Flight times spatially distributed ions

The Chevron channel plate ion detector assembly of an imaging atom-probe can also be replaced by a position sensitive particle detector combined with a data processor, as reported by Cerezo etal.5s (A position sensitive detector was used earlier for the purpose of field ion image recording and processing.59) With such a detector both the chemical identity and the spatial origin on the emitter surface can be found for each field evaporated ion. This position sensitive atom-probe can be used to study the spatial distribution of different ion species on the emitter surface as well as inside the bulk of the emitter with a spatial resolution nearly comparable to the FIM. For such a purpose, one carries out the field evaporation at an extremely slow rate so that no more than one ion is detected from the entire field ion emitter surface in each pulsed field evaporation. From the flight time of the ion its chemical species is identified, and from the location of the detector where the ion is detected the spatial origin of the ion is located. With a fast data processor, a two-dimensional distribution of chemical species on the tip surface can be... 

Schlag ct have obtained second-order solutions for the flight times of spatially distributed ions accelerated in a dual-stage extraction system. Using the notation shown in Figure 2.6, their relationship between the distances and. xijp is ... 

Time of flight ion probes (TOF SIMS) have unique capabilities not found in other mass spectrometers. A pulsed ion beam, typically either cesium or gallium, ejects atoms and molecules from the sample. Ionized species are accelerated down the flight tube and the arrival time in the detector is recorded, giving the mass of the species (see discussion of time-of-flight mass analyzers above). TOF SIMS instruments used in cosmochemistry have spatial resolutions of <1 pm. They are used to determine elemental abundances in IDPs and Stardust samples. The spatial distribution of elements within a small sample can also be determined. TOF SIMS instruments can obtain good data with very little consumption of sample. 

Unfortunately ions produced in the source are formed under less than ideal conditions (Fig. 21). Ions with the same m/z may have a distribution of kinetic energies consequently, the ions have different flight times and arrive at the detector at different times (Fig. 21A). Furthermore, ions with the same m/z can be formed at different locations in the source (Fig. 21B). This spatial distribution... 

There is a point along the flight tube, called the space focus plane , where the spatial distribution is minimized for ions of any given m/z. If the space focus plane is not at the detector then, after travelling through the space focus plane, ions with the same m/z will have different flight times to the detector (Fig. 21B). 

Time-of-flight (TOP) mass spectrometers are ideally suited for use with the MALDI technique because of their theoretically unlimited mass range, high ion transmission, and for the pulsed nature of the laser used in this method. Mass resolution is the ability of an instrument to separate the signals from ions of similar mass, expressed as the mass of a given ion divided by the full width at half maximum of the peak (fwhm). Resolution in MALDl-TOP MS is mainly restricted by the ionization process, rather than by instrumental limits, because the ions have a certain time-span of formation, a spatial distribution, and a kinetic energy spread. ... 

Mass spectrometry imaging with time-of-flight secondary ion mass spectrometry (ToF-SIMS) has revealed the spatial distribution of chemicals on a surface (1). When applied to biological samples, this method offers spatial information on biologically relevant small molecules (<1,000 Da). Indeed, ToF-SIMS imaging has been shown to be a powerful analytical tool for mapping the distribution of lipids on a cell membrane at the single-cell... 

The Space-Focus Plane. The early time-of-flight mass spectrometers utilized electron impact (El) to ionize volatile molecules in the gas phase. Ionization in the gas phase results in considerable uncertainties in the initial position of the ions in the extraction field that are less problematic in the desorption (PDMS and MALDI) instruments in use today. Sdll, it is useful to consider this problem (and the means by which it is corrected), since spatial distributions do exist when ions are desorbed from irregular or insulating surfaces. In addition, it is likely that compact EI-TOF instruments will continue to be developed (and commercialized) for environmental monitoring or as gas-chromatographic detectors. 

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) (Meeus et al. 2013 Scoutaris et al. 2012 Shard et al. 2009) Analysis of surface chemical composition of a sample (top 1-2 nm layer) Mapping of spatial surface distribution of mixture components, 3D chemical distribution and miscibility, relative drug distribution in multi-polymeric system Depth profiling by surface sputtering (removing dotted surface monolayers) Spatial resolution Umits the estimation of phase-separated domain... 

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