Kovats indices

Fig. 3. (a) Flame ionization detector (fid) response to an extract of commercially processed Valencia orange juice, (b) Gas chromatography—olfactometry (geo) chromatogram of the same extract. The abscissa in both chromatograms is a normal paraffin retention index scale ranging between hexane and octadecane (Kovats index). Dilution value in the geo is the -fold that the extract had to be diluted until odor was no longer detectable at each index. 

Kinetics, of secondary equilibria, 159 Kovats index, 280 Kozeny-Carman equation, 6... 

Figure 1. Loadings of molecular descriptors and sensory sweet score on two PlS factors. 1 = log k, 2 = Kovats index on OVIOI and (3) Caibowax-20M, 4 = molecular weight, S = dipole moment, 6 = ionization potential, 7 = electron energy, 8 = heat of formation, 9 = zero-order connectivity, 10 = first-order connectivity, 11 = first-order connectivity/n Y = sensory sweet score.

Figure 2.15—Graphical measurement of Kovats index (/ = lOO/jj) on a column in the isothermal mode.The equivalent carbon number nx is obtained using the logarithm of the corrected retention time tLX). When using a temperature program, a linear relationship can be obtained using a corrected formula. However, this is achieved with a lower precision.

In practice, the retention index is simply derived from a plot of the logarithm of the adjusted retention time versus carbon number times 100 (Figure 4.4). To obtain a retention index, the compound of interest and at least three hydrocarbon standards are injected onto the column. At least one of the hydrocarbons must elute before the compound of interest and at least one must elute after it. A plot of the logarithm of the adjusted retention time versus the Kovats index is constructed from the hydrocarbon data. The logarithm of the adjusted retention time of the unknown is calculated and the Kovats index determined from the curve (Figure 4.4). 

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. 

Figure 4.4. Plot of logarithm of adjusted retention time versus Kovats index.

Takacs and co-workers (3,4) calculated the Kovats index for paraffins, olefins, cyclic hydrocarbons, and homologs of benzene on the basis of molecular structures. The index was divided into three additive portions atomic index, bond index, and samplestationary phase index components. 

In extension, some retention index scales were proposed to mimic the Kovats index in GC. Alkanes, n-alkylbenzenes, alkan-2ones, alkylary] ketones, nitroalkanes, or polynuclear aromatic hydrocarbons were the advocated solutes. None of these scales is reliable, and observed indexes are not stable with variation in eluent composition, which precludes their use as a Kovats scale. 

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. 

Standardization of retention times. This is the first step. Retention times of our odor zones have to be normalized by those of n-alkanes used as standards. To do that, the mixture of alkanes, which can be directly obtained from different chromatographic suppliers, is injected in exactly the same conditions in which the GC-O experiment was carried out. As nearly all the GC-O experiments use temperature gradients, the Retention Index or Linear Retention Index - not Kovats Index according to the lUPAC (lUPAC 1997) - will be determined with the formula... 

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. 

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. 

Peak compound Kovats indexS MW mass spectral data. 

Kovats Indices. The Kovats Index of a substance X, referring to a... 

Kovats index of substrate X at temperature T and refer-ing to the separation phase P... 

Corrected retention time used to calculate Kovats index... 

Figure 2.22 Graphical measurement of Kovats retention index (/= lOOn ) on a column in the isothermal mode. The number of equivalent carbons n, is found from the logarithm of the adjusted retention time t of X. The chromatogram corresponds to the injection of a mixture of 4 n-alkanes and two aromatic hydrocarbons. The values in italics match the retention times given in seconds. By injecting periodically this mixture the modifications to the Kovats indexes of these hydrocarbons permits the following of the column s performance. The calculations for retention indexes imply that the measurements were effected under isothermal conditions. With temperature programming they yield good results to the condition to adopt an adjusted formula, though this entails a reduction in precision.

To evaluate the behaviour of a stationary phase, a comparison of the Kovats indexes for five reference compounds belonging to different structural classes is made on the studied phase as well as on squalane, chosen as the reference standard phase for this calculation. The five indexes on a column using squalane, the only reproducible apolar phase since it is formed from a pure material, have been established once and for all (Table 2.1). 

The five McReynolds constants for a given stationary phase are obtained by calculating the differences observed for each of the substances tested between their Kovats indexes on squalane (/squaiane) at corresponding to the stationary phase being studied (/phase) ... 

Peak no. Constituent Kovats Index DB-WAX Peak Area, Rel. Amt., %... 

Figure 20-4 illustrates how a Kovats index can be obtained. Basically, it is done this way. Two straight chain hydrocarbons are found that differ by one carbon and that bracket the sample compound. The retention volumes of the reference alkanes are converted to logs and are assigned values 100 times the number of the carbons (C4 = 400 Cj = 500). The sample then has a value in between. 

TATP is a relatively volatile explosive and readily produces vapours that can be detected. Shown in Figure 13 is a typical chromatogram comparison which establishes the Kovats index for TATP of 1115 using a zNose equipped with a db5 column. 

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... 

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. 

Kovats Index for silane dimers used to analyze commercial materials or used as Internal standards. Column SE-30... 

The Kovats Retention Index for a specific organic compound (i.e., the analyte of interest on a specific GC column) is nothing more than a relative retention time The index is in relation to aliphatic hydrocarbons. For example, the hydrocarbon iso-octane, or 2,2,4-trimethylpentane if gas chromatographed, would probably have a retention time between that of the straight-chained alkanes, -octane Cg and n-nonane C9. The Kovats Index for n-octane is 800 and that for n-nonane is 900. Iso-octane would then have a Kovats Index somewhere in between, such as 860. The Kovats Index is calculated from a GC chromatogram using the retention volume for the analyte whose index is sought, and the retention... 

Bearing in mind that the compounds forming a perfume have a low boiling point, GC has been the most widely used technique in the perfume industry. In this sense, a perfume, after appropriate sample pretreatment, is analyzed using GC with a flame ionization detector (FID), and the Kovats index (KI) for each peak is experimentally determined. The identification is carried out by comparing the... 

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