Nonmechanical pumping of liquids is of key importance for applications ranging from biomedical lab-on-a-chip systems to morphing mechanical structures. In this paper, we report a new, reversible micropumping and pressurization system, with no moving parts, that uses only modest external power. This new "e-pump" operates via a type of electro-osmosis (EO) in which a charge imbalance is created electrochemically across a cation-selective membrane, cations migrate to balance the charge, and solvent is transported across the membrane, along with the mobile cation. No gas is produced in this electrokinetic pumping system, so only liquids are involved in pumping and pressurization. In this proof-of-concept study, an aprotic solvent (dimethylformamide) was chosen to select the specific cation that is migrating (tetrapropylammonium ion). To date, pressures up to 23 atm have been successfully demonstrated.

The performance of ion-exchange membranes under pressure differential conditions is important for a number of separations and for fuel cell applications. Fluid transport properties under pressure are also key to understanding electrochemically-driven pumping and pressurization systems. The physicochemical properties and performance of perfluorosulfonic acid membranes varies considerably with conditioning treatment. In this study, hydraulic permeability and water uptake are measured as a function of membrane treatment for Nafion® membranes that are contacted with aqueous electrolyte solutions. Several conditioning/contacting solution treatments are evaluated including exchanged cation, manner of cation exchange, anion effect, and heat treatment. For Nafion® 117 membranes, these treatments resulted in water content ranging from 3.8 to 20.2 water molecules/fixed sulfonate site and hydraulic permeability ranging from 3.78×10−16 to 1.79×10−13m/sPa. Similar to water uptake, a systematic decrease in hydraulic permeability is observed with the alkali series Li+>Na+>K+. Interestingly, the as-received H+ form and the Li+ form have nearly identical water content but the permeability of the H+ form is a factor of 6–7 higher than the Li+ form, regardless of heat treatment. Evaluation based on an equivalent pore model and Poiseuille flow shows good agreement for pore radii ranging from 1.0 to 2.0nm.

Electrokinetic flow provides a mechanism for a variety of fluid pumping schemes. The design and characterization of an electrochemically driven pump that utilizes porous carbon electrodes, iodide/triiodide redox electrolytes, and Nafion membranes is described. Fluid pumping by the cell is reversible and controlled by the cell current. Chronopotentiometry experiments indicate that the total available fluid that can be pumped in a single electrolysis without gas evolution is determined solely by the initial concentration of electrolyte and the applied current. The magnitude of the fluid flow at a given current is determined by the nature of the cation in the electrolyte and by the water absorption properties of the Nafion membrane. For 1 M aqueous electrolytes, pumping rates ranging from 1 to 14 microL/min were obtained for current densities of 10-30 mA/cm2 of membrane area. Molar volume changes for the I3-/I- redox couple and for the alkali cation migration contribute little to the observed volumetric flow rates; the magnitude of the flow is dominated by the migration-induced flow of water.

This review focuses on the emerging role of sulfated cyclodextrins in the capillary electrophoretic (CE) separation of chiral analytes. Since being introduced as enantioselective agents for CE in 1995, these anionic additives have continued to demonstrate remarkable application universality. The broad spectrum of chiral compounds successfully separated using this approach includes acidic, basic, neutral, and zwitterionic species. This impressive array of analyte structures is derived from a growing diversity of compound classes including pharmaceuticals, plant extracts, biomarkers, herbicides, alkaloids, fungicides, and metal ions. Moreover, literature reports highlight the minimal optimization required to achieve a successful separation. Based on these findings, sulfated cyclodextrins appear to be well suited for the development of a more universal, comprehensive separation strategy for chiral compounds. This review explores this proposition by beginning with the structure and migration properties of sulfated cyclodextrins, using applications to highlight the separating power of this technique and ending with a pragmatic, comprehensive separation strategy.

Polymer formation within ordered monolayer and thin-film assemblies is measured using fluorescence yield near-edge spectroscopy (FYNES). This spectroscopic method measures soft X-ray absorption in the 270−330-eV energy range, probing core-electron transitions to unfilled molecular orbitals by detecting the resultant fluorescence emission. By observing transitions in the carbon 1s−π* region (280−286 eV), the diminution of monomer and formation of polymer are measured directly and simultaneously. In contrast with spectroscopic methods measuring partial electron yield, film damage caused by neutralization of sample charging is eliminated with FYNES. Using FYNES, monomer-to-polymer conversion efficiencies for ultrathin films and monolayer assemblies are successfully measured for the first time. Measured conversion efficiencies for diacetylene-containing assemblies is near 80%, demonstrating remarkable agreement among ultrathin films, surface-attached configurations, and previous studies of thicker film struct...

X-ray fluorescence spectroscopy is demonstrated here as a novel, element-specific detector for capillary electrophoresis. Monochromatic 10 keV X-rays from a synchrotron light source are used to excite core electrons, causing emission of characteristic Kalpha X-ray fluorescence (XRF) lines. Using this technique, XRF energies provide elemental identification, while XRF intensities can be used to quantitate the metal composition of each eluent. An X-ray transparent polymer coupling is used to create a window for the on-line, X-ray detection. This coupling contributes no measurable extra-column variance, and electrophoretic mobilities for the metal complexes used as model solutes are highly reproducible. The combination of XRF detection with capillary electrophoresis (CE-XRF) creates the first on-line detection system that is element-specific, nondestructive, and directly applicable to a broad range of applications including nonelectroactive species. CE-XRF is successfully demonstrated here for high binding-constant complexes of Fe(III), Co(II), Cu(II), and Zn(II). Within a single injection, electropherograms are obtained for each element of interest, with the element identity obtained directly from the emission energy. In contrast with ICPMS, this detection technique is directly on-line and does not require volatilization of the eluent. As a result, element-specific detection is not limited by the sample or the buffer volatility or atomization efficiency. Simultaneous XRF and UV absorbance detection can be used to provide an on-line determination of metal/chelate ratios. Although XRF detection limits are presently only in the 0.1 mM (0.5 ng) range, both collection geometry and incident intensity have yet to be optimized. Further optimization is expected to enhance this detection limit by another 2-3 orders of magnitude. As a result, the advent of XRF detection combined with the separating power of CE presents new possibilities for on-line, element-specific analysis.

Stable isotopes are used as tracers to follow the fate of elements in organisms. This article describes a technique to determine nitrogen isotope ratios of ammonium in enriched water for isotope dilution experiments using direct-injection cation-exchange HPLC with post-column derivatization and measurement of retention time shifts.

In this study, retention processes in reversed-phase liquid chromatography were examined in situ along an optically transparent column by means of laser-induced fluorescence. Using a homologous series of derivatized fatty acids as model solutes, the capacity factors were systematically measured as a function of temperature in the range from 10 to 60-degrees-C and pressure in the range from 980 to 4,430 psi. A phase transition was observed for the octadecylsilica stationary phase as temperature was decreased and as pressure was increased. From evaluation of its thermodynamic properties during this phase transition, the stationary phase appears to be more similar to a randomly oriented alkane solvent than a highly ordered system such as a phospholipid membrane. Because this phase transition occurs within the range of routine operating conditions in liquid chromatography, it has important implications for both fundamental and practical applications.

A selective fluorescence quenching method using nitromethane as the quenching agent is systematically studied in the absence and presence of absorption effects. Nitromethane is found to quench the emission intensity of polynuclear aromatic hydrocarbons (PAHs) with six-membered rings, while the emission of PAHs with five-membered rings is essentially unaltered. However, fluorescence attenuation caused by primary and secondary absorption may result in errors in the determination of dynamic quenching constants. In this work, a modified Stern-Volmer relationship is developed to distinguish and to compensate mathematically for absorption effects. Utilizing this expression, the Stern-Volmer quenching constant (Kq) is determined to be 125 and 0.15 M−1 for pyrene and fluoranthene, respectively, using nitromethane as the quenching agent in methanol at ambient temperature. Because of the large difference in quenching constants, this analytical methodology is applied for the class-selective identification of PAHs in coal-derived fluids by microcolumn liquid chromatography with laser fluorescence detection.

In liquid chromatographic separations, it is necessary for the solute zone to travel from a nonretentive injection value onto a retentive packed bed. Although it is often assumed that this injection process has little effect on chromatographic performance, experimental evaluation and verification of this important phenomenon have been hindered by the inability to measure fundamental separation parameters directly in the inlet region. With the advent of optically transparent columns and laser-induced fluorescence detection, the direct and accurate measure of the movement and dispersion of solute zones along the column is now feasible. In situ monitoring of the solute zones as they traverse the chromatographic column is accomplished by positioning one detector prior to the packed bed and five detectors directly on the column itself. Upon injection onto the column, a decrease in zone length variance and a concomitant increase in solute concentration are measured as a function of solute capacity factor. Good agreement is seen between experimental measurements and theoretical predictions based on a simple steady-state model of the abrupt change in solute retention in the inlet region. Variation in the injection profile and in the resulting on-column zone profile is also measured as a function of the injection solvent composition. Although solute retention is nearly constant along the column under these conditions, the length variance changes markedly with relatively small changes in the composition of the injection solvent. However, when the injection solvent differes greatly from the mobile phase, changes occur in the zone profile that cannot be predicted on the basis of a simple increase in retention upon injection. These results have important implications in both the routine practice of chromatography, where the composition of the injection solvent is often altered to improve solute resolution, and in the experimental determination of fundamental separation parameters, where the precise control of the zone profile introduced onto the column is essential.

Although the mobile phase is generally considered to be incompressible in liquid chromatography, the local pressure appears to have a significant effect upon solute retention. In this investigation, laser fluorescence detection was utilized to measure solute retention directly along a reversed-phase packed capillary column with a methanol mobile phase. For derivatized fatty acids, n-C10:0 to n-C20:0, the capacity factors increased systematically by +9.3% to +24.4% under normal operating pressure conditions (1,500 to 5,000 psi). Whereas the magnitude of this increase in solute retention seems surprising, it appears to correlate well with the unified theoretical model developed by Martire.

An anomalous response is often observed in high-sensitivity absorbance detection upon a rapid change in solvent composition. This detector artifact appears to be related to refractive index gradients arising when the injection solvent differs from the mobile phase or during solvent programming. Because this anomalous response is often of the same magnitude as the true solute absorbance, misinterpretation of the acquired chromatogram may result. Characterization of this refractive index artifact has been accomplished by modelling the Z-pattern flow cell as a dynamic lens, with constantly changing refractive index profiles both radial and axial to the direction of flow. A ray-tracing algorithm is utilized to predict the final image diameter and the apparent absorbance caused by the changing refractive index conditions. Initial predictions for an ideal, delta function injection show excellent correlation with the experimentally observed response for a commercially available absorbance detector. Simulations are extended to include the more complex axial profiles commonly encountered in liquid chromatography, including those arising from non-ideal injection conditions as well as solvent programming. Predictions based on this model show good agreement with the shape, direction and magnitude of the observed detector response. In addition to aiding in the characterization of solute peaks, this ray-tracing model has direct implications for the accurate interpretation of system peaks in liquid chromatography. Although investigations described here are limited to the Z-pattern cell, this dynamic lens model may be adapted to the evaluation or improved design of any flow cell of interest.

An anomalous signal is often observed upon sample injection in both liquid chromatography and flow-injection analysis using high-sensitivity absorbance detectors. This characteristic detector response, unrelated to sample absorbance, appears to arise from the change in refractive index within the flowcell. The factors affecting the refractive index gradient have been incorporated in a ray-tracing model, where the flowcell is regarded as a dynamic lens. The response predicted by this model correlates well with experimental measurement of the general shape, magnitude, and direction of the refractive index artifact. The proposed model should have wide ranging implications for both flowcell design and chromatographic interpretation of these anomalous signals.

Nonequilibrium interactions, which are known to influence broadening throughout the column, become very important at the exit of the chromatographic column. Theoretical predictions, derived herein, show a substantial increase in length variance and decrease in concentration when the solute is detected off-column. The discrepancy between on-and off-column values for variance and concentration is predicted to increase markedly with capacity factor. Experimental investigation of elution nonequilibrium is accomplished by measuring the length variance and concentation for a homologous series of model solutes both on-and off-column by laser-induced fluorescence. Excellent agreement of experimental measurement with theoretical predictions is seen for solutes with capacity factors ranging from one to nine. These results have interesting implications for the general elution problem in chromatographic separations.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTDual on-column fluorescence detection scheme for characterization of chromatographic peaksChristine E. Evans and Victoria L. McGuffinCite this: Anal. Chem. 1988, 60, 6, 573–577Publication Date (Print):March 15, 1988Publication History Published online1 May 2002Published inissue 15 March 1988https://pubs.acs.org/doi/10.1021/ac00157a016https://doi.org/10.1021/ac00157a016research-articleACS PublicationsRequest reuse permissionsArticle Views45Altmetric-Citations19LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose Get e-Alerts