Ion focusing

Morrison, J.D. Ion Focusing, Mass Analysis, and Detection, in Gaseous Ion Chemistry and Mass Spectrometry, Fu-trell, J.H., editor John Wiley Sons New York, 1986 pp. 107-125. 

Fig. 1.28 Schematic of a quadrupole-time of flight instrument. Quadrupole qO is used for collisional cooling and ion focusing.

The two most common instruments for these experiments are a triple quadru-pole mass analyser [34] and a quadmpole time of flight (Q-TOF) spectrometer [35]. The triple quadmpole instrument comprises two conventional quadmpole analysers usually referred to as Q1 and Q3 separated by a third known as Q2, which acts as an ion-focusing device and a collision cell. All the scan modes below (Figure 6.13) can be carried out on a triple quadmpole instmment. 

Sector field analyzers are magnetic or electric sector fields or a combination of the two with ion focusing and separation properties used in static mass spectrometers. 

A small positive potential on the repeller plate of the ion source pushes ions toward the analyzer tube, and a small potential on the ion focus plates creates a focused beam. High voltage ( 1 000-10 000 V) between the ion acceleration plates imparts a high velocity to ions as they are expelled from the bottom of the ion gun. 

A new generation of quaclrupole-based ICP-MS instruments offer an ion focusing and molecular ion destruction device named hexapole, collision cell or dynamic reaction cell depending on the manufacturer [17]. This device increases the transmission and destroys molecular ions with varying efficiency. An example of molecular ions and their impact on the analysis of bromine is shown in Fig. 3-... 

The use of mass spectrometry (MS) as a detection system is inevitable in the evolution of any separation method, especially CE where the liquid flow rate ( 1 ml/min) is compatible with conventional mass spectrometers. The combination of a high-efficiency liquid-phase separation technique, such as capillary electrophoresis, with MS detection provides a powerful system for the analysis of complex mixtures. Analyte sensitivity and the mass spectrum obtained depend on the electrospray ionization (ESI) voltage, ion-focusing parameters, and buffer composition. In general, the greatest sensitivity is obtained by employing conditions that facilitate desolvation and minimize cluster formation.47 Three ways of interfacing for CE-MS... 

Figure 7 shows the structures of the ion optics with the ionization chamber on the left and the entrance of the mass-analyzer on the right, which are described in the next section. The arrangement and shape of these structures were also computed via ion trajectory simulations. A result is shown in Fig. 8. At an extraction voltage (IE) of -25 V and separator input voltage (WFP) of 0 V an ion focus voltage (IFO) of 67 V generates the highest ion yield for the separator.

Extraction and acceleration of electrons and ions, focusing, and energy filtering of the ion beam rely on stable but tunable electric fields. Different potentials have to be applied to the electrode structures to generate these fields. 

By varying Uwo the focus point of ion beam is varied. Again the maximum ion current against t/IFO is the optimum. The voltage is varied between 10 and 120 V with the optimum typically between 30 and 75 V. Figure 22b shows a typical result for the ion focus variation. 

As shown in Figure 19-2, neutral and ionic species desorbing from a biomolecular film can be analyzed by mass spectrometry. Whereas ions that emerge from the film can be focused by electrostatic lenses located in front of the mass spectrometer (MS), neutral species spread in all directions. So, to obtain reasonable signals for the desorbing neutral species, they are usually ionized close to the target surface by a laser [28] and then the resulting ions focused into the MS (i.e., in the example... 

Fig. 2. Perspective cross sectional view of the temperature controlled drift cell after Kemper and Bowers design [22] (a) cooling line, (b) cell body, (c) buffer gas inlet, (d) cell end cap, (e) drift guard ring, (f) ion entrance hole, (g) ion focusing lens, (h) ceramic ring, (i) ion exit hole, (j) ceramic rod holding guard rings, (k) ceramic rods holding cell assembly...

---