Ultrahigh-pressure liquid chromatography (UHPLC) is a method of liquid chromatography utilizing sub-2-microm particles packed into capillary columns 25 to 100 cm long. Columns of this length packed with particles this fine require operation with pressures from 1,000 to 7,000 bar (15,000 to 100,000 psi). The advantages of this technique are high separation powers (theoretical plate counts from 100,000 to 300,000) and run times from a few minutes (isocratic) to a few hours (long gradients). This review discusses the background and theoretical basis of UHPLC, practical aspects of UHPLC hardware, examples of separations, future areas for research in UHPLC, and techniques that are both competitive with and complementary to UHPLC.

Capillary liquid chromatography at ultrahigh pressures.

There has been much recent activity in the theory and application of separation by flotation, both in the United. States and abroad. This review primarily addresses new developments in foam flotation, colloid flotation and precipitate flotation. The work in ore flotation represents a major effort and would itself require a lengthy article. Work from ore flotation which significantly impacts on the areas of interest listed above is included here, however. Foam separation of biological materials and the microflotation of bacteria are not routinely discussed in this review. A few recent review articles on these areas are included at the end of the Review Articles and Books Section to afford the interested reader a start on that literature. The development and use of flotation separation techniques for waste treatment and water reuse over the last few years are included in this article.

Separation by Flotation

Silver recovery from aqueous streams using ion flotation

Interactions are analyzed between an ionic surfactant and nonsurface-active ions (colligends) of opposite charge being separated in foam fractionations. Surfactant selectivity for competing colligends is determined in terms of models based on surfactant—colligend ion pair formation in the feed solution to a foam fractionation unit, based on colligend-surfactant counterion exchange at the gas-solution, bubble interfaces, and based on surface exchange coupled with ion pair formation in the bulk solution. Accurate, continuous-flow, single-equilibrium-stage foam fractionation data for NO− 3, BrO− 3, CIO− 3, and I−, each versus Br−, the counterion of the ethylhexadecyldimethylammonium cation, are used to discriminate among the models. Based on a detailed statistical analysis of the selectivity coefficients determined by two interaction models for each of the four colligends, the hypothesis of colligend-counterion exchange at the gas-solution interface is shown to be valid and that of solution ion pair...

Models for Interactions between Ionic Surfactants and Nonsurface-Active Ions in Foam Fractionation Processes

It has been determined that ion-water and cation-surface active anion interactions govern the ion flotation selectivity, when a sublate was not formed in the bulk solution. The flotation selectivity orders of anions were correlated with the experimentally-determined viscosity coefficients, whereas the flotation selectivity orders of cations were correlated with their ionic potential values. It has been found that, if sublate precipitation takes place in the bulk solution, the surface charge of sublate was the key factor influencing the floatability of an ion. Basic properties of sublates, e.g. the solubility and electrokinetic potential in aqueous solution have been determined. Ion flotation kinetics, using cobalt(II) and cobalt(III) ions as an example, is also considered in this study.

Mechanism of Selective Ion Flotation

An experimental study is presented of the continuous flow, foam fractionation of cyanide complex anions: Ag(CN)2 − and Au(CN)2 − from 1.0 to 8.0 × 10−5 M aqueous solutions. The quaternary ammonium surfactant, hexadecyltriethylammonium iodide, is modeled as a soluble ion exchanger. The selectivity coefficients were determined for Ag(CN)2 − vs I− equal to 2.71, and for Au(CN)2 − vs I− equal to 10.85. These coefficients are discussed in terms of the physicochemical properties of cyanide complex anions.

Zofia Rudnik

Papers

Selectivity Coefficients for Ag(CN)2 − and Au(CN)2 − from Continuous Foam Fractionation with a Quaternary Ammonium Surfactant