Sandwich chambers

Apart from such trough chambers there are also S-chamber systems (small chambers, sandwich chambers) with deliberately reduced vapor volumes, which are specially suited to adsorptive separations. Such chambers are available for vertical and horizontal development (Fig. 58). Different separation results are naturally obtained in trough and S-chambers [8]. 

Sandwich chamber 126,127 Sapogenins 43,195, 206, 411 -, steroid 69, 206 -, trifluoroacetates 69 Saponins 7,109, 411, 430 Sarcosine 435 Scandium cations 144 Scanners, optical trains 30, 39 S-chamber see Sandwich chamber Scintillators 12 Sebacic acid 178,233,249, 308 Selectivity... 

The discussed effects, such as evaporation and adsorptive saturation, are prevented by placing a counter plate at a distance of one or a few millimeters from the chromatographic layer. The development with such a reduced vapor phase in the so-called sandwich chambers (S-chambers) can improve the separation. The glass-backed 20 X 20 cm plate forms one wall of the chamber with the adsorbent facing inward. A glass plate with spacers, called counter plates, is clamped to this plate and forms the other wall of the chamber (Figure 5.31, left [32]). 

As mentioned earlier, the preceding chromatogram development on the full length of the 20-cm plate takes a lot of time. To overcome this problem, the development of chromatogram on a short distance with simultaneous evaporation of the mobile phase from the exposed part of the chromatographic plate can be very conveniently performed by means of horizontal chambers. The mode was introduced by Perry [23] and further popularized by Soczewinski et al. [24,26], using a horizontal equilibrium sandwich chamber. 

Sandwich chambers can be nsed for 2-D separation on a large scale, rather than the traditional 2-D technique. The possibihty of using solvents of different selectivities improves the separation. 

Figure 7.2 Relation between the solvent front position and tine for (1) an enclosed layer with forced-flow development, (2) an exposed layer in a saturated chamber with capillary controlled flow, (3) a covered layer (sandwich chamber) with capillary controlled flow, and (4) an exposed layer in an unsaturated atmosphere with capillary controlled flow. (Reproduced with permission from ref. 30. Copyright Or Alfred Huethlg Publishers).

Cabon tetrachloride, n-hexane, chloroform, ACN, acetone, THF, pyridine, acetic acid, and their various mixtures were applied as mobile phases for adsorption TLC. Methanol, 1-propanol, ACN, acetone, THF, pyridine and dioxane served as organic modifiers for RP-TLC. Distilled water, buffers at various pH (solutions of and dipotassium hydrogen phosphate or potassium dihydrogen phosphate) and solutions of lithium chloride formed the aqueous phase. Carotenoids were extracted from a commercial paprika sample by acetone (lg paprika shaken with 3 ml of acetone for 30 min), the solution was spotted onto the plates. Development was carried out in a sandwich chamber in the dark and at ambient temperature. After development (15 cm for normal and 7cm for HPTLC plates) the plates were evaluated by a TLC scanner. The best separations were realized on impregnated diatomaceous earth stationary phases using water-acetone and water-THF-acetone mixtures as mobile phases. Some densitograms are shown in Fig.2.1. Calculations indicated that the selectivity of acetone and THF as organic modifiers in RP-TLC is different [14],... 

Another type of ascending TLC, which is often used where chamber saturation is necessary, is that carried out in a sandwich chamber (Fig.3.3). The TLC plate is sandwiched between two glass plates with only a minimum of air-space above the surface of the adsorbent. Vapor saturation can be achieved in a short time with much less solvent than is required by conventional TLC chambers. Several sandwich chambers are available from Analtech, Brinkmann, Desaga, Eastman-Kodak, etc. 

Newman-Howells Associates manufactures a sandwich chamber that is overpressurized and thus allows the control of flow. And, finally, Harrison Research makes a preparative device that uses centrifugal force to move the mobile phase through a radial plate. 

The possibility of zonal sample dosage in equilibrium conditions (after the front of mobile-phase and continuous-chromatogram development, which is provided by a horizontal sandwich chamber) was utilized by Glowniak et al. [3] in preparative chromatography of simple coumarins and furano-coumarins found in Archangelica fruits, performed with a short-bed continuous development (SB-CD) technique. 

Superior coumarin compounds separation with use of the described flat sandwich chambers was achieved with gradient chromatography on silica gel and stepwise variation of polar modifier concentration in mobile phase, as less polar solvents (hexane, cyclohexane, toluene, or dichloromethane) and polar modifiers (acetonitrile, diisopropyl ether, ethyl acetate) are used. 

The chromatographic chamber commonly used in TLC can be divided into two categories. A normal chamber (N chamber) is a chromatographic chamber of a large volume with a distance of gas phase in front of a thin-layer greater than about 3 mm. Conversely, a sandwich chamber (S chamber) is a small chromatographic chamber having a small volume and a distance of gas space in front of the thin layer plate less than 3 mm. 

Developing chambers for horizontal development were originally conceived as sandwich chambers (S-chambers) in which the plate, with the layer imdemeath, is placed above the counter-plate at a distance of 0.5 mm. Without this counter-plate, a filter paper soaked in solvent can be placed at the bottom of the chamber, enabling development to be performed in a saturated chamber. Chromatography can be performed with only a few ml solvent in horizontal chambers, which is, of course, economic and reduces disposal problems. However, the question arises how much of a four- or five-component solvent system is actually used if only 2 ml is consumed in the development process in the 5 x 5 cm horizontal chamber ... 

In contrast to the N-chamber, completely different conditions exist in a sandwich chamber or S-chamber. This type of chamber, introduced by Stahl, is hardly used in TLC nowadays, but is often described in the literature as an example of the influence of the chamber atmosphere. 

Sandwich chamber (narrow developing chamber for vertical development in which the TLC plate is fixed to a second glass plate)... 

B. Sandwich chambers (S-chambers) The sandwich chamber is very thin to accommodate a single 20 X 20 cm plate and minimal solvent for ascending chromatography. S-chambers with a plain glass cover plate are considered unsaturated, whereas saturated conditions are achieved by employing a counter plate coated with sorbent (often cellulose) and soaked with solvent. S-chambers offer advantages over rectangular tanks, that is, faster gas-phase equilibration and excellent reproducibility both due to the much smaller internal volume when compared with N-tanks [51]. 

This technique has other advantages compared with TLC and HPTLC, e.g. high-speed sample development, low consumption of eluant and reduced diffusion of analyte spots. Newman-Howells Associates and Supelco market horizontal sandwich chambers which are overpressurised in the latter system the chromatoplates can be eluted simultaneously from both ends, thus doubling sample throughput. 

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