Modeling of anti-Langmuirian peaks in micellar electrokinetic chromatography: Benzene and naphthalene

Ionic Liquid-Liquid Chromatography: A New General Purpose Separation Methodology

Ionic liquids can form biphasic solvent systems with many organic solvents and water, and these solvent systems can be used in liquid-liquid separations and countercurrent chromatography. The wide range of ionic liquids that can by synthesised, with specifically tailored properties, represents a new philosophy for the separation of organic, inorganic and bio-based materials. A customised countercurrent chromatograph has been designed and constructed specifically to allow the more viscous character of ionic liquid-based solvent systems to be used in a wide variety of separations (including transition metal salts, arenes, alkenes, alkanes, bio-oils and sugars).

Electrokinetic chromatography and mass spectrometric detection using latex nanoparticles as a pseudostationary phase

The utility of novel latex nanoparticles as pseudostationary phases for electrokinetic chromatography with UV and mass spectrometric detection is demonstrated. The nanoparticles are synthesized using ab initio RAFT (reversible addition-fragmentation chain transfer) in emulsion polymerization, which yields small (63 nm) particles with a narrow size distribution, a hydrophobic core, and an ionic shell. The nanoparticles are shown to provide efficient and selective separations, with retention and separation selectivity dominated by hydrophobic interactions. The nanoparticles are highly retentive, such that they are effective at relatively low concentrations. Addition of the nanoparticles to the background electrolyte at these concentrations has a minor effect on the noise with UV detection, no measurable effect on the separation current, and minor effects on analyte ionization efficiency during electrospray ionization. The nanoparticles do not cause fouling or degradation of the electrospray-mass spectrometer interface even after several weeks of use. The combination of online sample preconcentration via sweeping and selective mass spectrometric detection yields low detection limits (10-16 ppb), particularly for more hydrophobic compounds.

Comparison of micellar isotherms of benzene determined by headspace gas chromatography and micellar electrokinetic chromatography

The possibility is discussed that micellar isotherms determined by vacancy-micellar electrokinetic chromatography (vacancy-MEKC) differ from isotherms in electrolyte-free surfactants due to thermodynamic effects of buffer. Also discussed is the possibility that they are biased at high solute concentrations by solubilization-induced changes of electrical conductivity. Such bias could invalidate a theory on peak asymmetry of neutral solutes in MEKC that is based on thermodynamic interpretation of the isotherms. To evaluate these possibilities, the nonlinear concave upward isotherm of benzene in a pH 7.0, 0.0060 M sodium phosphate buffer containing 50 mM sodium dodecyl sulfate (SDS) was measured by headspace gas chromatography. Of interest is the finding that benzene is more stable in the surfactant-free buffer than in water. The isotherm was compared to that previously measured by vacancy-MEKC in the same buffer and 10, 30, or 50 mM SDS. No difference was found between the isotherms. However, the isotherm indeed differed from that of benzene in buffer-free 50 mM SDS, which was also determined and agreed favorably with previous results. A partial explanation is given for the independence of the vacancy-MEKC isotherm of solubilization-induced conductivity changes.

Origin of peak asymmetry and isotherm nonlinearity in micellar electrokinetic chromatography: Variation of peak shape with buffer concentration

The diffuse fronts and sharp rears of peaks of nitrobenzene (nbz) solubilized at high concentrations in 50 mM SDS and 2.5, 25, and 50 mM sodium tetraborate buffers were modeled in MEKC by measurements of, and fits to, concave upward isotherms, and by numerical solution of the continuity equation. The isotherms varied with buffer concentration, with the smallest limiting slope and largest curvature found for the 50 mM tetraborate buffer. The Brunauer, Emmett, and Teller isotherm described the peak profiles in all buffers, with symmetrical peaks observed at sub- and low-mM levels of nbz, anti-Langmuirian peaks observed at 10-20 mM levels, and aquiline peaks resembling curved noses observed at 20-30 mM levels. The variation of the partition coefficient with nbz and buffer concentrations was shown to result from nonideal thermodynamics. High-buffer concentrations salt out nbz from the mobile phase, as quantified by a mobile-phase activity coefficient related to the Setchenov constant of nbz in sodium tetraborate. The activity coefficient of nbz in SDS micelles was shown to resemble that measured by other researchers for benzene in micelles of sodium octyl sulfate, i.e. it decreases with increasing solute concentration and increases with electrolyte concentration. Many examples from the literature are discussed, in which the variation of the intramicellar activity coefficient with solute mole fraction is consistent with peaks having diffuse fronts and sharp rears.