Quadrupole compared with

From Figure 49.2, it can be seen that the quadrupole assembly provides a potential well to contain the ions in their journey along the main quadrupole axis. The potential well of the quadinpole has not very steep sides and, compared with steep-sided hexapoles or higher -poles or ion tunnels, the quadrupole is not as efficient as the others in containing ions inside the rod assembly. 

In principle GD-MS is very well suited for analysis of layers, also, and all concepts developed for SNMS (Sect. 3.3) can be used to calculate the concentration-depth profile from the measured intensity-time profile by use of relative or absolute sensitivity factors [3.199]. So far, however, acceptance of this technique is hesitant compared with GD-OES. The main factors limiting wider acceptance are the greater cost of the instrument and the fact that no commercial ion source has yet been optimized for this purpose. The literature therefore contains only preliminary results from analysis of layers obtained with either modified sources of the commercial instrument [3.200, 3.201] or with homebuilt sources coupled to quadrupole [3.199], sector field [3.202], or time-of-flight instruments [3.203]. To summarize, the future success of GD-MS in this field of application strongly depends on the availability of commercial sources with adequate depth resolution comparable with that of GD-OES. 

A triple-quadrupole mass spectrometer with an electrospray interface is recommended for achieving the best sensitivity and selectivity in the quantitative determination of sulfonylurea herbicides. Ion trap mass spectrometers may also be used, but reduced sensitivity may be observed, in addition to more severe matrix suppression due to the increased need for sample concentration or to the space charge effect. Also, we have observed that two parent to daughter transitions cannot be obtained for some of the sulfonylurea compounds when ion traps are used in the MS/MS mode. Most electrospray LC/MS and LC/MS/MS analyses of sulfonylureas have been done in the positive ion mode with acidic HPLC mobile phases. The formation of (M - - H)+ ions in solution and in the gas phase under these conditions is favorable, and fragmentation or formation of undesirable adducts can easily be minimized. Owing to the acid-base nature of these molecules, negative ionization can also be used, with the formation of (M - H) ions at mobile phase pH values of approximately 5-7, but the sensitivity is often reduced as compared with the positive ion mode. 

Ion trap MS is particularly suited for chemical structure elucidation, as it allows for simultaneous ion storage, ion activation and fragmentation, and product ion analysis. The fragmentation pathway of selected ions and the fragmentation products provide information on the molecular structure. Compared with triple-quadrupole and especially with sector instruments, the ion trap instrument provides more efficient conversion of precursor ion into product ions. However, the CID process via resonance excitation, although quite efficient in terms of conversion yield, generally results in only one (major) product ion in the product-ion mass spectrum. MS/MS with a quadrupole ion trap offers a number of advantages ... 

The Rh-Ir vector defines the z-axis, and the NN bridges define the x- and y-axes. The N and P atoms are solely electron donors, but the CO ligand involves a lot of re-back-bonding. Thus the quadrupole matrix, which has large components if there is an electric field gradient at the Ir nucleus, is rotated 45° compared with the g-matrix principal axes. 

The results of energy partitioning in Li+... OH2 obtained with a number of different basis sets are listed in Table 3. Since intermolecular overlap is small in these kind of complexes (Table 1), we expect the electrostatic model to be a good approximation for classical contributions to the total energy of interaction. Indeed, ZlE cou is to a good approximation proportional to the dipole moment of the water molecule calculated with the same basis set. This can be seen even more clearly in Table 4 where zIEcou is compared with ion-dipole and ion-quadrupole energies obtained from the classical expression of the multipole expansion series 45,95-97) ... 

In the following section, we describe the case of adsorption of a Sn complex onto a palladium oxide suspension. In an alkaline medium (a basic PdO hydrosol), chlorides in the SnCL complex are substituted in the coordination sphere of tin(IV) by hydroxo anions, which are in excess, yielding the stannate Sn(OH)g complex. The Sn Mossbauer spectroscopy spectrum of a bimetallic sol (frozen in liquid nitrogen) is compared with a true stannic solution. At the same tin concentration, it shows the changes in the Sn environment due to adsorption onto the PdO surface (Fig. 13.27). The isomer shift S is found to be close to zero for the stannate solution and increases when contacted with the PdO suspension, indicating a modification of the coordination sphere of tin. The increase in 5 can be correlated to an increase in the core level electronic density of tin. The quadrupole splitting A, is related to a modification of the symmetry of the close environment of tin, due to adsorption of Sn(OH)g complexes onto the PdO colloidal nanoparticles. 

Mossbauer spectra have been measured for various tektites, as well as for both natural and synthetic iron-bearing silicate minerals. These results are reported and compared with other similar studies available in the literature. The ratios of the intensities of the appropriate Mossbauer lines have been used to determine the ferric-ferrous ratios where possible. The spectra of the ferrosilite-enstatite series of pyroxenes show four lines which are interpreted as two quadrupole split doublets, and the ratio of the intensities of these lines indicates the degree of ordering in filling the available metal ion sites. Similar studies on the fayalite-forsterite series of olivines are also reported. 

Present M sbauer Studies of Natural Pyroxenes and Olivines. Table IX gives the major element chemical compositions of the silicate minerals examined in this study. Table X compares the Mossbauer parameters of these minerals, while Figures 9-13 show representative Mossbauer spectra. Fayalite (Figure 9) is the only olivine in this group. The two lines are, however, somewhat broadened (0.35 and 0.39 mm./ sec.) compared with the width of natural iron foil lines observed with our source (0.24 mm./sec.) and suggest the near coincidence of two quadrupole-split doublets resulting from Mi and M2 sites. Analysis of this "two-line spectrum into a four-line spectrum in the manner described by Evans et al (11) could possibly yield parameters for the two iron sites, but this was not undertaken since both lines appear symmetric. The "two-line quadrupole splitting of 2.78 mm./sec. is somewhat smaller... 

In Table 7 we compare the ZPVCs for the dipole and quadrupole polarizabilities of HF. In the same way as for LiH, we have calculated the vibrational averages for each method with two different wavefunctions - one obtained from the PEC of the same or related method as used in the calculation of the property curve and the other obtained from the loo CAS PEC. Compared with the equivalent results for LiH we observe significant differences between the calculations on the two molecules. Eirst of all the vibrational corrections are smaller than in LiH but roughly in the same ratio as the polarizabilities. The influence of the PEC is larger than in LiH. 

The two main types of mass spectrometers used for analysis and detection of explosives are the quadrupole and the ion trap. These two types of mass analyzers are relatively small, when compared with magnetic sector instruments. They can be miniaturized to make mobile detectors weighing less than 15 kg. 

The above-mentioned nonlinear optical effects can be described by the perturbation of the electromagnetic held intensity under the electric dipole approximation. Actually, this approximation is broken in optical near-helds. Hence, a perturbation effect of multipole such as electric quadrupole or magnetic dipole should also be considered, although such a higher-order effect is normally negligible. Indeed, electric quadrupole contributions can be comparable with electric dipole contributions... 

Table 3 shows measurements of this effect, taken from a paper by Dixon and Bloembergen (4). The experimental results are compared with calculated values of the effect of the electric field on the quadrupole resonance frequency in which it was assumed that no jr-bonding is present. These calculations are rather crude but predict the correct order of magnitude for the Carbon-Chlorine bond. For the SiCl bond, however, the experimental Stark splittings are both much less than those for the C—Cl bond and almost an order of magnitude less than the calculated value and once more a ready rationalisation of this phenomenon lies in An—pn bonding. 

Figure 1. Photoabsorption cross section for the dipole plasmon in axially deformed sodium clusters, normalized to the number of valence electrons N - The parameters of quadrupole and hexadecapole deformations are given in boxes. The experimental data [39] (triangles) are compared with SRPA results given as bars for RPA states and as the strength function (49) smoothed by the Lorentz weight with A = 0.25 eV. Contribntions to the strength function from p =0 and 1 dipole modes (the latter has twice larger strength) are exhibited by dashed curves. The bars are given in eVA. ...

One further advantage of the magnetic sector compared with the quadrupole is that ion transmission, an hence sensitivity, are much greater for comparable resolution. 

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