On the Production of Flux Vortices and Magnetic Monopoles in Phase Transitions

We examine the basic assumptions underlying a scenario due to Kibble that is widely used to estimate the production of topological defects. We argue that one of the crucial assumptions, namely the geodesic rule, although completely valid for global defects, becomes ill defined for the case of gauged defects. We address the issues involved in formulating a suitable geodesic rule for this case and argue that the dynamics plays an important role in the production of gauge defects.Comment: 9 pages, in LATEX, UMN-TH-1028/92, TPI-MINN-92/20-

Microextraction techniques combined with capillary electrophoresis in bioanalysis

Over the past two decades, many environmentally sustainable sample-preparation techniques have been proposed, with the objective of reducing the use of toxic organic solvents or substituting these with environmentally friendly alternatives. Microextraction techniques (MEs), in which only a small amount of organic solvent is used, have several advantages, including reduced sample volume, analysis time, and operating costs. Thus, MEs are well adapted in bioanalysis, in which sample preparation is mandatory because of the complexity of a sample that is available in small quantities (mL or even μL only). Capillary electrophoresis (CE) is a powerful and efficient separation technique in which no organic solvents are required for analysis. Combination of CE with MEs is regarded as a very attractive environmentally sustainable analytical tool, and numerous applications have been reported over the last few decades for bioanalysis of low-molecular-weight compounds or for peptide analysis. In this paper we review the use of MEs combined with CE in bioanalysis. The review is divided into two sections: liquid and solid-based MEs. A brief practical and theoretical description of each ME is given, and the techniques are illustrated by relevant application

New trends in fast and high-resolution liquid chromatography: a critical comparison of existing approaches

Recent developments in chromatographic supports and instrumentation for liquid chromatography (LC) are enabling rapid and highly efficient separations. Various analytical strategies have been proposed, for example the use of silica-based monolithic supports, elevated mobile phase temperatures, and columns packed with sub-3μm superficially porous particles (fused core) or with sub-2μm porous particles for use in ultra-high-pressure LC (UHPLC). The purpose of this review is to describe and compare these approaches in terms of throughput and resolving power, using kinetic data gathered for compounds with molecular weights ranging between 200 and 1300g mol−1 in isocratic and gradient modes. This study demonstrates that the best analytical strategy should be selected on the basis of the analytical problem (e.g., isocratic vs. gradient, throughput vs. efficiency) and the properties of the analyte. UHPLC and fused-core technologies are quite promising for small-molecular-weight compounds, but increasing the mobile phase temperature is useful for larger molecules, for example peptides. Figure Recent progress in HPLC technology to increase throughput and resolving powe

High-Throughput Screening of Drugs of Abuse in Urine by Supported Liquid-Liquid Extraction and UHPLC Coupled to Tandem MS

A qualitative method, involving supported liquid-liquid extraction (SLE) and ultra high pressure liquid chromatography coupled to tandem mass spectrometry (UHPLC-MS-MS), was developed for the rapid tentative identification of various drugs of abuse in urine. In this study, 28 drugs and metabolites were covered by the screening procedure. Before analysis, urine samples were extracted by SLE and good extraction recoveries were obtained for most investigated compounds. The UHPLC strategy was then selected for the rapid separation of amphetamines, cocaine, opiates and related compounds in urine. Using columns packed with sub-2µm particles, analysis time was reduced down to 2min, while maintaining acceptable performance. Finally, the detection was by tandem MS operating in the single reaction monitoring (SRM) mode. The most intense transition was selected for the different drugs and SRM dwell times set at 5ms, to maintain sufficient data points across the narrow UHPLC peaks. The tentative identification of the drugs of interest, including amphetamines, opiates and cocaine, was based on both, retention times and mass spectrometry information. With the proposed method, limits of detection were estimated at about 1ngmL−1 and the applicability was assessed by successfully analyzing several samples of drug abusers. Finally, this study demonstrates the potential of UHPLC coupled to tandem MS for the rapid screening of drugs of abuse in urin

Light Stops in the MSSM: Implications for Photino Dark Matter and Top Quark Decay

We consider the viability of the minimal supersymmetric standard model with a light ($m_{\tilde t_1} <$ 45 GeV) stop. In order for its relic abundance to be cosmologically significant, the photino as dark matter must be quite close in mass to the stop, $(m_{\tilde t_1} - m_{\tilde \gamma}) \simeq 3 - 7$ GeV. However, as we show, the photino despite its low mass is virtually undetectable by either direct or indirect means. We also discuss the implications of these masses on the top quark branching ratios.Comment: 11pages, LaTex, UMN-TH-1309/9

Fast log P determination by ultra-high-pressure liquid chromatography coupled with UV and mass spectrometry detections

Ultra-high-pressure liquid chromatography (UHPLC) systems able to work with columns packed with sub-2μm particles offer very fast methods to determine the lipophilicity of new chemical entities. The careful development of the most suitable experimental conditions presented here will help medicinal chemists for high-throughput screening (HTS) log P oct measurements. The approach was optimized using a well-balanced set of 38 model compounds and a series of 28 basic compounds such as β-blockers, local anesthetics, piperazines, clonidine, and derivatives. Different organic modifiers and hybrid stationary phases packed with 1.7-μm particles were evaluated in isocratic as well as gradient modes, and the advantages and limitations of tested conditions pointed out. The UHPLC approach offered a significant enhancement over the classical HPLC methods, by a factor 50 in the lipophilicity determination throughput. The hyphenation of UHPLC with MS detection allowed a further increase in the throughput. Data and results reported herein prove that the UHPLC-MS method can represent a progress in the HTS-measurement of lipophilicity due to its speed (at least a factor of 500 with respect to HPLC approaches) and to an extended field of application. Figure The UHPLC approach described here greatly enhanced the time required for log P determination (5' min by compound using UV detection) and, at least, 8 compounds measured in a 5' run when Mass Spectrometry detection in used. These developments offer to medicinal chemists a high-throughput method to estimate the lipophilicity of NCE