Stationary phase Kovats Retention

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... 

In contrast to the Kovats relationship, retention indices depend only on the stationary phase and not on the column dimensions or the flow rate of the carrier gas. In practice, compound X is injected with the two bracketing alkanes to ensure that the experimental conditions are uniform. 

In a GPC experiment a mixture of n-alkanes (up to n carbon atoms, where n represents a variable number) and butanol (CH3CH2CH2CH2OH) were injected onto a column maintained at a constant temperature and whose stationary phase was of silicone-type material. The equation of the Kovats straight line derived from the chromatogram is log Jr = 0.39n — 0.29 (where /r the adjusted retention time is in seconds). The adjusted retention time of... 

Pyrido[l,2-a]pyrimidin-4-one, its 6,7,8,9-tetrahydro derivatives, and their monomethylated derivatives were characterized by gas-chromatographic (GC) retention indices measured on apolar and medium-polar stationary phases (91MI4). Kovats retention indices of alkyl-substituted 4//-pyrido[ 1,2-a]pyrimidines-4-ones and some 6,7,8,9-tetrahydro... 

De Beer et al. [501] reported an extensive comparative study of the chromatographic behaviour of methyl esters and pentafluorobenzyl esters of these substances. They correlated retention data (Kovats retention indices) on nine stationary phases with the structure of the derivatives and with the polarity of the stationary phases with the aim of utilizing these dependences for identification purposes. However, much more significant is the better separation obtained with pentafluorobenzyl esters and the possibility of increasing the sensitivity of the analysis. 

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),... 

The simple rule for the prediction of the possibility of GC analysis of organic compounds is based on the reference data of their boiling points. If any compound can be distilled without decomposition at the pressures from atmospheric to 0.01-0.1 torr, it can be subjected to GC analysis, at least on standard nonpolar polydi-methylsiloxane stationary phases. In accordance with this rule, most of the monofunctional —OH compounds (alcohols, phenols) and their S analogs (thiols, thiophenols, etc.) may be analyzed directly. The confirmation of chromatographic properties of any analytes must be not only verbal (at the binary level yes/no ) but also based on their GC Kovats retention indices as the most objective criteria for example ... 

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. 

Weckwerth solute descriptors (Table LI) are five solute parameters based on —> Kovats retention index on seven of GC stationary phases for 53 compounds [Weckwerth, Vitha et al, 2001]. 

The chromatogram that gives the Kovats relationship for a given stationary phase, can also serve to evaluate the performances of a column. For this, the separation number also known as the trennzahl number (TZ) is calculated from expressions 2.5 or 2.6. The two retention times occurring in these relationships relate to two successive alkanes differing by one carbon number (n and n + I atoms) or to two compounds of a similar type. The separation number indicates how many compounds could be baseline separated reasonably well by the column in the interval of retention time of these two reference compounds. The alkanes... 

Retardation factor in TLC, PC Corrected retardation factor Retention index, Kovat s RI Resolution between adjacent peaks Signal output from detector Separation number Stationary phase Time... 

Note that the Kovats Retention Index is independent of the chemical nature of the column. If we were to chromatograph the five mono-aromatics cited above on a purely nonpolar stationary phase such as Squalane (a C30 aliphatic hydrocarbon), we would generate a series of I values for each compound. If we then chromatograph these same compounds on a more polar stationary phase such as diethyleneglycolsuccinate (DEGS), we would obtain an entirely different set of I values. We calculate the difference in Kovats Indices as follows ... 

To find the Kovats index for a given solute on a given stationary phase, a few members of the paraffin homologous series are chromatographed and plotted. Then the solute is run under the same conditions and its Index value is determined from the graph. It is best if the paraffins chosen bracket the retention volume of the analyte. If the flow rate is kept constant during the gathering of these data, then adjusted retention times can be plotted. Alternatively, the index can be calculated from equation 4,... 

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. 

Let us return to our discussion regarding the determination of the polarity of stationary phases by beginning with an example using Kovats retention indexes. From McReynolds [3] we find that toluene has a Kovats retention index of 773 on the nonpolar phase squalane and 860 on the more polar dioctylphthalate. The difference in these indexes, 87, provides a measure of the increased relative polarity of dioctylphthalate relative to squalane. The difference can be designated as A/. 

Using ordinary detectors (e.g., FID, TCD, BCD, NPD), the identification of particular chemical agents is usually performed by comparing the retention indices of the substances being identified and the standard agent measured on at least two columns containing stationary phases of different polarities. Under isothermal conditions, the Kovats indices are applied and, under temperature programing, the equation of Van den Dool and Kratz is applied.f - ... 

Fig. 2 Predicted Kovats retention indices for various analytes in gas-liquid chromatography with squalane and polyethylene oxide stationary phases. See text for description of the analyte molecules...

The stationary phases requirements of selectivity and higher thermal stability then became more clearly defined the process of stationary-phase selection and classification became logical after the studies of McReynolds (28) and Rohrschneider (29,30) were pubUshed, both of which were based on the retention index (31). The Kovats retention index procedure and McReynolds constants are discussed in detail in the following section. Kovats retention indices today remain a widely used technique for reporting retention data, while every stationary phase developed for packed and capillary GC has been characterized by generation of its McReynolds constants. 

Flavomet is based on articles pubhshed since 1984 (though data has apparently not been added since 2004) concerning the use of gas chromatography-olfactometry (GC-O) to detect odorants in natural products. Therefore, to be included in Flavomet, an odorant must have been detected in a natural product or real environment by some form of quantitative GC-O, e.g., dilution analysis (Aroma Extraction Dilution Analysis or CharmAnalysisTi, or perceived irrtensity analysis (e g., Osme), or detection frequency analysis (e.g., SNIFF). The database comprises more than 730 flavor molecules (identified by CAS registry nrrmber) for which both Kovats and ethyl ester-based GC retention indices are provided (forrr stationary phases, varying in polarity) as well as characteristic odor note descriptions. 

Kovats retention index procedure and the McReynolds and Rohrschneider constants are discussed in detail in the following sections. The Kovats index remains a widely used technique for reporting retention data, and every stationary phase developed for packed and capillary gas chromatography has been characterized by its McReynolds constants. 

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