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Distance of flight: a new paradigm for MS TOFMS distinguishes ions of difconditions required for focusing (a) Field-free region ferent m/z according to the time are different. In TOFMS, ions required for accelerated ions to are accelerated by constant-energy Detector reach a detector at a fixed distance extraction, in which a constant from the ion source. Distance-ofelectric field is applied during flight MS (DOFMS) is a new type the entire time ions are leaving of MS that reverses this detection the source. In contrast, DOFMS Ion mirror concept: an array of detectors requires constant-momentum spaced throughout the drift region extraction, a rarely used form of could record the distance that ion acceleration in which a strong (b) Push plate ions travel during a fixed time. electric field is applied for a duraField-free region DOFMS has not yet been impletion less than that required for mented, but in the November 15 ions to exit the source. “Conissue of Analytical Chemistry (pp stant-momentum extraction gives 8650–8661), Christie G. Enke all ions the same quick shove in Array of de and Gareth S. Dobson of the the forward direction,” Enke says. tecto Detection rs University of New Mexico lay the Thereafter, the ions maintain pulse foundations for the method by their initial spatial dispersion, determining the conditions for ion which limits the resolution to that Ion mirror focus. comparable to quadrupole mass In TOFMS, an electric-field analyzers. Enke thinks that spatial pulse accelerates the ions, which (a) In TOFMS, ions receive constant-energy acceleradispersion might be minimized traverse a field-free region with tion, traverse an ion mirror, and are detected in order of with the use of MALDI, an ionm/z-dependent velocities and are m /z by a detector that is located at a fixed distance. (b) In ization technique in which a laser recorded by a high-speed detecDOFMS, ions receive constant-momentum acceleration, ionizes molecules on a planar tor. In contrast, DOFMS uses an traverse an ion mirror, and are detected all at the same solid matrix. With energy disperarray of charge detectors spaced time (but at different distances) by an array of detectors. sion compensated for and spatial throughout the field-free region, dispersion minimized, very high parallel to the direction of ion flight. Moreover, ions moving backward at the mass resolutions would be possible. At a specific time of flight, a detection time of extraction must stop and turn DOFMS offers several advantages pulse is applied to a push plate, which around before accelerating in the forover TOFMS, including simpler elecsimultaneously drives all ions to the deward direction. tronics and increased ion detection tectors below. Spatial and kinetic-energy dispersion rate, says Enke. Masses are physically Achieving high-resolution mass must be minimized or compensated for separated from each other on different spectra in both TOFMS and DOFMS to focus ions of the same m/z at the detectors, allowing signal accumulais complicated by the fact that ions of same distance (TOFMS) or at the same tion over multiple ion-packet releases, identical m/z have different positions time (DOFMS). In TOFMS, kineticwhich would improve the S/N and dyand velocities in the source. “Velocityenergy dispersion is minimized and namic range of detection. Furthermore, based mass separation is like having the spatial dispersion is compensated for by DOFMS could be applied to isotope ions run a race,” says Enke. “But unlike the use of an ion mirror. An ion mirror separation and isotope ratio MS. human races, the ions are not all lined is a decelerating field that causes ions to Now that Enke and Dobson have deup or standing still when the starting stop, turn around, and be reflected back termined the optimal energy focus congun [the extraction pulse] goes off.” to the mirror entrance. Higher-energy ditions, the main challenge to bringing The term “spatial dispersion” refers to ions penetrate farther into the mirror; DOFMS to fruition is the development the distribution of ions of the same m/z this increases their flight times and alof array detectors. “The technology for in the source—some will be closer to lows lower-energy ions of the same m/z array detection has reached the place the source exit, whereas others will be to catch up. where this is a very practical possibility,” farther away. Like all molecules in a gas, DOFMS also benefits from an ion Enke says. “Nevertheless, the detection vapor-phase ions also have an initial enmirror, but through a series of mathtechnology must be adapted to this new ergy dispersion, with thermal velocities ematical calculations and simulations, type of mass spectrometer.” a described by a Boltzmann distribution. Enke and Dobson determined that the —Laura Cassiday 8830
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