Qualitative versus Quantitative Approaches

We started this review with a brief discussion of some apparent dilemmas that we face in chemistry qualitative versus quantitative approaches, observables versus non-observables, structural criteria versus properties as criteria for characterization of aromaticity, chemical graph theory versus quantum chemistry, Clar 6/j rule versus Huckel An+ 2 rule, and hydrocarbons versus heteroatomic systems. As we have seen, most of the mentioned dilemmas are man-made and reflect inbred biases of different circles of chemists. It is not uncommon to come across critics with strong opinions and weak arguments, and it would be a waste of time to try to point out to them the other side of the coin . Max Planck apparently experienced difficult times before his quantum constant was accepted, as is reflected in the following quotations ... 

Based on experimental observations, the theoretical models considered throw light on the origin of simple periodic phenomena and complex oscillations, including chaos, in biochemical systems. Besides illustrating the variety of molecular mechanisms producing oscillations at the cellular level, the models allow us to delineate in a qualitative and quantitative manner the conditions in which sustained oscillations occur. The theoretical approach thereby contributes to a better understanding of the physiological roles of such rhythms. Furthermore, in such an approach the relative frequency of occurrence of simple versus complex patterns of oscillations becomes amenable to quantitative assessment. Finally, theoretical models for biochemical and cellular... 

All consequences of this relation have been verified both qualitatively and quantitatively. If one of the reactants is uncharged (say = 0), = o- If both reactants are of the same charge, a plot of log (i/h) versus - 7 is a straight line of positive slope. These correct conclusions would not be predicted by a naive approach that would write the following erroneous expression for the rate ... 

Figure 2. Current mathematical representations of metabolism utilize a hierarchy of descriptions, involving different levels of detail and complexity. Current approaches to metabolic modeling exhibit a dichotomy between large and mostly qualitative models versus smaller, but more quantitative models. See text for details. The figure is redrawn from Ref. 23. See color insert.

Every ionization method exhibits compound-dependent ionization efficiencies (Chap. 2.4). Whether a specific compound is rather preferred or suppressed relative to another greatly depends on the ionization process employed to deliver the ions to the mass analyzer. These circumstances require a careful calibration of the instrument s response versus the sample concentration for correct quantitation [6,7,50]. While relative signal intensities are perfect for qualitative analysis, i.e., for compound characterization, some means of measuring absolute intensities would be preferred in quantitation. Basic considerations on how to approach a quantitative analysis by mass spectrometry are given below [51-54]. Readers interested in a treatment of all aspects of quantitative analysis by mass spectrometry may refer to the highly recommended book by Boyd, Basic, and Bethem [50]. 

The students were then directed to read a well balanced review of quantitative versus qualitative analysis of risk (Apostolakis 2004), and a good account of possible approaches for assessing and managing risks from extreme events (Bier et al. 1999). 

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