Recent progress in atmospheric pressure ionization mass spectrometry

Atmospheric pressure ionization (API) is used as an interface in liquid chromatography-mass spectrometry and capillary electrophoresis-mass spectrometry. In API, quasi-molecular ions of biological molecules in solution are produced at atmospheric pressure. There have recently been several significant advances in spray ionization techniques, which use a solution spraying process. This article gives an overview of three spray ionization techniques: electrospray, ion spray, and sonic spray. It presents their characteristic features and describes the ion formation processes from charged droplets. Furthermore, the charged droplet formation process for each technique is described in terms of the non-uniformity in positive and negative ions in solution as well as in terms of the atomization process.