ESI greatly enhanced the ability to characterize protein and peptides by MS. ESI requires the sample of interest to be in solution- flow in to the ionization source region. To ionize the sample, high voltage is applied, result in the sample becoming positively or negatively charged; however, positive ionization is used primarily in the analysis proteins and peptides. As it exits the spray tip, the solution produces submicrometer-sized droplets containing both solute and analyte ions. The sample is then desorbed of solute prior to entering the analyzer region
