Quenching prevention techniques

Because Si — Ti absorption has a very small molar absorption coefficient, we would expect (because of the inverse relation between 8 and T0) the Ti state to have a much greater luminescent lifetime than the same molecules in the Si state. As a result of this longer lifetime, the Ti state is particularly susceptible to quenching, such that phosphorescence in fluid solution is not readily observed as the Ti state is quenched before emission can occur. This quenching in solution involves the diffusion together of either two Ti molecules or the Ti molecule and a dissolved oxygen molecule or some impurity molecule. In order to observe phosphorescence it is necessary to reduce or prevent the diffusion processes. The techniques most often used are ... 

GC-MIP systems have been investigated in considerable detail. Because of the low power of the plasma, it is easily quenched if the normal, atomic spectrometric sample introduction techniques, such as nebulisation, are used. Capillary columns overcome this problem as they require only low flow rates and small sample sizes more compatible with stable plasma operation. The capillary columns can be passed out of the oven, down a heated line, and the end of the column placed in the plasma torch just before the plasma, thus preventing any sample loss. A makeup gas is usually introduced via a side arm in the torch to sustain the plasma (Fig. 4.1, Greenway and Barnett, 1989). Other dopant gases can also be added in this way to prolong the lifetime of the torch and improve the plasma characteristics. 

To prevent accumulation of ash and slag in the water circuit, some water is discharged continuously. In a combined chemical and physical treatment sulfides, cyanides and suspended solids (ash) are removed. A concentrated sludge has to be filtered off and disposed of. The run-off water from the filters is stripped of ammonia and after pH-adjustment sent to a biological treatment unit. Uhde [532] has developed an alternative route for soot treatment in which the soot is filtered off and subjected to combustion, and the filtered water is recycled to the quench and scrubbing circuits. Let down water is treated as described above. This soot treatment technique avoids not... 

Nearly all materials can be prepared in the amorphous state. The principle involves rapid quenching, which prevents the material from crystallizing in an ordered manner. This can be achieved in a number of ways, including supercooling (cooling below the freezing point), vapour deposition and gelation. The majority of these methods lead to materials which cannot be easily characterized by normal techniques such as X-ray diffraction. 

Probably the simplest method to determine phase diagrams is to hold a carefully prepared mixture of known composition isothermally at elevated temperatures until equilibrium is achieved, quench the sample to room temperature rapidly enough to prevent phase changes during cooling, and then examine the specimen to determine the phases present. The latter is usually carried out by using a combination of X-ray diffraction and microscopy techniques. 

---