Stationary Phase

Although alumina, cellulose, and C2 and C18 reversed-phase pre-coated preparative plates are available, silica has been most widely used by far. The silica materials commonly used for PPC have coarse particle sizes (average & 25 |im) and their distribution range (between 5 and 40 |im) is also wide; Figure 2

Pre-coated HPTLC TLC

1 I Silica for TLC

1 I Silica for TLC

Particle size (|xm)

Figure 2 Particle sizes and particle-size distributions of silica stationary phases.

Particle size (|xm)

Figure 2 Particle sizes and particle-size distributions of silica stationary phases.

compares the quality of pre-coated analytical (TLC and HPTLC) and preparative plates, and that of silica for TLC. The advantage of making one's own preparative plates is that any desired thickness (< 10 mm) or layer composition (incorporation of salts, buffers, etc.) becomes feasible. Pre-coated 20 cm x 20 cm or 20 cm x 40 cm preparative plates with layer thicknesses 0.5, 1.0, 1.5, and 2 mm have the advantage of much higher reproducibility. It is generally accepted that higher resolution can be achieved on a thinner preparative layer (0.5-1.0 mm) and the resolution is much more limited on a higher capacity (1.5-2 mm) layer. The loading capacity of a preparative layer increases with the square root of the thickness, practically without loss of separating power so that as a rule of thumb, the loading capacity of a 0.5 mm layer is approximately half that of a 2 mm layer.

Preparative plates are commercially available with or without preadsorbent zones. The preadsorbent zone (generally 4 cm width) serves as a holding zone for the sample until development begins. Soluble compounds migrate with the mobile phase front through the preadsorbent zone and are concentrated in a narrow band as they enter the chromatographic layer, thus improving the resolution. The materials used to manufacture these concentrating zones are kieselguhr or inert silica. Resolution can also be significantly increased by using a layer-thickness gradient that contains a wedge-shaped silica layer ranging in thickness from 0.3 mm at the bottom to 1.7 mm at the top, with an adjacent 700 |im preadsorbent layer for sample application. The cross-sectional area traversed by the mobile phase front increases during migration through the tapered layer, so the cross-sectional flow per unit area is highest at the bottom of the layer and decreases towards the solvent front. As a result the lower portion of a zone moves faster than the upper portion, keeping each component focused in a narrow band. Theoretical separations on a preparative plate, without and with preadsorbent, and compared with a tapered plate, are depicted in Figure 3A-C. The improved resolution as a result of the greater local mobile phase velocity clearly suggests the wider use of a preadsorbent layer or a layer-thickness gradient if at all possible.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook


Post a comment