Linear ion traps are finding new applications in many areas of mass spectrometry. In a linear ion trap, ions are confined radially by a two-dimensional (2D) radio frequency (RF) field, and axially by stopping potentials applied to end electrodes. This review focuses on linear ion trap instrumentation. Potentials and ion motion in linear multipole fields and methods of ion trapping, cooling, excitation, and isolation are described. This is followed by a description of various mass discrimination effects that have been reported with linear ion traps. Linear ion traps combined in various ways with three-dimensional (3D) traps, time-of-flight (TOF) mass analyzers, and Fourier transform ion cyclotron resonance mass spectrometers are then given. Linear ion traps can be used as stand alone mass analyzers, and their use for mass analysis by Fourier transforming image currents, by mass selective radial ejection, and by mass selective axial ejection are reviewed.

Hydroxysulfinyl radical (1), hydrogensulfonyl radical (2), and dihydroxysulfane (6) were generated in the gas phase by collisional reduction of the corresponding cations and studied by the variable-time and photoexcitation methods of neutralization-reionization mass spectrometry and by ab initio and RRKM calculations. Radicals 1 and 2 were thermodynamically and kinetically stable. Two rotamers of 1, syn-1 and anti-1, were found computationally to be local energy minima. The computations suggested a complex potential energy surface for dissociations of 1. The minimum-energy reaction path was the rate-determining isomerization to 2 followed by fast loss of H • to form SO 2 . Direct H • loss from 1 was kinetically disfavored. Cleavage of the S-OH bond in 1 was highly endothermic and became kinetically significant at excitations >325 kJ mol -1 . In contrast to ab initio/RRKM predictions, 1 formed by vertical reduction of hydroxysulfinyl cation (1 + ) dissociated mainly to OH • and SO, whereas loss of H • was less significant. Both dissociations showed microsecond kinetics as established by variable-time measurements. Photoexcitation of nondissociating 1 opened the H-loss channel, whereas collisional excitation did not change the branching ratio for the H • and OH • loss channels. The experimental results pointed to the formation of a large fraction of metastable and dissociative excited electronic states of 1 upon vertical electron transfer. Radical 2 was cogenerated with 1 by vertical reduction of a mixture of 1 + and 2 + produced by highly exothermic protonation of SO 2 with H 3 + . Pronounced loss of H • from 2 occurred following collisional neutralization in accordance with RRKM predictions. Dihydroxysulfane (6) was stable following collisional neutralization of the cation-radical 6 •+ . The G2(MP2) potential energy surface predicted the isomerization to hydrogensulfinic acid (7) followed by loss of water to be the lowest-energy dissociation of 6. RRKM calculations showed the 6 f 7 isomerization to be the ratedetermining step. Cation-radical 6 •+ also eliminated water through unimolecular isomerization to a stable nonclassical isomer, OS •+ ‚‚‚H 2 O (9). The thermochemistry of the neutral and ionic systems is discussed. The important role of excited electronic states in the formation of radicals by vertical electron transfer is emphasized.

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Modeling of ion motion and experimental investigations of ion excitation in a linear quadrupole trap with a 4% added octopole field are described. The results are compared with those obtained with a conventional round rod set. Motion in the effective potential of the rod set can explain many of the observed phenomena. The frequencies of ion oscillation in the x and y directions shift with amplitude in opposite directions as the amplitudes of oscillation increase. Excitation profiles for ion fragmentation become asymmetric and in some cases show bistable behavior where the amplitude of oscillation suddenly jumps between high and low values with very small changes in excitation frequency. Experiments show these effects. Ions are injected into a linear trap, stored, isolated, excited for MS/MS, and then mass analyzed in a time-of-flight mass analyzer. Frequency shifts between the x and y motions are observed, and in some cases asymmetric excitation profiles and bistable behavior are observed. Higher MS/MS efficiencies are expected when an octopole field is added. MS/MS efficiencies (N 2 collision gas) have been measured for a conventional quadrupole rod set and a linear ion trap with a 4% added octopole field. Efficiencies are chemical compound dependent, but when an octopole field is added, efficiencies can be substantially higher than with a conventional rod set, particularly at pressures of 1.4 ϫ 10 Ϫ4 torr or less. . The most widely discussed distortion is the "stretched" ion trap [2], which has the end cap electrodes moved out so that the distance to the end cap, z 0 , is increased over that of an ideal field, z 0 ϭ r 0 ⁄ ͙ 2, where r 0 is the distance from the center to the ring electrode. It has been argued that the addition of higher order multipole fields of the correct sign to 3-D traps improves MS/MS efficiency [1c, 1f, 2], and allows faster ejection at the stability boundary [2,3], to give higher scan speeds and improved mass resolution.There is increasing interest in using linear quadrupoles as ion traps, both as stand alone mass analyzers with radial [4] or axial [5] ejection, or in combination with other mass analyzers (for a recent review see [6]).There is also interest in trapping and exciting ions for MS/MS at the relatively low pressures typical for operation of the last mass analyzing quadrupole in triple quadrupole systems, ca. 3 ϫ 10 Ϫ5 torr [7]. Addition of higher multipoles to a linear ion trap might be expected to provide benefits similar to those seen with 3-D traps. Douglas and coworkers [8] have shown that an octopole field can be added to a linear quadrupole by using rod sets with rods equally spaced from the central axis but with one pair of rods different in diameter than the other pair, as shown in Figure 1. The electric potential within this rod set is given to a good approximation bywhere x is the distance from the center towards a smaller rod, y is the distance from the center towards a larger rod, r 0 is the distance from the center to any rod, and U and V rf are the amplitudes...

The title radicals were produced by femtosecond collisional electron transfer in the gas phase and studied by the methods of variable-time neutralization−reionization mass spectrometry combined with fast-beam laser photoexcitation and G2(MP2) ab initio/RRKM calculations. The methylsulfonyl radical (CH3SO2 •, 1) was calculated to be bound by 59 kJ mol-1 against the lowest-energy dissociation to CH3 • and SO2 at 0 K and to have a heat of formation Δ H f,298(1) = −211 ± 4 kJ mol-1. When formed by vertical electron transfer, radical 1 dissociated rapidly due to a large Franck−Condon energy, E FC = 141 kJ mol-1. The reverse addition of CH3 • to the sulfur atom in SO2 had a potential energy barrier of 1.3 kJ mol-1 and Arrhenius parameters, log A = 12.19 and E a = 5.4 kJ mol-1. The calculated addition rate constant, k 295 = 1.7 × 1011 cm3 mol-1 s-1, was in excellent agreement with the previous measurement of Simons et al. The methoxysulfinyl radical (CH3OSO•, 2) was calculated to exist as an equilibrium mixture of syn (2s) and anti (2a) conformers. The Boltzmann-averaged heat of formation of 2 was calculated as Δ H f,298(2) = −230 ± 4 kJ mol-1. Vertical neutralization of ions 2s + and 2a + produced substantial fractions of stable 2s,a. Dissociating 2s,a formed CH3 • and SO2 through unimolecular isomerization to 1. Direct dissociation of the C−O bond in 2s,a to form CH3 • and SO2 was calculated to have a large activation barrier (152 kJ mol-1 from 2a) and did not compete with the isomerization to 1, which required 111 kJ mol-1 from 2a. Photoexcitation of 2s,a resulted in a slightly increased formation of 2s,a +. This was interpreted with the help of CIS/6-311+G(3df,2p) calculations as being due to the formation of a bound excited B state of 2s upon electron transfer. The B state was photoexcited at 488 and 514.5 nm to high Rydberg states which were predicted to have large cross sections for collisional ionization. The A state of 2s was calculated to be bound but photoinactive. The C through E states of 2s were unbound and predicted to dissociate exothermically to CH3OS and (3P)O.