Multichannel analyser gating

By solving the inverse problem, the fluorescence lifetime r of a fluorophore can be determined independently from the dependence Nn(tdei). In the experiment the fluorescenoe signal is measured in relative units. For comparison of the experimental data with the theoretical ones it is necessary to normalize the obtained experimental curve to the fluorescence intensity at some fixed time delay. This procedure, the fluorimeter capabiUties in the kinetic mode and the corresponding theory can be found elsewhere (Banishev et al., 2006). The difference of such variant of kinetic fluorimetry from the conventional time-resolved fluorimetiy is that the fluorescence is excited by a pulse with rather long duration ( 10 ns), and for fluorescence registration an optical gated multichannel analyser is used. Whereas, the conventional time-resolved fluorimetry (Lakowicz, 1999) is based on the analysis of fluorescence decay curves after the excitation pnilse, whose duration is much shorter than the lifetime of a fluorophore in the excitation state (picoseconds). 

Figure 7.36 Diagram of a single photon counting apparatus for time-resolved spectroscopy. L, pulsed light source S, sample P, photodiode F, filter or monochromator D, photomultiplier R, voltage ramp driver G, gate Cj, C comparators M, multichannel analyser...

Figure 1. Experimental system used in the time resolved absorption measurements. (EL=excimer laser, KrF, 248nm DG=delay generator OMA=optical multichannel analyser MC=monochromator and gated diode array detector C=cell X=xenon flash lamp L=lenses )...

Figure C3.1.5. Schematic diagram of an intensifier-gated optical multichannel analyser (OMA) detector. The detector consists of a microchannel plate (MCP) image intensifier followed by a 1024-channel Reticon photodiode array. Light dispersed across the semitransparent photocathode ejects photoelectrons. These are accelerated toward the entrance of the microchannels by the gate pulse. The photoelectrons collide with the channel walls to produce secondary electrons, which are accelerated in turn by the MCP bias voltage to produce further collisions and electron multiplication. Electrons leaving the microchannels are further accelerated by the phosphor bias voltage.

The normal experimental technique is to scan rapidly through the velocity range and repeat this scan many times imtil data of the required accuracy has been accumulated. The Doppler motion is provided by an electromechanical drive system controlled by a servo -amplifier. Usually, the source is attached to the drive shaft and driven either in a saw-tooth or a triangular constant acceleration wave form. The transducer is coupled to a multichannel analyser operating in the multiscaler mode, and the servo-amplifier is controlled by the channel advance frequency. The dwell time in each channel, corresponding to a specific velocity increment, is 100 ps, and while the channel gate is open it accepts pulses from the detector. 

Describe the construction and use of an optical multichannel analyser system with time-gating possibihty. 

New types of detectors, such as OMA (optical multichannel analyser), SMA (spectrum multichannel analyser) and MCPD (multichannel photodiode array), are able to record the total emission spectrum by a single shot after flash excitation, so that acquisition of time-resolved emission spectra becomes much easier. One uses appropriate gating techniques, synchronized to the excitation flash, to control the time scale in the data acquisition. At the moment semiconductor based detectors are still less sensitive than photomultiplier (FMT) detectors. 

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