These include Atmospheric Pressure Chemical Ionization, Atmospheric Pressure Laser-induced Acoustic Desorption Chemical Ionization (AP/LIAD-CI), Atmospheric Pressure Photoionization (APPI), Electrospray ionization (ESI) and Laser Desorption Ionization.
Atmospheric pressure chemical ionization (APCI), a widely used ionization technique in mass spectrometry, is similar to atmospheric pressure photoionization (APPI). It is advantageous for its ability to easily ionize and detect nonpolar or slightly polar species.
All mass spectrometers require the molecules to be in the gas phase and charged (ionized either positive or negative). In this technique, UV light photons are used to ionize sample molecules. The technique works well with nonpolar or low-polarity compounds not efficiently ionized by other ionization sources.
Electrospray ionization (ESI) is a popular technique in mass spectroscopy for ionizing samples before they are measured. ESI works well with heavier compounds and is therefore often used in proteomics (analysis of proteins) and petroleomics (analysis of petroleum).
MALDI, a method of ionization that has been widely used since its introduction in the late 1980s, has been of great help to scientists studying polymers, proteins and other heavy molecules. This is how it works.