Electrodes with electrochemical dimensions as small as 10 angstroms have been fabricated and used for electrochemical studies. These nanometer-scale electrodes have enabled the measurement of electron-transfer rate constants, k(het), that are two orders of magnitude faster than k(het) values accessible with any other electrochemical method.

A steady-state model describing photofacilitated transport in liquid membranes is presented. The model can be used to consider photoactive carriers with a wide range of thermodynamic and kinetic properties in order to calculate photoinduced transport of solutes down their concentration gradient (photomodulation) and against their concentration gradient (photopumping). The description of transport in these systems is generalized by combining the relevant physical constants (diffusion coefficients, rate constants, concentrations, molar absorptivities, light intensity, etc.) into dimensionless parameters. Detailed descriptions of photomodulation and photopumping are presented for the case where the carrier has properties that are optimal for downhill transport in the dark (thermal transport). Absorption of light by the carrier and carrier solute complex can cause downhill transport to increase by 50% or to decrease by a factor of 4 depending on the light intensity. Photopumping can be maintained against a 10-fold concentration gradient. The transport efficiencies for photomodulation and photopumping are also discussed.

Extractive electrospray ionization mass spectrometry (EESI-MS) has been shown, in other laboratories, to be a useful technique for the analysis of aerosols from a variety of sources. EESI-MS is applied here, for the first time, to the analysis of secondary organic aerosol (SOA) formed from the reaction of ozone and a-pinene. The results are compared to those obtained using atmospheric pressure chemical ionization mass spectrometry (APCI-MS). The SOA was generated in the laboratory and merged with electrospray droplets. The recovered ions were directed towards the inlet of a triple quadrupole mass spectrometer. Through the use of a denuder to remove gas phase compounds, the EESI-MS technique was found to be effective for measuring the major ozonolysis products either in particles alone or in a combination of vapor phase and particulate products. Due to its relatively simple setup and the avoidance of sample collection and work-up, EESI-MS shows promise as an excellent tool for the characterization of atmospherically relevant particles.

Real-time in situ mass spectrometry analysis of airborne particles is important in several applications, including exposure studies in ambient air, industrial settings, and assessing impacts on visibility and climate. However, obtaining molecular and 3D structural information is more challenging, especially for heterogeneous solid or semisolid particles. We report a study of extractive electrospray ionization mass spectrometry (EESI-MS) for the analysis of solid particles with an organic coating. The goal is to elucidate how much of the overall particle content is sampled, and determine the sensitivity of this technique to the surface layers. It is shown that, for NaNO particles coated with glutaric acid (GA), very little of the solid NaNO core is sampled compared to the GA coating, whereas for GA particles coated with malonic acid (MA), significant signals from both the MA coating and the GA core are observed. However, conventional ESI-MS of the same samples collected on a Teflon filter (and then extracted) detects much more core material compared to EESI-MS in both cases. These results show that, for the experimental conditions used here, EESI-MS does not sample the entire particle but, instead, is more sensitive to surface layers. Separate experiments on single-component particles of NaNO, GA, or citric acid show that there must be a kinetics limitation to dissolution that is important in determining EESI-MS sensitivity. We propose a new mechanism of EESI solvent droplet interaction with solid particles that is consistent with the experimental observations. In conjunction with previous EESI-MS studies of organic particles, these results suggest that EESI does not necessarily sample the entire particle when solid, and that not only solubility but also surface energies and the kinetics of dissolution play an important role.

A steady-state model describing photofacilitated transport in liquid membranes is used to consider photoactive carriers with a wide range of thermodynamic and kinetic properties in order to investigate active transport against a concentration gradient (photopumping). Most experimental and theoretical studies have focused on systems in which strongly binding forms of the carrier and carrier−solute complex absorb light to convert to weakly binding forms. For such systems, absorption of light can result in transport against a concentration gradient greater than a factor of 5. However, the maximum photoefficiencies are less than 0.01%. Membranes containing carriers in which the weak forms of the carrier and complex are photoactive exhibit similar photopumping performance with an order of magnitude higher photoefficiencies. This study provides guidelines for the selection of carriers for future experimental investgations of photofacilitated liquid membranes.

A steady-state model describing photofacilitated transport in liquid membranes is used to consider photoactive carriers with a wide range of thermodynamic and kinetic properties in order to calculate photoinduced modulation in the transport rates of solutes, a phenomenon known as photomodulation. Most experimental and theoretical studies have focused on systems in which strongly binding forms of the carrier and carrier−solute complex absorb light to convert to weakly binding forms. For such systems, absorption of light can cause photomodulation increases up to a factor of 3. However, the maximum photoefficiencies are only a few percent, where photoefficiency is defined as the difference between the flux under illumination and in the dark divided by the incident photon flux. Conversely, if the weakly binding forms of the carrier and complex are photoactive, photomodulation increases of a factor of 20 can be achieved, along with photoefficiencies well over 10%. This study provides guidelines for the selection of carriers for future experimental investigations of photofacilitated liquid membranes.

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A steady-state model describing photofacilitated transport in liquid membranes under double illumination is presented. The model allows for the exploration of the effects of a wide range of thermodynamic and kinetic carrier properties on the control of photoinduced transport rates of solutes, called photomodulation. Most previous experimental and theoretical studies have explored the illumination of only the feed or sweep side of the membrane, while this study examines the effects of illuminating both sides simultaneously. Under double illumination, solute transport rates can be as much as five times greater than those measured in the dark and 2.5 times greater than rates obtained under single illumination. Carriers that are predominantly in the weakly binding form in the dark generally provide slightly better performance at lower light intensities than do carriers that are predominantly in the strongly binding form in the dark. The greatest enhancement in solute transport under double illumination is seen for carriers with very slow interconversion rate constants between the strongly and weakly binding forms. These results provide guidelines to help those studying photofacilitated membranes select or design photoactive molecules that will act as optimal carriers in liquid membranes under double illumination.