Alkanes, retention index

There are fewer studies on the retention mechanism for open tubular columns but the theory presented for packed columns should be equally applicable. For films of normal thickness open tubular columns have a large surface area/volume ratio and the contribution of interfacial adsorption to retention will be significant for those solutes with limited solubility in the stationary phase [42]. Interfacial adsorption was shown to affect the retention reproducibility of polar solutes on non-polar stationary phases of different film thicknesses and the retention index values of solutes on polar stationary phases due to liquid phase interfacial adsorption of the n-alkane retention index standards [184,188,193-197]. 

Kovat s retention index for a normal alkane is 100 times the number of carbons thus... 

Kovat s retention index a means for normalizing retention times by comparing a solute s retention time with those for normal alkanes, (p. 575)... 

Smith, R. M. and Finn, N., Comparison of retention index scales based on alkyl aryl ketones, alkan-2-ones and 1-nitroalkanes for polar drugs on re-versed-phase high-performance liquid chromatography,. Chromatogr., 537, 51,1991. 

Reference compounds, n- Alkanes with an even number of carbon atoms. Retention index 2314 [71]. 

A retention index (RI) was used to allow comparison of compound retention data between columns of different equipment, e.g. GC and GC-MS. Straight chain hydrocarbons (alkanes) were assigned an index of 100 times the number of carbon atoms in the molecule. The RI s of all compounds within all the extracts were calculated by using the difference between the retention indices of the alkane eluting before and after that compound ... 

The studies with barbiturates revealed that the logarithm of the retention time is linearly related to the octanol/water partition coefficients [66,67]. It has been observed that the retention index of the drug is linearly related to the octanol/water partition coefficient (logP), and that results are very close to that of the 2-keto alkane standard (solid line in Fig. 15.9). 

Kovat s retention indexes analychem Procedure to identify compounds in gas chromatography the behavior of a compound Is Indicated by Its position on a scale of normal alkane values (for example, methane = 100, ethane = 200). ko-vats ri ten-... 

Figure 11.5 shows the structures of some of the major components in peppermint oil. The use of the retention index system is illustrated in Figures 11.6 and 11.7 for peppermint oil run in comparison with n-alkane standards on both a weakly polar OV-5-type column and a polar carbowax column. 

This index was applied to correlations with gas chromatographic retention index, boiling points, standard enthalpies of formation in gas phase, heats of solution, refractive indices, theoretically calculated total surface area of alkanes. 

Without changing the tuning of the instrument, a compound X is now injected onto the column. The new chromatogram obtained allows the calculation of the Kovats retention index of X on the specific column used. This index is obtained by multiplying the equivalent alkane carbon number that has the same retention time as X by 100. 

The Kovats retention index, /, for a linear alkane equals 100 times the number of carbon atoms. For octane, / = 800 and for nonane, 1 = 900. A compound eluted between octane and nonane (Figure 23-7) has a retention index between 800 and 900 computed by the formula... 

Retention index relates the retention time of a solute to the retention times of linear alkanes. 

Wehrli and Kovats (1) introduced the concept of the retention index to help confirm the structure of organic molecules. This method utilizes a series of normal alkanes as a reference base instead of one compound as in the relative retention method. Identification can be assisted with the use of the retention... 

Use the data from the n-alkanes to convert each time into a retention index (see Support Protocol). 

Figure G1.3.1 Determination of the retention index (Rl) of an unknown compound C in an aroma extract (A) by comparing with a series on n-alkanes (B) analyzed under the same GC-conditions. 

In extension, some retention index scales were proposed to mimic the Kovats index in GC. Alkanes, n-alkylbenzenes, alkan-2ones, alkylary] ketones, nitroalkanes, or polynuclear aromatic hydrocarbons were the advocated solutes. None of these scales is reliable, and observed indexes are not stable with variation in eluent composition, which precludes their use as a Kovats scale. 

Zarei, K. and Atabati, M. (2005). Predictions of GC retention indexes for insect-produced methyl-substituted alkanes using an artificial neural network and simple structural descriptors. J. Anal. Chem., 60, 732-737. 

Rohrschneider [205,210] has developed a scheme for the characterization of stationary phases for gas chromatography. The scheme is based on the retention index (/). The retention index is a dimensionless retention parameter, designed to be independent of flow rate, column dimensions and phase ratio. The retention index of a solute is defined as 100 times the number of carbon atoms in a hypothetical n-alkane, which shows the same net retention time as that solute. This definition is illustrated in figure 2.2. By plotting the logarithm of the net retention time against the number of carbon atoms in n-alkanes, a straight line is obtained. The net retention time for a solute may then be located on the vertical axis, and the retention index found on a horizontal scale, which represents 100 times the scale for na... 

Figure 2.2 Illustration of the definition of the retention index in GC. ncis the number of carbon atoms in n-alkanes, / the retention index and t R the net retention time. 

In eqns.(3.21) and (3.22) i denotes the solute and n and n +1 the preceding and the following n-alkane, respectively (see figure 2.2). It is seen that there is a hyperbolic relationship between the retention index and the temperature Although over small sections of the hyperbola a linear approximation is often used, this is not a sound basis for temperature optimization, especially not since a straight line can easily be obtained by plotting In (k/T) vs. 1/T(eqn.3.10). 

Figure 3.6 Variation of retention with the composition of the stationary phase in GLC. Stationary phase styrene-butadiene polymer blends and copolymers, the butadiene fraction is plotted on the horizontal axis, (a) Specific retention volumes for three n-alkanes and benzene. V is proportional to the capacity factor, (b) the retention index for benzene. The solid line is calculated from the straight lines in figure 3.6a. The circles (polymer blends) and triangles (copolymers) represent experimental data. Figure taken from ref. [310], Reprinted with permission.

A dimensionless molecular retention index Me parameter can be defined as the sum of Mr (relative molecular weight) and a structural increment W. Contained in W are all the additive contributions of the functional groups (see Eq. 4-52) which differ from a hypothetical n-alkane with the same Mr value. According to definition, the W values of the n-alkanes are always equal to zero. In this manner it is possible to estimate the partition coefficients of any given organic compound between a gas and any given liquid or polymer with help of additive structural increments. 

A most important contribution to the above means of identification is the Kovats retention index system [28]. The Kovats retention index of a compound is 100 times the number of carbon atoms in a hypothetical n-alkane that would display in the given system the same retention as the compound in question. Hence, the retention index system essentially is also based on the regularities between the retention data and number of carbon atoms in homologous compounds. The concept of the Kovats retention index system is illustrated by the model in Fig. 3.7, which shows a plot of log A) values for homologous compounds of the type CH3(CH2) X and for n-alkanes against carbon number. It is apparent that the retention index of, e.g., C2H5X is 560, i.e., 7(C2HSX) = 200 +... 

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