Double mass spectrometer

In essence, a guided-ion beam is a double mass spectrometer. Figure A3.5.9 shows a schematic diagram of a griided-ion beam apparatus [104]. Ions are created and extracted from an ion source. Many types of source have been used and the choice depends upon the application. Combining a flow tube such as that described in this chapter has proven to be versatile and it ensures the ions are thennalized [105]. After extraction, the ions are mass selected. Many types of mass spectrometer can be used a Wien ExB filter is shown. The ions are then injected into an octopole ion trap. The octopole consists of eight parallel rods arranged on a circle. An RF... 

Charge Exchange and Ion-Molecule Reactions Observed in Double Mass Spectrometers... 

Tn a double mass spectrometer several types of ion-molecule reactions - can be observed (a) charge exchange, A++B- A + B+, often followed by dissociation of B+ (b) transfer of part of A+ or B (e.g., proton transfer or hydride ion transfer) during the collisions (c) reactions at increased pressure in the collision chamber. 

Figure 1. Double mass spectrometer of perpendicular type used in Stockholm (3, 26)...

Figure 2. Double mass spectrometer of perpendicular type used in Baltimore (45). Radius of mass spectrometer A is only 1 cm. which makes possible work at very low velocities of the ions A (Journal of the American Chemical...

Figure 3. Double mass spectrometer of longitudinal type used in Chicago...

Ion-molecule reactions can be investigated in a double mass spectrometer in two ways (a) In the collision between the incident ion and the gas molecule, transfer of part of one of these structures can take place. The pressure in the collision chamber must be low (b) The pressure in the collision chamber is increased. The slow incident ions ionize the gas molecules by charge exchange. Then ion-molecule reactions take place between the ionized gas molecules or their fragment ions and other gas molecules. 

Widespread interest has developed during the last few years in a new analytical technique which uses a tandem electrostatic acclerator as one element of a double mass spectrometer [1-19]1. With this technique, individual atoms of many nuclear species from most areas of the periodic table can be identified with good efficiency and in the presence of an almost zero background of unwanted atoms and molecular species of the same mass. 

Fig. 11. Double mass spectrometer for investigation of dissociation after charge exchange. The ions A in mass spectrometer A move in a vertical plane and the ions B move in a horizontal plane. The permanent magnet core is shown only in magnet A.

E. Lindholm, Charge Exchange and Ion-Molecule Reactions Observed in Double Mass Spectrometers," in Ref. la, p. 1 and Mass Spectra and Appearance Potentials Studied by Use of Charge Exchange in a Tandem Mass Spectrometer, in Ref. Id, p. 457. 

Vance and Bailey [94] also carried out measurements of velocity distributions of product ions using a double mass-spectrometer system similar to the one used by Giese and Maier [95] combined with the retardation product-ion analysis by Menendez et al. [96]. They studied the charge transfer and dissociative charge transfer reactions of Hj and N2 with H2, as well as the reaction + H2 H3 + H, and the results yielded considerable new information on the reaction mechanism. 

This chapter is concerned principally with two-body reactions between negative ions and neutral molecules at low energies in the gas phase. Introductory material on the formation and disappearance of negative ions is also included. Emphasis is placed on results arising from the use of mass spectrometer ion sources or double mass spectrometers as the experimental technique. Additional information on negative-ion-neutral reactions may be found in Chapters 7, 8, 11, 15, and 16 of this book. 

This chapter is largely restricted to the study of negative-ion-neutral reactions using the mass spectrometer ion source and double mass spectrometer techniques, and only these methods are discussed in detail in this section. 

Double mass spectrometer systems were developed in order to give mass-analyzed beams of reactant ions having narrow energy distributions and widely variable kinetic energy, while retaining the obvious advantage of mass spectrometric analysis of the product ions. Several variations on this basic technique have been developed and are described in Chapters 10-12. An example is shown in Fig. 1 and described below. 

The experimental results reported in this paper were obtained using a longitudinal double mass spectrometer system that has been described previously. This apparatus, shown schematically in Fig. 1, consists of a 2.54 cm radius, 90 degree magnetic sector mass spectrometer with electron impact ion source for production of... 

J. F. Paulson, F. Dale, and S. A. Studniarz, Study of ion-neutral reactions with a time-of-flight double mass spectrometer, Int. J. Mass Spectrom. Ion Phys. 5 113 (1970). 

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