L. J. Cline. Love

Effects of restricted mass transfer on the efficiency of micellar chromatography

Abstract A model treating restricted mass transfer in micellar chromatography based on the kinetics of adsorption—desorption of the solute on the stationary phase and entrance—exit rates from micelles in the mobile phase is proposed. This model provides an explanation for the lower chromatographic efficiency of micellar mobile phases relative to conventional mobile phases. Based on our studies, the efficiency of micellar chromatography can be optimized by use of elevated temperatures, low flow-rates and minimal micelle concentrations. All of these experimental variables are targeted at maximizing the kinetic rate parameters.

Room-temperature phosphorescence, sensitized phosphorescence and fluorescence of licit and illicit drugs enhanced by organized media

Abstract The ability of microscopically organized media, in the form of surfactant micelles and α- and β-cyclodextrins, to enhance the luminescence phenomena of several licit and illicit drugs is discussed. Because physiological samples are not often amenable to direct spectrometric measurements without pretreatment, the applicability of these organized media to liquid chromatography is also considered. Fluorescence enhancements for certain hallucinogenic drugs such as N,N -dimethyltryptamine, mescaline and ibogaine are seen in cyclodextrin media compared to conventional, homogeneous solutions. Heavy-atom substituted sodium dodecyl sulfate micelles induce phosphorescence from cationic and/or hydrophobic drugs at room temperature in fluid solution; drugs such as propranolol, diflunisal, naphalozine, and selected quinoline derivatives can be determined conveniently. Sensitized phosphorescence is observed for several drugs including brethine, cocaine, didrate, estradiol, meprobarbital, methaqualone, phenobarbital, and sulfanilamide; it can be enhanced markedly when micellar solutions are used as the solvent. The energy-transfer step is facilitated by the organizing ability of the micelle; limits of detection can be decreased by over two orders of magnitude compared to homogeneous solvents. Sensitized phosphorescence can also be measured in cyclodextrin solutions, but the detectability is inferior to that in micellar media. Which form of organized medium is superior for determination of drugs is discussed.

Determination of drugs in untreated body fluids by micellar chromatography with fluorescence detection.

Direct serum and urine injection, without sample extraction or protein precipitation steps, into a liquid chromatographic system using sodium dodecyl sulfate (SDS) with 10% added propanol as the mobile phase, is described for measurement of drug levels. The ability of SDS micelles to form soluble protein-SDS complexes, with no on-column precipitation, provides a simple, rapid method for routine determination of quinine, quinidine, propranolol, morphine and codeine at concentration levels found in serum and urine following administration of therapeutic doses. Absolute limits of detection ranged from 0.2 to 6 ng. Variation of the surfactants mobile phase concentration allows control of selectivity and analysis time, although a minimum concentration is required to prevent protein precipitation. Chromatographic efficiencies are improved by the addition of propanol to the micellar mobile phase, and sensitivities improved by use of fluorescence detection. The sensitivities are more than adequate for therapeutic drug monitoring of concentration ranges normally encountered in serum and urine.

Direct injection of untreated serum using nonionic and ionic micellar liquid chromatography for determination of drugs

The chromatographic behaviour of nonionic micelles used as the mobile phase in liquid chromatography is similar to that of anionic and cationic micellar mobile phases, with reversal of retention order as the micelle concentration is varied. This behaviour follows the prediction of the model developed for anionic surfactants. Nonionic micelle chromatographic interactions are simplified by reducing the extent of electrostatic interactions with the solute and, as a result, their retention dependencies more closely follow the mathematical predictions. Drugs in blood serum samples are quantitatively determined by direct injection of the serum onto the chromatographic column, with no column clogging or pressure build-up. The results are similar to those found with anionic micellar mobile phases, but are in contrast to the protein precipitation observed with cationic micelles. Solute-micelle equilibrium constants and the critical micelle concentration of Brij-35, determined chromatographically, are reported. The potential usefulness of nonionic micelles for the determination of theophylline, paracetamol, phenobarbital, carbamazepine, quinine, quinidine, morphine, codeine and cocaine is demonstrated.

Optimization of selectivity in micellar chromatographic procedures for the determination of drugs in urine by direct injection.

Selectivity was optimized for the determination of drugs in urine by direct injection micellar chromatography through changes in specific mobile phase parameters. The rôle of mobile phase pH and the type of surfactant used for mobile phase preparation was investigated. The retention of the urine matrix was found to be minimal between pH 5.5 and 7.5. The non-ionic surfactant, polyoxyethylene 23 lauryl ether (Brij 35), was found to be the surfactant of choice for the separation of drugs from urine. Favourable retention of both the urine background and the analyte was achieved with this surfactant. Micellar mobile phases of optimal composition were used to develop chromatographic procedures for the determination of furosemide, hydrochlorothiazide and propranolol in urine. Good accuracy (98-102% of drug recovered), precision (1-4% RSD) and linearity were obtained for all methods. Limits of detection for all drugs were adequate.

Fluorescence enhancement of several hallucinogenic drugs in cyclodextrin media

The fluorescence characteristics of selected hallucinogenic drugs dissolved in solutions of α-cyclodextrin and β-cyclodextrin are reported. Fluorescence intensity enhancements in cyclodextrin media relative to aqueous solution range from 1.2 to 4.0, probably because inclusion of the drugs into cyclodextrin increases the quantum yield. Calibration graphs are linear over 2–3 orders of magnitude; limits of detection are 6–13 μg l−1 for ibogaine and N,N-dimethyltryptamine. The mescaline derivatives show limits of detection in the 0.8–1.4 ppm range. The role of cyclodextrin in enhancing the fluorescence intensities and some of the criteria for this fluorescence enhancement are discussed.

Determination of sun-screen agents in cosmetic products by micellar liquid chromatography

A micellar liquid chromatographic method was developed for the quantitative determination of sun-screen agents in cosmetic products. The qualitative analysis of parabens is also feasible. Excellent linearity was obtained (r = 0.999) and recoveries were generally greater than 98%. A variety of commercial formulations were analyzed.

Recent advances and future prospects in fluorescence and phosphorescence spectroscopy

Abstract Various approaches which manipulate the physical and chemical microenvironment of lumiphors are outlined and examples given of surfactant, microcrystalline and cyclodextrin molecular organization. These methodologies can give improved selectivity, and in some cases, enhanced fluorescence and/or phosphorescence intensities. Complimentary methods of generating luminescence via targeted energy transfer, namely, sensitized room temperature phosphorescence and chemiluminescence, are briefly discussed. The power of non-clasical luminescence techniques to produce more useful analytical results is illustrated for micelle-enhanced/sensitized room temperature phosphorescence, a liquid chromatographic/micelle-stabilized phosphorescence detector, and synchronous wavelength scanning/second derivative/micelle-stabilized phosphorescence. The combination of computer-assisted instrumentation and organized chemical microenvironments to obtain the total luminescence spectral profile should provide attomolar sensitivities, and selectivity without prior separation of mixtures on a more routine basis in the near future.

Luminescence properties of polycyclic aromatic hydrocarbons in colloidal or microcrystalline suspensions

Abstract The luminescence properties of polycyclic aromatic hydrocarbons in colloidal suspension in water are compared and contrasted to like concentrations dissolved in methanol. Spectral changes noted in the colloidal state involve apparent quantum yield, self-absorption, excimer formation and room temperature phosphorescence that is oxygen insensitive. All of these effects are constant down to the insolubility limit of each compound in water and this feature can be employed to differentiate dissolved or suspended matter without matrix manipulation.

Mixed organized media: effect of micellar−cyclodextrin solutions on the phosphorescence of phenanthrene

Abstract Intense room-temperature phosphorescence (RTP) from phenanthrene occurs when it is included within the β-cyclodextrin (β-CD) cavity in the presence of 1,2-dibromoethane (DBE). The phosphorescence spectral profile of phenanthrene in β-CD exhibits considerably higher vibronic resolution than that observed in 70:30 Na/Tl dodecyl sulfate (Na/TlDS) micellar solutions, and the wavelength maximum is blue-shifted by 7 nm. Below the critical micelle concentration (CMC), in a mixed system of β-CD; DBE and Na/TlDS, phosphorescence is observed characteristic of that induced by cyclodextrin. Upon addition of more surfactant, but still below the CMC, the phosphorescence intensity from the β-CD microenvironment is enhanced. This is interpreted in terms of favorable effects produced by aggregation of the surfactant monomers at the open ends of the β-CD torus and/or partial inclusion which serves to reduce phenanthrene-water contact. This increases the hydrophobicity of the phosphors microenvironment and further shields it from quenching effects. At surfactant concentrations above the CMC, phenanthrene is transferred from the β-CD to a micellar environment, as evidenced by sharp changes in its phosphorescence spectral profile, wavelength maximum, and sensitivity to quenching by oxygen. Results indicate that DBE also prefers to reside in the micellar aggregate over β-CD, and this severely diminishes micelle-induced RPT by heavy-atom quenching.