Peptide Purity: HPLC & Mass Spec Explained

A Certificate of Analysis (COA) is the only objective evidence of what is in a vial. Knowing how to read one separates serious researchers from buyers who assume marketing claims.

The Four Mandatory Sections

A complete COA contains:

1. Identity, compound name, sequence, lot number, molecular formula, molecular weight.
2. Purity, RP-HPLC chromatogram with % area at 220 nm.
3. Mass confirmation, ESI-MS or MALDI-TOF result matching the theoretical [M+H]+.
4. Quantitative analysis, net peptide content (NPC), water content, counterion content.

RP-HPLC Purity

Reverse-phase high-performance liquid chromatography separates the target peptide from impurities. The chromatogram shows peaks; the % area of the main peak at 220 nm is the purity figure quoted on the COA.

• •≥98% is the standard threshold for research-grade material.
• •≥95% is acceptable for early development work.
• •Anything below 95% should be rejected for serious experiments.

Watch for adjacent shoulder peaks, these often indicate diastereomers or single-residue deletions that can have biological activity different from the parent peptide.

Mass Spectrometry

ESI-MS provides the monoisotopic mass. The reported [M+H]+ should match the theoretical mass to within ±0.5 Da (or much tighter on high-resolution instruments). A mass that is 18 Da low suggests dehydration; +16 Da suggests oxidation; ±1 Da differences usually mean isotope misassignment, not a real problem.

Net Peptide Content (NPC)

This is the most overlooked number on a COA. A vial labeled "10 mg peptide, 99% HPLC purity" might still only contain 7 mg of actual peptide by mass. The remaining 3 mg is:

• •Water (1–10%)
• •TFA counterion (5–25% for unmodified peptides)
• •Salts and residual solvents

NPC is determined by amino acid analysis or quantitative NMR. If your dosing precision matters, you must use the NPC value, not the gross vial weight.

TFA Counterion Correction

Most synthetic peptides are isolated as TFA (trifluoroacetate) salts. Each basic residue (Arg, Lys, His, N-terminus) carries one TFA molecule. For a peptide with three basic groups, TFA can account for 15–25% of the powder mass.

For sensitive cell-based work, request a counterion exchange to acetate, TFA can have its own biological effects at high concentrations.

Water Content (Karl Fischer)

Lyophilized peptides typically contain 2–8% residual water. Higher values accelerate degradation during storage.

Putting It Together, A Worked Example

A vial labeled:

• •HPLC purity: 99.2%
• •Mass: matches theoretical
• •Net peptide content: 78%
• •Water: 4%
• •TFA counterion: 18%

Means: of every 10 mg of powder, 7.8 mg is actual peptide, of which >99% is the correct sequence. Dose your experiments using 7.8 mg per 10 mg vial.

Red Flags on a COA

• •No chromatogram image, only a number.
• •Mass spectrum missing or "data on file."
• •No lot number or analysis date.
• •Net peptide content not reported.
• •Generic boilerplate language with the compound name swapped in.

A trustworthy supplier will publish the actual chromatogram and MS spectrum per lot. Anything less is marketing, not chemistry.

HPLC, What the Chromatogram Shows

Reversed-phase HPLC (RP-HPLC) passes a peptide sample through a C18 column with a water/acetonitrile gradient. Peptides elute in order of hydrophobicity. A UV detector at 220 nm (peptide bond) or 214 nm records absorbance versus time, producing the chromatogram.

Key elements to inspect:

• •Target peak, usually the tallest, at the reported retention time
• •Peak area %, the purity number on the COA
• •Impurity peaks, sequence variants, truncations, deletions
• •Baseline drift, poor baseline suggests column contamination or a bad method

A clean chromatogram shows one dominant symmetric peak with baseline separation from any minor peaks.

Mass Spec, Confirming Identity

Electrospray Ionization (ESI) or Matrix-Assisted Laser Desorption/Ionization (MALDI) ionizes the peptide and measures its mass-to-charge ratio. The COA should report:

• •Theoretical monoisotopic mass, calculated from sequence
• •Observed mass, measured from the sample
• •Delta, the difference, ideally within ±1 Da

Common deltas and what they mean:

• •+16 Da, methionine or tryptophan oxidation
• •+22 Da, sodium adduct (usually harmless artifact)
• •+42 Da, acetylation
• •−18 Da, dehydration

Combined Interpretation

HPLC without mass spec tells you the sample is pure but not necessarily the right peptide. Mass spec without HPLC tells you the target is present but not necessarily dominant. A trustworthy COA carries both.

Frequently Asked Questions

Why 220 nm for HPLC detection?

220 nm is the absorbance maximum of the peptide bond, giving uniform detection across sequences regardless of aromatic content.

What purity threshold is standard?

≥95% for research use; ≥98% for premium suppliers; ≥99% for peptides where impurities would confound sensitive assays.

Can HPLC alone confirm identity?

No. Two different peptides can co-elute. Mass spec is required for identity confirmation.

What is a "shoulder peak"?

A partially resolved second peak fused to the target, usually a deamidation product or a diastereomer. It inflates the target area and lowers effective purity.