Molecular size dependent response

Discrimination using Molecular Size-Dependent Response Transients ZnO is one... 

Finally, the rate of response of a coated sensor is temperature dependent. When measurements are made under conditions of equilibrium between free and sorbed analyte, changes in kinetics present no problem unless the response becomes too slow for a chosen application. In some cases, however, the rate of response can be used to identify the species being detected an example is the molecular size-dependent diffusion of organic solvents into some polymer films [35]. In this case, failure to accurately measure and/or control temperature could lead to misidentification of the analyte. 

The significant intrinsic limitation of SEC is the dependence of retention volumes of polymer species on their molecular sizes in solution and thus only indirectly on their molar masses. As known (Sections 16.2.2 and 16.3.2), the size of macromolecnles dissolved in certain solvent depends not only on their molar masses but also on their chemical structure and physical architecture. Consequently, the Vr values of polymer species directly reflect their molar masses only for linear homopolymers and this holds only in absence of side effects within SEC column (Sections 16.4.1 and 16.4.2). In other words, macromolecnles of different molar masses, compositions and architectures may co-elute and in that case the molar mass values directly calculated from the SEC chromatograms would be wrong. This is schematically depicted in Figure 16.10. The problem of simultaneous effects of two or more molecular characteristics on the retention volumes of complex polymer systems is further amplifled by the detection problems (Section 16.9.1) the detector response may not reflect the actual sample concentration. This is the reason why the molar masses of complex polymers directly determined by SEC are only semi-quantitative, reflecting the tendencies rather than the absolute values. To obtain the quantitative molar mass data of complex polymer systems, the coupled (Section 16.5) and two (or multi-) dimensional (Section 16.7) polymer HPLC techniques must be engaged. 

Published values of BAFs for HOCs, summarized by Swackhamer and Skoglund (10), range from 18 to 1,000,000. The variety of methods used in these studies prevents direct comparison, so it is unclear what factors are responsible for the large variation in BAFs. Several hypotheses have been proposed to explain these variations in accumulation and their deviations from KoW-based predictions. One hypothesis (II, 12) proposed that a lack of complete reversibility in the partitioning process is responsible for the deviations. A second (13-16) theory attributed the deviations to the presence of a third phase (colloids or dissolved organic matter) and the inability to accurately separate the dissolved and sorbed states. A third (17) proposal is that partitioning is dependent on sorbent concentration. And finally, a fourth (18, 19) hypothesis holds that this deviation is a function of the effects of molecular size and shape on cellular transport. 

From the view point of the assessment, the quality of an HPLC separation in response to changes in different system variables, such as the stationary phase particle diameter, the column configuration, the flow rate, or mobile phase composition, or alternatively, changes in a solute variable such as the molecular size, net charge, charge anisotropy, or hydrophobic cluster distribution of a protein, can be based on evaluation of the system peak capacity (PC) in the analytical modes of HPLC separations and the system productivity (Peff) parameters in terms of bioactive mass recovered throughput per unit time at a specified purity level and operational cost structure. The system peak capacity PC depends on the relative selectivity and the bandwidth, and can be defined as... 

The raw data in gel permeation chromatography consist of a trace of detector response, proportional to the amount of poly mer in solution, and the corresponding elution volumes. A typical SEC record is depicted in Fig. 3-8. It is normal practice to use a set of several columns, each packed with porous gel with a different porosity, depending on the range of molecular sizes to be analyzed. 

A polymer sample may consist of a mixture of species whose compositions differ enough to affect the responses of both the concentration-dependent detector and the molecular-weight-sensitive detector in a multidetector system. Examples are mixtures of different polymers or copolymers (Chapter 7) whose composition is not independent of molecular size. Conventional GPC cannot be used reliably to characterize such mixtures, but an on-line viscometer can be employed to measure molecular weight averages independent of any compositional variations [25]. Remember, of course, that such data characterize the mixture as a whole, and not just the major component. 

Because of the fundamental importance of solvent-solute interactions in chemical reactions, the dynamics of solvation have been widely studied. However, most studies have focused on systems where charge redistribution within the solute is the dominant effect of changing the electronic stale.[I,2] Recently, Fourkas, Benigno and Berg studied the solvation dynamics of a nonpolar solute in a nonpolar solvent, where charge redistribution plays a minor role.[3,4] These studies showed two distinct dynamic components a subpicosecond, viscosity independent relaxation driven by phonon-like processes, and a slower, viscosity dependent structural relaxation. These results have been explained quantitatively by a theory of solvation based on mechanical relaxation of the solvent in response to changes in the molecular size of the solute on excitation.[6] Here, we present results on the solvation of a nonpolar solute, s-tetrazine, by a polar solvent, propylene carbonate over the temperature range 300-160 K. In this system, comparisons to several theoretical approaches to solvation are possible. 

In the second part of this work we review our theoretical and experimental works to obtain enhanced two-photon cross-sections by using the super-linear response of centrosymmetric monomers that are coherently coupled. In this alternative approach, the nonlinear material consists of an assembly of nonsubstituted /r-electron systems that are coupled by dipole-dipole interactions. The monomer two-photon term is a pure transition dipole term ( UQ,jU,2). Typical materials can be molecular aggregates, nanocrystals, oligomers, and dendrimers. The dipole-dipole interactions determine the size dependency of optical properties, and in particular of two-photon cross-sections. 

The polymers produced by Ziegler-Natta polymerization normally have very wide molecular weight distributions. The polydispersity index PDI (= Myj/Mn) is 5-20 for polyethylene and 5-15 for polypropylene. The cause of the wide dispersity is not precisely known. Some workers believe that the propagation reaction becomes diffusion controlled after a few percent conversion and it is this which is responsible for the large dispersity. Some other workers believe that the rate constants are dependent upon the molecular size. 

Fig. 20.5 Using bioassay-guided fractionation to identify the molecular size of female sex pheromone. The dependent measure is the number of male blue crabs out of 72 tested that performed courtship stationary paddling. Stimuli were male or female urine fractionated into the indicated molecular sizes (S small <500 Da, M medium 500-1,000 Da, L large >1,000 Da, a mixture of S+M+L Mix) and positive control (unprocessed pubertal female urine = Urine) and a negative control (.SW sea water). Friedman ANOVA shows an overall difference in the responsiveness to these stimuli (P < 0.0001, n = 10). An asterisk marks stimuli that elicit significantly more males to respond compared to the sea water control (Wilcoxon post hoc tests, P < 0.05)...

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