Nanovolume Gas-Free Hydroxide Eluent Generator for Open Tubular Ion Chromatography.

A gas-free KOH eluent generator (EG) with 210 nL of internal volume is described. It utilizes a two-membrane configuration where there is a single CEM layer on one side and a single BPM layer on the other side for use in open tubular ion chromatography systems with typical back pressures < 50 psi. At a flow rate of ∼190 nL/min, the 10-90% gradient rise time is 3.5 min. The device shows good linearity between applied current and concentration of KOH generated, which is stable over extended periods. The overall system reproducibility (that includes contributions from any changes in flow rate), as judged by the relative standard deviation (RSD) of the retention times of individual separated ions in repeat measurements (n = 6), ranged from <0.5% for isocratic to <1.2% for gradient elution schemes. Perceptible current flow and KOH production in the BPM-based EG begins at subelectrolytic applied voltages, prompting us to look more closely at exact field strength necessary for field-enhanced dissociation of water. An increase in the specific conductance of pure water is noticeable by a field strength of 105 V/m.

Flavin Nitroalkane Oxidase Mimics Compatibility with NOx/TEMPO Catalysis: Aerobic Oxidization of Alcohols, Diols, and Ethers.

Biomimetic flavin organocatalysts oxidize nitromethane to formaldehyde and NOx-providing a relatively nontoxic, noncaustic, and inexpensive source for catalytic NO2 for aerobic TEMPO oxidations of alcohols, diols, and ethers. Alcohols were oxidized to aldehydes or ketones, cyclic ethers to esters, and terminal diols to lactones. In situ trapping of NOx and formaldehyde suggest an oxidative Nef process reminiscent of flavoprotein nitroalkane oxidase reactivity, which is achieved by relatively stable 1,10-bridged flavins. The metal-free flavin/NOx/TEMPO catalytic cycles are uniquely compatible, especially compared to other Nef and NOx-generating processes, and reveal selectivity over flavin-catalyzed sulfoxide formation. Aliphatic ethers were oxidized by this method, as demonstrated by the conversion of (-)-ambroxide to (+)-sclareolide.

Direct Photothermal Measurement of Optical Absorption in a Flow System.

We describe here a simple photothermal detection scheme in a flow stream based on the temperature difference upstream and downstream of the point of illumination. We use a single, two-junction 25 µm diameter thermocouple to measure the temperature change. The baseline standard deviation in the dark is ∼0.001 °C that increases up to 0.0016 °C depending on the illumination source. We demonstrate the detection of several chromatographically separated dyes both with a 1.5 mm and a 0.1 mm i.d. detection cell, respectively, with a white LED and a solid-state laser source. With an inexpensive 660 nm, 19 mW laser as the light source, the estimated detection limit for methylene blue (MB) was 30 nM, corresponding to 120 amol in the illuminated volume. The dimerization constant of MB and the quantum efficiency of the monomer was determined.

Capillary Scale Admittance and Conductance Detection.

Admittance detection, more commonly called capacitively coupled contactless conductivity detection, is widely used. While the true conductance of a solution is linear with concentration up to ∼1 mequiv/L, the admittance signal is nonlinear. In small-bore capillaries and highly resistive solutions, such as in suppressed open tubular ion chromatography (SOTIC), the admittance signal is exponentially related to concentration. Solution contact conductivity detection is common in microfluidics, but no true conductivity measurement efforts have been made with conventional capillaries in the last two decades. We examine five solution contact cell geometries: (A) wire/disk electrodes perpendicular to the flow direction, (B) annular tubular electrodes facing the flow (that exits through the center tube), (C) ring-disk electrodes facing the flow that exits through the annulus, (D) coplanar parallel wire/disk electrodes facing the flow that exits around the electrodes, and (E) planar electrodes separated by a thin insulating layer with a through-hole in which the liquid flows. Present limitations of the available components as well as the designs themselves do not allow type A and B cells to reach low enough dispersion levels. Types C-E all produce the same efficiency for chloride in a SOTIC setup (12000 ± 200 plates/m), limited by dispersion in the suppressor. Further choice and refinement will be dictated by the availability of a lower dispersion suppressor. All such contact conductance results were linear with concentration, and initial results suggest that attainable LODs will be competitive with those from benchtop ICs.