Figure 2.

MALDI and ESI Ionization. In MALDI ionization, samples are co-crystallized with an organic matrix on a MALDI plate. A pulsed laser irradiates the co-crystals. This induces rapid heating and desorption of ions into the gas phase. Ions pass through the mass analyzer, such that the smaller peptides reach the detector before the larger peptides (time of flight; TOF). The detector registers the numbers of ions at each individual mass-to-charge (m/z) value, and then the peptide mass fingerprint is generated. In ESI-MS, sample molecules (peptides) are ionized directly in solution. The peptide solution passes through a heated capillary device, and droplets of solution are then sprayed into a vacuum chamber containing a high-strength electric field. The resulting ions pass through a mass analyzer and reach the detector, generating complex spectra with multiply charged ions.