Extraction Region

WavUengtti 337nm Pulse width 15nste Pulse energy 4mJ(M x.) 

FIGURE 3.8 Schematic diagram of the coaxial reflectron time-of-flight mass spectrometer used by Tanaka et al. (Reprinted with permission from reference 14). 

FIGURE 3.9 Schematic diagram and simulated ion trajectories for aTOF mass spectrometer (a) with a conventional two-stage mirror, and (b) with the three-stage mirror designed by Short and Todd. (Reprinted with permission from reference 15). 

The ideal reflectron would provide kinetic energy correction to infinite order. One can easily show that this would occur if the reflectron was designed such that In this case, the flight time of an ion in the reflectron can be calculated  

FIGURE 3.10 Time-of-flight mass spectra for Na ions as a function of m3/ m2 ratio for (a) conventional two-stage configuration, (b) optimal temporal focusing, and (c) overcompensation. (Reprinted with permission from reference 15). 

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Figure 4.3. Diagram of the extraction region of a liquid metal ion source.

The main factor affecting orthogonal TOF is the duty cycle. This is how often ions can be pulsed into the TOF analyzer. When waiting between extraction pulses, ions are being wasted (they pass through the extraction region and are lost) and so the ideal is obviously to have this process as near to continuous as possible. The factor limiting the duty cycle is the time taken by the slowest... 

Figure 5.8 Schematic representation of a chip-based solid phase extraction-MEKC device, (a) Layout of the entire device and (b) expanded view of the extraction region of the device. The dotted lines represent the direction of fluid flow during extraction the solid lines signify flow during elution/injection (Narrow channels are 55 pm wide, column chamber is 210 pm wide, with all channels 15 pm deep.) [87].

This modulation can be achieved by preacceleration modulation, that is, by the formation of ion packets prior to extraction. A portion of the ion beam that is similar in length to that extracted in the right-angle design is selected by a set of parallel plates and allowed to travel into the extraction region, where the ions are extracted for mass analysis. In theory, any optical device that would allow a portion of the ion beam to enter the extraction region would work in this case. 

Alternatively, the difference in the energies of those ions present within the extraction region at time of a repeller event from those ions not selected for mass analysis can be exploited. By applying a potential barrier of the appropriate dimensions immediately prior to the detector, ions within the continuum that do not have sufficient energy do not pass this barrier, and those ions contributing to signal are detected. The different means of modulation are examined in more detail in a later section. 

Notable among the instrumental design characteristics is the use of direct current (dc) quadrupole lenses to limit ion-beam width spatially within the extraction region. These lenses, along with the use of an ion reflectron, provided resolving powers of approximately 1600 (FWHM) [40]. An illustrative spectrum of Pb and Bi obtained with this instrument is provided as Fig. 12.10. 

In the first instrument, sensitivity and limits of detection were constrained by the total ion beam current measured after the extraction region of the TOF-MS [42]. Although pulser circuitry limited the repetition rate to 7.1 kHz and a duty factor of 3.5%, detection limits ranged from 0.03 to 3 ppb using ultrasonic nebuliza-tion [16]. Mahoney et al. later reported on improvements to the same orthogonal ICP-TOF-MS [29]. The use of a commercial skimmer cone and quadrupole doublet in the extraction optics led to an increase in primary ion current from 2 nA to 50 nA [29]. This, along with electronic improvements allowing a 16-kHz spectral-repetition rate, dropped limits of detection to 1-10 ppt when ultrasonic nebuliza-tion was utilized. 

Several strategies were employed in order to reduce this effect. Myers et al. described the use of a pulsed ion optic to limit the access of ions to the extraction region [28]. By application of a voltage pulse to one of the quadrupole optics, the ion beam was swept across the slit at the end of the primary optic chain. By correct choice of delay parameters, the extraction region was filled with ions only immediately prior to a repeller event. Mahoney et al. described a different approach that relied on differences in the energies of signal and continuum background ion populations [42]. 

Currently, the state of Acre in western Brazilian Amazonia has nearly half the area of extractive reserves and setdements in Amazonia (Table 1). Of the 1.7 million ha in reserves and settlements within the state, Chico Mendes Extractive Reserve is the largest, covering 970,000 ha. It is located in the Brazil nut-rich southeastern portion of Acre and it is this extractive region that is the focus of our subsequent discussions. 

Fig. 11 Various chromatograms from APCI LC/MS analyses of an elastomer solvent extract (region of greater expansion).

In this source the gas inside a tube is ionized by radio-frequency 100 Watts, 10— 100 MHz). A magnetic field concentrates the discharge on the extraction region. The extraction takes place due to a potential difference between the discharge and the beam extractor. The source works very conveniently for gases. For some applications it has the drawback that the energy spread of the ions produced is larger compared with other sources. 

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