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Mass Spectrometry Instrumentation
Published in Grinberg Nelu, Rodriguez Sonia, Ewing’s Analytical Instrumentation Handbook, Fourth Edition, 2019
Yuan Su, Li-Rong Yu, Thomas P. Conrads, Timothy D. Veenstra
The first TOF analyzers operated in a linear mode, in which ions were continually extracted from the source region and sent through the flight tube to the detector (Chernushevich et al., 2001; Shevchenko et al., 2000). This mode does not provide the highest resolution, as ions with the same m/z will have varying velocities, because they have different initial energies and positions as they move from the source to the analyzer region. Two developments solved this problem. To correct these deficiencies, reflectron TOF (Cornish and Cotter, 1993), which focuses ions with the same m/z values and allows them to strike the detector, and time pulsed-laser ionization with delayed extraction, in which there is a slight delay between the ionization of the sample and the extraction of the ions into the flight tube, were developed (Brown and Lennon, 1995; Juhasz et al., 1996). Delayed extraction allows all the ions to have an equal start time, enabling ions of equal m/z to reach the detector simultaneously.
Post extraction inversion slice imaging for 3D velocity map imaging experiments
Published in Molecular Physics, 2021
Felix Allum, Robert Mason, Michael Burt, Craig S. Slater, Eleanor Squires, Benjamin Winter, Mark Brouard
Several experimental approaches to ‘slice’ imaging exist, generally with a common goal of stretching the ion cloud along the time-of-flight axis prior to its arrival at the detector. The detector can then be gated over the arrival time of the central slice of the sphere. A widely applicable technique, DC slicing, was developed independently by Townsend et al. [16] and Lin et al. [17]. In these methods, additional electrode(s) are incorporated into the ion optics assembly to create a lower potential difference in the extraction region, whilst still bringing about velocity mapping conditions. Consequently, there is a greater turnaround time for fragments produced with negative velocities along the time-of-flight axis, and so the arrival time spread, for a given ion increases. Typically, is of the order of several hundred nanoseconds, with the gate time of the detector, on the order of tens of nanoseconds, giving a slicing resolution, of ∼10%. Recently, significant progress has been made in correcting for the effects of finite slicing resolution using the finite slice analysis technique [8,19]. In an alternative approach to slice imaging introduced by Kitsopoulos and coworkers [15], a delay between ionisation and extraction is introduced by pulsing the (gridded) extraction electrodes from ground. During this delay, the ion cloud expands under field-free conditions, leading to a larger ion sphere impinging on the detector following velocity mapping, with a greater . The ability to effectively slice image over large interactions using the delayed extraction technique has proven beneficial in studies of scattering at surfaces [20]. In both DC and delayed extraction slice imaging, varying the time window over which ion events are recorded results in imaging sequential slices through the three-dimensional velocity distribution. By incorporating a fast timestamping detector, many such slices can be recorded simultaneously, as demonstrated previously using a Pixel Imaging Mass Spectrometry camera [21–23] or a Timepix detector [24–27]. Similar techniques involving the correlation of events recorded by a camera and a digitised high-resolution time-of-flight trace have similarly realised the goal of three-dimensional imaging at high count rates [27–30].