The Orbitrap is an ion trap mass analyzer that consists of two outer electrodes and a central electrode, which enable it to act as both an analyzer and detector. Ions entering the Orbitrap are captured through "electrodynamic squeezing," after which they oscillate around the central electrode and in between the two outer electrodes. Different ions oscillate at different frequencies, resulting in their separation. By measuring the oscillation frequencies induced by ions on the outer electrodes, the mass spectra of the ions are acquired using image current detection. GC-MS is a technique which combines separation capabilities of GC with detection and identification capabilities of MS. Two ionization modes commonly employed are electron ionization (EI) and chemical ionization (CI). In EI, a beam of high energy electrons emitting from a filament strikes the stream of sample molecules. As a result, ionization takes place. Whereas in CI, the sample molecules collide with a stream of pre-ionized reagent gas causing ionization. In tandem MS (MS/MS or MSn), precursor ions undergo further fragmentation.