There are many types of ionization techniques are used in mass spectrometry methods. The classic methods that most chemists are familiar with are electron impact (EI) and Fast Atom Bombardment (FAB). These techniques are not used much with modern mass spectrometry except EI for environmental work using GC-MS. Electrospray ionization (ESI) - ESI is the ionization technique that has become the most popular ionization technique. The electrospray is created by putting a high voltage on a flow of liquid at atmospheric pressure, sometimes this is assisted by a concurrent flow of gas. Atmospheric Pressure Chemical Ionization (APCI) - APCI is a method that is typically done using a similar source as ESI, but instead of putting a voltage on the Electrospray Tandem Mass Spectrometry Newborn Screening itself, the voltage is placed on a needle that creates a corona discharge at atmospheric pressures. Matrix Assisted Laser Electrophoresis is a technique of ionization in which the sample is bombarded with a laser.