Time-of-flight, velocity analysis

Methods for analysis of the particle size distribution in the aerosol cloud include techniques such as time of flight measurement (TOE), inertial impaction and laser diffraction. Dynamic light scattering (photon correlation spectroscopy) is confined to particles (in suspension) in the submicron range. In addition to the size distribution, the particle velocity distribution can be measured with the Phase Doppler technique. 

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. 

Fig. 3. A schematic view of a crossed-molecular beam apparatus used for studying the reactions of chlorine atoms with halogen molecules. The mass spectrometer detector is rotatable about the scattering centre for measuring the angular distributions of the reaction products whose recoil velocities are determined by time-of-flight analysis. (Reproduced from ref. 558 by permission of the authors and the American Institute of Physics.)...

The intensity of signal transmitted to the detector is greatly improved by using time-of-flight methods instead of mechanical velocity selectors. The beam of product molecules is chopped into a sequence of short pulses and the molecules then travel a known distance before being detected. The time-of-arrival spectrum at the detector gives the velocity distribution of the products [30]. This method of velocity analysis is now widely used in studies of crossed-beam reactions [111]. 

Capillary isoelectric focusing can be applied as a micropreparative tool for protein analysis by matrix-assisted laser desorption time-of-flight mass spectrometry (MALDI-TOF-MS) [69,70]. The exact timing of the collector steps in the interface is based on determining the velocity of each individual zone measured between two detection points close to the end of the capillary. During the collection a sheath flow fraction collector is used to maintain the permanent electric current. 

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. 

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