HPLC and mass spec — what each one actually proves.

If you've spent any time reading certificates of analysis from peptide suppliers, you've noticed the variation. Some show a chromatogram. Some show a mass-spec readout. Some show both. Some show neither and just publish a percentage. The difference matters more than the standardization implies.

What HPLC actually measures

High-performance liquid chromatography separates the compounds in a sample by how strongly they interact with a stationary phase as a mobile phase pushes them through a column. For peptides, the standard setup is reverse-phase HPLC: a non-polar column (typically C18) and a gradient of water-to-acetonitrile with trifluoroacetic acid as an ion-pairing agent. Compounds elute in order of hydrophobicity. The detector — usually UV at 214 nm for peptide bonds — measures absorbance over time.

What you get is a chromatogram: a curve where the x-axis is retention time and the y-axis is signal intensity. A clean peptide produces one tall, narrow main peak with maybe a few small impurity peaks scattered nearby. Purity is calculated as the area of the main peak divided by the total integrated area of all peaks above the baseline. That's where the percentage on a COA comes from.

What HPLC doesn't tell you

It doesn't tell you what the main peak is. It tells you the sample contains one dominant compound and that compound elutes at a specific retention time. If you have a reference standard, you can co-inject and confirm the retention times match. If you don't, you have evidence of purity, not evidence of identity. A clean chromatogram of the wrong molecule looks identical to a clean chromatogram of the right one.

What mass spec adds

Mass spectrometry ionizes the sample and measures the mass-to-charge ratio of every ion produced. For peptides, electrospray ionization (ESI-MS) is the standard. Each peptide has a known molecular weight and produces a characteristic isotope envelope. The m/z spectrum of a clean peptide preparation matches the calculated molecular weight to within a few parts per million on modern instruments ^[1].

MS is the identity confirmation HPLC alone can't provide. The two techniques are complementary: HPLC shows you the sample is one compound and quantifies how much; MS shows you that compound is the molecule the label claims. Either one alone leaves a gap.

How to read a COA

• Look for both an HPLC trace and a mass-spec readout. If only one is shown, ask why.
• On the chromatogram, the baseline should be flat between peaks. A noisy or sloped baseline suggests the gradient was poorly resolved — not necessarily a quality issue but worth noting.
• Main peak symmetry matters. A symmetric peak suggests good column performance and no co-eluting impurities. A tailing peak can hide a second compound underneath.
• On the mass spec, the dominant ion should match the calculated [M+H]⁺ or [M+nH]ⁿ⁺ for the peptide. Adducts (sodium, potassium) are common and benign.
• Compare the reported purity percentage against the visible chromatogram. The math should add up — a 99.5% claim from a chromatogram with a meaningful 1% impurity peak doesn't.

Why some brands skip mass spec

Mass spec is more expensive and more specialized than HPLC. A reverse-phase HPLC instrument at a contract lab costs roughly $15–40 per injection; an ESI-MS run is closer to $80–200 depending on resolution and the lab. For high-volume peptide production, suppliers cutting costs often run HPLC every batch and reserve MS for a quarterly identity check, or skip it entirely after the initial production lot was characterized.

That's a defensible decision for established peptides — once you've run MS on the production process and confirmed the synthesis is making the right molecule, ongoing HPLC purity checks are sufficient most of the time. It becomes a problem when the supplier doesn't tell you that's what they're doing.