Orthogonal extraction

FIGURE 7.10 Schematic diagram of the orthogonal-extraction time-of-flight mass spectrometer. (Reprinted with permission from reference 22). 

FIGURE . 12 Orthogonal-extraction time-of-flight mass spectrum of tris-2,4,6-(pentadecaflu-oro-heptyl)-1,3,5-triazine (PFHT). (a) Complete mass spectrum, (b) Expansion of the isotopic peaks for the M-H ion. (Reprinted with permission from reference 24). 

FIGURE 7.13 Schematics of (a) the API-TOF mass spectrometer with an ion mirror and (b) the ion-storage modulator. (Reprinted with permission from reference 25). 

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Molecular beam sample introduction (described in section (Bl.7.2)). followed by the orthogonal extraction of ions, results in improved resolution in TOP instruments over eflfrisive sources. The particles in the molecular beam typically have translational temperatures orthogonal to the beam path of only a few Kelvin. Thus, there is less concern with both the initial velocity of the ions once they are generated and with where in the ion source they are fonned (since the particles are originally confined to the beam path). 

In ToF-MS, the ion source is pulsed to create packets of ions. In the conventional procedure, the system waits for all the ions in a packet to reach the detector before injecting the next packet of ions. Complications arise when ToF-MS is coupled to a continuous ion source. Such coupling is therefore often accomplished by the orthogonal extraction approach, in which a segment of the ion stream is accelerated orthogonally by a push-out pulse. However, in this process, up to 95 % of the information contained in the ion steam is lost. Recently, Hadamard transform time-of-flight mass spectrometry (HT-ToFMS) was developed to couple continuous ion... 

A. N. Verentchikov, W. Ens, and K. G. Standing. Reflecting Time-of-Flight Mass Spectrometer with an Electrospray Ion Source and Orthogonal Extraction. Anal. Chem., 66(1994) 126-133. 

Verentchikov A.N., Ens W., and Standing K.G. (1994), Reflecting time-of-flight mass spectrometer with an electrospray in source and orthogonal extraction, Anal. Chem. 66(1), 126. 

Shen, J. X. Wang, H. Tadros, S. Hayes, R. N. Orthogonal extraction/ chromatography and UPLC, two powerful new techniques for bioana-lytical quantitation of desloratadine and 3-hydroxydesloratadine at... 

Estimation of the sensitivity for the orthogonal extraction and alternative on-axis instruments is quite straightforward. The velocity (v) of the ion beam formed by skimming the supersonic expansion within the ICP-MS interface can be calculated as... 

The coupling of an ICP ionization source to a TOF-MS was first reported by Myers and Hieftje [24]. A schematic diagram of their instrument is provided as Fig. 12.9. Similarly to other ICP-MS instruments, it utilized a three-stage differentially pumped interface in order to realize the microtorr pressures required for TOF-MS from mean-free path considerations. This instrument was based on an orthogonal-extraction geometry and utilized a 1.6-m total flight path, which, at a 2-kV acceleration potential, dictated a maximum spectral repetition frequency of 20 kHz. 

Pulsed discharges may be utilized to analyze nonconductive samples. Steiner et al. described the use of a pin-type pulsed rf-GD in conjunction with a linear orthogonal extraction TOF-MS [58]. Figure 12.24 depicts differences in the spectra obtained 4.0 msec after the power plateau and 7.0 msec after the plateau. As expected, the sputtered species (Si) in the discharge are found in the afterpeak at 7.0 msec (see Fig. 12.24). The resolution and sensitivity of this early system were limited and improvements were planned in the input optical system of the instrument. 

Figure 27 Plot of signals vs. steering plate (Y) potential for differing masses in the LECO on-axis time-of-flight mass spectrometer. Compare to Fig. 12.9, obtained with an orthogonal-extraction, time-of-flight mass spectrometer (TOF-MS).

Orthogonal extraction, improvements in TOF/MS resolution, and improvements in electronics have created a new application for TOF/MS in accurate mass measurements [70,71]. Accurate mass applications have traditionally been associated with high-resolution magnetic sector and FTMS systems. The ability to provide accurate mass measurements (+5 ppm) with more cost-effective instrumentation has created potential opportunities in the pharmaceutical industry. Such applications can offer improved selectivity for single-stage instrumentation and metabolite or impurity/degradation product identification, particularly when standard compounds are not available, such as in early drug discovery. 

An older book that describes the basic principles of TOE, and several innovative techniques, including orthogonal extraction, post source decay, and delayed extraction. 

The orthogonal extraction allows running the source in unpulsed and pulsed RF modes. Furthermore, when... 

Separation of ions with the same mass but different surface areas is thus possible. Small dense ions quickly traverse the IM cell and enter the mass spectrometer where a pulsed voltage applied to the orthogonal extraction region deflects the ion beam into a time-of-flight mass spectrometer equipped with a reflectron, which measures the ions m/z directly and their ion mobility... 

More recently, delayed orthogonal extraction has been utilized to observe peptide fragmentation on MALDI time-of-flight instnmients. Figure 7.9 shows the high irradiance MALDI mass spectrum of melittin obtained by Standing et al., following an I8-ps delay. 

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