Susan V. Olesik

Electrospun fibers for solid-phase microextraction.

A method of producing solid-phase microextraction (SPME) fibers based on electrospinning polymers into nanofibrous mats is demonstrated. Using this method the polymer mat is attached to a stainless steel wire without the need of a binder. While applicable to any polymer that can be electrospun, a polymeric negative photoresist, SU-8 2100, is used for this initial study. SPME devices composed of carbon nanofibers are also illustrated by pyrolyzing SU-8 to produce amorphous carbon. Nonpolar compounds, benzene, toluene, ethylbenzene, and o-xylene (BTEX) and polar compounds, phenol, 4-chlorophenol and 4-nitrophenol are extracted under headspace SPME conditions. Extraction efficiencies are compared to commercial polydimethylsiloxane (PDMS), polydimethylsiloxane/divinylbenzene (PDMS/DVB), and polyacrylate (PA) fibers. For both the nonpolar and polar compounds, the carbon nanofiber based phases demonstrated enhanced or comparable (o-xylene only) extraction efficiencies. Distribution constants, K, for benzene on the electrospun fibers are of greater or similar magnitude to those of the compared commercial fibers and increase with carbonization temperature. Finally, the measured detection limits for all the organic compounds are similar to those measured with other SPME gas chromatography-flame ionization detector (GC-FID) methods with a large linear dynamic range (3 orders of magnitude) for quantification.

Determination of methanol/CO2 and acetonitrile/CO2 vapor-liquid phase equilibria using a variable-volume view cell

Abstract Methanol/CO 2 phase behavior was investigated previously by a number of researchers. However, the entire composition range at moderate temperatures was not mapped in a single study. Extensive acetonitrile/CO 2 vapor-liquid phase information has not been reported. The goals of this research were twofold: (1) to demonstrate the accuracy and precision of visual observation for vapor-liquid phase boundary determination using a high-pressure, variable-volume view cell with the methanol/CO 2 system relative to the data available, and (2) to examine phase behavior of methanol/CO 2 and acetonitrile/CO 2 , commonly used binary fluids for supercritical fluid chromatography and extraction, from 0–1.00 mole fraction CO 2 and over a 25–100°C temperature range. Results for methanol/CO 2 were found to be very comparable to the literature values.

Technique for Ultrathin Layer Chromatography Using an Electrospun, Nanofibrous Stationary Phase

A technique for creating devices for ultrathin layer chromatography (UTLC) using an electrospinning method is described. The devices use a nanofibrous stationary phase with fiber diameters that are 400 nm. Separations of mixtures of laser dyes and mixtures of steroidal compounds were performed to illustrate the capabilities of these new UTLC media. The complete analyses were found to require very little development time and require less solvent than typical TLC methods. The efficiency of the separations was substantially improved compared to that determined using commercial phases. The retention properties and efficiency of the technique are discussed as are the effects of mat thickness and mobile phase composition on the chromatographic properties of the devices.

The potential and challenges of elemental speciation by capillary electrophoresis-inductively coupled plasma mass spectrometry and electrospray or ion spray mass spectrometry☆

Abstract Capillary electrophoresis-inductively coupled plasma mass spectrometry (CE-ICP-MS) and electrospray (ES) or ion spray (IS) mass spectrometry (MS) are recently introduced techniques for elemental speciation. Both techniques have the potential for rapid elemental speciation with low detection limits. Examples of the use of CE-ICP-MS for elemental speciation of positive, neutral and negative species are discussed. Issues in interfacing CE and ICP-MS are considered briefly. The potential advantages and disadvantages of laminar flow in CE-ICP-MS are examined. Potential difficulties in CE-ICP-MS including loss of sample, chemical matrix effects and changes in speciation during separation are discussed. The interpretation of ES or IS-MS spectra and analysis of complex mixtures are considered. Calibration and chemical matrix effects are assessed. Potential pitfalls of interpreting bare metal ion spectra as elemental analysis are discussed. The need for fundamental understanding of the processes that control ES and IS-MS signals is examined. High conductivity samples currently present difficulties for CE-ICP-MS or ES and IS-MS.

Electrospun glassy carbon ultra-thin layer chromatography devices

The development and application of electrospun glassy carbon nanofibers for ultra-thin layer chromatography (UTLC) are described. The carbon nanofiber stationary phase is created through the electrospinning and pyrolysis of SU-8 2100 photoresist. This results in glassy carbon nanofibers with diameters of approximately 200-350 nm that form a mat structure with a thickness of approximately 15 microm. The chromatographic properties of UTLC devices produced from pyrolyzed SU-8 heated to temperatures of 600, 800, and 1000 degrees C are described. Raman spectroscopy and scanning electron microscopy (SEM) are used to characterize the physical and molecular structure of the nanofibers at each temperature. A set of six laser dyes was examined to demonstrate the applicability of the devices. Analyses of the retention properties of the individual dyes as well as the separation of mixtures of three dyes were performed. A mixture of three FITC-labeled essential amino acids: lysine, threonine and phenylalanine, was examined and fully resolved on the carbon UTLC devices as well. The electrospun glassy carbon UTLC plates show tunable retention, have plate number, N, values above 10,000, and show physical and chemical robustness for a range of mobile phases.

Femtosecond laser micromachining of dielectric materials for biomedical applications

Techniques for microfluidic channel fabrication in soda-lime glass and fused quartz using femtosecond laser ablation and ablation in conjunction with polymer coating for surface roughness improvement were tested. Systematic experiments were done to characterize how process variables (laser fluence, scanning speed and focus spot overlap, and material properties) affect the machining feature size and quality. Laser fluence and focus spot overlap showed the strongest influence on channel depth and roughness. At high fluence, the surface roughness was measured to be between 395 nm and 731 nm RMS. At low fluence, roughness decreased to 100 nm–350 nm RMS and showed a greater dependence on overlap. The surface roughness of laser ablation was also dependent on the material properties. For the same laser ablation parameters, soda-lime glass surfaces were smoother than fused quartz. For some applications, especially those using quartz, smoother channels are desired. A hydroxyethyl methacrylate (HEMA) polymer coating was applied and the roughness of the coated channels was improved to 10–50 nm RMS.

Chiral separation by simultaneous use of vancomycin as stationary phase chiral selector and chiral mobile phase additive

Improved chiral selectivity was observed for numerous compounds when vancomycin was added to the mobile phase on a Chirobiotic-V column. This chiral mobile phase additive (CMPA) is the same chiral selector as that bonded to the stationary phase of this Chirobiotic-V column. A substantial increase in the difference in enthalpy of transfer, deltadeltaH, and in the difference in entropy of transfer, deltadeltaS, for two enantiomers was observed when vancomycin was used as both the mobile phase and the stationary phase chiral selector. The importance of mobile phase composition, analytical column, CMPA concentration was investigated. Also, higher resolution was observed for the separations of acidic compounds when a fluidity enhancing solvent, such as fluoroform, was added into the mobile phase. However, the most commonly used fluidity enhancement solvent, CO2, was ineffective.

Chiral separations performed by enhanced-fluidity liquid chromatography on a macrocyclic antibiotic chiral stationary phase.

The use of enhanced-fluidity liquid chromatography (EFLC) for chiral separations was demonstrated on a macrocyclic antibiotic column, Chirobiotic-V. This technique was compared to high performance liquid chromatography (HPLC) and supercritical fluid chromatography (SFC) for the separation of chiral compounds in normal-phase mode. The highest resolution was always observed for EFLC condition. Higher efficiency and shorter retention time were also observed for most separations with portions of CO(2) in the range of 0-50 mol %. Larger amounts of CO(2) caused efficiency to decrease and retention time to be prolonged. For some separations, the temperature was elevated to bring the mobile phase to the supercritical condition. Improved efficiency was obtained in SFC, whereas resolution and selectivity were worse. The use of EFLC in reversed-phase chiral separations was also tested. Enantiomer resolution improved under the EFLC condition. For the tested methanol/H(2)O mixture, fluoroform provided more significant improvements in chromatographic performance than CO(2) when used as a fluidity enhancing liquid. The use of EFLC instead of HPLC also caused a markedly lower pressure drop across the column for commonly used flow rates. The low-pressure drop will allow the use of longer columns or multiple columns to increase the total efficiency of the separation. Since chiral columns are often inefficient, this attribute may be very important for chiral separations.

Reversed-phase high-performance liquid chromatography using enhanced-fluidity mobile phases

Abstract The use of enhanced-fluidity liquids in reversed-phase HPLC separations is characterized. Enhanced fluidi liquids are commonly used liquids with high proportions of low viscosity fluids, such as carbon dioxide, added. When carbon dioxide is added to the methanol-water mobile phase, substantially lower plate heights and time of analysis are achieved without losing mobile phase solvent strength. The results indicate that these improvements are caused by the combination of the increased diffusivity of the enhanced-fluidity solvents and the ability of carbon dioxide to readily break hydrogen bonds in the methanol-water mixtures.

Aligned electrospun nanofibers for ultra-thin layer chromatography

The fabrication and implementation of aligned electrospun polyacrylonitrile (PAN) nanofibers as a stationary phase for ultra-thin layer chromatography (UTLC) is described. The aligned electrospun UTLC plates (AE-UTLC) were characterized to give an optimized electrospun mat consisting of high nanofiber alignment and a mat thickness of ~25 μm. The AE-UTLC devices were used to separate a mixture of β-blockers and steroidal compounds to illustrate the properties of AE-UTLC. The AE-UTLC plates provided shorter analysis time (~2-2.5 times faster) with improved reproducibility (as high as 2 times) as well as an improvement in efficiency (up to100 times greater) relative to non-aligned electrospun-UTLC (E-UTLC) devices.