Time-of-flight Mass Spectrometers ToF-MS

The Bendix Corporation s (USA) time-of-flight mass spectrometer (ToF-MS) with a pulsed electron impact ion source was one of the first commercial non-magnetic sector mass spectrometers on the analytical market as described by Wiley in 1956.37 

ToF mass spectrometers as dynamic instruments gained popularity with the introduction of matrix assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI) as effective pulsed ion sources for the soft ionization of large biomolecules (up to 10s dalton) due to their high ion transmission.38 ToF mass spectrometers, quadrupole analyzers and/or magnetic sector fields can be combined in tandem mass spectrometers (MS/MS) for the analysis of organic compounds. 

The axial time-of-flight ICP-MS was introduced in 1998 by LECO (Renaissance, LECO, St. loseph. Ml) and has been commercially available for several years. A schematic of the ICP-ToE-mass spectrometer is shown in Eigure 5.11. The analytical performance of this mass spectrometer, which allows 25 000 spectra to be measured per second and is thus faster than a quadrupole ICP-MS, 

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This fundamental equation explains that the velocity of heavier ions (iq of ions with mass m,) is lower than of lighter ions (v2 of ions with mass m2, with m, > m2). Equation (10) is used directly in time resolved measurements, for example in time-of-flight mass spectrometers (ToF-MS). The charged ions of the extracted and accelerated ion beam are separated by their mass-to-charge ratio, m/z, in the mass analyzer. Mass-separated ion beams are subsequently recorded by an ion detection system either as a function of time or simultaneously. Mass spectrometers are utilized for the determination of absolute masses of isotopes, atomic weights, relative abundance of isotopes and for quite different applications in survey, trace, ultratrace and surface analysis as discussed in Chapters 8 and 9. 

Another type of dynamic mass spectrometer is the time-of-flight (ToF) analyzer. In 1946, Stephens presented his concept of the linear time-of-flight mass spectrometer (ToF-MS) as the simplest mass separation technique at an American Physical Society meeting in Cambridge, MA.49 Cameron and Eggers first published the design and showed mass spectra for linear ToF-MS in... 

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).

Problems with contamination and losses of volatile boron compounds during sample preparation have limited the reliable documentation of boron concentrations in human tissue and body fluids. A complex technique involving a porous graphite column—inductively coupled plasma-atomic emission spectrophotometry (ICP-AES)— and an ICP time of flight mass spectrometer (TOF-MS) has been developed for investigations of boron neutron capture in cancer therapy. Adaptation of this method to nutritional studies of boron should be possible. 

Figure 18 Schematic of a time-of-flight mass spectrometer (TOF/MS).

FIGURE 4.4 Schematic diagram of the SELDI Ciphergen mass spectrometer. After sample preparation, the ProteinChip arrays are analyzed by a laser-desorption/ionization (LDI) time-of-flight mass spectrometer (TOF MS). The TOF MS measures the molecular weights of the various proteins that are retained on the array. For comparison purposes, the software associated with the SELDI Ciphergen instrument is capable of displaying the resultant data as either a spectral, map, or gel view. 

Over the past few years, GCxGC has been utilised in air and aerosol analysis. Particularly, GCxGC in combination with a fast acquisition mass spectrometer, for example, time-of-flight mass spectrometer (ToF MS) with a unit-mass resolution, provides extremely high analytical resolution with mass spectral information. Thus, the GCxGC-ToF MS is an exceptionally powerful tool in the air and aerosol analysis. It should be noted, however, that, as already mentioned in previous chapters, the system produces large amounts of data, and it is difficult to identify compounds from these datasets even with the structural nature and the mass spectral information. Therefore, automated procedures for data handling have been developed for identification purposes. 

Price has reported the use of pulsed laser pyrolysis to generate conditions, analogous to those behind the flame front in a real fire, at the surface of a polymer sample. The temporal behaviour of the species escaping from the reaction zone created in this way at the polymer surface is monitored by the very fast scanning time-of-flight mass spectrometer (ToF-MS). The ToF-MS is able to provide a complete mass spectral analysis every 25 ps. This identifies the species involves and indicates their relative concentrations. 

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