Vario chambers

FIGURE 5.17 Analytical preselection of a suitable selectivity of the mobile phase on silica gel using the CAMAG HPTLC Vario Chamber. 

In the early stages of method development, when a suitable mobile phase has not yet been identified, the analyst will literally prepare nearly a 100 different mobile phases and plates to obtain the necessary separation. The HPTLC Vario Chamber system allows the user to take one HPTLC plate and cut it into six mini plates and expose them to six mini wells of mobile phase simultaneously. The analyst simply prepares a mixture of the components to be resolved and applies this solution to the six mini plates as a spot. This is a huge time saver since six mobile phases can be evaluated at a time and necessary dilutions can be serially prepared, (see Fig. 13.12). 

To select suitable gradient conditions, TLC experiments are performed using an elu-otropic series based on dichloromethane-methanol mixtures. By means of a Camag Vario chamber (Fig. 4.24), the eluent combinations following Tab. 4.9 have been tested. 

A useful tool for method development and optimization of chromatographic parameters is the VARIO Chamber (CAMAG, Switzerland). On 10 x 10 cm HPTLC plates, which have been previously scored to form six parallel tracks, up to six different solvents or six different conditions can be investigated. The chamber can also be operated in saturated, unsaturated, or sandwich mode, thus enabling convenient transfer of the optimized separation conditions to a HDC. 

The Vario chambers, which can be used in sandwich or tank configurations, allow selection of the optimal vapor-phase conditions and are best suited for working with the PRISMA optimization system (Chapter 6) (Nyiredy, 1992). 

Geiss, F., and Schlitt, H, (1968). A new and versatile thin layer chromatography separation system (KS-Vario-Chamber). Chromatographia I 392-402. 

The Vario-KS Chamber and HPTLC-Vario-Chamber (Camag) are horizontal chambers that have a wide variety of operational modes and applications. The plate is placed layer down over a tray with various compartments, which can hold different solvents, humidity-control liquids, and volatile acids and bases whose vapors will impregnate and condition or preload the layer. Developing solvent is in a separate tray and is transferred to the layer by a wick. The Vario chamber can be used to test six mobile phases side-by-side on one plate for solvent optimization, to determine if layer preequilibration (preloading) is advantageous, to ascertain if S- or N-chamber configuration is best, and to test different humidity conditions. 

A precise mastery of the chromatographic process also requires that the relative humidity be controlled. There are sufficient examples demonstrating that reproducible development is only possible if temperature and relative humidity are maintained constant. The influence of the latter on chromatographic behavior can be investigated using the Vario KS chamber (Fig. 59). When the relative humidity IS altered it is possible that not only the zone behavior will be changed but also the order of the zones on the chromatogram (Fig. 60). 

The selection of a mobile diase for the separation of simple aixtures may not be a particuleurly difficult problem and can be arrived at quite quickly by trial and error. Solvent systems can be screened in parallel using either several development chambers or a device like the Camag Vario KS chamber, which allows the simultaneous evaluation of a number of solvents by allowing each of these to migrate along parallel channels scored on a single TLC plate [8]. However, whenever the number of components in a mixture exceeds all but a small fraction of the spot capacity for the TLC system, a more systematic method of solvent optimization is required. 

Related to the linear development chamber is the Vario-KS chamber [8,195]. The plate is also developed in the horizontal position in a sandwich configuration. However, this chamber is... 

Discontinuous gradient of the stationary phase can be obtained easily using an ordinary spreader. The trough is divided into separate chambers filled with suspensions of mixtures of adsorbents. The carrier plates are covered in the usual way [1]. Another method of formation of gradients of stationary-phase activity is the use of a Vario-KS chamber, which permits adsorption of various vapors on the adsorbent surface or to control the activity of adsorbent. 

The Vario-KS chamber (Camag, Fig. If) is used for optimization of developing conditions for 10 x 10 cm plates by simultaneously testing of up to six different mobile phases and vapor equilibration conditions with N or S chamber conditions. Also, the Vario-KS chamber may be used for optimization of developing conditions for 20 X 20 cm TLC plates. The thin-layer plate (1) is laid down on the support (2). The eluent in the reservoir is connected to the thin layer of adsorbent by means of a filter paper strip. Under the thin-layer plate, there are many troughs (3) filled with solvents for preconditioning of the adsorbent layer. The chamber is tightly sealed by two clamps (4). 

Fig. 1 Different TLC sandwich chambers a, diagram of the S chamber b, diagram of the saturation S chamber c, diagram of the Camag sandwich cover plate d, diagram of the Camag horizontal developing S chamber e, diagram of the Desaga H-separation chamber f, diagram of the Camag Vario-KS chamber.

An optimization procedure based on the Geiss (1987) structural approach uses a Vario KS chamber (Chapter 7) with three strong solvents, methyl-r-butyl ether, acetonitrile, and methanol, that are diluted with a weak solvent such as 1,2-dichloroethane to produce a series of mobile phases ranging in e values from 0.0 to 0.70 in 0.05 increments. Once the appropriate solvent strength is determined, the separation is fine-tuned by blending solvent mixtures of this strength but with different selectivity (Szepesi and Nyiredy, 1996). 

Simple and inexpensive PTFE horizontal S-chambers for 5 X 5 cm or 10 X 10 cm plates are shown in Figure 7.1. The chambers are covered with a 4-mm-thick glass cover, and a glass frit rod is used to transport the mobile phase to the layer. Other commercial chambers for horizontal TLC include the Camag horizontal development chamber (Section IIIC), the Vario-KS chamber (Section IIIF), and the BN-chamber (no longer available commercially). For construction of a chamber suitable for horizontal TLC, see Stahl (1969) and Dzido (1990). 

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