Deans’ switches

There are three phases in the operation of a Deans switch ... 

Overall, the technical complexity of the Deans switch system is considerably greater than that of a mechanical switching valve and it is accepted that reliability and ease of use is reduced as the system complexity increases. For many compound types, however, the completely non-intrusive nature of the Deans method offers sufficient advantages to justify its application. However, the use of modern electronic pressure and flow controls integrated into the overall computer control of the chromatographic system does now make the operation of Deans switches significantly easier or more reliable than has been reported in its earlier applications. 

Figure 3.7 shows some early examples of this type of analysis (39), illustrating the GC determination of the stereoisomeric composition of lactones in (a) a fruit drink (where the ratio is racemic, and the lactone is added artificially) and (b) a yoghurt, where the non-racemic ratio indicates no adulteration. Technically, this separation was enabled on a short 10 m slightly polar primary column coupled to a chiral selective cyclodextrin secondary column. Both columns were independently temperature controlled and the transfer cut performed by using a Deans switch, with a backflush of the primary column following the heart-cut. 

Figure 10.4 shows a schematic representation of the multidimensional GC-IRMS System developed by Nitz et al. (27). The performance of this system is demonstrated with an application from the field of flavour analysis. A Siemens SiChromat 2-8 double-oven gas chromatograph equipped with two FIDs, a live-T switching device and two capillary columns was coupled on-line with a triple-collector (masses 44,45 and 46) isotope ratio mass spectrometer via a high efficiency combustion furnace. The column eluate could be directed either to FID3 or to the MS by means of a modified Deans switching system . 

Nevertheless, a number of gas chromatographic applications exist, epecially those for the determination of crude oil indicators. Such indicators are used as geochemical parameters for the thermal history of the crude as well as to indicate the possible relationship between crudes from different wells. These indicators comprise a number of isomeric aromatic species, such as the individual alkylnaphthalenes (44, 45), the individual Cio-mono-aromatics or the individual C9-mono-aromatics. The ratio between these isomers gives a definite indication of the crude oil. In general, these systems use a Deans switching unit to make a heart-cut, which then is focused, reinjected and separated on a second column with a different polarity. 

The simplest type of two-dimensional gas diromatography for heart cutting or trace enrichment using a packed precolumn and a capillary column is shown in Figure 8.16 [205]. Almost any modem gas chromatograph could be converted into a similar unit with the addition of a few auxiliary components. Preliminary separation t2dces place on the packed column, the effluent from which is directed either to a vent or to the capillary inlet by the Deans switch, nie effluent reaching the capillary inlet is split three ways. One portion passes to a detector used to monitor the preseparation, a second portion enters the cap Bry column and is... 

Figure 8.16 A simple two-dimensional gas chrtwatograph for packed column fractionation or enrichment using a Deans switch and intermediate trap for transfer to a capillary column. (Reproduced with permission from ref. 205. Copyright Dr. Alfred Huethig Publishers).

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