Electric field quadrupole

The electron distribution of the helium atom in field-free space is, of course, spherically symmetric. The atom has, however, a large polarizability in a quadrupole electric field, which we may ascribe to the partial orientation of the prolate ellipsoid. 

Figure 8.26. Geometry of the electrodes for (a) a dipole electric field, (b) a quadrupole electric field.

The principles of a beam maser spectrometer are illustrated in figure 8.46. Beam maser spectroscopy depends upon the ability to create artificially a large population difference (and inversion) between two energy levels. As we know from our discussion of electric resonance, a quadrupole electric field achieves just this for levels which are... 

Quadrupole Electric field gradient Symmetry of charge... 

In modem ion trap mass spectrometry, weak higher-order multipole and/or dipole electric fields are always superimposed upon the quadrupole electric field. The weak superimposed higher-order multipole and/or dipole electric fields are found to enhance the mass spectral performance with respect to mass resolution, charge capacity, scan speed, and to chemical mass shift. In fact, all commercial ion trap mass spectrometers are non-linear ion traps [7-9]. In the Finnigan ion trap instrument, the positions of the two end-cap electrodes are stretched out from the theoretical positions to add the higher-order multipoles. In the Bruker ion trap, the profiles of the end-cap electrodes are crafted to create the superimposed higher-order multipoles. In the Varian 4000 ion trap, a dipole and higher-order multipoles are superimposed upon the quadrupole field by a switchable electric circuit [10],... 

The effective well depth in a Paul trap is another concept that is frequently met in the literature. The hyperboloid shapes of the electrodes were chosen so as to produce an ideal quadrupole electric field when an RF potential is applied to the ring electrode and the two end cap electrodes are grounded. Such a field in turn produces a (truncated) parabolic potential well (V a r or z ) for the confinement of ions (recall that the field strength Ej = dV/dr and similarly for z) thus hyperboloid shapes were chosen because the motions of particles in parabohc potential wells is a very well understood problem in mechanics. The wells are truncated at the physical boundaries of the trap to define well depths for the electrical potential given by ... 

The polarizabilities of N2, O2, and Ar are nearly the same (1.74, 1.58, and 1.63 in units of 10 " cm, respectively), and are all nonpolar. Consequently, they adsorb nearly the same on all sorbents except zeolites. The fact that zeolites can distinguish between N2 and O2 was observed as early as 1938 (Barrer, 1937 1938). Barrer reported values for heats of adsorption of N2 on chabazite as high as 8 kcal/mol. The high heats of adsorption were subsequently explained quantitatively in terms of the quadrupole-electric field gradient interactions (Drain, 1953 Kington and Macleod, 1959). The unique adsorption properties of zeolites derive from the fact that their surfaces are composed of negatively charged oxides with isolated cations that are located above the surface planes. Despite... 

Similarly, the energy of a quadrupole electric field gradient with components... 

However, the use of highly reactive gases such as ammonia and methane can lead to more side reactions and potentially more interferences unless the by-prodncts from these side reactions are rejected. The way around this problem is to ntilize a lower-order multipole, such as a quadrupole, inside the reaction/collision cell and nse it as a mass discrimination device. The advantages of using a quadrupole are that the stability boundaries are much better defined than a hexapole or an octapole, so it is relatively straightforward to operate the quadrupole inside the reaction cell as a mass or bandpass filter. Therefore, by careful optimization of the quadrupole electrical fields, unwanted reactions between the gas and the sample matrix or solvent, which could potentially lead to new interferences, are prevented. This means that every time an analyte and interfering ions enter the reaction cell, the bandpass of the... 

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