Detection efficiency of product ions

The above tests on the primary ion beam are routine. The major uncertainty in the longitudinal tandem technique is the detection efficiency of product ions. This detection efficiency can be calculated if the complementary crossed-beam experiment has been performed to yield the contour map, i.e., both the angular distribution and the velocity distribution. As has been emphasized both earlier in this chapter and in Chapter 12, this is the proper approach to the problem, but the necessary data are rarely available. The alternative is to measure the detection efficiency and, in the absence of the angular distribution data, this must be done before an accurate cross section can be obtained. In practice, it rarely is done because the measurements are not straightforward. This absence constitutes a major source of possible error in most determinations of excitation functions by the longitudinal tandem technique. 

The problem of the detection efficiency of product ions has two aspects. Do the ions escape from the collision chamber, and, if so, do they enter the mass analyzer Considering the second aspect first, it is exceedingly difficult to measure the transmission efficiency through the focusing and accelerating system without angular analysis. Conceptually, the most effective way to avoid the problem is to measure the differential cross... 

The measurement of the detection efficiency of product ions is treated in a recent paper from Friedman s laboratory.DC electric fields were used in the collision chamber to expel any ions which may be back-scattered in the laboratory. Data similar to Fig. 2 of this chapter are shown for the neon charge-transfer reaction in Fig. 2c of the paper concerned. " ... 

Fig. 1.103. TPR spectra of the catalytic formation of CeHe, C4H6, and C4H8 for a defect-rich MgO thin film, Pdi, Pd4, Pde, Pdg, Pdis, Pd2o, and Pdso. The relative ion intensities are corrected with the relative detection efficiencies of the experiment, and scale with the nnmber of formed product molecules per cluster...

Finally, mention must be made of the type of control experiment which has often been considered sufficient in the past, namely the reproduction of an accepted rate constant for a standard reaction, usually CH4 (CH4,CH3)CH5 . Such reproduction establishes that detection efficiencies of reactant and product ions are equal for this reaction, but this may not be so for other reactions where the initial product velocity distributions differ. Second, it may well not reveal the presence of errors which cancel one another (see Section 3.2.2). Third, this particular reaction is unsuitable as a standard since its rate constant is rather insensitive to Ef, accordingly, it cannot be used to establish the homogeneity of the field within the chamber, a necessary condition if [Eq. (11)] is to be identified with k(Vf) [Eq. (15)]. 

The possibility of comparing disappearance rate constants of reactant ions with appearance rate constants of product ions offers obvious advantages for the characterization of reaction pathways. In general, however, the task of collecting useful rate data is harder at high pressures than at low pressure, since additional complications arise. Additional assumptions are also necessary, for example, that it is valid to plot the normalized mass spectrum as a function of pressure. Surprisingly, perhaps, in view of what has been said about the uncertainty of detection efficiencies, this particular procedure seems to work rather well. ... 

The second and third examples have been selected because these reactions are in principle among the simplest reaction rates to measure. Problems of product ion detection efficiency are eliminated by the collision dynamics, whereby both the reactant and the product ions exhibit very similar velocity and angular distributions, irrespective of the collision energy. However, the reactant ion is formed in two states by electron impact and the cross sections for these states are significantly different. Rate parameters will therefore depend on the relative populations of these two states. In this case, both the chemists techniques (mass-spectrometer ion source and ion cyclotron resonance) and some of the physicists ... 

ECD is not an efficient process. At least one charge is neutralized as a result of electron capture. Furthermore, a given precursor ions cleaves into any one of a rather large number of possible product ions, with the result that a high abundance of precursor ions is required in order to detect a particular product ion. An ultimate limit of ca 30% conversion of precursor to product ions exists. 

In the case of processes occurring in a reactor, the molecules can be delivered directly via transfer line systems or picked up by carrier gases to the inlet of a mass spectrometer. If the reaction occurs in ambient conditions, the molecules can be sampled by atmospheric pressure ion sources. If the ionization, transmission, and detection efficiencies of the reactants and the products of Equation 10.6 are available, their concentrations can be estimated from... 

However, an alternative method of SIM is still commonly used in lipidomics, particularly in the platforms associated with LC-MS, where high duty cycle instruments such as Q-ToF-type mass spectrometers are employed. In this case, a product-ion analysis at any moment of elution time could be performed for certain ions above a preset threshold for identification of these species (i.e., data-dependent acquisition), while a mass spectrum in the full MS mode, which detects both miz values and intensities of the ions between the mass ranges of intoest at the eluent time, is acquired over the entire elution time period for quantification. Owing to the very high scan rate, high sensitivity, and very fast and efficient acquisition of full product-ion mass spectra with the Q-ToF-type instruments over QqQ-type mass spectrometers, multiple acquisitions can be recorded at an elution time for identification of the relatively abundant species. The combination of elution time, m/z value, and a number of product-ion mass spectra provides reasonably accurate information about the chemistry of lipid species. 

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]. 

The X-ray spectrum observed in PIXE depends on the occurrence of several processes in the specimen. An ion is slowed by small inelastic scatterings with the electrons of the material, and it s energy is continuously reduced as a frmction of depth (see also the articles on RBS and ERS, where this part of the process is identical). The probability of ionizii an atomic shell of an element at a given depth of the material is proportional to the product of the cross section for subshell ionization by the ion at the reduced energy, the fluorescence yield, and the concentration of the element at the depth. The probability for X-ray emission from the ionized subshell is given by the fluorescence yield. The escape of X rays from the specimen and their detection by the spectrometer are controlled by the photoelectric absorption processes in the material and the energy-dependent efficiency of the spectrometer. 

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