Oxidation methods, comparison problems

Collins and Williams [30] have recently described a modification of Ehr-hardt s earlier photochemical method [57], which offers the practical advantages of speed, convenience, and the potential for real-time analyses. However, until the accuracy of the results is established, the method will not receive general acceptance. Collins and Williams [30] examined the completeness of oxidation of their photo-oxidation system using three independent methods, but pointed out that while essentially complete oxidation was indicated, definitive proof was lacking. A more satisfactory solution to the problem might be found through comparison of results of the photo-oxidation method with the dry-combustion method, which most analysts are willing to accept as complete [96]. 

Other methods of identification include the customary preparation of derivatives, comparisons with authentic substances whenever possible, and periodate oxidation. Lately, the application of nuclear magnetic resonance spectroscopy has provided an elegant approach to the elucidation of structures and stereochemistry of various deoxy sugars (18). Microcell techniques can provide a spectrum on 5-6 mg. of sample. The practicing chemist is frequently confronted with the problem of having on hand a few milligrams of a product whose structure is unknown. It is especially in such instances that a full appreciation of the functions of mass spectrometry can be developed. 

The detection of products derived from the N-oxygenation of C=N functionalities presents many problems, which illustrate difficulties that are associated with the isolation, identification and quantification of small amounts of water-soluble metabolites. Spectrophotometric methods19 as well as differential pulse polarographic techniques20 previously used to determine oximes, nitrones and N-oxides frequently lack sensitivity and/or specificity. Improved analytical methods for the quantification of these N-oxy compounds include chromatographic techniques taking into account the chemical peculiarities of the individual N-oxygenated C=N functionalities. These procedures usually require the chemical synthesis of authentic material for comparison with data obtained with the isolated metabolites, and also for the construction of calibration curves. 

Concluding remarks. - From the above it is clear that neither the problem of the structure of supported vanadium oxide nor that of the special role of Ti02 as a support have definitively been solved. Further work on these and related problems is certainly necessary. As suggested above a more widespread use of some of the experimental methods by different research groups, allowing a comparison of the results, offers important advantages in this respect. 

Other analytical problems to which the direct comparison method has been applied include the determination of mixed iron oxides in the oxide scale on steel [14.10], the beta phase in titanium alloys [14.11], and mixed uranium and plutonium carbides [14.12]. 

A final comment is required about the problem of the description of isolated TM atoms on oxide surfaces in general. TM atoms have complex spin states which are not properly described within the DFT approach. Spin-polarized calculations provide a way to take into account the spin properties of the system but some details of the interaction may be described incorrectly. In this respect, the use of wave function-based methods is particularly important for benchmarks and comparisons. This problem is less severe when one considers small clusters where several electronic states exist separated by small energies. But one should be well aware of the fact that the treatment of isolated atoms, dimers, and very small aggregates with DFT methods requires spin-polarized approaches and special care in evaluating the results [199,200]. 

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