Log adjusted retention

Graph of log adjusted retention times of n-alkanes versus carbon number. The open circles show log retention times of n-alkane standards. The retention of an unknown (full circle) can be determined from the graph as shown. 

As predicted by Eq. 7.1, a plot of log k or log adjusted retention time versus log tx is linear for the trivalent lanthanides (Fig. 5.2) as well as for most of the divalent metal ions. By adjusting pH and the concentration of complexing ligand, log oli can be varied and the retention time either increased or decreased. 

It has long been known that within a homologous series, a plot of the logarithm of adjusted retention time (carbon atoms is linear, provided the lowest member of the series is excluded. Such a plot for C4 to C9 normal alkane standards is shown in Figure 27-18. Also indicated on the ordinate are log adjusted retention times for three compounds on the same column and at the same temperature. Their retention indexes are then obtained by multiplying the corresponding abscissa values by 100. Thus, the retention index for toluene is 749, and for benzene it is 644. 

An / value of a component can be determined by spiking a mixture of n-alkanes with the component(s) of interest and chromatographing the resulting mixture under the specified conditions. A plot of log-adjusted retention time versus retention index is generated, and the retention index of the solute under consideration is determined by extrapolation, as depicted in Figure 3.5 for... 

In a GPC experiment a mixture of n-alkanes (up to n carbon atoms, where n represents a variable number) and butanol (CH3CH2CH2CH2OH) were injected onto a column maintained at a constant temperature and whose stationary phase was of silicone-type material. The equation of the Kovats straight line derived from the chromatogram is log Jr = 0.39n — 0.29 (where /r the adjusted retention time is in seconds). The adjusted retention time of... 

SP is some free energy related solute property such as a distribution constant, retention factor, specific retention volume, relative adjusted retention time, or retention index value. Although when retention index values are used the system constants (lowercase letters in italics) will be different from models obtained with the other dependent variables. Retention index values, therefore, should not be used to determine system properties but can be used to estimate descriptor values. The remainder of the equations is made up of product terms called system constants (r, s, a, b, I, m) and solute descriptors (R2,7t2, Stt2, Sp2 log Vx). Each product term represents a contribution from a defined intermolecular interaction to the solute property. The contribution from cavity formation and dispersion interactions are strongly correlated with solute size and cannot be separated if a volume term, such as the characteristic volume [Vx in Eq. (1.6) or V in Eq. (1.6a)] is used as a descriptor. The transfer of a solute between two condensed phases will occur with little change in the contribution from dispersion interactions and the absence of a specific term in Eq. (1.6) to represent dispersion interactions is not a serious problem. For transfer of a solute from the gas phase to a condensed phase this... 

Since the ratio of adjusted retention volumes is inversely proportional to the log of the ratio of solute vapor pressures according to the integrated form of the Clausius-Clapeyron equation, we can conclude that,... 

The maximum retention factor (kQ) is related to the log P value and k and k are the retention factors of the cationic and anionic forms, respectively. The pKa values are known, and the retention factor in a given eluent can therefore be predicted in reversed-phase liquid chromatography using an alkyl-bonded silica gel or polystyrene gel column. The separation conditions can be adjusted according to their logP and pKa values by the selection of a suitable organic modifier concentration and the pH of the eluent.3,4... 

To characterize the relative gas-chromatographic retentions of condensed aromatics and heteroaromatics, inclu g thienothiophenes, benzo[b]thiophene, dibenzothiophene, naphthobenzothiophenes, and anthrabenzothiophenes, a system of indices. In, was proposed, In this system a series of similar linearly condensed hydrocarbons (such as benzene, naphthalene, anthracene, tetracene, pentacene,...) was used as a reference scale. The logarithm of the corrected retention volume (adjusted to 0°), log Ft, depends linearly upon the number of condensed benzene rings (z) in the molecule, both in the polar and nonpolar phases. In is expressed by Eq. (58) ... 

Sensitivity analyses were conducted to address the question of whether there is an optimum pretreatment condition that could be used for carbonic acid pretreatment. Laboratory results from Yourchisin and van Walsum (7) and McWilliams and van Walsum (6) indicate that optimal reaction severity would be in the range of Log(R ) = 4.2, where the severity R0 is as defined by Overend and Chornet (10). Relatively high temperature (220°C) and short retention time (4 min) were chosen to achieve this severity, since reactor costs dominate the economics and longer retention times would increase capital costs significantly. Pretreatment temperatures and pressures were varied for this study to determine the sensitivity of the cost to adjustments in these parameters. 

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