Alternative Sensitizer Molecules

To provide a stable attachment of a sensitizer to a wide-bandgap oxide semiconductor and to allow for a fast electron injection from the excited state of a sensitizer molecule to the conduction band of the oxide, covalent links are preferable as opposed to simple coordination or van der Waals bonds [22]. On the other hand, very acidic sensitizer molecules favor corrosion of ZnO and thereby lead to inefficient charge transfer at the interface and to a low cell stabihty [94]. Carboxylic or 

A further fundamental prerequisite is a high extinction coefficient in the visible range of the solar spectrum. The absorption spectrum has to be chosen such that either a broad band is provided by a given sensitizer or more than one sensitizer has to be used to efficiently cover the solar spectrum (panchromatic sensitization) [95-97]. 

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Current developments in MS have been directed to the application of alternative ionization methods for sensitive molecules, to the use of tandem systems (Durand et al. 1992), and to the development of interfaces with chromatographic systems (Arpino 1990 Niessen et al. 1991 Arpino et al. 1993). Some examples include the following. 

It has been demonstrated that liposomes, micelles, certain oil emulsions, and macromolecules (e.g., serum albumin) will keep the sensitizer molecules in a monomeric form (Jori, 1992 Hoebeke, 1995). The fact that most sensitizers are heavily charged and in many cases present as zwitterions, combined with their extensive polycyclic nature, makes some of these compounds particularly difficult to incorporate in a simple emulsion or liposome preparation. The zwitterions, in particular, can be almost insoluble in hydrophilic and lipophilic media. Alternative formulation approaches may therefore be required for this group of sensitizers. 

Peroxidases Enzymatic labeling is a milder alternative to chemical oxidants. It can be accomplished without deactivation or degradation. It is thus suitable to label very sensitive molecules, such as biomolecules. The labeling yields are close to those obtained with chloramine-T and iodogen, and no carrier-added radioiodination can be obtained (Davidson, 1987). 

The reaction can be driven in the desired direction by using a carbonyl compound that is a good acceptor of hydrogen, that is, one that is easily reduced. Quinone and fluorenone have been utilized for this purpose. Alternatively, an excess of a hydrogen acceptor can be used. Since the reaction conditions are nonacidic, this method is valuable for very acid-sensitive molecules. 

Thermal Conductivity Detector In the thermal conductivity detector (TCD), the temperature of a hot filament changes when the analyte dilutes the carrier gas. With a constant flow of helium carrier gas, the filament temperature will remain constant, but as compounds with different thermal conductivities elute, the different gas compositions cause heat to be conducted away from the filament at different rates, which in turn causes a change in the filament temperature and electrical resistance. The TCD is truly a universal detector and can detect water, air, hydrogen, carbon monoxide, nitrogen, sulfur dioxide, and many other compounds. For most organic molecules, the sensitivity of the TCD detector is low compared to that of the FID, but for the compounds for which the FID produces little or no signal, the TCD detector is a good alternative. 

Cl is an efficient, and relatively mild, method of ionization which takes place at a relatively high pressure, when compared to other methods of ionization used in mass spectrometry. The kinetics of the ion-molecule reactions involved would suggest that ultimate sensitivity should be obtained when ionization takes place at atmospheric pressure. It is not possible, however, to use the conventional source of electrons, a heated metallic filament, to effect the initial ionization of a reagent gas at such pressures, and an alternative, such as Ni, a emitter, or a corona discharge, must be employed. The corona discharge is used in commercially available APCI systems as it gives greater sensitivity and is less hazardous than the alternative. 

When a multistep synthesis is being undertaken with other sensitive functional groups present in the molecule, milder reagents and reaction conditions may be necessary. As a result, many alternative methods for effecting interconversion of the carboxylic acid derivatives have been developed and some of the most useful reactions are considered... 

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