Temperature programming retention index

The approach described by Arey and co-workers [54], which uses n-alkanes as reference solutes in both dimensions, is acceptable for nonpolar stationary phases, since the difference between the temperature-programmed retention index and the isothermal retention index for terpenes does not generally exceed 1% [49]. However, there is a marked temperature dependence of retention indices on polar polyethylene glycol stationary phase columns [55], so this approach may not be suitable for the "reversed polarity" column GCxGC arrangement that is sometimes employed for essential oil analysis. 

Berngard, A., Colmsjo, A., and Wrangskog, K. (1994) Prediction of temperature-programmed retention indexes for polynuclear aromatic hydrocarbons in gas chromatography. 

Van Den Dool, H. and P.D. Kratz (1963), Generalization of the retention index system including linear temperature programmed gas-liquid partition chromatography . J. Chromatogr, Vol. 11, p. 463. 

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 Chapter 8 we will see that programmed temperature GC results in a regular, linear relationship between retention time and carbon number. Under those circumstances logs should not be used in Eq. (8) and in the retention index plot. The increase in temperature decreases the partition coefficients and effectively removes the logarithmic dependence of I. 

Hale MD, Hileman FD, Mazer T, Shell TL, Noble RW, Brooks JJ (1985), Anal. Chem. 57 640-648.. .Mathematical modeling of temperature programmed capillary gas chromatographic retention indexes for polychlorinated dibenzofurans"... 

Retention index There are two types of retention index, namely the isothermal retention and the hnear temperature programming index. Both indices express the retention characteristics of a chemical compound analysed by GC to the retention of the homologous series of normal ahphatic hydrocarbons analysed under identical conditions. Under both retention index systems, a chemical compound is bracketed by two ahphatic hydrocarbons that are assigned a retention index value corresponding to the number of carbon atoms in the hydrocarbon molecule multiphed by 100. The isothermal retention index, RL, is defined as a logarithmic interpolation between two successive ahphatic hydrocarbons eluted just prior to and just after the compound A under the isothermal GC conditions, and is calculated for compoxmd A as follows ... 

The retention time of the non-retained hydrocarbon methane is represented as tcH4. Similarly, the temperature programming index, Rip is experimentally determined by equation (23) ... 

There are many books and other sources that discuss the retention index from theoretical and practical aspects. One comprehensive book is "The Sadtler Standard Gas Chromatography Retention Index Library" [15]. This series of book (from Volume 1 to Volvune 4) provides detailed data on the retention indices for more than 2,000 compoimds under varying isothermal and temperature programming GC conditions for the purpose of the general identification of unknown compoimds. 

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.

The Kovats retention index [137], using n-alkanes as a series, is most commonly used for both internal and external data comparisons. With certain precautions in mind, this retention system is usable for temperature-programmed runs. Within the field of biological investigations, fatty acid esters [133] and steroidal hydrocarbons [134] were also used to standardize the retention data within a compound class. However, the use of the so-called methylene units [135] is basically identical with the Kov its system. 

When a compound is submitted to spectroscopic techniques, the response usually affords some information about its structure. If the retention time in GC depends, as previously mentioned, on the interactions between stationary phase and analyte, then it should be related to the chemical structure of the analyte. The presence of retention patterns correlated with similar structures in simple cases is well known for chromatographers the regularity of the retention times for n-alkanes (and other homologous series), both in isothermal and temperature-programmed modes, is a basis of the retention index concept as already explained in Section 3.1.1). 

The programmed-temperature retention index is used when linear temperature programmes are carried out and defined as ... 

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