Retention index determination

C. Rostad and W. Pereira. Kovats and Lee retention indexes determined by gas chromatog-raphy/mass spectrometry for organic compounds of environmental interest. Journal of High Resolution Chromatography and Chromatography Communications, 9(6) 328-334,1986. 

The retention index calculation of optically active compounds can be considered as a troublesome issue due to complex inclusion complexation retention mechanisms on CD stationary phases if a homologous series, such as the n-alkanes, is used, the hydrocarbons randomly occupy positions in the chiral cavities. As a consequence, n-alkanes can be considered as unsuitable for retention index determinations. Nevertheless, other reference series can be employed on CD stationary phases, such as linear chain FAMEs and FAEEs. However, retention indices are seldom reported for optically active compounds, and publications refer to retention times rather than indices. 

The distribution of tritium in compounds 137-150 can be determined by tritium NMR spectroscopy without chemical manipulations115. The structure retention index relationship (SR IR)116 has been used for identification of unknown radioactive peaks and to differentiate by-products from radioimpurities from extraneous sources. 

McReynolds used the retention index of certain solutes to compare different stationary phases and to assess their selectivity compared with a reference liquid phase, squalane. Squalane is considered to be non-polar and any increase in the retention index of the selected solute on the test column compared to squalane may be considered to be due to the greater polarity of that solvent. McReynolds constants have been determined for all stationary phases using a range of solutes of varying polarity (Table 3.8) and may be used to assist in selecting an appropriate stationary phase. 

The value of each phase constant (i.e., X, Y, Z, U, and S ) is determined by subtracting the retention index of the probe on a squalane stationary phase (Isq) from the retention index of the probe on the stationary phase being characterized (Ijp). For example, the phase constant of benzene (X ) would be calculated as shown in Equation 4.2. 

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). 

For synthetic rubbers, the mill homogenization is generally omitted, although it is specified as an alternative where the appropriate evaluation procedure requires it before measuring Mooney viscosity. Different conditions are given for specific polymers. The mill method of ISO 248 for determining volatile matter is specified but the oven method may be substituted if the material sticks to the mill rolls. Regardless of which volatile matter method is used, the mill procedure is required to dry samples for any chemical tests needed - unless this is not possible. To the uninitiated at least, this is not a model of clarity. Vulcanization characteristics are determined for synthetic rubbers, but not plasticity retention index. 

ISO 2930,1995. Rubber, Raw Natural - Determination of plasticity retention index (PRI). 

There are many samples that can be injected into a gas chromatograph in order to determine what aroma active volatile is present or to quantify a particular one (Maarse and van der Heij, 1994). Unfortunately, many of the protocols that were followed in the past did not rely on meaningful standards or retention indexing, or failed to use GC sniffing or the more formal GC/O procedures to establish that the peaks detected had odor activity however, if the object of the analysis is to simply monitor one or two odorants known to contribute positively or... 

Figured.1.4 An FID chromatogram of concentrated extract of the same Concord grape essence shown Figure G1.1.3, drawn to display the data on a linear retention index scale. By simply comparing the index of a peak with the data listed in the flavornet, the odorants that have similar retention indices can be determined. Notice how large the methyl anthranilate peak is, but still no convincing peak for p-damascenone, even though both compounds have the same odor activity (intensity).

Figure G1.3.1 Determination of the retention index (Rl) of an unknown compound C in an aroma extract (A) by comparing with a series on n-alkanes (B) analyzed under the same GC-conditions. 

The hydrocarbon standard provides a universal scale (retention index, RI) for the characterization of volatile odorants. Since a single determination may require the use of more than one instrument (GC, GC-O, GC-MS), it is crucial that every time a run is made using new operating conditions, a new calibration is recorded using the hydrocarbon standard. This is the only way that RI data from one instrument can be compared with that from another provided that the stationary phase is the same. RIs do not vary with the operating conditions, while retention times do. (See more on the use of hydrocarbon standards in unitgu.)... 

Retention index of odorants on the stationary phase used in the chiral analysis (single oven) or on the precolumn (MDGC), and on more than one stationary phase if found to be necessary for the identification of odorants, determined by using a hydrocarbon standard, described above and in unitgu. 

The extract of the volatiles is separated by high resolution gas chromatography (HRGC) and the odor of the compounds is assessed by sniffing the effluent of the GC column in parallel with the FID-detection. This technique allows the detection of odor-active volatiles, the determination of their odor qualities and, most important, the combination of these sensory data with an analytical parameter, the retention index (RI). In Figure 2, the results of... 

Spieksma, W., Luijk, R., Govers, H.A.J. (1994) Determination of the liquid vapour pressure of low-volatility compounds from the Kovats retention index. J. Chromatogr. A, 672, 141-148. 

The retention index (RI) was determined on a 30 m x 0.32 mm fused silica column (Supelcowax 10 Supelchem Germany). Structure not established by MS. 

The GC retention index data must be supported by additional determinations such as an independent measurement or extrapolation from a series of homologous chemicals where applicable. The identification of chemicals from CWC-related samples must be supported by other data, for example, GC/MS. 

J. Takacs, M. Rockenbauer and I. Olacsi, Determination of the relationship between retention index and column temperature in gas chromatography through the temperature-dependence of the net retention volume, 7. Chromatogr., 42, 19-28 (1969). 

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