HPLC vs. mass spec: what peptide testing actually measures
Peptide COAs usually reference two methods: high-performance liquid chromatography (HPLC) and mass spectrometry (MS). They answer different questions, and neither one is enough on its own.
HPLC answers: how pure is it?
In reversed-phase HPLC, the dissolved sample is pushed through a column packed with a nonpolar material (commonly C18). Different molecules stick to the column more or less strongly, so they come out at different times. A UV detector, usually set around 214–220 nm where peptide bonds absorb light, records each one as a peak.
Purity is the main peak's area as a percentage of the total peak area. If the target peptide accounts for 99.2% of the detected area, the purity is reported as 99.2%.
What HPLC can miss
- Impurities that come out at the same time as the main peak (co-elution) get counted as the main peak.
- Anything that does not absorb at the detection wavelength, such as water or many salts, is invisible.
- HPLC does not identify the main peak. A pure peak could still be the wrong molecule.
Mass spec answers: is it the right molecule?
Mass spectrometry ionizes the sample (commonly by electrospray, ESI, or MALDI) and measures the mass-to-charge ratio of the ions. The observed mass is compared with the theoretical molecular weight of the expected sequence. A match confirms identity. A mismatch can reveal a deletion, a missing protecting-group removal, or oxidation.
Mass spec is excellent at identity but is not a reliable way to quantify purity by itself, because different molecules ionize with different efficiency.
Why the best COAs use both
Together, HPLC and MS tell you the material is the right molecule and that it makes up the stated share of what was detected. LC-MS combines both in one run, linking each HPLC peak to a mass. When you read a COA, look for a purity result and an identity result, plus the chromatogram and mass spectrum behind them.