Time focusing devices

Time Focusing Devices. The resolution of the TOF analyzer is limited by the initial velocity spread of the ions. However, there are powerful devices that can compensate for this velocity distribution, and the most widespread techniques at present are the electrostatic ion reflector (electrostatic mirror) and time-lag focusing (delayed extraction). 

Experimental equipment for X-ray diffraction methods has improved enormously in recent years. CCD detectors and focusing devices (Goepel mirror) have drastically reduced the data acquisition time. Cryogenic systems have been developed which allow structural studies to be extended down to the liquid helium temperature range. These developments have had important implications for SCO research. For example, fibre optics have been mounted in the cryostats for exploring structural changes effected by light-induced spin state conversion (LIESST effect). Chaps. 15 and 16 treat such studies. 

Fig. 4.10. Variation of potentials in pulsed ion extraction with time. The lens stack acts as angular focusing device for the ion beam. By courtesy of Bruker Daltonik, Bremen.

The two most common instruments for these experiments are a triple quadru-pole mass analyser [34] and a quadmpole time of flight (Q-TOF) spectrometer [35]. The triple quadmpole instrument comprises two conventional quadmpole analysers usually referred to as Q1 and Q3 separated by a third known as Q2, which acts as an ion-focusing device and a collision cell. All the scan modes below (Figure 6.13) can be carried out on a triple quadmpole instmment. 

Figure 16.6—Linear time of flight (TOF) and principle of the reflectron. 1) Sample and sample holder 2) MALDI ionisation device 3 and 3 ) extraction and acceleration grid (5 000 V potential drop) 4) control grid 5) multichannel collector plate 6) electron multiplier 7) signal output. The bottom figure shows a reflectron, which is essentially an electrostatic mirror that is used to time-focus ions of the same mass, but which have different initial energies. This device increases resolution, which can attain several thousand.

At present, efforts are being focused on the development of shortcut methods for the measurements of tars. The purpose of these revisions are to simplify the sampling and characterisation procedure (both in time and devices), but still to provide accurate and reproducible information reducing safety hazards and minimising time and cost of operation. 

Figure 16.6 A simplified schematic of a time of flight spectrometer and the principle of the ion reflector (reflectron). (1) sample and sample holder (2) MALDI ionization device by pulsed laser bombardment (3 and (3 ) ions are formed between a repeUer plate and an extraction grid (PD 5000V) then accelerated by an other grid (4) control grid (5) microchannel collector plate (6) signal output. Below, a reflectron, which is essentially an electrostatic mirror that is used to time-focus ions of the same mass but which have initially different energies. The widths of the peaks are of the order of 10 and the resolution ranges between 15 to 20 000.

Figure 6. Experiments on time-resolved tracking of the shape of the gas-liquid interface during the process of formatiorr of a single bubble in a microfluidic flow-focusing device. From a video recording of the process of break-up, we extract the projection of the interface on the x-y plarre (plane of the microfluidic device). We then extract the minimum width of the neck as a functiorr of time (Adapted Ifom Ref [21]).

Temporal Distributions. These include actual distributions in the time of ion formation as well as limitations of ion-detection and time-recording devices. The classical example of the former was the 1955 instrument of Wiley and McLaren in which ions were formed in the gas phase by a pulsed electron beam with a pulse width of 1 to 5 ps. In this case, temporal focusing was achieved by forming the ions... 

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