Precoated layers

LB films prepared from a 3 2 charge-transfer complex of TMTTF and TCNQ and transferred to cadmium-arachidate (five layers) precoated sheets of polyethylene terephthalate ESR... 

A two-layer precoat must be prepared on the filter rack prior to starting filtration. The second layer activates the filtration cycle. The first, mechanical, layer is made using a coarse adjuvant (permeability above 1 Darcy), with the possible addition of 10% of a cellulose-based product. The quantities... 

The sorbent or stationary phase (type of sorbent brand batch number layer thickness particle size binder in layer precoated or homemade type of backing size of channels, if any activation method and temperature impregnation, if any)... 

All the thin-layer plates discussed above consist of a uniform sorbent layer. Precoated layers introduced in this section are, on the contrary, combinations of different types of layers. Specific advantages of these precoated layers with concentrating zones can be summarized as falling into three categories ... 

In units designed to use a precoat filter aid, the dmm can be evacuated over the fiiU 360° and fitted with an advancing knife system that continuously shaves off the deposited soHds together with a thin layer of the precoat. The precoat has to be renewed periodically. 

Dynamic membranes are concentration—polarization layers formed in situ from the ultrafiltration of coUoidal material analogous to a precoat in conventional filter operations. Hydrous zirconia has been thoroughly investigated other materials include bentonite, poly(acryhc acid), and films deposited from the materials to be separated (18). 

Pretreatment of membranes with dynamically formed polarization layers and enzyme precoats have been effective (12,13,39). Pretreatment with synthetic permeates prevents startup instabiUty with some feed dispersions. 

In cake or surface filtration, there are two primary areas of consideration continuous filtration, in which the resistance of the filter cake (deposited process solids) is veiy large with respec t to that of the filter media and filtrate drainage, and batch pressure filtration, in which the resistance of the filter cake is not veiy Targe with respect to that of the filter media and filtrate drainage. Batch pressure filters are generally fitted with heavy, tight filter cloths plus a layer of precoat and these represent a significant resistance that must be taken into account. Continuous filters, except for precoats, use relatively open cloths that offer little resistance compared to that of the filter cake. 

There are some solids, however, which form a less permeable cake, even in veiy thin layers. With these sohds, the resistance of the deposited cake will be veiy high when compared to that of the precoat bed, and the slope of the filtrate cui ve will be -t-0.5 for all v ues of form time. 

Construction is similar to that of other drum filters, except that vacuum is applied to the entire rotation. Before feeding slurry a precoat layer of filter aid or other suitable solids, 75 to 125 mm (3 to 5 in) thick, is apphed. The feed slurry is introduced and trapped in the outer surface of the precoat, where it is removed by a progressively advancing doctor knife which trims a thin layer of solids plus precoat (Fig. 18-126). The blade advances 0.05 to 0.2 mm (0.002 to 0.008 in) per revolution of the drum. When the precoat has been cut to a predefined minimum thickness, the filter is taken out of service, was ned, and freshly precoated. This turnaround time may be 1 to 3 h. 

Filter aids may be applied in one of two ways. The first method involves the use of a precoat filter aid, which can be applied as a thin layer over the filter before the suspension is pumped to the apparatus. A precoat prevents fine suspension particles from becoming so entangled in the filter medium that its resistance becomes exces-sive. In addition it facilitates the removal of filter cake at the end of the filtration cycle. The second application method involves incorporation of a certain amount of the material with the suspension before introducing it to the filter. The addition of filter aids increases the porosity of the sludge, decreases its compressibility, and reduces the resistance of the cake. In some cases the filter aid displays an adsorption action, which results in particle separation of sizes down to 0.1 /i. The adsorption ability of certain filter aids, such as bleached earth and activated charcoals, is manifest by a decoloring of the suspension s liquid phase. This practice is widely used for treating fats and oils. The properties of these additives are determined by the characteristics... 

In precoating, the prime objective is to prevent the filter medium from fouling. The volume of initial precoat normally applied should be 25 to 50 times greater than that necessary to fill the filter and connecting lines. This amounts to about 5-10 lb/100 fF of filter area, which typically results in a 1/16-in. to 1/8-in. precoat layer over the outer surface of the filter medium. An exception to this rule is in the precoating of continuous rotary drum filters where a 2-in. to 4-in. cake is deposited before filtration. The recommended application method is to mix the precoat material with clear liquor (which may consist of a portion of the filtrate). This mixture should be recycled until all the precoat has been deposited onto the filter medium. The... 

Table 25 Summary of the pH values of some layer materials of precoated plates, determined as 10% aqueous suspensions (duplicate determination two different TLC/HPTLC plates from the same bateh).

All commercially available precoated plates are manufactured with great care. But they are active layers which, on account of the numbers and structures of their pores, possess a very large internal surface area, on which water vapor and other volatile substances can condense, particularly once the packaging has been opened. In order to prevent this as far as possible the precoated plates are packed with the glass or foil side upwards. 

H. E. Hauck and W. lost, Sorbent materials and precoated layers in thin-layer cltromatography , Chromatogr. Sci. 47 251-330 (1990). 

Polymerization in aqueous solution of acrylamide can also be fulfilled in thin layers (up to 20 mm) applied on a steel plate or a traveling steel band. Polymerization is initiated by persulfates, redox system, UV or y radiation. Polymerization proceeds in isothermal conditions as the heat of polymerization is dissipated in the environment and, additionally, absorbed by the steel carrier. Nonadhesion of the polymer to the carrier is ensured by the addition of glycerol to isopropyl alcohol or by precoating the steel band with a film based on fluor-containing polymers. This makes polymerization possible at a high concentration of the monomer (20-45%) and in a wider process temperature range. This film of polyacrylamide is removed from the band, crushed, dried, and packed. 

Sometimes filtration can be improved by using filteraids. These filteraids, which are based on diatomaceous earth, improve the porosity of a resulting filter cake leading to a faster flow rate. Before filtration a thin layer is used as a precoat of tire filter (normally standard filters). After that a mix is made with the harvest broth and filtration is started. 

Merck precoated silica gel F254 plates, layer thickness 0.25 mm., were used. Developing with 10 1 (vjv) benzene-acetone and visualizing with ultraviolet light, the product appears at Rf 0.58 0.67. Normally the product is found in the lower third of the column, and occasionally some is found in the last fractions of eluent collected during development. However, since the exact position of this material on the column depends critically on the way in which the column is packed, a thorough check of all fractions is advisable. 

The submitters also tried to purify the crude product by distillation (130-145° at 0.005 mm.), but under these conditions decomposition took place. It is possible to substitute thick-layer chromatography for the dry column. Using Merck silica gel F254 precoated plates, layer thickness 2 mm., and developing with 10 1 (vjv) benzene-acetone, the submitters report a 73% yield of pure product. 

The literature reports [a]n +23.2° (c = l, aqueous 5N hydrochloric acid). The product was analyzed by the submitters. Analysis caleulated for C5H11NO2S C, 40.25 H, 7.43 N, 9.39 S, 21.49. Found C, 40.14 H, 7.42 N, 9.50 S, 21.52. The product was homogeneous according to thin-layer chromatograms on precoated silica gel G plates purchased from Analtech, Inc., Newark, Delaware, and developed with the following two solvent systems (solvents, volume ratios of solvents in the same order) 1-butanol-acetic acid-ethyl acetate-water, 1 1 1 1, Rf 0.49 1-butanol-acetic acid-pyridine-water, 15 3 10 12, R/0.51. 

Plates with 0.5- to 2-mm layer thickness are normally nsed for increased loading capacity. Layers can be self-made in the laboratory, or commercially precoated preparative plates are available with silica gel, alumina, cellulose, C-2 or C-18 bonded siliea gel, and other sorbents. Resolution is lower than on thinner analytical layers having a smaller average partiele size and particle size range. Precoated plates with a preadsorbent or eoneentrating zone faeilitate application of sample bands. 

Manufacturers of TLC materials and accessories are well prepared to satisfy the needs for professionally performed PLC. High-quality precoated preparative plates are available from a number of eommercial sources. Alternatively, less expensive or specialty preparative plates ean be homemade in the laboratory, and loose sorbents and coating devices ean be purehased for this purpose. More-or-less-automated devices can also be purehased for band application of higher quantities of sample solutions to preparative layers. At least for some users, sophisticated densitometric and other instrumental techniques are available as nondestructive tools for preliminary detention and identification of separated compounds in order to enhance the effieiency of their isolation. The only aid still missing, and maybe the most important of all, is a comprehensive monograph on PLC that might encourage and instruct many potential users on how to fully benefit from this very versatile, efficient, relatively inexpensive, and rather easy to use isolation and purification technique. This book was planned to fill that void. 

Chapter 3 through Chapter 8 deal with the basic aspects of the practical uses of PLC. Chapter 3 describes sorbent materials and precoated layers for normal or straight phase (adsorption) chromatography (silica gel and aluminum oxide 60) and partition chromatography (silica gel, aluminum oxide 150, and cellulose), and precoated layers for reversed-phase chromatography (RP-18 or C-18). Properties of the bulk sorbents and precoated layers, a survey of commercial products, and examples of substance classes that can be separated are given. 

Sorbent Materials and Precoated Layers for Straight Phase... 

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