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for Carbowax 20M) can be used for deposition, impregnation or as grafted phases, for temperatures not exceeding 260°C, depending on the type (Figure 2.8).

      Column catalogues present optimized phases for special applications: separation of sulfur products, chlorinated pesticides, permanent gases, triglycerides, PAHs, petroleum products or fatty acid esters. Some columns accept high temperatures up to 450°C (e.g. DEXSIL 400 or PETROCOL). The original applications include simulated distillation in the oil industry, replacing conventional distillation, which can take up to 100 hours per analysis.

      

      2.6.3 Ionic Liquids (IL)

Schematic illustration of polarity scale of stationary phases in GC and summary of the three types of stationary phase.

      

      2.6.4 Chiral Stationary Phases

      An organic molecular compound, whose formula demonstrates a centre of asymmetry, leads at our scale, i.e. macroscopic, to a mixture of the two possible enantiomers R and S, in equal quantities or not. Their physical properties are identical, but their behaviour regarding polarized light is what distinguishes them, hence the name of optical activity. In space, these two forms cannot be superimposed. One is the mirror image of the other.

      The separation of optical isomers is an important application of chromatography. GC optimizes separations more quickly than other chromatographic techniques (see Chapters 4, 5, and 6). Therefore, it holds a privileged spot in this field, even though the selectivity factor for enantiomer pairs remains close to 1.

      Two separations methods exist: direct process and indirect process.

      Direct process

      The optical purity of the analyte refers to its enantiomeric excess (e.e.), calculated from the following relationship, where SR and SS refer to the areas of the two enantiomer peaks:

      (2.2)upper O p t i c a l normal upper P u r i t y left-parenthesis e period e period percent-sign right-parenthesis equals 100 StartFraction StartAbsoluteValue upper S Subscript upper R Baseline minus upper S Subscript upper S Baseline EndAbsoluteValue Over upper S Subscript upper R Baseline plus upper S Subscript upper S Baseline EndFraction

      According to this formula, a pure chemical compound present as a racemic mixture will yield two peaks equal in size, each corresponding to an enantiomer. Its optical purity will thus be equal to 0.

Schematic illustration of an example of a separation with a chiral phase which contains grafted cyclodextrins.

      Indirect process

      Unlike the direct process, this process consists in creating a chemical reaction between the analyte and a chiral reagent, prior to the injection, to form diastereomers. As these have different physical properties, they should be separable with conventional phases. This longer process is not used as often as the direct one. It may cause a partial racemization of the analyte.

      After possible chemical transformation, cyclodextrins are either deposited on the internal surface of the capillary column or incorporated into a poly(dimethylsiloxane)‐type polymer or grafted onto the silanol functions of silica via a short carbonated chain. These can be used up to 200°C. Above this temperature, analytes can racemize.

      As a reminder, the other chiral vectors for GC, much less in use, are crown ethers and some asymmetrical diamides, which are progressively being abandoned (Figure 2.10).

      2.6.5 Solid Stationary Phases

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