Retention index system

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. 

Injection (GC) 258 interface (LC/GC) 801 Retention index system (GC) 176 choice of standards 181 isothermal 176... 

Since members of a homologous series have incremental boiling point differences and if the amount of any homolog in the moving gas phase is related to vapor pressure at the temperature of the experiment, plots of log k vs. carbon number should also be a straight line. (The enthalpy of vaporization increases monotonically with carbon number.) This in fact is observed in gas-liquid equilibrium separation systems. It is the basis of retention index systems pioneered by Kovats for qualitative identification. 

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. 

The above-given Martin equation form the basis for the Kovats retention index system in gas chromatography as well as for several HPLC retention prediction schemes. It must be noted here that the relationships between retention parameters and carbon numbers are usually linear at some limited range of the aliphatic chain length up to 6-8 carbon atoms in reversed-phase HPLC [491. 

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 application of this concept in chromatography requires the introduction of some extra components with previously known (postulated) retention indices [RI = /(f )] into the samples being analyzed. Their peaks form a mobile coordinate system for the recalculation of tp. data of the target analytes. Hence, the establishment of any retention index system needs the following ... 

If we consider the last relationship as the function y = f x) in Eq. (2), we come to the final equation of the Kovats Retention Index System ... 

Zenkevic, LG. Kovats Retention Index System. In Encyclopedia of Chromatography, Cazes, J., Ed. Marcel Dekker, Inc. New York, Basel, 2001 466-470. 

Retention index There are two types of retention index, namely the isothermal retention and the hnear temperature programming index. Both indices express the retention characteristics of a chemical compound analysed by GC to the retention of the homologous series of normal ahphatic hydrocarbons analysed under identical conditions. Under both retention index systems, a chemical compound is bracketed by two ahphatic hydrocarbons that are assigned a retention index value corresponding to the number of carbon atoms in the hydrocarbon molecule multiphed by 100. The isothermal retention index, RL, is defined as a logarithmic interpolation between two successive ahphatic hydrocarbons eluted just prior to and just after the compound A under the isothermal GC conditions, and is calculated for compoxmd A as follows ... 

Elizalde-Gonzalez MP, HutflieB M, Hedden K (1996) Retention index system, adsorption characteristics and structure correlations of polycyclic aromatic hydrocarbons in fuels. JHigh Res Chrom 19, 345-352. 

Vassilaros DL, Kong RC, Later DW, Lee ML (1982) Linear retention index system for polycyclic aromatic compounds. Critical evaluation and additional indices. J Chrom 252, 1-20. 

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),... 

G. Castello, Retention index systems alternatives to the w-alkanes as calibration standard, /. Chromatogr., A, 1999, 842, 51-64. 

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. 

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