The retention index system

Because of the lack of quantitativeness of the Regular Solution Theory and large amount of effort and computing power required for the UNIFAC method, yet another way will be taken here. This way leads to values using simple means which can adequately estimate values for the most important practical cases. The method described in this section is based on the potential already recognised in gas chromatography that the partition of a substance between a gas and a polymer liquid can be estimated based on its structural increments and these can be used as characteristic quantities for identification. 

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. 

From the definition of the molecular retention index Me(i) for a substance i (Pirin-ger et al., 1976)  

The corresponding partition coefficients of two neighboring n-alkanes with z and z + 1 carbon atoms are designated with K[G/L)(z) and K /L)(z + 1), and one can assume for a homologous series of n-alkanes with good approximation that the ratio  

An analogous expression can be used for the partition coefficient K[G/P)(i) of / between gas G and polymer P  

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Van Den Dool, H. and P.D. Kratz (1963), Generalization of the retention index system including linear temperature programmed gas-liquid partition chromatography . J. Chromatogr, Vol. 11, p. 463. 

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. 

A great number of stationary phases are listed in catalogues and it is sometimes difficult to choose the best column for a particular analysis. The chemical nature of the phases and their polarities do not always allow one to predict which column will be optimal for a given separation. Therefore, a technique called the retention index system has been developed with the use of reference compounds whose retention factors differ with different stationary phases. Using retention indices obtained on columns of different stationary phases, it is possible to characterise a compound and facilitate its identification. 

The selectivity of a stationary phase for a particular compound can be measured in terms of the degree to which the retention index differs from the retention index obtained with a nonpolar column. As shown in Figure 3.10, the retention index for benzene was 649 on the squalane column. If the experiment is repeated in exactly the same manner but using dinonylphthalate as a stationary phase, the retention index is 733. This increase, Al, of 84 units of retention index indicates that the dinonylphthalate column will retard the benzene slightly more than will the squalane column. Under the same conditions, a highly polar phase such as SP-2340 would give a retention index of 1169 which would be a Al of 520 units. An excellent review of the retention index system has been presented by Ettre (22). 

Kovats, E. 1965. Gas chromatographic characterization of organic substances in the retention index system. Adv. Chromatographia 1 229. 

M.V. Budahegyi, E.R. Lombosi, T.S. Lombosi, G. Tarjan, I. Timar and J.M. Takacs, Twenty-fifth anniversary of the retention index system in gas-liquid chromatography, J. Chromatogr., 271, 213-307 (1983). 

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 +... 

Both of these approaches used in the characterization of stationary liquid-phase polarities by means of retention indices have been further explored and expanded [104, 259-261]. For a review on the characterization of solvent properties of phases used in gas-liquid chromatography by means of the retention index system, see reference [344]. Similar methods for the characterization of solvent polarity in liquid-liquid and liquid-solid chromatography can be found in references [105-107] cf also Section A-7 and Tables A-10 and A-11 in the Appendix. 

Extensions of the definition of a retention index were done by choosing gradient temperature conditions [42] or methyl esters as standards instead of normal hydrocarbons. Tables for retention indexes of certain substances have been published [43]. However, the retention index system has limited utility for the GC analysis of pyrolysates due to the complexity of such samples. 

The retention index system has the advantage of being based on readily available reference materials that cover a wide boiling range. In addition, the temperature dependence of retention indexes is relatively small. In 1984 Sadtier Research Laboratories introduced a library of retention indexes measured on four types of fused-silica open tubular columns. The computerized format of the database allows retention index searching and possible identity recall with a desktop computer. Measurement of retention indexes is the basis of the Rohrschneider-McReynolds scheme for classification of stationary phases in GC (see Section 27C-4),... 

Kovats, E. (1965). Gas Chromatographic Characterization of Organic Substances in the Retention Index System. Ado. Chromatogr., Vol.l, pp. 229-247, ISSN 00652415. 

Van den Dool, H. Kratz, P. (1963). A Generalization of the Retention Index System Including Linear Temperature Programmed Gas-Liquid Partition Chromatography.. Chromatogr., Vol.ll, pp. 463-471, ISSN 0021-%73. 

The retention index system of Kovats is one way to enhance the quantitative elution information of a compound on a particular column. It essentially relates the elution of a compound to that of a reference series of compounds, in this case the -alkanes. The retention index I is formally calculated under... 

For interlaboratory comparisons, the retention index appears to provide the best method for documenting the GC properties of any compound. The retention index system compares retention of a given solute (on a logarithmic scale) with the retention characteristics of a set of standard solutes that are the members of a homologous series ... 

Kovats, E. Gas chromatographic characterization of organic substances in the retention index system. In J. C. Giddings and R. A. Keller, Eds., Advances in Chromatography, p. 229—247. New York Marcel Dekker. 1966. 

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