Parameter experimental retention-determining

Knowing the experimental retention times, the previous equation allows the calculation of experimental concentration on the solid phase. Parameters of adsorption isotherms, can then be determined by fitting experimental and calculated concentrations. 

The proposed competitive adsorption model was implemented and used to calculate the band profiles of cyclopentanone with mobile phases of different compositions. One more adjustable parameter, an equilibrium constant for additional interactions, was introduced in order to match calculated and experimental retention of cyclopentanone. Figure 15.3 compares some calculated and experimental band profiles of cyclopentanone for mobile phases containing different concentrations of methanol in the mobile phase. In general, the agreement observed with either methanol or acetonitrile is good. However, like as any other complicated model, there are many parameters which must be determined by fitting the experimental data to the model and stiU, at the end, the calculated retention times of the solute(s) must be adjusted using a last empirical parameter in order to match the experimental retention times. There are no independent ways to verify that these parameters are correct. Therefore, in practice, the use of a more simplified model remains preferable. 

The retention parameter A is the critical link between the experimental retention time given by equation 3 (or equation 4) and the particle properties that determine the force F exerted on the particles by the field. The specific particle properties that are characterized by this relationship depend on the type of field applied. For a sedimentation field, F is related to the particle mass m by... 

Perturbations to retention ratios due to particle-wall interactions may be described in terms of the semiempirical parameter 6 having units of length. A measure of 6, which is independent of field strength, may be obtained with the aid of a simple plot of determined experimental retention data, over a range of field strength, via the equation ... 

Recalling that a separation is achieved by moving the solute bands apart in the column and, at the same time, constraining their dispersion so that they are eluted discretely, it follows that the resolution of a pair of solutes is not successfully accomplished by merely selective retention. In addition, the column must be carefully designed to minimize solute band dispersion. Selective retention will be determined by the interactive nature of the two phases, but band dispersion is determined by the physical properties of the column and the manner in which it is constructed. It is, therefore, necessary to identify those properties that influence peak width and how they are related to other properties of the chromatographic system. This aspect of chromatography theory will be discussed in detail in Part 2 of this book. At this time, the theoretical development will be limited to obtaining a measure of the peak width, so that eventually the width can then be related both theoretically and experimentally to the pertinent column parameters. 

Thus, experimental data may yield a value for the parameter which is not the equilibrium constant for binding of the hetaeron to the surface even if the hetaeron was strongly bound to the surface and eluite interaction with the bound hetaeron played a significant role in determining the magnitude of retention. 

The variance of the instrumental spreading function, i.e. the spreading factor of monodispersed polymer in a SEC column was determined experimentally with narrow MWD polystyrene standard samples by the method of simultaneous calibration. The dependence of the spreading factor on the retention volume deduced from a simple theoretical approach may be expressed by a formula with four physically meaningful and experimentally determinable parameters. The formula fits the experimental data quite well and the conditions for the appearance of a maximum spreading factor are explicable. 

Chromatographic Parameter-Relationships Correlations between Kov/ and various chromatographic parameters (CGP), such as HPLC retention time and thin-layer chromatography (TLC) capacity factors, allow the experimental estimation of Kow [19]. Usually, the CGP-A ow correlation is evaluated for a calibration set of compounds with accurately known K0w values. The Kow of a new compound can then be estimated by determining its CGP under the same experimental conditions as those used for the calibration set. 

Table 3.10b lists the optimization parameters that may be used in LLC. Clearly, the polarities of the two phases largely determine retention and selectivity. The exact composition of (preferably) the mobile phase may be varied to optimize the separation (i.e. variations in the nature and the concentration of mobile phase components, without substantial variations in the polarity). Even if the temperature is not a major optimization parameter, adequate temperature control is required in all LLC experiments. Therefore it may be experimentally straightforward to exploit temperature as a secondary optimization parameter. 

To further check the consistency of the method, we compare the copolymer s diffusion coefficient in THF, calculated from equation 3, with an independent measurement. Using our experimentally determined values of My and [ry], equation 3 predicts a D value of 9.85 X 10 cm /s this is in good agreement with the value of 9.79 X 10 cm s measured independently using NMR by workers at Exxon. (A similar comparison in toluene cannot be made because an independent value of D in toluene is not available.) A treatment of the propagation of errors is summarized in Table II. Here, uncertainties in My and Xa propagated by estimated uncertainties in the independent variables are listed. The largest uncertainty comes from the Dt values, but accurate retention data are also critical. For example, a 2% uncertainty in retention parameter X translates to a 9-10% uncertainty in My and a 5-8% uncertainty inXA. 

X parameter, a perfect correlation was not to be expected. From the correlation curve so-established the swelling of experimental vulcanizates could be predicted with reasonable accuracy from the easily determined x parameter. The GC method diould be particularly valuable for testing experimental samples, whenever only small amounts are available. For non-crosslinked materials, the magnitude of the x parameter is a direct indication of the solubility of the polymer in any given solvent. The dividing line between solvents and non-K>lvent of the polymer can be drawn at approximately x = 0.5, the smaller the values of x below this limit, the better the solvent. It is a simple matter to measure the retention characteristics of a series of probes and thus determine a suitable >lvent for any pdymer. It should be noted, however, that the temperature at which tfie GC determination is possible (T>Tg+ 50) may sometimes ermeed the temperature of interest and the dependence of x on temperature may have to be assessed. 

An alternative approach uses the polarity index, P proposed by Snyder. This is based upon experimentally determined gas chromatographic retention of three test solvents on a large number of stationary phases. The test solvents selected are ethanol, 1,4-dioxane and nitromethane. As well as an overall polarity index (P), three other parameters are calculated, Xe (a proton acceptor parameter), xproton donor parameter) and x (a strong dipole parameter). 

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