"HPLC / MS" appears on every credible peptide Certificate of Analysis, but the two instruments answer completely different questions. HPLC measures how much of the sample is one dominant species. Mass spectrometry confirms what that species is. Either result alone is a partial answer. Together they close the gap.
HPLC answers "how much"
High-Performance Liquid Chromatography separates the components of a sample by pushing them through a column that retains them at different rates. The compound that dominates the sample elutes as the largest peak on the resulting chromatogram. A UV detector at 220 nm measures the area under every peak, and purity is expressed as the target-peak area as a percentage of total peak area.
The strength of HPLC is quantitation. If the chromatogram shows one large peak and a scattering of small ones, and integration gives 99.1%, that is the purity. What HPLC cannot tell you is whether the compound in that dominant peak is the compound you ordered. Two peptides with different sequences but similar hydrophobicity elute at similar retention times. HPLC alone cannot distinguish them.
Mass spec answers "what"
Mass spectrometry ionizes the sample and measures the mass-to-charge ratio of every ion that passes through the analyzer. For a peptide of known sequence, the theoretical molecular weight can be calculated to four decimal places. If the observed mass in the spectrum matches that calculated mass within instrument tolerance (typically ± 0.1 Da for ESI-MS), the identity is confirmed.
The strength of MS is identity, not quantitation. A spectrum showing the right mass tells you the target compound is present — but not whether it dominates the sample. A batch that is 60% target peptide and 40% synthesis byproducts of similar mass would still show a strong peak at the expected m/z. Identity confirmed, but the batch is unusable.
Two questions, one answer each
A single-assay COA leaves one of the two questions unanswered:
- HPLC alone — you know the sample is 99% one thing, but not what that thing is.
- MS alone — you know the target compound is present, but not whether it is the majority of the vial.
- HPLC + MS — 99% of the sample is one species, and that species is the compound you ordered. The vial matches the label.
Why the combination is the standard
Modern LC-MS instruments couple the two techniques directly: the HPLC eluent flows into the mass spec, and every peak in the chromatogram gets a mass spectrum attached. That gives you retention time, area, and mass in one run — the full identity + purity picture for the target peak and for every impurity it separated from.
On the AminoVita Research COA, both methods appear on every batch. When you see "HPLC / MS" listed together, that is the paperwork saying: we measured how much, we confirmed what, and both answers came from the same batch that shipped to you.