The most common type of MS associated with a GC is the quadrupole mass spectrometer. Hewlett-Packard (now Agilent) markets it under trade name "Mass Selective Detector" (MSD). Another relatively common detector is the ion trap mass spectrometer. Additionally one may find a magnetic sector mass spectrometer. However these particular instruments are expensive and bulky and not typically found in high-throughput service laboratories. Other detectors are used such as time of flight (TOF), tandem quadrupoles (MS-MS) (see below), or in the case of an ion trap MS. The n indicates the number mass spectrometry stages.