Field free drift tubes

Since pulsed laser ionization produces well defined packets of ions and electrons, TOF analysers (which essentially are magnetically shielded, electric-field-free drift tubes with apertures and an electron multiplier) can readily be used. TOF resolution for slow electrons can approach 3 meV, and throughput is similar to that of electrostatic analysers operating without an extraction field (i.e. a detection efficiency < 1%). The kinetic energy is obtained from the flight time, which is proportional to the reciprocal velocity, (KE) /2, whereas the resolution varies as (KE)/2. Thus, the resolution for 1-5 eV electrons is comparable to that for electrostatic analysers, but degrades seriously for 5-10 eY electrons. 

For this kind of analyser, a discrete pulse of molecular ions is injected into a field-free drift tube , measuring the time taken for ions to reach the detector (see Figure 3.3). This method of analysis is particularly suited to MALDl ionization methods, and can be used for quite large macromolecules. As before, the separation of molecular ions is on the basis of their mass/charge ratio mjz). 

Figure 9.19 A pulse of ions of two different m/z values enters the field free drift tube of a TOE mass spectrometer at time 1. The large white circles have m/z > than the small dark circles. As they travel down the tube, the lighter ions move faster, and by time 3, have been separated from the heavier ions.

Time-of-flight (ToF) mass spectrometers base their ion separation on the flight-time of ions in a field-free drift tube. Ions enter the ToF between two plates, a pulsed source (Figure 3.38a) with a large potential difference (not to be confused with the ion source) accelerates the ions. 

As shown in Section 11B-3, in TOF instruments, positive ions are produced periodically by bombardment of the sample with brief pulses of electrons, secondary ions, or laser-generated photons. The ions produced in this way ate then accelerated into a field-free drift tube by an electric field pulse of 10 to 10" V (see Figure 20-15). Separation of ions by mass occurs during the transit of the ions to the detector located at the end of the tube. Because all ions entering the tube have the same kinetic energy, their velocities in the tube vary in-... 

MALDI is compatible with a time-of-flight (TOF) mass spectrometer in which the ions are released at one point and accelerated by an electric field. The masses of the ions are then determined by measuring the time required for them to traverse a fixed distance in a field-free drift tube [26]. 

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