Field free drift region

The mass separated, pulsed, and focused primary ions with the energy of 1 -25 keV, typically liquid metal ions such as Ga, Cs, and O", are used to bombard the sample surface, causing the secondary elemental or cluster ions to emit from the surface. The secondary ions are then electrostatically accelerated into a field-free drift region with a nominal kinetic energy of ... 

Reflectron time-of-flight mass spectrometers can be used to generate fragment spectra. Before the introduction of delayed extraction, it was observed that the ions had a tendency to disassemble into fragments while they were traveling to the reflectron (Spengler et al., 1992). This behavior is called postsource decay (PSD). The ions collide with matrix ions in the gas phase when they are accelerated with several kilovolts. Their decay occurs in the first field-free drift region. 

Tire TOI mass analyzer uses the differences in transmit time through a drift region to separate ions of different masses. It operates in a pulsed mode, so ions must be produced or extracted in pulses. An electric field accelerates all ions into a field-free drift region, and lighter ions have a higher velocity than heavier ions and reach the detector at the end of the drift region sooner. 

The factor in curly brackets is the normalised probability that A is produced between t and t -I-dt from A,.,., with energy . The second exponential gives the probability that A does not decay in the interval tj — f. The relevant time f, of this experiment is the time the cluster passes through plate PI in fig. 2. Note that it is not the time to pass through P2, as all ions which decay between PI and P2 are not focused onto one mass peak, but contribute to a broad background. The probability F that A fragments into A , -E A in the field-free drift region (and that Af, can be focused on one peak by the TOP spectrometer) is... 

Fig. 3.5. Schematic representation of a time-of-flight mass analyser with reflectron and external ion storage for time-delay extraction. Ions that enter the field-free drift region migrate to the detector at a rate that is dependent on their m/z ratio. The reflectron lenses compensate for variations in kinetic energies of the injected ions these variations would otherwise produce broadened peaks and loss of spectral resolution.

The field-free drift region in our instrument has a length of 80 cm, while the ion reflector is 12 cm long. This results in a total time-of-flight of about 130 xs for ions of 700 eV and m/z 500. As an ion detector, channel plates in tandem configuration are used. The flight tube is pumped by a 350 Ls l turbomolecular pump. 

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