual BPC-157 content is 9 mg, and the other 1 mg is unknown impurities. For research, impurities can introduce confounding variables. For any use involving biological systems, contaminants like endotoxins can cause inflammatory responses unrelated to the peptide itself.

The Role of Third-Party Labs

Credible COAs must come from independent, third-party laboratories rather than from the peptide vendor's laboratory. There is a conflict of interest if the vendor tests their own products. The COA should provide the name of the laboratory that carried out the analysis, the date of analysis, the product lot or batch that corresponds to the product label, and details of the analysis. COA without the laboratory name and the batch number cannot be verified.

HPLC: The Primary Purity Method

How High-Performance Liquid Chromatography Works

High-performance liquid chromatography (HPLC) is a method of separating components of a mixture by pumping a sample through a column of stationary phase material. Different chemicals react to the stationary phase differently, causing them to move at different speeds in the column. This creates a chromatogram, which illustrates the amount of each component. In testing peptide purity, the target peptide should show as one major peak with an insignificant number of impurities shown by other peaks.

Reading an HPLC Chromatogram

Important metric necessary for calculating purity is purity percentage, which involves dividing area of the target peptide peak by the total area of all peaks. If we obtain the result of 98.5%, it means that 98.5% of UV-absorbing matter in the sample is indeed our target peptide, while 1.5% is something else. Research-grade peptides should have a purity of ≥98%, while pharmaceutical-grade peptides may have purity of ≥99%. Purity below 95% raises doubts regarding synthesis quality.

Mass Spectrometry: Identity Confirmation

Molecular Weight Verification

Mass spectrometry (MS) measures the molecular weight of the peptide with high precision. BPC-157 has a known molecular weight of 1419.5 g/mol. If the MS result shows 1419.5 (±0.5), the identity is confirmed. If it shows a different value, the vial does not contain the labeled peptide. MS is the identity test, it tells you that the compound is what it claims to be. HPLC is the purity test, it tells you how much of it there is relative to impurities. Both are needed for a complete picture.

What MS Cannot Tell You

While mass spectrometry is a tool for verifying the correctness of the molecular weight, none of the parameters pertaining to concentration, purity, or the amount of possible impurities in the sample can be evaluated since mass spectrometry cannot provide information about whether impurities in question are ionized (e.g., moisture, solvents, salts, etc.). Mass spectrometry cannot tell the difference between properly folded and poorly folded proteins even if they have the same mass. In a nutshell, mass spectrometry is a helpful but insufficient technique for answering the question of whether or not one deals with the correct substance, while it does not give the answer to the question of how pure the sample is and how much active peptide is in the vial.

Additional Tests on a COA

Endotoxin (LAL) Testing

The Limulus Amebocyte Lysate (LAL) test detects bacterial endotoxins — lipopolysaccharides from gram-negative bacteria that can cause fever, inflammation and immune activation. LAL testing is critical for any peptide that will be used in biological systems. Research-grade peptides should test below the endotoxin limit (typically <0.5 EU/mg). Not all vendor COAs include LAL testing; its presence is a mark of higher quality control.

Residual Solvent Analysis

The process of peptide synthesis involves the utilization of organic solvents (such as dimethylformamide (DMF), dichloromethane (DCM), trifluoroacetic acid (TFA), and acetonitrile), which should be eliminated in purifying the resulting product. The identification of residual solvents through gas chromatography or headspace analysis is used to ensure the solvents remain even below pharmacopoeial thresholds. Trace solvents are found in the allowable quantities, but high concentrations suggest ineffective purification processes.

Amino Acid Analysis

AAA validates the amino acid composition of the peptide by hydrolyzing the peptide into individual amino acids and analyzing them for amounts. The amino acid analysis confirms that the peptide contains the right amino acids and their ratios as they ought to be. Although it is the most direct confirmation of the sequence integrity, it is also the most costly analysis and is usually not included in standard certificates of analysis.

How to Read a Peptide COA