Sequential Recording Techniques

Multiplexed multidetector systems can also be used in laser scanning microscopy to obtain lifetime images in several emission wavelength intervals and for different excitation wavelength. Please see Sect. 5.7, page 129 and Sect. 5.6, page 121. 

Sequential recording, also known as double kinetic mode [353] or time-lapse recording , adds one or two additional dimensions to the photon distributions recorded by multidetector operation and multiplexing. Controlled by its internal clock oscillator, the sequencer switches through a specified number of memory blocks. Each memory block contains the photon distributions of all detectors and multiplexing channels. Sequential recording in a multidetector system is illustrated in Fig. 3.7. For sake of simplicity, multiplexing has been omitted. 

If the stimulation of the system under investigation is repeatable a large number of triggered sequences can be accumulated. With accumulation, a sufficient number of photons per step is obtained even for sequencing rates faster than the count rate. The different time scales are illustrated in Fig. 3.8. Again, multiplexing was omitted for simplicity. 

Because the sequence is controlled by the TCSPC hardware, it is possible to achieve extremely fast and accurate stepping, down to less than a microsecond per data block. Of course, sequential recording can also be achieved by software control of a TCSPC device, and advanced TCSPC devices in fact include operating modes for recording software-controlled sequences. However, modem computers are far from being real-time systems. Stepping faster than 100 ms per step becomes inaccurate, which makes an accumulation of software-controlled sequences impossible. 

In some TCSPC devices a Continuous Flow mode is implemented to record a virtually unlimited number of waveforms. The sequencer of the continuous-flow mode uses two independent data memory banks see Fig. 3.9. After a trigger pulse 

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AlAs/GaAs. Sample preparation and laser melting techniques are found in reference [62], Images of Al, Ga, Si, and O were sequentially recorded with a resistive anode encoder as the sample was depth profiled with Cs" and positive secondary ions in a Cameca IMS 5f in the microscope mode of operation. Two-dimensional cross sections of the Al, Ga, Si, and O images from a 60 pm rectangular slice centered around the 4 pm laser stripe were then computer generated and are shown in Fig. 4.30a. Depth profiles of Al, Ga, and O from selected 4 pm areas were also generated in the non laser-striped area in Fig. 4.30b and in the laser-striped area in Fig. 4.30c. Samples similar to the one in Fig. 4.30 have been analyzed with TEM, SEM, and electron dispersive X-ray spectroscopy (EDS) in addition to SIMS for full characterization [62]. 

Other types of mass spectrometer may use point, array, or both types of collector. The time-of-flight (TOF) instrument uses a special multichannel plate collector an ion trap can record ion arrivals either sequentially in time or all at once a Fourier-transform ion cyclotron resonance (FTICR) instrument can record ion arrivals in either time or frequency domains which are interconvertible (by the Fourier-transform technique). 

Electronic techniques can generate a larger number of pulses in a specified time and are therefore more accurate than mechanical devices. At the receiving end, the pulses are used to determine the state of a series of bi-stable networks. These are scanned and reset sequentially and the total number of pulses recorded. 

D-Fluorescence spectroscopic analysis has also been used for analysis of terrestrial and aquatic HS. Figure 16.40 shows an example of topographic and contour plot of 3D-fluorescence spectrum. In this case, the Fluorescence spectroscopy involved scanning and recording 17 individual emission spectra (260-700 nm) at sequential 10-nm increments of excitation wavelength between 250 and 410 nm (Parlanti et al., 2002). The authors used this technique to obtain structural information about HS and also used it in studies concerning their transformation processes. They reported that there were five major fluorescent components in bulk seawater based on 3D-fluorescence spectroscopy. They defined a and a (excitation at 330-350 nm and emission at 420-480 nm excitation at 250-260 nm and emission at... 

In HR-CS AAS, although the simultaneous measurement of two elements or more is only possible within the simultaneous recorded wavelength interval, the reference element technique can be used essentially without compromises for FAAS, where nonspecific interferences are quite common. The fast-sequential... 

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