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Emerging Biomedical Analysis
Published in Lawrence S. Chan, William C. Tang, Engineering-Medicine, 2019
The way to improve the mass-resolving power in a TOF is to increase the length of the ion path. Due to the physical limitation of an instrument, an extended ion path is generally achieved by using a reflectron. A reflectron is a reversed electric field that is placed on the opposite side of the accelerating electric field in the flight tube. Ions are “reflected” (slowed, stopped and accelerated to the reverse direction) in the reflectron. Therefore, the ion path is doubled in a given flight tube by using one reflectron. Theoretically, there is no upper limit of the mass-resolving power in a TOF mass analyzer as long as a sufficient number of reflectrons are configured within the mass spectrometer. However, the excessive length of the ion path results in a significant reduction of sensitivity and mass range (Toyoda et al. 2003). Current commercialized reflectron TOF instruments are usually equipped with one or two reflectrons and can routinely attain a mass-resolving power of > 20,000 and a mass accuracy of 5 ~ 10 ppm (Fig. 4).
Basics Of Gas Chromatography Mass Spectrometry System
Published in Raquel Cumeras, Xavier Correig, Volatile organic compound analysis in biomedical diagnosis applications, 2018
William Hon Kit Cheung, Raquel Cumeras
TOF base MS analyzers are widely applied in the field of metabolomics for the untargeted profiling experiments (Dunn, 2008; Rudnicka, 2011; Skogerson, 2011; Tsugawa, 2011). In TOF mass analyzers (Guilhaus, 1995; Mamyrin, 2001), ions are pulsed into a region known as a flight tube where they are accelerated under known electric field strength within a vacuum. The ions are focused by a series of ion optics and directed towards the detector. Based on the time required for the ions to traverse the length of flight tube (the transient time) and be detected, the original mass of the ion can be determined accurately. As the packet of ions travels towards the detector, it may become defocused or spread out due to acceleration effect of the electric field over a large macromolecular distance, resulting in reduced mass resolution and sensitivity. In order to off-set the spread of the ions, a reflectron system is incorporated into all modern TOF system. A reflectron system serves two main functions: (1) to focus and redirect the flight path of the ions towards the detector, and (2) increase the distance available within the flight tube without significantly increasing its size. The longer available flight path created by the refectron system also has the added benefit of enhancing the overall mass resolution. A single stage reflectron consists of a series of stack electrodes generating a static homogeneous electric field, as the packet of ions encounters the reflectron, slight differences in the acceleration and spread of the incoming ion is adjusted and refocused. Ions of different acceleration energy will impinge into the reflectron’s electric field at varying degree; higher energy ions will penetrate further into the reflectron electric field before being redirected outwards relative to lower energy ions of the same packet. As a result, all the ions within the same packet will exit the reflectron as a more spatially uniform packet and be directed towards the detector. Single and dual-stage reflectron (Mamyrin, 2001) systems have been developed to greatly enhanced the mass range and mass accuracy of TOF base MS system. In order for TOF mass analyzer to generate reproducible and accurate mass measurements, it is critical that the system is properly calibrated prior to analysis. The mass accuracy of the system is constantly being monitored and adjusted by continuously infusing a reference solution of known composition and concentration into the system (lock mass solution).
Optical spectrum of the cyanoadamantane radical cation
Published in Molecular Physics, 2023
Parker B. Crandall, Viktoria D. Lovasz, Robert Radloff, Simone Stahl, Marko Förstel, Otto Dopfer
Herein, we report the first optical spectrum of AdCN+, which is recorded in the large range from 275 to 1200 nm by EPD spectroscopy. To reduce broadening factors such as hot bands, collisions, or solvation effects, the experiments are carried out in the gas phase under ultra-high vacuum conditions and utilise a cryogenic 22-pole ion trap cooled to 5 K (BerlinTrap) [27,37]. By doing so, we ensure that only the ground vibrational level of the ground electronic state of the cryogenic ions is populated before they are exposed to the laser pulse to excite electronic transitions. Resulting photofragments as well as the depletion of the parent ion are measured by a reflectron time-of-flight mass spectrometer and analysed as a function of the photon frequency. TD-DFT calculations are carried out to aid in the assignments of the observed spectral transitions.
Bayesian model calibration for vacuum-ultraviolet photoionisation mass spectrometry
Published in Combustion Theory and Modelling, 2022
James Oreluk, Leonid Sheps, Habib Najm
This work focuses on VUV-PIMS probing of Cl atom-initiated propane oxidation as an example. A comprehensive analysis of the instrument is found in Sheps et al. [6]. Gaseous mixtures containing dilute fuel (propane), O, radical precursor (in this case Cl), and buffer gas (He) flow continuously into a high-pressure photolysis flow reactor, held at a constant temperature and pressure. The gas mixture inside the flow reactor is instantaneously and uniformly irradiated by a photolysis laser pulse every 250 ms, which dissociates Cl. The nascent Cl atoms abstract H atoms from propane to produce propyl radicals, initiating a sequence of oxidation reactions. The reacting gas mixture exhausts out of the flow reactor into a VUV-PIMS apparatus, where a tunable vacuum ultraviolet beam ionises the incoming gas. These ionised species are detected by a reflectron TOF mass spectrometer, producing experimental ion counts . The gas is fully replenished before the next photolysis laser pulse and the reactor returns to its initial state. This experiment is repeated over thousands of photolysis cycles, accumulating statistics on the time and energy-dependent mass spectrum.
Clustering and multiphoton effects in velocity map imaging of methyl chloride
Published in Molecular Physics, 2021
I. S. Vinklárek, J. Rakovský, V. Poterya, M. Fárník
Quite often in the literature, if the images from isolated molecules exhibited only sharp rings, the appearance of a central feature in the images was automatically associated with cluster effects [13–18]. Unambiguous cluster effects could be observed in experiments, where beams of relatively large clusters were generated and individual molecules were deposited on the clusters later on in a pickup cell before the photodissociation [19–21]. However, in other than pickup experiments the assignment of blurred central features might be more ambiguous, especially in VMI experiments without the capability of mass spectrometry with a sufficient mass range, resolution and sensitivity. Other effects, such as multiphoton processes, may generate central features in the images as well. We presented an example of ethanethiol [22], where we combined the VMI with reflectron time-of-flight mass spectrometry to demonstrate that a broad central blob in images resulted from multiphoton processes while the photodissociation in clusters yielded very sharp zero-kinetic-energy peak.