Characterization strategies

The main characterizing strategies are the multivariate approach to the problem, searching for relevant information, model validation to build models with predictive power, comparison of the results obtained by using different methods, and definition and use of indices capable of measuring the quality of extracted information and the obtained models. 

The following discussion focuses exclusively on what are termed small molecules in the industry, namely compounds with a molecular mass on the order of one 1000 Da or less. The study of proteins, polymers, and other such macromolecules by NMR warrants an entirely different approach that is beyond the scope of this book. Similarly, we will restrict our discussion to liquid-state NMR spectroscopy, since the quantity of sample available for the characterization of a degradant or impurity is typically far below reasonable amounts needed for most solid-state NMR techniques. The overall characterization strategy we will use for NMR is shown in Figure 25. 

The next step is to compare the data and structural information obtained by NMR to any other information gathered. More specifically, is the structural information obtained by NMR consistent with any proposed structures If not, identify the particular areas of inconsistency to propose new structures. Even if a structure is identified to be consistent with the NMR data, it is important to consider if any other structures may also be consistent with the data. As mentioned, NMR cannot distinguish magnetically equivalent nuclei, and the characterization strategy outlined here does not explicitly detect nuclei other than carbon and hydrogen. One may also use spectral simulations to lend credibility to a specific structure.However, simulations should not be taken as proof of structure but rather be used to suggest plausible structures. The structure proposed based on the NMR data must be consistent with the total molecular mass, as determined by mass spectrometry. This is especially important for distinguishing monomers from dimers, trimers, etc. 

Yu, L., Reutzel, M. Stephenson, G.A. (1998) Physical characterization of polymor[4iic drugs an integrated characterization strategy, Pharm. Sci. Technol. Today, 1, 118-127. 

On another front, to accomplish electrical measurments of individual NWs, free of parasitic effects, and to develop competitive sensors, various fabrication and characterization strategies have been evaluated. For instance, low-current measur ent protocols have been found to aUow the devices to operate long-term withont degradation of their performance (Hemandez-Ramirez et al. 2007a, b). Thus the present state of development of NW-based technologies has led to complete and weh-controUed characterization of proof-of-concept devices, which were previously imattainable (Comini et al. 2009). 

FIGURE 29 The overall characterization strategy for structural characterization of impurities and degradants by NMR is shown schematically above. 

Advincula R, Xia C, Onishi K, Tarenekar P, Deng S, Baba A, Knoll W (2002) Conjugated polymer network ultrathin films by electropolymerization of precursor polymers polymer design, characterization strategies, and devices. PolymPrepr43(2) 514—515... 

Characterization can be defined as determining some characteristic or property of a material in a defined and reproducible way. The characterization is often used in a comparative manner so it is relative to a previous measurement. This type of characterization should be precise, not necessarily accurate (Sec. 11.3.1). Characterization can be at all levels of sophistication and expense. Several questions should be asked before a characterization strategy is defined. 

---