Temperature dependence, Kovats Indices

A useful 2ilternative to the Kovats system is the Lee retention index (isotherm2il and temperature dependent), based on the polynuclear aromatic hydrocarbon (PAH) standard compounds naphthalene (/ = 200), phenanthrene (/ = 300), chrysene (/ = 400), and picene or benzo(g,h,i)perylene (/ = 500). Isothermal (and temperature-dependent) Kovats and Lee retention indices for many compounds are tabulated in the NIST Chemistry Web Book (Ref. 8). 

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. 

The Kovats index has become a popular method for reporting GC data, replacing the absolute retention parameters. McReynolds [3] has published a reference book of self-consistent indices for 350 solutes on 77 stationary phases at two temperatures. From these data it can be seen that the Kovats index is not very temperature dependent and that adjacent members of any homologus series will have index values dMering by about 100 units. Using this approximation, one can estimate the index for any chemical if the index for one member of its homologous series is known. 

The same operational conditions ideally should result in the production of the same retention time for a given solute. However, because the Rj of a solute depends on the stationary phase, temperature, and flow rate, it is not always possible to obtain the same retention time. Retention indices have been developed to address this issue for expressing analytical results in a more uniform and reproducible manner. The Kovats index (KI) uses -parafhn hydrocarbons as a homologous series. KI can be measured on polar (e.g., Car-bowax 20M) or on an apolar (e.g., OV-101, DB-1, or DB-5) stationary phases. Theoretically, isothermal conditions should be maintained to calculate Kovats indices. In practice, however, temperature programming also can be applied. The Ethyl Ester Index System has been developed mainly with polar stationary phases. The homologous series used are ethyl ester standards starting from ethyl formate to ethyl hexadecanoate. 

The suitability of a stationary phase for a particular application depends on the selectivity and the degree to which polar compounds are retarded relative to what their retardation would be on a completely non-polar stationary phase. Since retention time is a function of temperature, flow-rate, stationary phase type and loading or film thickness it cannot be used to relate the retention characteristics of one column to another. Various retention index methods have been described such as evaluating the partition and separation properties of solute-stationary phase systems. Kovats (1958) devised a system of indexing chromatographic retention properties of a stationary phase with respect to the retention characteristics of n-alkanes, alkanes being used as reference materials since they are non-polar, chemically inert and soluble in most common stationary phases [8-10]. The retention index (RI) for the n-alkanes is defined as... 

Starting up from the linear dependence between the logarithm of retention parameters of the members of a homologous series and the number, n, of carbon atoms in the molecule, at constant column temperature, Kovats [16—18] has defined another relative retention parameter — the retention index, I — which is very often employed in the qualitative characterization of a component. The retention time, displayed on the same cliroma-togram, or net retention volume, of the substance i to be identified lies between those of normal hydrocarbons with G and 0 -f-1 carbon atoms in the molecule. Thus, from the standpoint of retention characteristics, this substance may be regarded as a hypothetical normal hydro-... 

For complex mixtures, especially those analyzed with temperature programming (in GC) or gradient elution (in LC), it is impossible to select a single reference compound. The Kovats retention-index system [39], based on homologous series of reference compounds, provides an elegant solution in GC, one which has been widely accepted. In LC, the absence of suitable homologous series, and the fact that retention depends more than in the case of GC on the polarities of compounds and less on molecular weight makes the use of an index system impractical. 

The prediction of GC retention data is based mostly on the retention index introduced by Kovats. The measured retention time for an isothermal chromatogram is dependent on the flow rate of the eluent gas and the column temperature. To be able to compare the retention under different column conditions, the retention of n-alkanes is measured under the same conditions. The retention index of an n-alkane is defined as the number of C-atoms times 100. n-Hexane, for example, has an retention index of 600. The retention times of all other solutes are converted to retention indices according to... 

A 1.2 The Linear Retention Index is an extension of the Kovats concept to programmed temperature gas chromatography. The Linear Retention Index of a solute is dependent not only on the liquid phase but on other operating parameters as well. It is a useful indicator of relative retention of solutes on chromatographic systems operating under identical or nearly identical conditions. 

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