Does HPLC purity prove a peptide is the right compound?

No. HPLC quantifies how much of the peptide-related material is a single species; mass spectrometry is what confirms that species is the specified molecule. A certificate needs both.

Why is HPLC detection usually at 214 nm?

214 nm detects the amide bond and therefore all peptide-related species. Detection at 280 nm only sees aromatic residues and will under-report impurities that lack them.

Does a certificate of analysis expire?

It documents the material at the time of release rather than indefinitely. Long-stored stock used in quantitative work should be re-assayed rather than assumed to match its original certificate.

Analysis · 7 min read

HPLC and mass spectrometry: what each test actually proves

Why purity and identity are separate questions, what a chromatogram can and cannot show, and how to read the two analyses together on a certificate.

Two questions, two instruments

RP-HPLC answers a quantitative question: of the material that absorbs at the detection wavelength, what fraction elutes as a single peak at the expected retention time. Mass spectrometry answers a qualitative one: is the molecule present the molecule specified.

Neither substitutes for the other. A sample can be 99.5% pure by HPLC and be the wrong peptide entirely — purity says nothing about identity. Equally, a mass spectrum can confirm the correct molecular weight in a sample that is 60% related substances, because MS detects presence rather than proportion.

Reading a chromatogram properly

The number quoted as purity is the area of the main peak divided by the total integrated area, expressed as a percentage. That makes the integration baseline consequential: a baseline drawn generously under a shoulder can move a 96% result to 98.5% without any change to the material.

This is why the trace itself matters more than the number. Look for a symmetric main peak, a flat baseline, and whether close-eluting shoulders have been integrated separately or absorbed into the main peak. Detection is typically at 214 nm, which sees the amide bond and therefore all peptide-related species; 280 nm sees only aromatic residues and will under-report impurities lacking them.

What the mass result should show

The reported observed mass should match the theoretical monoisotopic or average mass within the instrument's stated tolerance — typically well under 1 Da for a peptide of this size on a modern instrument. Larger deviations warrant explanation.

Characteristic offsets are diagnostic. A result 18 Da low suggests dehydration or an unintended cyclisation; 42 Da high suggests residual acetylation; 16 Da high suggests methionine oxidation. A supplier that reports only 'MS confirmed' without the observed value has removed the only part of the result you could have checked.

What a certificate still does not tell you

Neither analysis addresses endotoxin, bioburden, residual solvents, or heavy metals. Neither establishes potency in a biological system. And crucially, neither carries forward in time: a certificate documents the material as it was at release, not as it is after nine months in a warm cupboard.

Treat the certificate as a release record with a date attached. For long-held stock in quantitative work, periodic re-assay is the only way to know the current state of the material.

Frequently asked questions

Does HPLC purity prove a peptide is the right compound?: No. HPLC quantifies how much of the peptide-related material is a single species; mass spectrometry is what confirms that species is the specified molecule. A certificate needs both.
Why is HPLC detection usually at 214 nm?: 214 nm detects the amide bond and therefore all peptide-related species. Detection at 280 nm only sees aromatic residues and will under-report impurities that lack them.
Does a certificate of analysis expire?: It documents the material at the time of release rather than indefinitely. Long-stored stock used in quantitative work should be re-assayed rather than assumed to match its original certificate.