APPROACHING THE PROBLEM

Step 2. Know your unknown Step 3. Pick the packing Step 4. Achieve the separation General tactics Easy as 1, 2, 3, 4 An example 

Differences in size of the sample components Normal and reverse phase Differences in stationary phases Differences in the mobile phases 

Differences in ionic functionality of the sample components What happens after choosing the LC packing  

The first rule of thumb to remember is that there is no single approach to developing a satisfactory separation. Most problems can ultimately be solved by more than one mode of LC. Therefore, a general strategy for the analyst to follow is presented. 

Define the goals of the separation. What degree of resolution needs to be achieved How many compounds need to be resolved It is crucial to have a clear statement of the problem to be solved. Is it necessary to obtain a quantitative separation of each compound present in a multicomponent mixture, or is there only one component of interest in the sample Is it necessary to achieve a general profile of the sample for routine screening purposes, or must there be a specific quantitative separation for collection and subsequent analysis by other methods  

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An alternative approach (78,79) is based on a set of possible reaction schemes that are used to generate potential new pathways. Under both approaches, the problem, in part, is how to evaluate the utiUty of a particular scheme. A computer-assisted approach to predicting potentially useful reactions has been developed (80). The union of existing capabiUties in modeling chemical stmctures with selecting reaction pathways has not yet taken place. 

In 1766 Smeaton designed an atmospheric steampumping engine for the New River Company however, when the engine was set to work, its performance was less than Smeaton had expected. Realizing his knowledge of steam engines was deficient, he approached the problem in the same manner as he did his study of water and wind power—by... 

Oldham and Mansfeld" approached the problem of linearity in a different way and their derivation avoids the approximation used by Stern and Geary. They conclude that although linearity is frequently achieved this is due to three possible causes (a) ohmic control due to the IR drop rather than control according to linear polarisation, (b) the similarity of the values of b, and be and (c) a predisposition by the experimenter to assume that the AE — Ai curves near must be linear. In a later paper Oldham and Mansfeld" showed that linearity of the AE — Ai curve is not essential and... 

Hall22 at the Sloan-Kettering Institute for Cancer Research has approached the problem of determining trace metals in biological tissues in a way that has the following noteworthy features (1) X-ray... 

The observations of the interconversion of allylic sulfenates and sulfoxides made by Braverman and Stabinsky34-38 are confirmed by the work of Mislow and coworkers44-47 who approached the problem from a different angle, namely, enhanced racemization of optically active allylic sulfoxides. 

Good thermal stability is a requirement for surfactants used in processes to enhance oil recovery. This applies most particularly to steam foam applications where surfactants such as AOS may be exposed to temperatures far above 100°C albeit for short times. Many authors have approached the problem of the thermal stability of a surfactant through a determination of the activation energy of the thermal degradation process. Once the activation energy is known, it can be used to estimate the rate of thermal degradation under various conditions. 

Each Example includes a stategy that describes how to approach the problem. After describing the solution, we offer an Extra Practice Exercise designed to help you master the particular type of problem while the example is fresh in your mind. The answers to these Extra Practice Exercises appear at the end of each chapter, along with the answers to the Section Exercises. 

This is a quantitative thermod mamics problem that asks us to determine entropy under nonstandard conditions using standard thermod3mamic data, so we apply the seven-step approach. The problem asks for two quantities the molar entropy of O2 gas in the pressurized tank and the entropy change when a sample of that gas undergoes a pressure change. 

This accounts for the considerable discrepancy between the alkene Z/E ratio found on work-up and the initial oxaphosphetan ais/trans ratio. By approaching the problem from the starting point of the diastereomeric phosphonium salts (19) and (20), deprotonation studies and crossover experiments showed that the retro-Wittig reaction was only detectable with the erythreo isomer (19) via the cis-oxaphosphetan (17). Furthermore, it was shown that under lithium-salt-free conditions, mixtures of (19) and (20) exhibited stereochemical drift because of a synergistic effect (of undefined mechanism) between the oxaphosphetans (17) and (18) during their decomposition to alkenes. 

After having considered the basics of preferred orientation, we now approach the problem of how to separate orientation from the intrinsic scattering intensity of the structural entities. After some reasoning the general relation for the observed intensity... 

In many cases, the comparison of a reaction accelerated by microwave irradiation has been made with the same reaction in an oil bath at the same bulk temperature. Unfortunately, there have been quite a few reports in the chemical literature that have not been conducted with such proper control of conditions and consequently a fair comparison is not possible. Nevertheless, using this MW approach, the problems associated with waste disposal of solvents that are used several fold in chemical reactions, and excess usage of chemicals are avoided or minimized. The discussion pertaining to the preparation of supported reagents or catalysts has not been included in this chapter because numerous review articles are available on this theme [14—22],... 

Pauling based electronegativity values on bond energies between atoms, but that is not the only way to approach the problem of the ability of atoms in a molecule to attract electrons. For example, the ease of removing an electron from an atom, the ionization potential, is related to its ability to attract electrons to itself. The electron affinity also gives a measure of the ability of an atom to hold on to an electron that it has gained. These atomic properties should therefore be related to the ability of an atom in a molecule to attract electrons. Therefore, it is natural to make use of these properties in an equation... 

Finally, development of drugs targeted toward modification of kinases and phosphatases required for activation or inhibition of particular transcription factors is a promising therapeutic approach. The problem with this paradigm is the specificity of the kinases and phosphatases, since they will often act enzymatically on multiple proteins (see Chs 24 and 25). Furthermore, such drugs often lack specificity and may interact with multiple kinases or phosphatases. 

You should attempt to solve a problem only after you have studied the appropriate sections in the textbook. If you try to circumvent this process by attempting to solve the problems without looking in the text, you will find yourself constantly flipping through the pages in the chapter to find the concepts you need to approach the problem. This search will, of course, be quite inefficient because you will not be familiar with the material in the chapter. Worse yet, you might simply look back for a sample question that is similar to the one you are working on. This latter technique does not help you learn how to problem solve it simply teaches you how to reproduce someone else s solution. 

Samples. Do not go on to a new sample until you thoroughly understand the example you are currently working on. If you do not understand a sample after considerable thought, ask your AP chemistry teacher or fellow classmates how they approached the problem. Be sure to work each sample with pencil and paper as you go through the book. Write out your answer to every free-response question. Practice makes perfect ... 

Some attempts to exploit sensor dynamics for concentration prediction were carried out in the past. Davide et al. approached the problem using dynamic system theory, applying non-linear Volterra series to the modelling of Thickness Shear Mode Resonator (TSMR) sensors [4], This approach gave rise to non-linear models where the difficulty to discriminate the intrinsic sensor properties from those of the gas delivery systems limited the efficiency of the approach. 

Prepare a problem-solving flow chart specific to this problem, similar to Fig. 21.4 for the Bulging Drum Problem. How might you have approached the problem differently ... 

The concept of an enzyme-substrate complex is fundamental to the appreciation of enzyme reactions and was initially developed in 1913 by Michaelis and Menten, who derived an equation that is crucial to enzyme studies. Subsequent to Michaelis and Menten several other workers approached the problem from different viewpoints and although their work is particularly useful in advanced kinetic and mechanistic studies, they confirmed the basic concepts of Michaelis and Menten. 

The medicinal chemist expects to find that a review written specifically for him will he mainly concerned with a discussion of structure/activity relationships. In the case of the antihypertensive activity of the (3-blockers such a discussion is not possible since no direct comparative activity data is available either from the laboratory or the clinic (the few comparative clinical studies are in no way definitive - see later). It is however possible to approach the problem somewhat indirectly and consider the influence of chemical structure on those pharma -cological properties which appear most relevant to the specific clinical effect. I propose therefore to -... 

Williams RJ, Davis DR. Differential nutritiona new orientation from which to approach the problems of human nutrition. Perspectives in Biology and Medicine 1986 29 199202. 

M. Reist, B. Testa, P. A. Carrupt, The Racemization of Enantiopure Drugs Helping Medicinal Chemists to Approach the Problem , Enantiomer 1997, 2, 147 - 155. 

Muda and Hanawa (MH) have approached the problem by considering the time variation of the quantities (r) = < r) cl,c, t)>, which are expectation values of products of creation and annihilation operators for site-centered orbitals >. The Schrodinger equation then leads to a set of first-order differential equations, viz.,... 

It is essential to characterize the reactant species in solution. One of the problems, for example, in interpreting the rate law for oxidation by Ce(IV) or Co(III) arises from the difficulties in characterizing these species in aqueous solution, particularly the extent of formation of hydroxy or polymeric species. We used the catalyzed decomposition of HjOj by an Fe(III) macrocycle as an example of the initial rate approach (Sec. 1.2.1). With certain conditions, the iron complex dimerizes and this would have to be allowed for, since it transpires that the dimer is catalytically inactive. In a different approach, the problems of limited solubility, dimerization and aging of iron(III) and (Il)-hemin in aqueous solution can be avoided by intercalating the porphyrin in a micelle. Kinetic study is then eased. 

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