External ions

Indeed, some early calculations had shown an explicit dependence of the proton transfer potential upon the presence of an external electric field. When placed in a field of 0.103 V/A directed along the H-bond direction, for example, an asymmetry of some 3 kcal/mol was introduced into the potential of a sample (H20-H+-0H2) system [32], and a Li+ ion was able to modify 

Some of the computed results are reported in Table 4 where the top line refers to the absence of any ion at all. The potential in this case is of course symmetric, with AE=0, and the two wells are separated by a barrier Et of 7.3 kcal/mol. When an ion is placed directly above the 0-0 midpoint, position 1, the potential retains its symmetry, and the barrier is scarcely affected, increasing by only 0.1 kcal/mol. Note that the effect is the same, regardless of whether one is dealing with cationic Na+ or anionic Cl . These two ions produce a very different perturbation, however, when shifted off-center, to position 2. The cation yields a negative value of AE, while lowering the transfer barrier, and Cl the opposite. The latter effect is the same, only stronger, when the ions are placed directly along the 0-0 axis in position 3. 

Energetic aspects (kcaj/mol) of proton transfer potential of (H20-H+-OH2) in presence of 

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The main feature of the new wave function is its polarization by the field of the ion. That is, the presence of the ion changes the effective ionization energies of HA and HB and tends to pull electrons toward HB. The effect of the external ion on the potential surface for bond breaking is shown in Fig. 1.4. 

The displacement is effected preferentially by a nucleophilic group already in the molecule. Thus, in the above reaction there is no evidence that OTs- is displaced by the external ion, OMe-, under the conditions used, nor is there ahy displacement of OTs- of a sugar sulfonate by OAc-, Buch as was found to occur when the sulfonate of a monohydric aliphatic alcohol was treated with potassium acetate.11... 

In anticipation of the development to operational status of the ion or direct counting systems, it would be helpful if we could compare these values with projected counting errors for the two types of direct counting systems being developed. Table 4 lists projections for the Rochester Van de Graaff facility [49] and the University of California Lawrence Berkeley cyclotron system employing an external ion source [31,50]. Table 4 also lists the sample sizes and approximate measurement periods for both systems. This data illustrates the potential extention in dating... 

The quadrupole ion trap (QIT) creates a three-dimensional RF quadrupole field to store ions within defined boundaries. Its invention goes back to 1953, [103-105] however, it took until the mid-1980s to access the full analytical potential of quad-mpole ion traps. [137-140] The first commercial quadmpole ion traps were incorporated in GC-MS benchtop instruments (Finnigan MAT ITD and ITMS). Electron ionization was effected inside the trap by admitting the GC effluent and a beam of electrons directly into the storage volume of the trap. Later, external ion sources became available, and soon a large number of ionization methods could be... 

Fig. 4.40. Schematic of a quadmpole ion trap, (a) QIT with external ion source (illustration stretched in z-direction) and (b) section in the rz-plane (in scale), (a) Reproduced from Ref. [144] by permission. lohn Whey Sons, 2000.

Fig. 4.44. Timing sequence used for mass-selective instability mode (about 1.5 cycles shown). With an external ion source the ionization time is replaced by the ion injection pulse. Reproduced from Ref. [150] by permission. Elsevier Science, 1984.

With external ion sources it became feasible to interface any ionization method to the QIT mass analyzer. [171] However, commercial QITs are chiefly offered for two fields of applications i) GC-MS systems with El and Cl, because they are either inexpensive or capable of MS/MS to improve selectivity of the analysis (Chap. 12) and ii) instruments equipped with atmospheric pressure ionization (API) methods (Chap. 11) offering higher mass range, and some 5-fold unit resolution to resolve isotopic patterns of multiply charged ions (Fig. 4.47). [149,162,172,173]... 

Ion traps, ICR eells as well as QITs, are best operated with the number of trapped ions elose to their respeetive optimum, beeause otherwise ion trajectories are distorted by eoulombie repulsion. Henee, external ion sourees in eombination with ion transfer opties eapable of eontrolling the number of ions injeeted are ideally attaehed to ion traps. Currently, MALDI [207] and ESI (Fig. 4.54) [192-194,199,208] ion sourees are predominating in FT-ICR work. The ion produetion may either be regulated by the souree itself, or alternatively, by some deviee to eolleet and store the desired amount of ions from that source until injection into the ICR. For that purpose, linear RF multipole ion traps are often employed (Chap. 4.4.6), [118,209] but other systems are also in use. [195] RF-only multipoles are eommonly used to transfer the ions into the cell (Chap. 4.4.4). For the injeetion, it is important to adjust the conditions so that the ions have low kinetic energy in z-direction in order not to overcome the shallow trapping potential. 

While some buffer gas is beneficial in case of QITs, ICR cells are preferably operated at the lowest pressure available. The typical path from an external ion source into the ICR cell is therefore characterized by multistep differential pumping to achieve some 10 -10 Pa in the cell. 

Ehlers, M. Schmidt, S. Lee, B.J. Grote-meyer, J. Design and Set-Up of an External Ion Source Coupled to a Quadrupole-lon-Trap Reflectron-Time-of-Flight Hybrid Instram t. Eur. J. Mass Spectrom. 2000, 6, 377-385. 

Mclver, R.T., Jr. Li, Y. Hunter, R.L. MALDl With an External Ion Source Fou-rier-Transform Mass Spectrometer. Rapid Commun. Mass Spectrom. 1994, 8, 237-241. 

Several other types of ion source under development should be watched closely for potential inorganic/organometallic use. Ionization by electrons from a Ni source has been used in an external ion source that is at atmospheric pressure 66,67) giving a reported sensitivity in the subpicogram range. 

EXTERNAL ION-PAIR RETURN HIDDEN RETURN Ion permeation across membrane, PERMEABILITY CONSTANT PERMEABILITY COEFFICIENT ION PUMPS... 

For dilute solutions, the Debeye-Huckel law (log 7 = —0.5zf/°5) indicates that 7 will be a constant for a given ionic strength /. Therefore, the same quantity of inert electrolyte, called the support electrolyte, must be added to the sample and to the series of standards to increase the concentration of external ions and stabilise the ionic strength. This addition of ISAB (Ionic Strength Adjustment Buffer) is intended to limit variation in 7. Under these conditions, the measured difference in potential only depends on the concentration of the ion to be analysed and is given by equation (18.3). 

On the other hand, as long as the external ions are few and hence typically not inside the Rydberg orbit, r> is the distance of the ion from the center of the core. Hence the contribution of an external ion goes the other way,... 

For many years, filament and PIG ion sources were placed in the center of the cyclotrons. However, the development of more complicated and powerful ion sources such as the ECR sources required more space than was available at the center of the machines. Present-day cyclotrons have external ion sources and a low-energy beam from an electrostatic injector is threaded into the center via an axial channel and inflector or via a radial channel in separated- sector machines (described below). 

Scheme 5 depicts Winstein s complete solvolysis mechanism.29 Ion-pair return can be from the intimate ion pair (ion-pair return or internal return), from the external ion pair (external ion-pair return), or from the free ions (external ion return). The term external return refers to the sum of external ion-pair return and external ion return. The special salt effect operates by diversion of the external.ion pair, probably through the mechanism shown in Equation 519, so that it can no longer... 

In certain favorable cases of rearranging systems, the occurrence of internal return distinct from external ion-pair return can be demonstrated without recourse to optical rotation or isotopic labeling experiments. See S. Winstein and A. H. Fainberg, J. Amer. Chem. Soc., 80, 459 (1958) S. Winstein, P. E. Klinedinst, Jr., and E. Clippinger, J. Amer. Chem. Soc., 83, 4986 (1961). 

Belov, M. E., Nikolaev, E. N., Alving, K., and Smith, R. D. (2001). A new technique for unbiased external ion accumulation in a quadrupole two-dimensional ion trap for electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Rapid... 

Pulsed-Valve Cl and CID-Exneriments. Chemical Ionization (Cl), self-CI (SCI), and direct or desorption Cl (DCI) experiments in FTMS can be done equally well with the differentially-pumped external ion source described below, or with a pulsed-valve single cell arrangement (5,6). In our experiments, we admit a pulse of reagent gas via a piezoelectric pulsed valve with a minimum opening time of about 2.5 ms (7). Unlike solenoid pulsed valves, the performance of piezoelectric pulsed valves is not disturbed by the strong magnetic field of 4.7 Tesla. 

SIMS. Secondary Ion Mass Spectrometry is particularly suited for ionization of nonvolatile, polar, and thermally labile molecules. Liquid SIMS, using liquid glycerol matrices, is best done in the differentially-pumped external ion source, because matrix effects and the high vapor pressure of glycerol make liquid SIMS unsuitable for single cell low-pressure FTMS. 

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