Logarithm-adjusted retention time

FIGURE 3.5 Plot of logarithm-adjusted retention time versus Kovats retention index isoamyl acetate at 120°C. 

In practice, the retention index is simply derived from a plot of the logarithm of the adjusted retention time versus carbon number times 100 (Figure 4.4). To obtain a retention index, the compound of interest and at least three hydrocarbon standards are injected onto the column. At least one of the hydrocarbons must elute before the compound of interest and at least one must elute after it. A plot of the logarithm of the adjusted retention time versus the Kovats index is constructed from the hydrocarbon data. The logarithm of the adjusted retention time of the unknown is calculated and the Kovats index determined from the curve (Figure 4.4). 

Figure 4.3. Logarithm of adjusted retention time versus carbon number.

Figure 4.4. Plot of logarithm of adjusted retention time versus Kovats index.

The most commonly used retention parameter in gas chromatography is the Kovats index. When the adjusted retention times are used to calculate Kovats indices, retention parameters are obtained which depend only on the column temperature and the stationaiy phase used. Kovats indices are highly reproducible, and with a well designed experimental technique and an accurate timing mechanism, an inter-laboratory reproducibility of one unit for larger values of Kovats indices and two units for indices below 400 is possible [14]. Instead of Kovats indices, sometimes in QSRR studies the logarithms of retention volumes of solutes are used. 

Figure 2.22 Graphical measurement of Kovats retention index (/= lOOn ) on a column in the isothermal mode. The number of equivalent carbons n, is found from the logarithm of the adjusted retention time t of X. The chromatogram corresponds to the injection of a mixture of 4 n-alkanes and two aromatic hydrocarbons. The values in italics match the retention times given in seconds. By injecting periodically this mixture the modifications to the Kovats indexes of these hydrocarbons permits the following of the column s performance. The calculations for retention indexes imply that the measurements were effected under isothermal conditions. With temperature programming they yield good results to the condition to adopt an adjusted formula, though this entails a reduction in precision.

It follows that the net retention volume and therefore the adjusted retention volume (Fr) and retention volume (Fr) vary as the logarithm of l/T. Similarly, adjusted retention time (/r) and retention time (Jr) will decrease logarithmically as temperature increases. 

It has lung been known that within a homologous series, a plot of the logarithm of adjusted retention time r = r the number of carbon atoms is lin-... 

RI values do not depend on column dimension or flow rate. They must be obtained in isothermal mode, and they are less dependent on temperature than other retention parameters (fc, K). Because there is a linear relationship between the logarithm of the adjusted retention times, obtained under isothermal conditions, of the components of a homologous series and their number of carbon atoms, n-alkanes that differ in two carbon atoms (z and z + 2) can be used as standards in Equation (3) with only a small loss in accuracy. Other homologous series can be used instead of n-alkanes when these compounds are not appropriate — for instance, in columns with high-polarity stationary phases. 

To perform the interpolation, the adjusted retention times of the n-alkanes must be known. This is often done by analyzing a separate mixture containing n-alkanes. This requires, however, automated injection. For manual injection, n-alkanes are added to the sample. In Equation (7.1), logarithmic interpolation is necessary because the... 

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. 

A 1.1.1 Based on the observation that under isothermal conditions the adjusted retention times of members of a homologous series increase logarithmically with increasing carbon number, the Kovats Retention Index is a number indicating (on a logarithmic scale) the retention of a com-... 

Retention index. I. A number relating the adjusted retention volume of a component A to the adjusted retention volumes of normal paraffins. Each n-paraffin is arbitrarily allotted, by definition, an index of one hundred times its carbon number. The index number of component A is obtained by logarithmic interpolation. 

The occurrence of hydrodynamic lift forces principally leaves two options to perform the FFF experiment either the field force is increased to offset the lift forces and confine the particles close to the wall [226], or they can be adjusted to allow the particles to gain a significant elevation above the wall, where they form hyperlayers. The first mechanism preserves the steric mechanism, whereas the latter is the mechanism of lift-hyperlayer FFF (see Sect. 2.8.2). From these considerations, it becomes clear that the mechanism of steric-FFF is more complicated than the normal mode operation, necessarily requiring a calibration prior to measurement. This calibration is performed using a double logarithmic plot of the retention time tr vs. the known hydrodynamic diameter dH of a standard particle. From the slope and intercept, one can obtain the calibration constants Sd and trl by using the equation [293] ... 

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