Initial velocities of ions

This reduces many of the complications arising from the need for spatial focusing. Also, the initial velocity of ions generated are invariably in the TOP direction. 

Juhasz, P. Vestal, M.L. Martin, S.A. On the Initial Velocity of Ions Generated by MALDI and Its Effect on the Calibration of Delayed Extraction-TOF Mass Spectra. 

S.A. (1997) On the initial velocity of ions generated by matrix-assisted laser desorption ionization and its effect on the calibration of delayed extraction time-of-flight mass spectra. J. Am. Soc. Mass Spectrom., 8, 209-217. 

Molecular beam sample introduction (described in section (Bl.7.2)). followed by the orthogonal extraction of ions, results in improved resolution in TOP instruments over eflfrisive sources. The particles in the molecular beam typically have translational temperatures orthogonal to the beam path of only a few Kelvin. Thus, there is less concern with both the initial velocity of the ions once they are generated and with where in the ion source they are fonned (since the particles are originally confined to the beam path). 

Therefore, in the RF mode, ions transmitted through the rod guide are subjected to (1) an oscillation in step with the variations of the RF field in the x,y-plane, (2) a drift or guided motion caused by the inhomogeneity of the RF field (x,y-plane), and (3) a forward motion (z-direction) due to any initial velocity of the ions on first entering the rod assembly. The separate motions... 

Similarly to quantitative determination of high surfactant concentrations, many alternative methods have been proposed for the quantitative determination of low surfactant concentrations. Tsuji et al. [270] developed a potentio-metric method for the microdetermination of anionic surfactants that was applied to the analysis of 5-100 ppm of sodium dodecyl sulfate and 1-10 ppm of sodium dodecyl ether (2.9 EO) sulfate. This method is based on the inhibitory effect of anionic surfactants on the enzyme system cholinesterase-butyryl-thiocholine iodide. A constant current is applied across two platinum plate electrodes immersed in a solution containing butyrylthiocholine and surfactant. Since cholinesterase produces enzymatic hydrolysis of the substrate, the decrease in the initial velocity of the hydrolysis caused by the surfactant corresponds to its concentration. Amounts up to 60 pg of alcohol sulfate can be spectrometrically determined with acridine orange by extraction of the ion pair with a mixture 3 1 (v/v) of benzene/methyl isobutyl ketone [271]. 

Chan, T.-W. D. Thomas, I. Colburn, A. W. Derrick, P. J. Initial velocities of positive and negative protein molecule-ions produced in matrix-assisted ultraviolet laser desorption using a liquid matrix. Chem. Phys. Lett. 1994,222,579-585. 

The mechanisms of ion formation in MALDI are a subject of continuing research. [30-34] The major concerns are the relationship between ion yield and laser flu-ence, [28,35] the temporal evolution of the desorption process and its implications upon ion formation, [36] the initial velocity of the desorbing ions, [29,37,38] and the question whether preformed ions or ions generated in the gas phase provide the major source of the ionic species detected in MALDI. [39,40]... 

Although the initial velocity of the desorbed ions is difficult to measure, reported values generally are in the range of 400-1200 m s" The initial velocity is almost independent of the ionic mass but dependent on the matrix. [33,36-38,46,50,51] On the other hand, the initial ion velocity is not independent of the compound class, i.e., peptides show a behavior different from oligosaccharides. [51]... 

The essential independence of mean ion velocities on the molecular weight of the analyte leads to an approximate linear increase of the mean initial kinetic energies of the analyte ions with mass. High-mass ions therefore carry tens of elec-tronvolts of translational energy before ion acceleration. [33,41,50] The initial velocity of the ions is superimposed onto that obtained from ion acceleration, thereby causing considerable losses in resolution with continuous extraction TOP analyzers, in particular when operated in the linear mode. 

Berkenkamp, S. Menzel, C. Hillenkamp, F. Dreisewerd, K. Measurements of Mean Initial Velocities of Analyte and Matrix Ions in Infrared MALDI-MS. J. Am. Soc. Mass Spectrom. 2002, 75, 209-220. 

Size of arrow indicates the size of initial velocity of the ion... 

However, since the velocities of ions before acceleration are non-zero, their total kinetic energy must take account of their initial kinetic energy, Eq, which is small and unknown ... 

Osaki (169) has studied the catal5dic oxidation of ferrous ion by ceruloplasmin, and reported non-linear Lineweaver-Burk plots explicable in terms of two K,a values 0.6, mM and 50 juM. This behavior could arise from the fact that ceruloplasmin may bind Fe(III) as it is known to bind several other metal ions quite strongly (770), and does not provide evidence for more than one type of active site. Ferrous ion is an excellent substrate of ceruloplasmin as evidenced by its very low K i. However, its F ax does not differ greatly from the values of all the substrates tested by Young and Curzon (167). From the data of Osaki (169) Fmax at pH 5.5 is 30e/Cu-min at 30 °C ( cat=3.5sec i). Using the activation energy of 10.9 Kcal/mole reported by Osaki and Walaas (171) for the initial velocity of Fe(II) oxidation the above Fmax can be corrected to 22 e/Cu-min at 25 °C. Thus, it appear that for all substrates of ceruloplasmin Fmax is remarkably independent of the nature of the substrate. 

A common problem to all Time-of-Flight mass spectrometers is the so called "turn around time" that is due to different initial velocities of the neutrals in the ion source. Even in a RETOF these kinetic energies can destroy the achievable mass resolution of the instrument. The low translational cooling, resulting from the supersonic beam expansion, yields small kinetic energy distributions of the neutral molecules /17/. 

Fourier transform mass spectrometry (ftms) is an ion cyclotron resonance method (3,4). This method is currently limited to masses less than 20,000. Thus, it works well for lower molecular mass polymers. Often it is used with MALDI. However, the MALDI process is so energetic that the initial velocity of the analytes can cause some intensity difficulties that make the method difficult to quantify. Because of the expense of the magnet required to obtain quahty data with this method, only a few laboratories use this method. 

Note The orbitrap is also an ion trap, but there is neither RF nor a magnet to hold ions inside. Instead, moving ions are trapped in an electrostatic field. The electrostatic attraction towards the central electrode is compensated by a centrifugal force that arises from the initial tangential velocity of ions very much like a satellite in orbit [206]. 

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