Chemical reactivity analytical applications

Analytical chemistry is more than a collection of techniques it is the application of chemistry to the analysis of samples. As you will see in later chapters, almost all analytical methods use chemical reactivity to accomplish one or more of the following—dissolve the sample, separate analytes and interferents, transform the analyte to a more useful form, or provide a signal. Equilibrium chemistry and thermodynamics provide us with a means for predicting which reactions are likely to be favorable. 

In recent years a simplifying attempt to overcome this complexity was to analyze carbon by TPD and to integrate the total CO and CO2 emission and to correlate the results with sample pretreatment and chemical reactivity [33]. The limited validity of such an approach is apparent. As is illustrated below, the chemically complex surfaces which are not described by such crude correlations are those with the highest catalytic activity. In applications of carbons as catalyst support it is immediately apparent that the details of the car-bon-to-metal interaction depend crucially on such details of surface chemistry. This explains the enormous number of carbon supports commercially used (several thousands). A systematic effort to understand these relationships on the basis of modern analytical capabilities is still missing. 

The native properties of high surface area, luminescence, tunable biostability, and reactive surface chemistry identify porous silicon as a potentially useful substrate for a variety of tasks. Presented below are several uses that have been developed by exploiting the chemical reactivity or changes effected by chemical interactions with the nanocrystallites. These applications range from use as a material or structural implant to use as a sensor or analytical support. 

There are various potential applications of photophysical phenomena in analytical chemistry. The relatively short lifetimes of most excited states, however, is a serious drawback to the construction of practical devices but studies which focus on finding ways to extend triplet lifetimes have now been described by Harriman et al. Kneas et al. have examined new types of luminescent sensor on polymer supports, and both Neurauter et al. and Marazuela et al. have designed sensors based on the ruthenium(II) polypyridine complex for the detection of carbon dioxide. A system, based on the formation of twisted intramolecular charge transfer states, has been devised for measuring the molecular weight of polymeric matrices (Al-Hassan et a/.), and the chemical reactivity at the interface of self-assembled monolayers has been assessed using fluorescence spectroscopy (Fox et al). 

In spite of the difficulties discussed above, the spectra of the cyclo-carbosilanes may be used in solving structural problems such as those associated with position isomerism in unsymmetrical methyl-substituted rings. This type of analytical application of nuclear magnetic resonance spectroscopy is particularly valuable for the carbosilanes, as the possibilities of establishing structures by chemical means are very restricted. The carbosilanes are not reactive and, unlike carbon compounds, undergo few reactions which yield information concerning their structures. In fact the structures of a number of compounds were first established with the aid of nuclear resonance. 

Although photo-QDNP spectroscopy has come of age by now, new applications still arise. As has emerged, the CIDNP effect connects diffusion, chemical reactivity, and spin evolution in a unique way it also combines the analytical potential of NMR spectroscopy with a sensitivity to species as short-lived as a nanosecond or even less. Hence, photo-ClDNP spectroscopy provides very diverse and deep insight into both chemical and physical processes, and yields information that is often inaccessible by other techniques. A method as powerful and versatile as this certainly deserves to be more widely known, and more frequently applied. 

Another analytical challenge presented to the researcher involved with thermal stability and catalytic reactivity of hydrocarbons relates to cracking of heavy fractions of petroleum. Thermal analysis is not discussed in this chapter. However, one should be aware that, although paraffins are considered to be chemically inactive, the application of certain analytical techniques may cause cracking, rearrangement and isomerization. 

For many fields of chemistry, lasers have become indispensable tools [1365-1370]. They are employed in analytical chemistry for the ultrasensitive detection of small concentrations of pollutants, trace elements, or short-lived intermediate species in chemical reactions. Important analytical applications are represented by measurements of the internal-state distribution of reaction products with LIF (Sect. 1.3) and spectroscopic investigations of collision-induced energy-transfer processes (Sects. 8.3-8.6). These techniques allow a deeper insight into reaction paths of inelastic or reactive collisions, and their dependence on the interaction potential and the initial energy of the reactants. 

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