The quadrupole ion trap, operated with a relatively high-pressure bath gas, occupies a unique place among mass analyzers. As an ion storage device, it shares many characteristics with the ICR instrument, but the use of a light bath gas and mass-selective instability for mass analysis contrasts with normal ICR operation. The bath gas facilitates the capture of ions injected into the oscillating quadrupole field and cools the ions to the center of the ion trap before mass-selective ejection. The bath gas also provides a conduit for heating ions. Mild collisional activation conditions can dissociate clusters, such as solvated analyte ions, formed by ES. Dissociation of covalently bound ions can be used to obtain structural information as part of a tandem MS experiment or to destroy polyatomic species in elemental analyses. The moderate background pressure requirements of the ion trap using mass-selective instability make it a natural mass analyzer to be coupled with high-pressure ionization methods. The ion trap can provide a significant advantage in efficiency over beam-type mass spectrometers, which translates into lower detection limits. Lower detection limits can be realized from superior duty cycle and MS/MS efficiency, although transmission for single-stage MS is comparable. The greatest advantage in efficiency can be obtained with weak ion beams that require long ion accumulation times and therefore high duty cycles. This, in part, makes the ion trap particularly attractive for ES. Bright ion sources can provide similar advantages if major ion beam components can be ejected during ion accumulation to minimize the deleterious effects caused by ion-ion interactions.(ABSTRACT TRUNCATED AT 250 WORDS)