The use of retention indices

The retention index of a compound obtained on a given stationary phase under given experimental conditions constitutes worthwhile information. However, if several indices of the same compound obtained on different stationary phases are available, better identification of this compound can be made. Because of the excellent reproducibility of retention times on modern chromatographs, this method is reliable for known control compounds. While obtaining retention indices does not constitute absolute identification of a compound, this method can be quite useful to identify unknowns if the proper retention indices tables are available on the most common stationary phases (Squalane, Apiezon, SE30, Carbowax 20M). However, the use of retention indices is now of lesser interest because of capillary columns that involve new stationary phases. This limits the information that can be obtained from the retention indices. Hyphenated techniques are currently more popular. They represent excellent methods for compound identification but depend on instruments that are more complex and more expensive. 

By far, mass spectrometry (MS) is the most popular detection technique for performing chromatographic studies of essential oils. The use of retention indices, in conjunction with GC/MS studies, is well established. Many laboratories use such procedures in their routine analyses to confirm the identities of unknown components. The identification of components is usually performed by comparing the mass spectra with an MS library. However, a feature of MS for essential oils is that mass spectra are not particularly unique in many cases because of the large numbers of isomers of the same molecular formula, but with different structures, that could exist. Therefore their mass spectra are similar and... 

However, as is well known, compounds such as isomers, when analyzed by means of GC-MS, can be incorrectly identi ed, a drawback that is often observed in essential oil analysis. As is widely acknowledged, the composition of essential oils is mainly represented by terpenes, which generate very similar mass spectra hence, a favorable match factor is not suf cient for identi cation, and peak assignment becomes a dif cult, if not impracticable, task (Figure 7.1). In order to increase the reliability of the analytical results and to address the qualitative determination of compositions of complex samples by GC-MS, retention indices can be an effective tool. The use of retention indices in conjunction with the structural information provided by GC-MS is widely accepted and routinely used to con rm the identity of compounds. Besides, retention indices when incorporated to MS libraries can be applied as a Iter, thus shortening the search routine for matching results and enhancing the credibility of MS identi cation [44]. 

In practice, qualitative analysis is made easier by the use of retention indices I [39] ... 

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 calculation of retention indices implies that measurements are made under isothermal conditions. When using temperature programming, good results can be obtained by substituting retention times in equation (2.7) by the corresponding logarithms. 

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... 

It should be stressed that for many compounds the yields were too low for identification experiments. It is, therefore, a prerequisite in the application of the AEDA on headspace extracts that the key odorants have already been identified in preliminary experiments and can be used as reference compounds to correlate the odor-active regions with the chemical structures on the basis of retention indices. 

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),... 

This work is an assessment of the possibihty of using topological indices with the HPLC technique. Every quoted work is a new aspect of the use of topological indices. Data from the scientific hterature indicate that the discussed topological indices can be used to predict and describe retention and lipophihc parameters obtained by HPLC for the different classes of organic compounds. 

The determination of retention indices in the first column is straightforward, using a suitable n-alkane mixture as reference. Equation (4) (LRI) is commonly used because the first column usually operates in temperature-programmed mode. The calculation of retention indices in the second column, which operates in isothermal mode, presents several problems, however. These problems will be addressed in Sections 3.1.2 and 3.4. 

A general advantage of QSRR is that the equations derived allow the prediction of retention indices for compounds structurally similar to those used to develop the model but not represented in the initial data set. An essential condition for derivation of significant quantitative relationships between retention data and molecular structure descriptors is the precision and reliability of the chromatographic data used. Although there are several potential sources of errors in routinely determined GC retention indices, the correlations obtained are often very promising and meaningful. 

Robinson, P.G. Odell, A.L. A system of standard retention indices and its uses. The characterization of stationary phases and the prediction of retention indices. J. Chromatogr. 1971, 57, 1-10. 

The EDBD variant of BP was used in several chemical kinetic studies. 2-154 This method gave much better estimates of kinetic analytical parameters than either nonlinear regression or principal components regression. EDBD has also been applied to multicomponent kinetic determinations and to the estimation of kinetic compartmental model parameters.i The EDBD method was found to offer increased modeling power for nonlinear multivariate data compared to partial least squares and principal components regression, provided the training set is extensive enough to adequately sample the nonlinear features of the data.i55 Finally, EDBD has been successfully applied to the prediction of retention indices, i ... 

Counterpropagation networks have been used in property prediction, QSAR, the prediction of retention indices, and Kovats indices in gas chromatography. " In the latter application these networks performed better than multilinear regression when the coefficient of determination (r ) was low but were worse when was high. Apparently, counterpropagation ANNs per-... 

The system of retention indices is also used for characterization of adsorbent selectivity (see Table 3-4). For example. Table 3-4 [28] provides values for McReynolds constants [29] at 200 °C using Chromasorb 106 as the reference adsorbent. The data in parentheses were obtained after aging the columns for 22 days. Polarity of polymer adsorbent changes greatly as follows from the data of Table 3-4. 

Peetre and Smith have developed equations for the calculation of retention indices for mixed tetraalkylsilanes using the same methods as they applied earlier for the tetraalkoxysilanes >. They discuss the significance of the sign of the charge on the terminal methyl carbon atom of the alkyl groups bonded to silicon in determining the magnitude of the retention index and AT value. 

Ellren et al202 used a slope detector that permitted the measurement of retention indices to within 0.1 second. Most of the work was performed using a Varian Model 1400 gas chromatograph with a flame ionization detector. As the temperature scale on this instrument could not be read to better than l C an auxiliary thermocouple and temperature gauge were used, which enabled the temperature to be measured to fO.l C (Mettler TM15). ... 

Motivated by the requirement of analysis and separation of polycyclic aromatic hydrocarbons, several authors have determined the retention indices of some of these compounds in chromatographic columns. Since the retention indices of many polycyclic aromatic hydrocarbons have not been determined yet, it is desirable to find the mathematical model about the relationships between the value of retention indices and molecular structure of polycyclic aromatic hydrocarbons. Some of these relationships have been studied by PLS. But it can be shown that support vector machine can give mathematical model with better prediction ability. Table 13.3 illustrates the experimental values of 33 polycyclic aromatic hydrocarbons and their molecular parameters [52 5]. Support vector regression has been used for this modeling work. 

Gas chromatography-mass spectrometry provides information of compounds by retention time from GC and mass spectral patterns from MS. Retention time is usually expressed by retention index that relates the retention time to a homologous series of reference compounds. Combined use of retention indices and mass spectra allows compound identification with high specificity. Overlapping GC compounds can be differentiated by the difference in their mass spectra. On the other hand, isomers with similar or identical MS can be differentiated by the difference in GC retention time. Thus, GC and MS complement each other for mixture analysis. 

The retention times of the additives which could be analysed by GC under the conditions described in Chapter II, are given in minutes. GC was performed using the normal hydrocarbons Cu, Cjd and C24 as retention time markers. This allows the calculation of retention indices which will facilitate the identification of additives. This calculation can be carried out by relating the measured retention time of the reference substances to the retention times of the hydrocarbons C12, C20 and C24. A possible formula is given by Guiochon and Guillemin [12]. All the additives were andysed under the same gas chromatographic conditions. 

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