A new instrumental concept, distance-of-flight mass spectrometry (DOFMS), is demonstrated experimentally. In DOFMS the mass-to-charge ratio of ions is determined by the distance each ion travels during a fixed time period; the mass spectrum is then recorded with a positionsensitive detector. The DOF approach provides a new way to separate and quantify components of complex samples. Initial results are demonstrated with a glow discharge ion source and a microchannel plate-phosphor screen detector assembly for atomic ion determination. This detection system demonstrated mass spectral peak widths of approximately 0.65 mm, corresponding to resolving powers of approximately 400-600 for a number of elemental samples.Key words: Mass spectrometry, Instrumentation, Glow discharge S ince the development of the first mass spectrograph over 90 years ago [1], numerous methods and instruments have been developed to separate and determine ions of varying mass-to-charge ratio (m/z). Currently, mass spectrometry (MS) can be accomplished by electrostatic and magnetic dispersion (sector-field MS), radio-frequency stability and filtering (quadrupoles and ion traps), resonance frequency determination (Fourier-transform ion cyclotron resonance and Orbitrap® MS), or velocity-based separation (time-of-flight MS). These mature MS technologies are routinely employed in a range of applications from biomolecule analysis to elemental isotope determination, and are often coupled with each other to achieve tandem MS analysis [2,3]. In the present paper, we describe the first implementation of a new form of MS, termed distance-offlight mass spectrometry (DOFMS), and suggest potential benefits of this new mass separation technique.Distance-of-flight mass spectrometry is akin to time-offlight mass spectrometry (TOFMS) in that both techniques separate ions of different m/z based upon an imparted m/zdependent velocity. In TOFMS, each ion is given the same energy (thus achieving a m/z-dependent velocity), and the m/z of each ion is calculated from the time required for it to traverse a known distance to a single detector. Conversely, in DOFMS the m/z of an ion is measured based on the spatial location of each ion at a specific time after the initial acceleration. As a useful analogy, DOFMS is to TOFMS as thin-layer chromatography is to elution chromatography (e.g., LC). TOFMS measures ions as they come off the "column", whereas DOFMS measures how far the ions travel after a specific separation time. Whereas TOFMS disperses ions in time, DOFMS disperses ions in space.The principle behind DOFMS is illustrated in Figure 1. To implement DOFMS, the primary ion beam is introduced into an orthogonal extraction region where ions are subjected to a brief electrostatic field, imparting identical momentum to each ion. This constant-momentum acceleration (CMA) is accomplished by limiting the temporal width and electrostatic field

Distance-of-flight mass spectrometry (DOFMS) is a velocity-based mass-separation technique in which ions are separated in space along the plane of a spatially selective detector. In the present work, a solid-state charge-detection array, the focal-plane camera (FPC), was incorporated into the DOFMS platform. Use of the FPC with our DOFMS instrument resulted in improvements in analytical performance, usability, and versatility over a previous generation instrument that employed a microchannel-plate/phosphor DOF detector. Notably, FPC detection provided resolution improvements of at least a factor of 2, with typical DOF linewidths of 300 μm (R((fwhm)) = 1000). The merits of solid-state detection for DOFMS are evaluated, and methods to extend the DOFMS mass range are considered.

Distance-of-flight mass spectrometry (DOFMS) offers the advantages of physical separation of ions, array detection of ions, focusing of initial ion energy, great simplicity, and a truly unlimited mass range. DOFMS instrumentation is similar to that of time-of-flight mass spectrometry (TOFMS) and shares its ion-source versatility, batch analysis, and rapid spectral-generation rate. With constant-momentum ion acceleration and an ion mirror, there is a time at which ions of all mass-to-charge values are energy focused at their particular distances along the flight path. A pulsed field orthogonal to the flight path drives the ions to reach the detector array at this specific time. Results from a 0.29-m proof-of-principle instrument verify the theoretically predicted energy focus and demonstrate how the range of mass-to-charge values that impinge on the detector array can be readily changed. DOFMS could be combined sequentially with TOFMS to enable simultaneous scanless tandem mass spectrometry.