Bonded phase Anion-exchange resins

These TLC plates have cation- or anion-exchange resins bonded to their surfaces. The resins are materials such as styrene-divinylbenzene copolymers having either quaternary ammonium or sulphonic acid groupings for ion exchange. They are particularly usefril for substmices of high molecular weight and for amphoteric materials. Strong acids or alkalis are usually used as mobile phases. 

The choice of the sorbent is dictated by the characteristics of both the analytes and their potential interferences. The sorbents most frequently employed here are silica, alkylsilane-modified silica (bonded phases), alumina, porous polymers (with and without ion-exchange groups) and carbon-based materials. One typical application is a method for the determination of hexavalent chromium in soils [10] using the on-line system depicted in Fig. 4.9. After USAL, the analytes in the leachate were directly determined or preconcentrated depending on their concentration. Concentration was performed by on-line solid-phase extraction using a laboratory-made minicolumn packed with a strong anion-exchange resin. The absolute limits of detection were 4.52 and 1.23 ng without and with preconcentration, respectively. 

Bonded-phase silica materials are the most widely used particles for SPE. Since the technology of bonded-phase silica packings for LC columns was already well developed, it was relatively easy for suppliers to offer similar materials in a form suitable for SPE. The most common types are listed in Table 1. Octadecylsilane, or sometimes octylsilane, particles are used for retention of relatively hydro-phobic organic analytes from predominately aqueous samples. A more polar particle, such as cyano-propylsilane, is selected for SPE of less hydrophobic analytes. Cation- and anion-exchange resins are used in small precolumns to concentrate sample ions from very dilute aqueous samples prior to their... 

Separation and quantitation of carbohydrate mixtures may be achieved using HPLC, a method that does not necessitate the formation of a volatile derivative as in GLC. Both partition and ion-exchange techniques have been used with either ultraviolet or refractive index detectors. Partition chromatography is usually performed in the reverse phase mode using a chemically bonded stationary phase and acetonitrile (80 20) in 0.1 mol U1 acetic acid as the mobile phase. Anion- and cation-exchange resins have both been used. Carbohydrates... 

The percent ring substitution (% RS) of the polymer with active sites affects catalytic activity. Polystyrenes with < 25 % RS with lipophilic quarternary onium ions are swollen in triphase mixtures almost entirely by the organic phase. Water reduces the activity of anions by hydrogen bonding. In most triphase nucleophilic displacement reactions onium ion catalysts with <25% RS are highly active, and those with >40% RS, such as most commercial ion exchange resins, are much less active. However, low % RS is not critical for the reactions of hydroxide ion with active methylene compounds, as commericial ion exchange resins work well in alkylation of active nitriles. 

In both cation and anion exchangers, the acid or base groups are chemically bonded to the resin matrix, and the resins have a high concentration of fixed negative or positive ions. These are balanced by mobile counterions such as Na+, H+, or Ca2+ for cation resins and CL, OH", or N03 for anion resins, so that electrical neutrality is always maintained in the resin particles. Ion exchange takes place when the activity of ions in the external solution differs from that of the mobile ions in the solid phase. For example, exposing a hydrogen-form resin HR to a solution with Na+ and H+ will result in diffusion of some Na+ ions into the resin and diffusion of some H+ ions into the solution. 

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