Timed interval recording, detection method

For either of these two methods, in which the measurement of the current is restricted to one selected potential, it is necessary to carry out a preliminary recording of the i — E curves in the reaction mixture at chosen time intervals. This makes it possible to choose the correct applied potential corresponding to the limiting current of the wave studiet. It is also advisable to record one complete i — E curve at the end of each kinetic run, in order to confirm that the applied voltage chosen really corresponded to the limiting current of interest. Such a current-voltage curve also allows unexpected reactions to be detected, if new waves appear. 

These methods of obtaining complete ultraviolet-visible spectra can be very time-consuming. However, rapid scanning spectrophotometers based on optical diode array detectors are now eormnereially available and are capable of recording complete spectra in very short time intervals (1 second or less). They can therefore be used to obtain spectra of separated components without stopping eluent flow or to detect at several wavelengths simultaneously. 

Using two detectors simultaneously, one has to detect the 122 keV y quanta coming from the Mossbauer source as a start signal. This actually marks the birth of the 14.4 keV Mossbauer excited level (see the decay scheme inO Fig. 25.7). Then, with the other detector, the 14.4 keV y quantum is detected, and the elapsed time measured (stop signal). With this method, Mossbauer spectra can be recorded in any time interval after the nuclear decay of Co. Recording characteristic X-rays following the electron capture, part of the aftereffect events can be filtered off. Due to the coincidence technique, very low activities can only be used, and therefore these measurements are rather time consuming (several weeks or months). 

Random position samplers are also available, which can randomly select, under programme control, sample containers in racks or on trays. The sizes of the sample plate and containers depend on the amount of sample required for the determination of each individual compound. Using the peak-recording principle 2-3 mL of sample usually is needed per analysis of each compound while the steady-state method uses twice the amount. A pick-up device controlled by a mechanical or electrical timer directs a thin stainless steel tube into the sample vessel and the sample is pumped into the analytical system for a pre-determined time interval. The pick-up is then lifted and moved into a rinsing solution reservoir filled with the appropriate zero water . Using the peak-detecting method, the rinsing time equals... 

The remote-echo detector is shown in Figure 11. In this method the electron spin echo at the end of the pulse sequence, which uses Vi < rnuclear coherence generator, is not recorded. Instead, at the time of echo formation an additional nil pulse transfers the electron coherence to longitudinal magnetization. The echo amplitude information can thus be stored for a time interval up to the order of T. After a fixed time delay h < T l, the z-magnetization is read out using a two-pulse echo sequence with a fixed time interval X2 > r. Remote echo detection can be applied to many experiments, including three-pulse ESEEM and HYSCORE, and thus can eliminate blind spots with an appropriate choice of small ri. Note, however, that it may suffer from reduced sensitivity due to the increased sequence time. 

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