Christine E. Evans

Improved chromatographic analysis of 15N:14N ratios in ammonium or nitrate for isotope addition experiments

Abstract Estimating nitrogen transformation rates in aquatic ecosystems by isotope dilution techniques is simplified by directly measuring nitrogen isotopic ratios for NH4+ in the water using high performance cation exchange liquid chromatography (HPLC). Modifications of HPLC conditions and implementation of a median-area method for retention time determination improved and linearized a previously reported sigmoid relationship between the retention time shift (RTshift) of the NH4+ peak and the ratio of [15NH4+]: [Total NH4+] in seawater fortified with 15NH4+. Increasing the temperature of the HPLC column from 47 to 85 °C increased mobile phase buffer flow rate relative to column back pressure, decreased the retention time for NH4+, and allowed the buffer pH to be optimized relative to the pK of NH4+. The use of median-area rather than maximum-height to define the retention time of NH4+ further improved the linearity (r > 0.995) of the relationship between the ratio [15NH4+]: [Total NH4+] and RTshift over the range of isotope ratios. Reduction of NO3− to NH4+ by adding zinc dust to acidified (pH 2) seawater or lakewater samples, followed by pH neutralization, and subsequent analysis of NH4+ isotope ratios by HPLC, extended application of the method to isotope dilution experiments with NO3−. Advantages of this direct-injection method over mass-measurement approaches traditionally used for isotope dilution experiments include small sample size and minimal sample preparation.

Diacetylene polymerization in self-assembled monolayers: influence of the odd/even nature of the methylene spacer

Abstract In this study, the interdependence between molecular architecture and photo-polymerization is evaluated for self-assembled monolayers containing diacetylenes. While such assemblies provide added robustness and versatility in molecular design, direct surface-attachment significantly limits the degrees of freedom within the monolayer structure. As a result, the polymer conjugation length as evidenced by the chromatic properties cannot be directly predicted from the extensive literature on Langmuir–Blodgett (LB) films. Indeed, a clear odd/even effect is observed in the polymerization properties of these surface-confined monolayers that is not evident in LB systems. Odd-numbered methylene spacers between the surface and the polymer backbone lead to the longer conjugation length blue form, whereas even-numbered spacers yield the shorter conjugation length red phase. This observation is consistent with simple modeling that indicates a release in localized strain upon polymerization for the odd-numbered spacer architectures. While spacers of both 5 and 9 methylene unit yield similar conversion efficiency to the blue-phase polymer, the kinetic profiles with UV exposure are quite different. The longer conversion times for the shorter spacer are attributed to lower freedom of movement within the monolayer to achieve the spatial alignment required for the topochemical polymerization.

Effect of pressure-induced ionization, partitioning, and complexation on solute retention in reversed-phase liquid chromatography

Abstract In contrast to supercritical fluid chromatography, pressure is not commonly considered an important parameter affecting solute retention in liquid chromatography. While it is true that the bulk compressibility of polar mobile phases is minimal for the modest pressures encountered in reversed-phase LC (

The synthesis of conjugated diacetylene monomers for the fabrication of polymerized monolayer assemblies

Abstract Symmetrical disulfides containing conjugated diacetylene groups for use in forming polymerized self-assembled monolayers have been fabricated using common synthetic techniques. In contrast with previous reports, the proposed synthetic route does not require the use of hexamethylphosphoramide (HMPA) or the incorporation of an ester moiety near the surface attachment site. Overall yields of 20–30% are readily achieved for this four-step synthesis.

Dual-Wavelength Resonance Raman Spectroscopy of Polydiacetylene Monolayers on Au Surfaces

In this paper, resonance Raman spectroscopy with dual-wavelength laser excitation is applied to study the chromatic phase properties of self-assembled diacetylene monolayers on atomically flat Au surfaces. Laser excitation at 632.8 nm is utilized to exclusively monitor the longer conjugation length blue-phase polydiacetylene (PDA) backbone structure, while laser excitation at 514.5 nm is applied to probe the shorter conjugation length red phase. This experimental design allows in situ measurement of the photophysics and chemistry of the polymerization process within a single molecular layer. These studies demonstrate the presence of multiple chromatic species of PDAs within the same monolayer structure. Similar to observations for analogous thin film and multilayer assemblies, systematic conversion from the higher conjugation length blue form of the polymer to the shorter conjugation length red form is observed upon extended UV irradiation. Moreover, a very large Raman dispersion of 130 cm−1/eV is measured, demonstrating the highly conjugated nature of these monolayer polymers. Finally, backbone planarity and strain during the polymerization process are assessed by using the correlation of double- and triple-bond stretching frequencies.

Effect of mobile‐phase composition on pressure‐induced shifts in solute retention for LC separations using β‐cyclodextrin stationary phases

Modest pressure - 350 bars has a significant impact on reversed-phase liquid chromatographic retention using b-cyclodextrin stationary phase. Demon- strated here for separations of the positional isomers of nitrophenol and naphthol as well as the enantiomers of hexobarbital and mephobarbital, the effect of small pressure perturbations on solute capacity factor ranges from q11 to y14%. These changes in retention arise from pressure-induced shifts in solute equilibria and are used to estimate the change in molar volume upon solute binding with b-cyclo- . dextrin DV . In this article, the effect of the mobile phase on pressure-induced comp shifts in solute retention is examined. Mobile-phase composition is observed to have a significant influence on the change in solute capacity factor with pressure and therefore on DV . As the composition of water in the mobile phase comp increases, the change in partial molar volume of complexation is observed to become more positive for both positional isomer and chiral separations. Although these studies imply that changes in solvent composition have a general effect on the observed DV and therefore on the change in pressure perturbation with comp

On-line X-ray fluorescence detection for capillary electrophoresis separations

In these investigations, capillary electrophoresis with on-line X-ray fluorescence detection (CE-XRF) has been demonstrated for the first time. The insertion of a polyethylene sample cell between fused-silica capillary segments enabled continuous XRF detection during electrophoresis with minimal additional band broadening. Detection limits in the 10−4 M range are currently feasible for the CE-XRF separation of metal complexes, and design advances will enhance detectability to the 10−5–10−6 M range, permitting studies of important environmental and biological samples.

Pressure-controlled high-performance liquid chromatographic study on the influence of rim chemistry on partial molar volume differences between free and complexed cyclodextrins

Pragmatic comparison of pressure dependent retention for differing cyclodextrin rim chemistries is assessed using controlled-pressure HPLC. For pressure differences of <300 bar, systematic shifts in solute capacity factor are observed for both native and methylated beta-cyclodextrin stationary phases. In addition to the importance of this observation for the practice of liquid chromatography, this technique can also be implemented in the fundamental determination of the influence of rim chemistry on the cyclodextrin partial molar volume both with and without solute inclusion. That is, pressure-controlled measurements provide a direct comparison between the partial molar volumes for native cyclodextrin (CD) and methylated cyclodextrin (MCD) in the presence and absence of the complexing solute (comp). Surprisingly, direct comparison of the measured partial molar volumes for the two rim chemistries indicates that the presence of neutral solutes does not contribute significantly to the volumetric component of complexation, V(comp,CD) - V(comp,MCD) approximately V(CD) - V(MCD). In contrast, their ionized counterparts are shown to exhibit marked rim chemistry differences in the partial molar volume of cyclodextrins with and without anion inclusion, V(comp(-),CD) - V(comp(-),MCD) < V(CD) - V(MCD). Not previously demonstrated by direct chromatographic measurement, these results have interesting implications for advancing the fundamental understanding of host-guest solvation properties.