Robert Weinberger

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.

Solvent and pH effects on peroxyolxalate chemiluminescence detection for liquid chromatography

Abstract The role of the mobile phase modifier, mobile phase water content and pH are described with regard to chemiluminescence (CL) intensity and lifetime for a post-column peroxyoxalate CL detection system. In non-aqueous mobile phases, the highest intensities and shortest lifetimes are observed with methanol as a modifier. In more aqueous solvents, the highes intensities are found with acetonitrile or isopropanol. A theory concerning solvent cage effects on the chemical excitor precursor is proposed to account for these observations. However, the role of pH is more significant thant the selection of the mobile phase modifier. The optimum CL with bis-(2.4,6-trichlorophenyl)oxalate as the chemical excitor is approimately 7.5. By properly buffering boh the organic and aqueous solvents, gradient elution with CL detection is highly practical.

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.

Use of Dynamically Coated Capillaries for the Routine Analysis of Methamphetamine, Amphetamine, MDA, MDMA, MDEA, and Cocaine using Capillary Electrophoresis

A rapid, accurate, precise, reproducible, economical, and environmentally gentle method using capillary electrophoresis (CE) is presented for the routine analysis of methamphetamine, amphetamine, MDA, MDMA, MDEA, and cocaine in seized drugs. The methodology uses a 32 cm by 50 microm capillary (length to detector 23.5 cm) with a commercially available buffer kit and diode array UV detection. Dynamic coating of the capillary surface is accomplished by flushing with base for 1 min, a proprietary polycation for 1 min, and then a proprietary polyanion for 2 min. This approach provides a relatively high and stable electroosmotic flow (EOF), even at low pHs. The background electrolyte (BGE) contains 75 mM phosphate buffer (pH 2.5) with the same polyanion as above. Using this methodology, amphetamine, methamphetamine, MDA, MDMA, MDEA, and an internal standard (n-butylamphetamine) are baseline resolved in less than 5 min. The run-to-run migration time %RSDs and peak area %RSDs are typically <0.3% and <2.1%, respectively. The day-to-day and capillary-to-capillary migration time %RSDs are <1.5% and <2.1%, respectively. The %RSDs of the relative migration times compared with the internal standard on a day-to-day and capillary-to-capillary basis are <0.2% and <0.06%, respectively. The linear dynamic range using peak areas range from 0.003 to 0.10 mg/mL. The correlation coefficients are >0.9998, with all calibration curves passing at or near the origin. Similar data are obtained for cocaine and its internal standard henyltoloxamine. None of the compounds usually encountered in illicit samples interfere with the target compound (e.g., methamphetamine and cocaine) or the internal standard. Quantitative results for synthetic mixtures and seized exhibits are in good agreement with actual values, and also with results obtained from other techniques. The relatively high EOF for the dynamically coated capillary system allows for the screening of basic, acidic, and neutral adulterants in drug seizures; identification is facilitated by the use of automated UV library searches.

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.

Determination of reducing and non-reducing carbohydrates in food products by liquid chromatography with post-column catalytic hydrolysis and derivatization comparison with refractive index detection

Post-column catalytic hydrolysis combined with 4-aminobenzoic acid hydrazide derivatization is employed for the determination of both reducing and non-reducing carbohydrates in a variety of complex sample matrices such as dairy products, processed foods and tobaccos. Comparison with refractive index detection shows the post-column method to be superior from the standpoint of selectivity, sensitivity and simplicity of sample preparation. Limits of detection are in the low-nanogram range. Quantitative results are presented for the determination of sugars in potato extracts. The method is also applicable for the separation and detection of carbohydrate oligomers.

Triplet State Processes in Aqueous Colloidal Dispersions of Polycyclic Aromatic Hydrocarbons at Room Temperature

A unique and facile means of producing room-temperature phosphorescence (RTP) from colloidal or microcrystalline suspensions of aromatic molecules in aqueous media is reported. Unlike previously reported RTP techniques, colloidal RTP is insensitive to quenching by dissolved oxygen. Delayed fluorescence was observed from several non-phosphorescent species. Potential uses of the technique are for the determination of the solubility of highly insoluble compounds and the ability to distinguish between suspended and dissolved matter.

Separation of phenols as neutral compounds by micellar electrokinetic capillary chromatography

Separation of phenols as neutral solutes by micellar electrokinetic capillary chromatography provides a quantitative linear dynamic range of 6000-13,000. Since the compounds are injected and separated as neutral solutes, the dispersive processes of anti-stacking and electrodispersion are eliminated. Optimized conditions allow for sub-ppm quantitation of trace impurities in the presence of the major components at various stages of the production of high purity phenols. The background electrolyte consists of 100 mM sodium dodecyl sulfate in 50 mM phosphate buffer pH 7. The method is precise, reliable, and the limits of detection are superior compared to HPLC by a factor of 20.