Raymond E. March

An Introduction to Quadrupole Ion Trap Mass Spectrometry

A concise introduction is presented to the theory and application of quadrupole ion trap mass spectrometry. The presentation of the theoretical treatment is based on a demonstration of the equivalence of the force acting on an ion in a quadrupole field and the force derived from the Mathieu equation; this equivalence permits the application of the solutions of Mathieu’s equation to the confinement of gaseous ions. Resonant excitation, collision-induced dissociation, mass spectrometry, tandem mass spectrometry and chemical ionization are discussed in the context of analytical applications. Sample calculations of the trapping parametersqzand βz, the axial secular frequency, mass range, mass range extension and the magnitude of the potential well depth are given.

Quadrupole ion trap mass spectrometry: a view at the turn of the century

A personal account of the quadrupole ion trap researches carried out in my laboratory and in collaboration with other laboratories. This account commences with the announcement, in 1983, of the first commercially available ion trap detector, manufactured by Finnigan MAT, and continues to the present day. Much of the ion trap mass spectrometry research that took place during the period following this announcement until 1994 has been discussed in detail in three volumes entitled Practical Aspects of Ion Trap Mass Spectrometry that were published in 1995. Except for those researches that impinged directly on our work during this period, no discussion of the contents of these three volumes is repeated here. The ion trap literature from 1994 to the present has been reviewed selectively so as to convey to the reader the dynamic nature of ion trap mass spectrometry and the wide variety of its application.

Quadrupole ion traps

The extraordinary story of the three-dimensional radiofrequency quadrupole ion trap, accompanied by a seemingly unintelligible theoretical treatment, is told in some detail because of the quite considerable degree of commercial success that quadrupole technology has achieved. The quadrupole ion trap, often used in conjunction with a quadrupole mass filter, remained a laboratory curiosity until 1979 when, at the American Society for Mass Spectrometry Conference in Seattle, George Stafford, Jr., of Finnigan Corp., learned of the Masters study of Allison Armitage of a combined quadrupole ion trap/quadrupole mass filter instrument for the observation of electron impact and chemical ionization mass spectra of simple compounds eluting from a gas chromatograph. Stafford developed subsequently the mass-selective axial instability method for obtaining mass spectra from the quadrupole ion trap alone and, in 1983, Finnigan Corp. announced the first commercial quadrupole ion trap instrument as a detector for a gas chromatograph. In 1987, confinement of ions generated externally to the ion trap was demonstrated and, soon after, the new technique of electrospray ionization was shown to be compatible with the ion trap.

Ion trap mass spectrometry

Abstract Enhanced trajectory control in the quadrupole ion trap, in the presence of helium buffer gas, has brought about enormous improvements in mass range (mass limits in excess of 70000 Da have been achieved), mass resolution (full width at half-maximum, 1 130000 at m/z 3510), and multistage tandem mass spectrometry (MS)n where n ⪕ 13. The compatibility of the ion trap with external ion sources, such as electrospray and atmospheric pressure discharges, has greatly extended the range of application of the ion trap. Brief descriptions of the diagrammatic representation of ion trajectory stability and ion trapping theory precede a review of axial modulation, mass-selective axial ion ejection, ion isolation, collision-induced dissociation, tandem mass spectrometry and dynamically programmed scans. The progress made during the past 3 years in all aspects of ion trap mass spectrometry is reviewed.

Ion Trap Mass Spectrometers

An ion trap mass spectrometer functions both as a mass spectrometer of considerable mass range and variable mass resolution and as an ion store in which gaseous ions can be confined. Unlike other mass spectrometers that operate at pressures o10xa0−6 Torr, the ion trap operates at 1 mTorr of helium. As a storage device, the ion trap acts as an ‘electric-field test-tube’ for the confinement of gaseous ions either positively charged or negatively charged. The confining capacity of the ion trap arises from a trapping potential well formed when appropriate potentials are applied to the ion trap electrodes. The ion trap functions as a mass spectrometer when the trapping field is changed, so that the trajectories of simultaneously trapped ions of consecutive specific mass/charge ratio become sequentially unstable, and ions leave the trapping field in order of mass/charge ratio. Upon ejection from the ion trap, ions strike a detector and provide an output signal.