Kovats retention index method

A compound (X) is now injected onto the column without changing the tuning of the instrument. The resulting chromatogram will enable 4, the Kovats retention index, to be calculated for X and the specific column employed this is equal to 100 times the equivalent number of carbon atoms of the theoretical alkane having the same adjusted retention time than X. Two methods can be used to find... 

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... 

However, relative retention volumes are more reproducible than individual retention volumes, so qualitative data should be reported on a relative basis. The retention index attributed to Kovats (see Chapter 4) has become a reliable method for reporting retention data. If a retention parameter is to be used for qualitative identification or classification, the Kovats retention index is a good method to choose. 

GC-MS is the method of choice for identification of the components in complex mixtures of tropane alkaloids combining the high resolution power of capillary columns with the specificity of the mass detectors. It allows identification of tropane alkaloids without using of standard compounds. Isomers (at positions C3 and C6/7) are tentatively identified by Kovats retention indexes and cochromatography with reference compounds [16, 17]. In some cases, tentative identification of new tropane alkaloids has been possible by GC-MS [16, 24—26]. In fact, this is the only way to assess the minor or trace compounds, which are difficult or impossible to isolate. More than 30 0 tropane alkaloids can be detected in a single species by GC-MS [19]. GC-MS chromatogram of alkaloid extracts of D. stramonium L. hairy roots and representative MS spectra are presented in Fig. 7.1. 

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... 

It has been shown that gas-Hquid chromatographic methods are particularly suitable for a quantitative characterization of the polarity of solvents. In gas-liquid chromatography it is possible to determine the solvent power of the stationary liquid phase very accurately for a large number of substances [98, 99, 259, 260]. Many groups of substances exhibit a certain dependence of their relative retention parameters on the solvation characteristics of the stationary phase or of the separable components. In determining universal gas-chromatographic characteristics, the so-called retention index, I, introduced by Kovats [100], is frequently used. The elution maxima of individual members of the homologous series of n-alkanes (C H2 +2) form the fixed points of the system of retention indices. The retention index is defined by means of Eq. (7-41),... 

Retention indices, RI method of expressing relative retention characteristics of a wide range of components, the most well known being the Kovat s index. This compares log retention time of an unknown compound with log retention times of the -alkanes eluted before and after. 

GC method/retention index library Retention index library nist ri 346 757 Kovats RI values for 82,868 compounds... 

Many factors can influence the Kovats index which can make it unreliable at times for the characterization of gas chromatographic behavior, although it generally varies less than the relative retention with temperature, flow, and column variation. However, for many it is the preferred method of reporting retention data. 

The usefulness of retention data from gas chromatography can be enhanced by reporting standardized times or retention indices (RI), which involves expressing retention in terms of a ratio of the retention time (RT) of an analyte to the RT of a standard. Retention scaling based on the Kovats (1965) method requires the chromatographic separation of a homologous series of normal paraffins, esters, and others, producing an index that is the ratio of the RT of an analyte minus the RT of a less retentive standard to the RT difference between... 

Some standard one-dimensional GC methods use reference peaks to help recognize drift [23]. For more widely varying chromatographic conditions, retention times for targets can be related using a linear retenhon index (LRI) [24], in which retention times are referenced relative to the retention times of marker compounds. A common LRI scheme uses the n-alkanes as marker points with indices equal to 100 times the carbon number (foUowing the Kovats index [24]) then the indices for peaks between marker points are computed using piecewise hnear interpolation. If retention-time windows are defined relative to marker p>eaks that can be located, then any linear retention-time transformation observed in the marker jjeaks can be apphed to the windows used for chemical identification. 

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. 

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