HPLC and Mass Spectrometry: How Peptide Purity and Identity Are Tested
HPLC and mass spectrometry are the two standard tests for research peptides. One tells you how pure a sample is, the other tells you what the molecule is.
Key takeaways
- Reversed-phase HPLC separates a peptide from its impurities and reports purity as a peak-area percentage.
- Mass spectrometry confirms identity by measuring molecular mass.
- Net peptide content differs from HPLC purity because water and counter-ions add weight.
Reversed-phase HPLC in brief
Most peptide purity testing uses reversed-phase high-performance liquid chromatography (RP-HPLC). The sample is pushed through a column packed with a hydrophobic material, commonly C18. A gradient of increasing organic solvent, often acetonitrile in water with a small amount of an ion-pairing acid, releases different molecules at different times depending on how hydrophobic they are. A UV detector, typically set around 214 to 220 nm where peptide bonds absorb, records each peak.
Reading a chromatogram
Each peak is a component of the sample. Purity is calculated as the area of the main peak divided by the total area of all peaks. A sharp, dominant main peak with small side peaks indicates a sample dominated by one species. Method details, such as the column, gradient and wavelength, should be stated, because different methods can resolve different impurities.
What HPLC cannot tell you
HPLC purity says nothing about what the main peak is. A pure peak of the wrong molecule would still look pure. It also only counts what the detector sees, so water and salts, which do not absorb UV light, are not included in the percentage.
Mass spectrometry for identity
A mass spectrometer ionizes molecules and measures their mass-to-charge ratio. Electrospray ionization (ESI) is common for peptides and often produces several charge states of the same molecule, such as [M+2H]2+ and [M+3H]3+. Software converts these into the molecular mass, which should match the theoretical mass calculated from the sequence.
Net peptide content and counter-ions
A lyophilized vial contains more than peptide. Residual water and counter-ions left from purification, commonly trifluoroacetate (TFA) or acetate, contribute to the weight. Net peptide content describes the fraction that is actual peptide and is often well below 100 percent even when HPLC purity is very high. It is measured by methods such as amino acid analysis or elemental nitrogen analysis.
Other tests you may see
- Endotoxin testing, relevant for cell-culture work.
- Sterility testing, relevant for sterile preparations.
- Amino acid analysis, which confirms composition and net peptide content.
- NMR, occasionally used for structural confirmation.
Put together, these results appear on a certificate of analysis.