Ionization efficiency factor

In formula (4.38) I stands for the threshold energy of ionization and tj(w) denotes the ionization efficiency factor. The latter does not have to be unity at hw> 1. For instance, in the case of SES (see Section II.B) rj(w) < 1, while for multiple ionization it can be greater than unity. 

In the ideal case for REMPI, the efficiency of ion production is proportional to the line strength factors for 2-photon excitation [M], since the ionization step can be taken to have a wavelength- and state-mdependent efficiency. In actual practice, fragment ions can be produced upon absorption of a fouitli photon, or the ionization efficiency can be reduced tinough predissociation of the electronically excited state. It is advisable to employ experimentally measured ionization efficiency line strengdi factors to calibrate the detection sensitivity. With sufficient knowledge of the excited molecular electronic states, it is possible to understand the state dependence of these intensity factors [65]. 

Carburization of rhenium filaments has been used to optimize Th and Pa ionization efficiency for TIMS analysis on single filaments (Esat 1995). ReC has a greater work function than Re metal, and elemental oxidation state is maintained in the reduced or metal state by the presence of carbon in the filament. Using this method and a mass spectrometer with improved ion optics, Esat (1995) was able to improve Th transmission and ionization efficiency by about a factor of 30 over conventional methods. Using more conventional mass spectrometry, Murrell et al. (personal communication) were able to improve ionization efficiency for Pa and Th by a factor of 5-10 over conventional graphite sandwich loads on Re filaments (Goldstein et al. 1989 Pickett et al. 1994). For Pa analysis, one drawback is that Pa and U ionization commonly overlap using this... 

None of the three theories used to calculate electron impact ionization cross sections could be considered to render the others obsolete. The BEB method gives the best fit to the functional form of the ionization efficiency curve for small molecules, it provides a better fit to the experimental data closer to the ionization threshold than the other methods, but it underestimates the maximum ionization cross sections for heavier molecules. The DM method provides a better fit to the ionization efficiency curves for the heavier molecules, especially for electron energies greater than max, but it tends to overestimate the cross sections for heavier molecules and it underestimates E for lighter molecules. The EM method performs as well as the other methods for the value of amax for the light molecules but underestimates the cross sections for heavy molecules by a factor similar to the overestimation of the DM method. The polarizability method outperforms the BEB and the DM methods for the calculation of and when combined with the value from the EM calculation reproduces the ionization efficiency curve as well as the BEB method. 

The equilibrium constants can be approximated by ratios of ion currents in some instances otherwise, the currents are converted to partial pressures by comparison with the evaporation of known amounts of a standard material. Various geometric corrections (K) such as the solid angle subtended by the sample at the orifice, the Clausing factor for orifice geometry, molecular cross-section (o-), which control ionization efficiency, and detector efficiency are included in the general relationship... 

Internal standardization circumvents the effects of time-variant instrument response, but does not compensate for different ionization efficiencies of analyte and standard. For internal standardization, a compound exhibiting close similarity in terms of ionization efficiency and retention time is added to the sample at a known level of concentration, e.g., an isomer eluting closely to the analyte or a homologue may serve for that purpose. It is important to add the standard before any clean-up procedure in order not to alter the concentration of the analyte without affecting that of the standard. For reliable results, the relative concentration of analyte and standard should not differ by more than a factor of about ten. 

ALIS measures the MS response of the ligand following its dissociation from the protein-ligand complex. Therefore, the magnitude of the MS response corresponds to the equilibrium concentration of the receptor-ligand complex concentration [ S] times the compound s MS calibration factor Cms, which depends on the ionization efficiency and other molecular properties of the ligand ... 

Utilizing ionization efficiency curves to determine relative populations of vibrationally excited states (as in the photoionization experiments) is a quite valid procedure in view of the long radiative lifetime that characterizes vibrational transitions within an electronic state (several milliseconds). However, use of any ionization efficiency curve (electron impact, photon impact, or photoelectron spectroscopic) to obtain relative populations of electronically excited states requires great care. A more direct experimental determination using a procedure such as the attenuation method is to be preferred. If the latter is not feasible, accurate knowledge of the lifetimes of the states is necessary for calculation of the fraction that has decayed within the time scale of the experiment. Accurate Franck -Condon factors for the transitions from these radiating states to the various lower vibronic states are also required for calculation of the modified distribution of internal states relevant to the experiment.991 102... 

Bias corrections determined from analysis of standards are applied to the samples under test. Use of such an average bias correction can be viewed only as an approximation to the truth so many factors contribute to bias that it is impossible to control them all. For example, as previously stated, the work function of a rhenium filament is determined by which crystal face is involved One way of loading samples on filaments is through use of single resin beads [56,57]. The beads are 100-200 xm in diameter, which is about the size of rhenium crystallites in a poly crystalline filament [17]. Clearly the work function applicable to the analysis in question may or may not be that operative when instrument calibration was carried out. Another parameter difficult to control in real-world conditions is sample purity, which also affects bias. It is impossible to purify all samples to the same degree, and contaminants adversely affect ionization efficiency low efficiency means higher filament temperatures, which in turn mean a different bias correction. These are only two of sundry variables that can affect ionization efficiency. 

Quantitation in ms/ms is comparable to quantitation in gc/ms or lc/ms. In each of these techniques a variety of factors, including those just mentioned in connection with ionization efficiency, make standard addition or internal standards desirable. Internal standards are commonly isotopically labeled analogs of the analyte. Several runs must be made to quantitate and ms/ms again has an advantage in speed over the slower chromatographic techniques. Precision and accuracy of quantitation has not been sufficiently compared but at this early stage in its development, ms/ms lags in these respects. 

SIMS is mostly employed for microlocal analytical investigations. For example, isotope ratio measurements by SIMS have been performed in nuclear forensic studies to determine the age of Pu particles." For the age determination of Pu particles, relative sensitivity coefficients (RSC) were determined as correction factors for the different ionization efficiency of Pu compared to U. The age of a sample of known origin calculated from Pu/ " and °Pu/ U ratios agreed well with the reported age of 2.3 years." SIMS was employed for oxygen isotope ratio measurements in three different uranium oxide microparticles of nuclear forensic interest by Betti s working group." The... 

In terms of ionization, the majority of large-scale phosphoproteomics strategies utilize ESI. The difficulty to effectively analyze phosphopeptides by ESI-based approaches is often attributed, among other factors, to selective suppression of phosphorylated peptides in the presence of unmodified peptides, and fo decreased ionization efficiencies of phosphopeptides relative to their non-phosphorylated counterparts. However, this notion has been challenged in a recent study (18) underscoring the... 

The Ion source Is a radio frequency vacuum discharge source and Is Illustrated In Figure 3. The Ionization efficiency of this source is uniform within a factor of three, due to the fact that the Ionizing energy available Is so much greater than the Ionization potential of any element. This Is one of the most attractive features of the technique, since It allows us to analyze every element on the periodic table without large variations In sensitivity. 

So mass may be an important factor, but overall the situation is likely to be more complex. For example, ionization efficiency could also have played a role, given (a) the evidence in several cases for ion implantation as the process by which noble gases were introduced into their host phases (e.g., SiC, diamond section Origins and History), and... 

The criteria for optimum performance of atmospheric pressure chemical ionization and electrospray are different. For electrospray, it is important to control the flow rate and composition of the sheath-flow liquid, the mobile phase flow rate, and the dimensions of the electrospray and sheath-flow capillaries, as well as their relative position with respect to each other. Sample ionization with carbon dioxide alone does not occur in the absence of sheath-flow liquid. For atmospheric pressure chemical ionization, apart from those factors that affect the performance of the nebulizer, the flow dynamics throughout the interface and temperature in the ionization region, are the most important parameters. Moistened air is sometimes used as a makeup gas to maintain stable operation of the atmospheric pressure chemical ionization interface with HaO ions supplementing solvent ions in the reaction gas plasma. The ionization efficiency of both methods is similar to liquid chromatography. 

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