Precoat Procedure

Precoat Procedure Precoat filtration tests are run in exactly the same manner as bottom-feed tests except that the leaf must first be precoated with a bed of diatomaceous earth, perhte, or other shave-able inert sohds. Some trial and error is involved in selecting a grade of precoat material which will retain the filtered solids to be removed on the surface of the bed without any significant penetration. During this selection process, relatively thin precoat beds of I to 2 cm are satisfactory. After a grade has been selected, bench-scale tests should be... 

To diminish any eventual residual adsorptive activity of the support that may still be present after deactivation by acid and alkaline washing, and by chemical procedures, Bohemen et al. introduced a precoating with 0.1 % PEG 400. The PEG molecules are thought to be adsorbed tightly to the residual active sites of the support. This precoating procedure was also used by Brochmann-Hanssen and Baerheim Svendsen in their gas chromatographic studies on... 

Precipitates or salts are used when corrosive liquor must be filtered, and where there is no available medium of sufficient fineness that is corrosion-resistant and will not contaminate the cake. In these cases, precipitates or salts are used on porous supports. In the filtration of caustic liquors, ordinary salt (sodium chloride) is used as the filter medium in the form of a precoat over metallic cloth. This procedure has the advantage that the salt medium will not be detrimental to either the cake or the filtrate if inadvertently mixed with it. 

An alkylamine linker is incorporated into either the 5 or 3 end of the capture probe and the probe is activated with ethylene glycol bis-succinimidylsuccinate. The activated capture probe is bound to polystyrene microtiter wells precoated with poly(Lys-HBr, Phe). Nearly all of the solid-phase probe coupled to the polystyrene by this procedure is available for hybridization (Running and Urdea, 1990). 

After the precoat is established the solids to be removed from the filter feed are trapped on the surface of the precoated bed. This thin layer of slime is removed by a knife which is caused to advance slowly towards the drum. The knife also removes a thin layer of the precoated bed so that a new surface of the filter medium is exposed. This procedure allows steady filtration rates to be achieved. 

One obvious procedural refinement would be to precoat the catalyst on the Teflon tape. This would allow low loadings to be delivered by length as opposed to mass measurements, or the tedious preparation of standard solutions. Accordingly, strips of tape were added to a solution of 16-Rf6 in CFaCeFii. The solvent was removed under an inert gas stream to give a yel-... 

Thin-layer chromatography (TLC), sometimes also called planar chromatography, employ a stationary phase immobilized on a glass or plastic plate and an organic mobile phase. It is a rather old technique whose application in residue analysis has been limited in the past by poor chromatographic resolution, inadequate selectivity, and insufficient sensitivity (49). This was due to inherent problems in the quality of the available stationary phase materials and in the uniformity of the layers prepared. Today, the availability of affordable, precoated plates with acceptable performance and consistency has led to the general acceptance of TLC as an efficient procedure for residue analysis (50). The method is used preferentially when analysts must process large numbers of samples in a short period of time (51). 

Protein A, obtainable as a purified product, is a Staphylococcus aureus bacterial cell wall protein that has the property of binding specifically to the Fc portion of IgG molecules (2) and can be used for precoating the grids prior to the ISEM procedure (7-9). This has the effect of increasing the amount of virus trapped to the grid by a modest amount but is useful when using antisera of low titre and can be used to detect distant serological relationships that would not be detected by normal ISEM (2). 

B. 2-Bromo-3-methylbenzoic acid. Caution This procedure must be carried out in a hood with a good draft, because poisonous hydrogen cyanide gas is evolved. In a 5-1. round-bottomed flask are placed 90 g. of potassium cyanide, 900 ml. of 2-ethoxyethanol (Note 3), 850 ml. of water, and the moist 2-bromo-4-nitrotoluene obtained above. A reflux condenser is attached, and the mixture is boiled for 16 hours (Note 4). To the hot, dark-red solution is then added 1.51. of water, and the mixture is acidified with concentrated hydrochloric acid. (Caution Hydrogen cyanide is evolved.) The acidified mixture is boiled for 15 minutes to expel hydrogen cyanide and then allowed to cool to 35-40°. Five grams of diatomaceous earth is stirred in, and the mixture is filtered through a Buchner funnel precoated with a little diatomaceous earth. The solid is discarded, and the filtrate is extracted three times with 200-ml. portions of chloroform. The chloroform extracts are combined and extracted with three 100-ml. portions of 5% ammonium carbonate solution. The basic extracts are combined, acidified with concentrated hydrochloric acid, and cooled in an ice bath. The oil which first forms soon crystallizes. 

Procedures were used in this work which could be converted to industrial process steps. Coal was refluxed with a phenol-catalyst mixture at atmospheric pressure. The hot solution was filtered through a tared filter which had been precoated with filter aid. Contents of the filter were washed, first with hot phenol-methanol mixture (20/1, v/v) to remove adhering depolymerized coal, and finally, with methanol to remove phenol and catalyst. The filter cake was dried at 10 mm Hg pressure and 140-160 C and weighed. 

Mass spectrometry is another detection technique widely used in neuropeptide analysis. Concentration sensitivities in CE-MS do not reach those obtained by CE-LIF nevertheless, tedious derivatization procedures are avoided. In addition, CE-MS has proven to be a powerful tool for structure elucidation as illustrated by the investigation of the in vivo metabolic fate of peptide E by Caprioli s group [12]. After microdialysis and in-line SPE, neuropeptides migrating out of the electrophoresis capillary were deposited directly onto a precoated cellulose target used in matrix-assisted laser desorption-time of flight (MALDI-TOF) MS subsequently. Structural information is then obtained along with the mass of the peptide(s). 

Precoat Test Procedure Precoat filtration tests are run in... 

Lipsky and Landowne dissolved both the polar phase (PEG) and the non polar phase in the same solvent and carried out both precoating and coating simultaneously. They found that this procedure reduced or eliminated tailing due to non-linear sorption isotherms, and attributed the effect to deactivation of the support. 

For the fabrication of devices, glass substrates (0.7 mm) coated with ITO (80 nm, 10-20 ohm/ square sheet resistance) were used. After routine cleaning procedures for the substrate with wet (acetone and isopropyl alcohol) and dry (UV-ozone) processes, a blended solution of organic materials was spin coated (700 rpm) on top of ITO, precoated with a poly(3,4-ethylenedioxythiophene) poly(4-styrenesulphonate) (PEDOT PSS) hole-injecting buffer layer. The basic organic solution consisted of a hole transporting material of N,N -diphenyl-... 

In the first edition of Stahl s laboratory handbook Durmschicht-Chromatographie published in 1962, equipment for coating glass plates is illustrated, and detailed procedures for the preparation of TLC plates are described. Even today, TLC sorbents for a manual coating operation are produced in an only shghtly modified form, especially where labor costs are small and time is a minor consideration. The layers obtained often show good selectivity, but do not achieve the separation efficiency obtained by precoated layers and seldom lead to reproducible results. 

Table 3 shows the most important commercially available precoated layers and some typical examples of their use. In the choice of the hardware for TLC it cannot be assumed that nominally identical sorbents from different manufacturers will lead to equal separations [13]. It follows that trade names and item numbers used in publications or testing procedures should not be regarded simply as surreptitious advertising, but are absolutely essential if reproducible results are to be obtained. This is especially true for vahdated methods. An example is given in Fig. 6, which shows separations of peppermint oils on TLC plates whose sorbent specifications in the catalogs of different manufacturers are identical. When the development heights are equal, the variation in separation efficiency is clearly visible, and different hRf values are obtained for the same substances (see Table 4). 

At this point, there usually follows a chapter about the pretreatment of the samples. However, in contrast to HPLC/GC, sample preparation for TLC is not considered to be quite as critical. As well as the use of precoated layers with a concentration zone (e.g. an application zone consisting of silica 50 000 and a separation zone of sihca gel 60 or RP-18 material) upon which the matrix constituents can often be held back by suitable choice of solvent system, a chromatogram that is imusable for lack of sample preparation is more rapidly rectified (use a different preparation method and a new plate ) than an irreversibly destroyed column. A detailed treatment of the subject of sample preparation would exceed the scope of the present book. In Section 9.4, rm-der the title Examples of GMP/GLP-Conforming Testing Procedures , we describe the extraction of a pharmaceutically active substance from a tablet and the working up of plant components from dry extracts. The reader is referred to other TLC textbooks [2,21] and to literature and brochures produced by manufacturers of articles for sample preparation [28, 29]. 

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