Capillary electrokinetic separations: Influence of mobile phase composition on performance

The composition of the mobile phase employed in capillary zone electrophoresis and the related technique, micellar electrokinetic capillary chromatography, is an important factor in determining separation performance. The influences of ionic salt, surfactant, and organic solvent mobile phase additives on separation efficiency, retention, and elution range are discussed and demonstrated. 23 refs., 2 figs., 2 tabs

Capillary electrokinetic separations with optical detection. Technical progress report, February 1, 1994--January 31, 1995

This multifarious research program is dedicated to the development of capillary electrokinetic separation techniques and associated optical methods of detection. Currently, research is directed at three general objectives. First, fundamental studies of pertinent separation and band broadening mechanisms are being conducted, with the emphasis on achieving rapid separations and understanding separation systems that include highly-ordered assemblies as running buffer additives. Second, instrumentation and methodologies associated with these capillary separation techniques are being advanced. Third, applications of these separation and detection systems should fill current voids in the capabilities of capillary separation techniques. In particular, it should be possible to perform rapid, highly efficient, and selective separations of hydrophobic compounds (e.g., higher MW polycyclic aromatic hydrocarbons (PAHs) and fullerenes), certain optical isomers, DNA fragments, and various pollutants including certain heavy metals

Capillary Electrophoresis - Optical Detection Systems

Molecular recognition systems are developed via molecular modeling and synthesis to enhance separation performance in capillary electrophoresis and optical detection methods for capillary electrophoresis. The underpinning theme of our work is the rational design and development of molecular recognition systems in chemical separations and analysis. There have been, however, some subtle and exciting shifts in our research paradigm during this period. Specifically, we have moved from mostly separations research to a good balance between separations and spectroscopic detection for separations. This shift is based on our perception that the pressing research challenges and needs in capillary electrophoresis and electrokinetic chromatography relate to the persistent detection and flow rate reproducibility limitations of these techniques (see page 1 of the accompanying Renewal Application for further discussion). In most of our work molecular recognition reagents are employed to provide selectivity and enhance performance. Also, an emerging trend is the use of these reagents with specially-prepared nano-scale materials. Although not part of our DOE BES-supported work, the modeling and synthesis of new receptors has indirectly supported the development of novel microcantilevers-based MEMS for the sensing of vapor and liquid phase analytes. This fortuitous overlap is briefly covered in this report. Several of the more significant publications that have resulted from our work are appended. To facilitate brevity we refer to these publications liberally in this progress report. Reference is also made to very recent work in the Background and Preliminary Studies Section of the Renewal Application

Detection and Characterization of Chemicals Present in Tank Waste

The goal of this three-year project is to develop and demonstrate novel multi-parameter micro-electro-mechanical system (MEMS) sensors that are robust and can be used to simultaneously detect the presence of target chemicals in a mixture, radiation emitted from radioactive materials and the heat generated by the absorption of photons of specific wavelength by the target molecules.The goal of this program is to study and develop effective methods of immobilizing chemical selective phases for improved microsensor performance 1. Investigations of Photo-Induced and Adsorption-Induced Stress in Micromechanical Structures and Photothermal Spectroscopy Microcantilevers respond to chemical stimuli by undergoing changes in their bending and resonance frequency even when a small number of molecules adsorb on their surface. In our present studies, we extended this concept by studying changes in both the adsorption-induced stress and photo-induced stress as target chemicals adsorb on th e surface of microcantilevers. For example, microcantilevers that have adsorbed molecules will undergo photo-induced bending that depends on the number of absorbed molecules on the surface. However, microcantilevers that have undergone photo-induced bending will adsorb molecules on their surfaces in a distinctly different way. Coating the surface of a microstructure with a different material can provide chemical specificity for the target chemicals. Therefore combining measurements of photo-induced and adsorption-induced stress in MEMS devices caused by target molecules with microcalorimetric spectroscopy both the presence and identity of target molecules can be determined. In addition, radioactive chemicals can also be identified by measuring the temperature changes of micromechanical sensors as the absorb emitted radiation. (i) Studies of Adsorption-Induced Stress in MEMS We investigated the effect of absorption of trace amounts of target molecules, 2- mercaptoethanol and diisopropyl methylphosphonate (DIMP), on micromechanical structures. Although gold coated surfaces adsorb DIMP effectively, the selectivity can be substantially improved by first coating the surface with self-assembled monolayers. The chemical selectivity of the layer is based on the interaction of Cu+2 bound to the MEMS surface by a carboxylate-terminated n-alkanethiol monolayer. Microcantilever MEMS devices with such surface coatings were exposed DIMP molecules by flowing a mixture of N2 and DIMP vapor in a chamber containing the microcantilever. The composite self-assembled monolayer coating transiently adsorbs molecules of DIMP vapor, which causes the microcantilever to bend. We found that our derivitized MEMS respond proportionally and reversibly to the presence of DIMP molecules in a way that is distinguishable from any response to common organic solvents such as ethanol, methanol, or acetone. During the exposure time, we observed no measurable change in the resonance frequency of the MEMS

Dispersion Characteristics in Disk-on-Pillar Array Nanostructures for Surface-Enhanced Raman Spectroscopy

In this paper, we analyze periodic disk-on-pillar nanoarrays as a platform for surface-enhanced Raman spectroscopy measurements. The nanostructure is a two-dimensional grating of silicon pillars covered by thin layers of silica and silver. The system supports both localized surface plasmons and surface plasmon polaritons. We investigate the dispersion characteristics of the nanoarray and present the relevant field distribution for each plasmon mode. The interaction between localized and propagating modes can be tuned to synergistically enhance the electric field, which results in larger surface-enhanced Raman signals. We find that utilizing this effect can generate Raman enhancements that are approximately 1000 times larger than that of an isolated pillar under the same excitation conditions