The most common type of mass spectrometer ( MS ) associated with a gas chromatograph ( GC ) is the quadrupole mass spectrometer , sometimes referred to by the Hewlett-Packard ( now Agilent ) trade name `` Mass Selective Detector '' ( MSD ) .
Hewlett-Packard ( now Agilent ) markets it under trade name `` Mass Selective Detector '' ( MSD ) .