Kovats index system

It is a common practice for qualitative analysis to be based on measurements of tr, this is especially true in those laboratories that run standards with each analysis. Nevertheless, is not the ideal parameter for identification purposes because it is a function of temperature, flow rate, and liquid-phase volume. (Indeed, the liquid-phase volume is continuously changing with time because of evaporation even its chemical composition can vary under the conditions of the experiment.) What is needed, then, is a parameter that is independent of all these factors. A very successful, but not perfect, solution is the Kovats index system, which relates the retention volume (or the retention time t ) of the unknown compound with that of M-hydrocarbons eluting before and after it. To each of a series of paraffins is attached an index I, given by... 

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. 

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

Chemical structure Name of component Occurrence in food or model systems Kovats-Index 08-13>... 

Qualitative analysis is enhanced if data are acquired on more than one system. For example, in GC it is fairly common and easy to run a sample on each of two columns that are chosen to be widely different in their polarities. The results can be plotted as net retention volumes or as Kovats index values on either linear or log scales as shown in Figure 6.5. In either case, straight lines result for homologous series, thus aiding qualitative identifications. The principle is simple the more data, the more reliable the analysis. 

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 +... 

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. 

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. 

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

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... 

Evans, M.B. Haken, J.K. Toth, T.J. Solute characterization in gas chromatography by an extension of Kovats retention index system. Dispersion and selectivity indices. J. Chromatogr. 1986, 351,155-164. 

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. 

Modern capillary GC is characterized by very good precision in retention time, and this allows the use of retention indices for peak identification. In a retention index system, the retention behavior of a particular solute is expressed in a uniform scale determined by a series of closely related standard substances. In the retention index scale developed by Kovats [61] for isothermal separations and by Van den Dool and Kratz [62] for temperature-programmed analyses, the fixed... 

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. 

To some extent, the retention time in a standardized GC analysis can be used for positive identification of the compounds analyzed. The most widely applied approach in this respect is the use of the Kovats retention index (RI) [13], The RI value of a sample peak is determined by comparing its retention time to retention times of closely eluting alkane standards, i.e., the alkanes that elute just before and Just after the sample peak. In this way, the RI is quite insensitive to small changes in experimental conditions. In the Kovats RI system, normal alkanes are assigned an RI value of 100 X the carbon number, e.g., /7-pentane has an RI of 500. In an isothermal separation, the RI of component i with retention time 4,/ (and net retention time of, is thus calculated from... 

A uniform system of retention classification according to a solute s relative location between a pair of homologous reference compounds on a specific column under specific conditions. It compares the time a compound is eluting from the column to the times of a set of standard compounds. The most common set of compounds used for retention indices are hydrocarbons typically from C5 to C30, separated in the order of boiling points, Kovats index. 

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