Chemical Analysis. Francis Rouessac

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Figure 2.18 Kovats retention index (I = 100n_x) on a column in isothermal mode. The equivalent number of carbons n_x is found from the logarithm of the adjusted retention time t′_R(X) of X. The chromatogram corresponds to the injection of a mixture of four n‐alkanes and two aromatic hydrocarbons. The values in italics indicate the retention times given in seconds. By injecting this mixture periodically, the modifications to the Kovats indexes of these hydrocarbons enable us to track the column’s performance. With temperature programming, we can still plot this relationship using an adjusted formula, though this entails a reduction in precision.

(2.5)

      or

(2.6)

      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.

It should be remembered that the above calculations for retention indexes imply that the measurements were made under isothermal conditions. With temperature programming, they still give good results when substituting retention times for the corresponding logarithms in Eq. (2.4).

      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

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 phase for this calculation. The five indexes on a column using squalane, the only reproducible nonpolar phase since it is formed from a pure material, have been established once and for all (Table 2.2).

The five McReynolds constants for a given stationary phase are obtained (Eq. (2.7)) by calculating the differences observed for each of the substances tested between their Kovats indexes on squalane (I_Squalane) and on the stationary phase being studied (I_Phase):

(2.7)

Table 2.2 McReynolds constants (∆I) for several stationary phases normalized to squalane.

The sum of the five calculated values, using Eq. (2.7), has been used to define the overall polarity of the phase under study.

      Each of the test compounds yields specific information regarding the stationary phase: benzene for СКАЧАТЬ