Chemical Analysis. Francis Rouessac

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Chemical Analysis - Francis Rouessac


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retention time tR of an alkane having n atoms of carbon, minus the dead time tM; a and b are numerical coefficients. The slope of the graph obtained depends on the overall performance of the column and the operating conditions of the chromatograph.

      This expression follows on from another linear relation seen in thermodynamics linking the variation in free energy and the equilibrium constant K (∆G = −RT ln K), for a homologous family of compounds in which each term differs from the preceding one by an additional CH2 unit. Since K = kβ, t prime Subscript normal upper R Baseline equals upper K t Subscript normal upper M Baseline slash beta ; thus log t prime Subscript normal upper R will increase with ln K for the homologous family: ln t prime Subscript normal upper R Baseline equals ln upper K plus ln normal left-parenthesis t Subscript normal upper M Baseline slash beta right-parenthesis .

      2.10.2 Kovats Retention Index

      A compound (X) is now injected onto the column without changing the settings of the instrument. The resulting chromatogram will enable Ix, the Kovats retention index, to be calculated for X on the specific column employed: this is equal to 100 times the equivalent number of carbon atoms nx of the ‘theoretical alkane’ having the same adjusted retention time as X. Two methods can be used to find the number nx of equivalent carbons of X.

      The first is based on the Kovats relationship obtained above (Figure 2.17). This leads to a calculation of nx (therefore Ix), using a spreadsheet, for example.

      In contrast to the Kovats regression line, the retention indexes depend only on the stationary phase and not on the settings of the chromatogram. In particular, they do not depend on retention times.

Schematic illustration of kovats retention index (I = 100nx) on a column in isothermal mode.

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      There are tables of retention indexes of compounds currently in general use on the most common stationary phases. If several retention indexes of the same compound obtained on different stationary phases are available, then this unique collection of values could then characterize the compound with greater certainty. However, identification by retention index is not as reliable as using coupled techniques such as GC/MS (see Section 2.7.1), which require more expensive equipment.

      Retention time locking. It is obviously difficult to identify compounds whose retention times are very close and whose mass spectra are almost identical (certain forms of isomers). A current method consists in selecting an internal standard or a compound known to be present in all of the samples to be analysed. Through the use of computer software, the value of its retention time is locked for the different analyses, even if these are undertaken on different apparatuses. The effect of this is to conserve the retention times of the other compounds of the mixture, facilitating their identification. This approach, which avoids use of retention indexes, is possible with modern GC instruments and is known as Retention Time Locking (RTL).

      2.10.3 McReynolds Constants for Stationary Phases


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