Electrospray Ionisation Mass Spectrometry (ESI-MS)
A mass spectrometry technique that ionises molecules from a liquid solution by applying high voltage. ESI-MS is especially useful for larger peptides and proteins and can provide detailed structural information including sequence confirmation and identification of post-translational modifications.
Technical Context
ESI process: peptide solution flows through a capillary at high voltage (2-5 kV) → charged droplets form at the capillary tip (Taylor cone) → solvent evaporation in heated desolvation gas → repeated droplet fission (Coulomb explosion) → fully desolvated multiply charged peptide ions. Key advantage: multiply charged ions ([M+nH]n+) bring large peptides into the m/z range of standard mass analysers. For a 4 kDa peptide with 4 charges: m/z = 1001. Mass analysers: quadrupole (unit mass resolution, good for quantification), time-of-flight (high resolution, high mass accuracy), Orbitrap (ultra-high resolution, <1 ppm mass accuracy), and ion trap (MSn capability for structural characterisation). LC-ESI-MS/MS is the workhorse of pharmaceutical peptide analysis: characterising sequences, detecting modifications, quantifying impurities, and measuring peptide concentrations in biological matrices (pharmacokinetic studies).