Preloading the layer

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. 

PRELOADING. Sorption of gaseous molecules by the layer prior to development with the mobile phase also called layer conditioning or preadsorption. 

Rate-zonal separations It can be used to separate particles of similar density according to size or to separate particles of different density and size as a function of their sedimentation coefficient, s. In its simplest form, the sample mixture is layered in a narrow band on top of a preloaded, homogeneous medium as shown in Figure 3. 

A wide road embankment is to be constructed over a site with a 6 m thick soft clay deposit, overlying a deep sand layer. To enable construction to be completed in 2 years and to minimize settlement of the road it is proposed to pre-load the soil. To avoid stability problems the pre-load is to be increased at a steady rate for 1 year, at which time the applied stress will be Q kPa. At that time some of the preload will be removed and the embankment stress will be thereafter held constant at 60 kPa. Road construction takes place 6 months after the pre-load is removed, and results in an increase of 20 kPa in the total stress. The stress, time history is shown in Figure 1. 

An important result of these experiments which were conducted for a total of about 3000 hours under different chemistry conditions (see below) was that in most of the cases cobalt release to the solution was favored compared to that of the other constituents of the specimen. A possible explanation for this result is that cobalt is retained in the oxide layers at a lower concentration than it was in the base material. However, this effect typically occurs only when the test solution is not preloaded with cobalt ions, while in the case of a preloaded solution the cobalt retention in the oxide layer is significantly higher. By contrast, chromium release from the specimens was observed to be very small under reducing conditions, due to chromite formation in the inner oxide layer in the course of transients leading to oxidizing conditions in the test solution, however, an enhanced chromium release was observed. 

Guinchard, C., Mason, J. D., Truong, T. T., and Porthault, M. (1982). Research on the best chromatographic system for separation of polar aromatic compounds on silica by different layer preloading for transposition to a column. J. Liq. Chromatogr. 5 1123-1140. 

STATIONARY PHASE. The solid sorbent layer, with or without any impregnation agent, preloaded vapor molecules, or immobilized mobile-phase components). 

Jost et al. (212) studied the use of TLC as a pilot technique for transferring retention data to column LC (HPLC). TLC is potentially an inexpensive and convenient method for this purpose if essentially identical phases with the same retention mechanisms are used. However, there are inherent procedural differences in TLC and HPLC, which make exact transfer of data questionable. These differences include a capillary mobile phase driving force in TLC, and forced flow with constant and adjustable rates in HPLC formation of mobile phase gradients (solvent demixing) when multicomponent solvents are used in TLC preloading of the stationary phase with components from the gas phase of the TLC solvent and the presence of binder in layers but not columns. 

The adsorbent layer can also be exposed to solvent vapors in special, sandwich-type chambers that permit various solvent vapors to contact different parts of the plate, resulting in an adsorbent activity gradient along the plate. This technique is called preloading (43) or vapor-progranuned gradient TLC (40). 

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