Fritted membrane

In order to generate a cross-flow of carrier liquid in a flow FFF channel, the walls of the channel must be permeable to the liquid. For this reason the walls of flow FFF systems are made up of frit-membrane combinations. Generally, the upper wall is a frit, and the lower wall is a frit covered by a membrane having the desired molecular-mass cutoff limits. 

The question of resistances on the low pressure side of the membrane has also been addressed. Pressure drops across filter paper and porous metal frit membrane supports used in these experiments were measured and found to be not large enough to account for the thickness effects [23]. Again, the greatest effect of this type of resistance is at high fluxes, and the largest thickness effect is at low flux conditions. 

Figure 11.18 Schematic diagram of an in-line SPE unit for CE using (a) polyester wool frits to hold the sorbent, or (b) a paiticle-loaded membrane. Reprinted from Journal of Capillary Electrophoresis, 2, A. J. Tomlinson and S. Naylor, Enhanced performance membrane preconcenti ation-capillary electrophoresis-mass spectiometi y (mPC-CE-MS) in conjunction with ti ansient isotachophoresis for analysis of peptide mixtures, pp 225-233, 1995, with permission from ISC Teclmical Publications Inc.

The separator is frequently a sintered glass frit, but it may also be any of a wide range of inert, porous materials such as celloton, vycor or porvic or an ion exchange membrane. A number of stable ion exchange membranes suitable for use in aqueous and non-aqueous solvents have become available in recent years. 

The frits in the cartridges are intended to retain stationary phase material in the separation channels (columns) while permitting the passage of the mobile phase during separations. Each frit is constructed from a permeable polypropylene membrane with an average pore size smaller than the... 

The titanosilicate version of UTD-1 has been shown to be an effective catalyst for the oxidation of alkanes, alkenes, and alcohols (77-79) by using peroxides as the oxidant. The large pores of Ti-UTD-1 readily accommodate large molecules such as 2,6-di-ferf-butylphenol (2,6-DTBP). The bulky 2,6-DTBP substrate can be converted to the corresponding quinone with activity and selectivity comparable to the mesoporous catalysts Ti-MCM-41 and Ti-HMS (80), where HMS = hexagonal mesoporous silica. Both Ti-UTD-1 and UTD-1 have also been prepared as oriented thin films via a laser ablation technique (81-85). Continuous UTD-1 membranes with the channels oriented normal to the substrate surface have been employed in a catalytic oxidation-separation process (82). At room temperature, a cyclohexene-ferf-butylhydroperoxide was passed through the membrane and epoxidation products were trapped on the down stream side. The UTD-1 membranes supported on metal frits have also been evaluated for the separation of linear paraffins and aromatics (83). In a model separation of n-hexane and toluene, enhanced permeation of the linear alkane was observed. Oriented UTD-1 films have also been evenly coated on small 3D objects such as glass and metal beads (84, 85). 

Fig. 3.94. Process and instrument flow sheet diagram PI, P2 pumps Wl, heat exchanger Bl, B2, glass reactors FI, F2, membrane cells B3, B4 safety PTFE cells F3, F4, HPLC filter frits. Reprinted with permission from A. Rehorek et al. [155].

In both types of liquid-membrane ISEs, the membrane acts as an inunis-cible phase boundary between the aqueous and non-aqueous solutions inside the ISE (see the schematic diagram presented in Figure 3.13). In order to minimize mixing, the liquid membrane is held in place by an inert, porous material such as a rigid glass frit or a flexible synthetic polymer - the choice will depend on the manufacturer rather than on experimental considerations. 

In fact, the movement of ions in and out of a half cell (for example, across a semipermeable membrane or frit) gives rise to an additional form of potential, the so-called junction potential . Such potentials are discussed in Section 3.6.5. 

In reality, most real cells are never fully at equilibrium because the two half cells are located on either side of a semipermeable membrane, sinter or frit, such as a thin separator of rubber or terracotta (see Figure 3.14). Because ions transfer, and because the two half cells comprise different electrolytes (e.g. in terms of different concentration, etc.), the compositions of the two half cells change with time. 

Reduction of analyte occurs at the cathode (on the right-hand side of the cell). Once formed, however, the reduced form of the analyte couple diffuses across the cell - it may also be swept along by the stirred solution - and/or be re-oxidized again at the anode. Clearly, a single molecule of analyte could be oxidized and reduced many times, thus leading to an artificially high charge at the coulometer. For this reason, the two halves of the coulometry cell should be separated if possible, e.g. with a semipermeable membrane or frit, or we should ensure that the product of electron transfer should be a solid, i.e. it is immobilized as soon as it is formed. 

Fig. 5.6. A flow-through electrode system for liquid-membrane ISEs [111] 1 - reference electrode 2 - hole through which the sample solution flows 3 - liquid ion-exchanger reservoir, 4 - triangular piece of a frit soaked with the liquid ion-exchanger.

FIGURE 3.9 Circulating water photoreactor system for determination of photomicro-biocidal activity under water flow conditions, a, reinforced membrane used in the study b, water jacket, continuous flow, infrared filter c, light source d, air pump e, bacterial air filter f, 3-way tap/pressure release g, 2-way taps h, frit for aeration J, peristaltic pump k, reservoir 1, ground glass Joints for ease of cleaning and sterilization (Bonnett et ai, 2006). 

For studies in aqueous solutions, the external reference electrode is often an Ag/AgCl/KCl electrode. Electrical contact with the solution is achieved using a disc-like membrane made of porous fritted glass. Because ions have a tendency to migrate across the membrane, a small potential Ej is generated by this liquid junction. This phenomenon can be minimised by inserting a saturated KC1 solution as a salt bridge. 

The reference electrode recommended for use in the AlCl3-NaCl melt is the A1(III)/A1 couple that is obtained by placing an aluminum wire in a tube containing NaCl(satd) melt (49.8-50.2 mol% AlCl3-NaCl at 175°C). The reference electrode tube is terminated at one end with a Pyrex frit [26] or a thin Pyrex membrane [27]. A platinum wire quasi-reference electrode is used by some researchers [28]. 

---