Kovats retention indexes, analysis

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... 

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. 

An unquestionable benefit of GC analysis is the possibility to correct retention time shifts easily by calculating retention time indices (Kovats index) or even by reanalyzing analytes on a second stationary phase resulting in a second confirmative index. 

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... 

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