Atmospheric Pressure Ionization (API) is a soft ionization technique that provides quasimolecular information. Two API methods are available at our lab: Electrospray Ionization (ESI) and Atmospheric Pressure Chemical Ionization.

Electrospray ionization (ESI)
ESI is a technique used in mass spectrometry to overcome the propensity of macromolecules to fragment. In ESI, as illustrated below, a liquid is pushed through a very small charged metal capillary by a carrier gas. The liquid contains the substance which is to be studied, the analyte, as well as a large amount of solvent, which is usually much more volatile than the analyte. The charge contained in the capillary transfers to the liquid which charges the analyte molecule. As like charges repel, the liquid pushes itself out of the capillary and forms a mist or an aerosol of small droplets about 10um diameter, to increase the distance between the similarly charged molecules. A neutral carrier gas is sometimes used to evaporate the neutral solvent in the small droplets, this in turn brings the charged analyte molecules closer together. The proximity of the molecules becomes unstable, however, and as the similarly charges molecules come closer together, the droplets once again explode. This process repeats itself until the analyte is free of solvent and is a lone ion. The lone ion will then continue along to a mass analyzer.

Atmospheric Pressure Chemical Ionization (APCI)
As the name implies, APCI is a technique which creates ions at atmospheric pressure. A sample solution flows through a heated tube where it is volatilized and sprayed into a corona discharge with the aid of nitrogen nebulization (see schematic illustration below). Ions are produced in the discharge and extracted into the mass spectrometer. APCI i s best suited to relatively polar, semi-volatile samples. An APCI mass spectrum usually contains the quasi-molecular ion, [M+H]+. This technique is used as an LCMS interface because it can accomodate very high (1 ml/min) liquid flow rates.