Electrospray Ionization (ESI) is a soft ionization technique used in mass spectrometry to produce ions from large molecules, such as proteins and nucleic acids. In this process, a high voltage is applied to a liquid to create an aerosol of charged droplets, which then evaporate to yield gas-phase ions. This method is particularly valuable in the analysis of biomolecules due to its ability to ionize large and polar compounds without fragmentation. ESI is widely used in proteomics and metabolomics, enabling researchers to study complex biological samples with high sensitivity and specificity.