Chromatographic plates

The chromatogram is freed from solvent, dipped in the reagent solution for 5 — 10 s and then heated to 120—150°C for 5 — 10 min. (Caution Remove perchloric acid from the back of the chromatographic plate ). 

Use of a bilayer plate (15) affords the special chromatographic possibility of being able to perform two different multidimensional separations on the same chromatographic plate (Figure 8.5), either with the same mobile phase or with mobile phases of different composition. 

Repeated chromatography of the sample in the same direction with the chromatographic plate being dried before all re-developments. 

Repeated chromatography of the sample in the first direction, followed by repeated development in the second direction, at right angles to the first. Again, the chromatographic plate is dried between re-developments. 

Chromatographic plates can be connected for both capillary-controlled and forced-flow planar chromatography (FFPC), i.e. irrespective of whether capillary action or forced-flow is the driving force for the separation. The first technique is denoted as grafted planar chromatography (31), while the second is known as long distance (LD) OPLC, which uses heterolayers (32, 33). 

Figure 8.17 depicts MD-PC performed on three different types of stationary phase (6). The three grafted chromatographic plates (Figure 8.17(a)) are clamped in lap-joint fashion with the edges of their stationary phases in close contact. The manner in which the three plates are prepared and the separation which can theoretically be achieved are also apparent from the schematic diagrams in Figures 8.17(b-d), in which the most polar stationary phase is phase A and the least polar is stationary phase C . 

LD)-OPLC, involves the serial connection of the chromatographic plates. By application of this technique, it is also possible to use different stationary phases of decreasing polarity during a single development, as shown in Figure 8.18(a). 

The place at which this transfer occurs is illustrated in Figure 8.19 as a thin circle on the lower chromatographic plate. Because the overpressure is uniform throughout the whole system, the compounds will be divided into two parts and migrate in both circular (outwards) and anticircular (inwords) directions. A hole at the centre of the... 

Satisfactory combination of more than three chromatographic plates for longdistance OPLC is always difficult, because only the bottom plate can be glass-backed the others must be aluminium-backed (32, 33). 

Naturally, several other possibilities can be used to increase the number of dimensions. Between the first and second developments, or sample, the characteristics of the chromatographic plate or the properties of the sample can also be modified. Although interfacing of on-line OPLC with one- or two-dimensional TLC is not particularly difficult, it is not yet widely practiced. It must be concluded that full exploitation of the versatility of MD-PC is at an early state of development as a consequence several significant changes in practice might be expected in the next few years (10). 

Figure 9. Infrared spectrum of pyrethrin I decomposition product isolated from thin-layer chromatographic plate...

It is often possible to carry out any clean-up step that may be necessary in the concentrating zone of a suitable chromatographic plate, in any event clean-up is less complex than for other forms of chromatography. 

It is often possible to increase the selectivity of detection by carrying out a sequence of reactions on one and the same chromatographic plate - a technique that is only possible in thin-layer chromatography. In principle it is possible to distinguish between two sorts of reagent sequence ( TYPE A and TYPE B ), which are discussed in this and the next volume. 

The mobile phase must not chemically affect or dissolve the chromatographic plate (support, adsorbent, and binder). 

For the same adsorbent, different mobile phases can be used according to the aim of chromatographic analysis. Rather than preparing an endless line of chromatographic plates of different thickness, it is easier to change the mobile phase up to the most convenient composition, keeping the same characteristics of the stationary phase. In the case of a hygroscopic adsorbent, the adsorbed water influences its activity. 

This consists in spotting the separation mixture at intervals on a chromatographic plate and then applying the selected solvents in the center of each spot by means of a thin capillary. The spots migrate radially, and different chromatographic behavior of spotted rnixmre can be observed [16,59]. 

FIGURE 6.2 Brenner and Niederwieser S-chamber (a) normal development, (b) continuous development 1 — mobile phase, 2 — wick, 3 — chromatographic plate with adsorbent layer face up, 4 — cover plate, 5 — support, 6 — heater, 7 — evaporation of the mobile phase. (From Brenner, M. and Niederwieser, A., Experientia 17, 237-238, 1961. With permission.)... 

FIGURE 6.8 Horizontal DS-II chamber (a) before development, (b) during development 1 — cover plate of the mobile phase reservoir, 2 — mobile phase reservoir, 3 — chromatographic plate facedown, 4 — body of the chamber, 5 — main cover plate, 6 — cover plates (removable) of the troughs for vapor saturation, 7 — troughs for saturation solvent, 8 — mobile phase, 9 — mobile phase distributor. 

Another construction in which the mobile phase is delivered to the center of the chromatographic plate with the syringe coimected to the stepping motor was reported by Kaiser [17] and manufactured by CAMAG (Figure 6.11). The sample to be separated can be spotted on the plate in its center or 1 to 2 cm around its center. 

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