The Time-of-Flight Mass Spectrometer

FIGURE 10.17 An illustration of a time-of-flight mass spectrometer. 

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The "time of flight" mass spectrometer has been used to confirm that this highly radioactive halogen behaves chemically very much like other halogens, particularly iodine. Astatine is said to be more metallic than iodine, and, like iodine, it probably accumulates in the thyroid gland. Workers at the Brookhaven National Laboratory have recently used reactive scattering in crossed molecular beams to identify and measure elementary reactions involving astatine. 

The pulsed circuitry of the time-of-flight mass spectrometer is ideally suited for this type of operation, and both Hand and von Weyssenhoff (12, 13) and Lampe and Hess (25) have reported experiments using such an instrument with variable delay between the ionizing and withdrawal pulses to study secondary processes. 

The time-of-flight mass spectrometer was usually operated in the positive mode in order to detect the cationic sodium adducts of the analyte molecules. The anions of H-acidic substances were detected in the negative mode. This mode significantly enhanced the signals of acidic compounds, e.g. free fatty acids compared with ester signals in the mass spectra of beeswax and tung oil. 

The basic idea is to build the time-of-flight mass spectrometer into two electrically well shielded sections, the acceleration-reaction section of very short length / and a field free flight section of very large length L, or... 

The time-of-flight mass spectrometer used in these studies has been described in... 

For free-clusters, the cluster size distribution can be measured by the time-of-flight mass spectrometer for cluster films deposited on substrate by the cluster beam, the measurement of size distribution and observation of nanostructure are mostly done using transmission electron microscopy (TEM). In this section we will focus on the latter and pay special attention to FePt, CoPt clusters which have high anisotropy Tl0 phase after annealing [43-45]. For the TEM observations, FePt, CoPt nanoclusters, produced in a gas-aggregation chamber, in which high pressure Ar gas ( 0.5-lTorr) was applied and cooled by LN2, were directly deposited onto carbon-coated films supported by Cu grids. 

Courtesy of the Perkin Elmer Corporation The Time of Flight Mass Spectrometer... 

After focusing the accelerating potential (V) is applied for a much shorter period than that used for ion production ca 100 nsec) so that all the ions in the source are accelerated almost simultaneously. The ions then pass through the third electrode into the drift zone and are then collected by the sensor electrode. The velocity of the ions after acceleration will be inversely proportional to the square root of the ion mass. With modern ion optics and Fourier transform techniques Erickson et al. (6) could sum twenty spectra per second for subsequent Fourier transform analysis. The advantage of the time of flight mass spectrometer lies in the fact that it is directly and simply compatible with direct desorption from a surface, and thus can be employed with laser desorption and plasma desorption techniques. 

Secondly, the matrix film takes up the solvent and thus incorporates the solutes into the matrix layer. The steel plate is then removed, leaving the matrix layer containing the solutes on the surface of the thin layer plate. The thin layer plate is then placed in the laser desorption apparatus shown in figure 46. The desorption device consists of a TLC plate holder, driven by a stepping motor, that can be traversed both in the (x) and (y) axis so that the plate can be precisely scanned in two dimensions. The plate is situated in the ion source of the time of flight mass spectrometer and by pulsing the laser beam the spectra can be obtained and, if necessary, accumulated in the usual... 

Mass, infrared, ultraviolet, and mnr spectrometry have been highly developed as aids to identification. In some gas-chromatographic techniques some of the sample corresponding to a peak must be condensed and examined in others the effluent can be monitored continuously with an instrument such as the time-of-flight mass spectrometer, whereby mass spectra corresponding to each peak can be recorded as it is eluted. Developments in the mass-spectrometric analysis of gas chromatography effluents have been reviewed by McFadden. ... 

The time-of-flight mass spectrometer was invented many years ago, but the performance of the modern version is greatly improved. A diagram of the time-of-flight mass spectrometer is shown in Fig. 7. In a time-of-flight mass spectrometer, the following relationship holds ... 

The Fourier transform ion cyclotron resonance (FT-ICR) trapping of mass-selected cluster ions is an important emerging technique for the study of ion cluster reactivity. " This technique offers very high resolution and sensitivity. An alternative approach has been used by Brucat et al. who demonstrated that the reactivity of cluster ions can be studied in an experimental configuration identical to that used for the study of neutrals, except that ions are detected directly by pulsed extraction in the time-of-flight mass spectrometer. Other experiments " are exploring the reactions of mass-selected cluster ions in beam-gas-cell or drift-tube type configurations. This approach avoids the problems of mass overlap and offers a direct probe of cluster and cluster-adduct stabilities. For further experimental details, the reader is referred to the references. 

Fig. 1.19. Scheme of the experimental setup for infrared multiphoton ionization or dissociation of clusters or of metal clusters-rare gas complexes. The charged and neutral clusters are directly emitted from the laser vaporization/supersonic expansion source. The beam passes a skimmer and is subsequently crossed by the tightly focused beam of the FELIX. At some time after the FELIX pulse is over, the time-of-flight mass spectrometer acceleration plates are pulsed to high voltage, and a mass spectrum is recorded in a standard reflectron setup. Also schematically depicted is the particular pulse structure of the FELIX light [126,127]... 

The sensitivity of fragmentation to the nature of resonances was shown for tetramethylsilane by using the photoion photoion coincidence technique (Morin et al. 1986). In this technique, fragmentation channels are identified by the coincidences of pairs of ions in the time-of-flight mass spectrometer. For tetramethylsilane, two resonances were observed in the ion pair yield curves. The lower energy resonance, below the ionization edge, had been shown from electron spectra to decay into a one-hole state, but the production... 

Primer extension reactions can be analyzed using matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDl-TOF MS). Specially prepared single-base primer extension products are deposited on MALDI targets, evaporated by a laser beam, and directed into the time of flight mass spectrometer. This sensitive, label-free method can discriminate which base (A, C, G, or T) was added to the primer at the very SNP position, and affords excellent error tracking. 

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