Stochastic methods for aerosol chemistry: a compact molecular description of functionalization and fragmentation in the heterogeneous oxidation of squalane aerosol by OH radicals.

The heterogeneous oxidation of organic aerosol by hydroxyl radicals (OH) can proceed through two general pathways: functionalization, in which oxygen functional groups are added to the carbon skeleton, and fragmentation, in which carbon-carbon bonds are broken, producing higher volatility, lower molecular weight products. An ongoing challenge is to develop a quantitative molecular description of these pathways that connects the oxidative evolution of the average aerosol properties (e.g. size and hygroscopicity) to the transformation of free radical intermediates. In order to investigate the underlying molecular mechanism of aerosol oxidation, a relatively compact kinetics model is developed for the heterogeneous oxidation of squalane particles by OH using free radical intermediates that convert reactive hydrogen sites into oxygen functional groups. Stochastic simulation techniques are used to compare calculated system properties over ten oxidation lifetimes with the same properties measured in experiment. The time-dependent average squalane aerosol mass, volume, density, carbon number distribution of scission products, and the average elemental composition are predicted using known rate coefficients. For functionalization, the calculations reveal that the distribution of alcohol and carbonyl groups is controlled primarily by the initial OH abstraction rate and to lesser extent by the branching ratio between secondary peroxy radical product channels. For fragmentation, the calculations reveal that the formation of activated alkoxy radicals with neighboring functional groups controls the molecular decomposition, particularly at high O/C ratios. This kinetic scheme provides a framework for understanding the oxidation chemistry of a model organic aerosol and informs parameterizations of more complex systems.

Oxidation of a model alkane aerosol by OH radical: the emergent nature of reactive uptake.

An accurate description of the evolution of organic aerosol in the Earth's atmosphere is essential for climate models. However, the complexity of multiphase chemical and physical transformations has been challenging to describe at the level required to predict aerosol lifetimes and changes in chemical composition. In this work a model is presented that reproduces experimental data for the early stages of oxidative aging of squalane aerosol by hydroxyl radical (OH), a process governed by reactive uptake of gas phase species onto the particle surface. Simulations coupling free radical reactions and Fickian diffusion are used to elucidate how the measured uptake coefficient reflects the elementary steps of sticking of OH to the aerosol as a result of a gas-surface collision, followed by very rapid abstraction of hydrogen and subsequent free radical reactions. It is found that the uptake coefficient is not equivalent to a sticking coefficient or an accommodation coefficient: it is an intrinsically emergent process that depends upon particle size, viscosity, and OH concentration. An expression is derived to examine how these factors control reactive uptake over a broad range of atmospheric and laboratory conditions, and is shown to be consistent with simulation results. Well-mixed, liquid behavior is found to depend on the reaction conditions in addition to the nature of the organic species in the aerosol particle.

Method of measuring the spatial resolution of a photoresist.

By analysis of the response of a high-contrast photoresist to sinusoidal illumination, generated interferometrically, one can extract a phenomenological modulation transfer function of the resist material, thereby characterizing its spatial resolution. Deep-ultraviolet interferometric lithography allows the resist response to be quantified at length scales below 100 nm. As an example, the resolution (FWHM) of the commercial resist UVII-HS is found to be approximately 50 nm. This simple method can be applied to materials under development for advanced photolithography with short-wavelength illumination.

Light-directed synthesis of high-density oligonucleotide arrays using semiconductor photoresists.

High-density arrays of oligonucleotide probes are proving to be powerful new tools for large-scale DNA and RNA sequence analysis. A method for constructing these arrays, using light-directed DNA synthesis with photo-activatable monomers, can currently achieve densities on the order of 10(6) sequences/cm2. One of the challenges facing this technology is to further increase the volume, complexity, and density of sequence information encoded in these arrays. Here we demonstrate a new approach for synthesizing DNA probe arrays that combines standard solid-phase oligonucleotide synthesis with polymeric photoresist films serving as the photoimageable component. This opens the way to exploiting high-resolution imaging materials and processes from the microelectronics industry for the fabrication of DNA probe arrays with substantially higher densities than are currently available.

Role of appendages in skin resistance and iontophoretic peptide flux: human versus snake skin.

PURPOSE: 1. The assessment of the role of hair follicles and sweat glands in skin resistance and percutaneous iontophoretic flux of 9-desglycinamide, 8-arginine vasopressin (DGAVP) by comparing two skin species: human stratum corneum which contained hair follicles, sweat and sebaceous glands, and shed snake skin which lacked all appendages. 2. The effect of 1-dodecylazacycloheptan-2-one (dodecyl-Azone, a lipid perturbing agent) on the iontophoretic DGAVP flux. METHODS: Iontophoresis in vitro was performed in a transport cell (0.79 cm2 area available for percutaneous transport) by 8-hours application of a pulsed constant current of 100 Hz, 50% duty cycle and 0.26 mA.cm-2 current density delivered by a pair of Ag/AgCl electrodes, of which the anode was facing the anatomical surface of the skin samples. RESULTS: The initial resistances of human stratum corneum and shed snake skin samples were of the same order of magnitude (20-24 k omega.cm2) and both skin species showed a comparable resistance-decrease profile during 8-hours iontophoresis, indicating that the resistances were mainly determined by the stratum corneum and not greatly influenced by the appendageal structures. The initial resistances of the skin samples pretreated with dodecyl-azone were less than 50% of the values of untreated samples. Because dodecylazone is known to perturb the ordering of the intercellular lipids, the effect of azone on the resistance confirms that the resistance mainly resides within the intercellular lipids of the stratum corneum. No correlation was found between the iontophoretic DGAVP-flux and the conductance of human skin. For shed snake skin, however, a good correlation was found, indicating that the iontophoretic permeability of human skin in vitro for a peptide such as DGAVP is, unlike shed snake skin, not related to its overall permeability to ions. While the initial resistances of both human and snake skin were in the same order of magnitude and showed the same declining profile during iontophoresis, the steady state iontophoretic DGAVP flux across human stratum corneum was approximately 140 times larger than through shed snake skin. These findings suggest that small ions follow pathways common to both skin types, presumably the intercellular route, while the peptide on the other hand is transported differently: across snake skin presumably along intercellular pathways only, but across human stratum corneum along additional pathways (most likely of appendageal origin) as well. This interpretation is supported by the observations made of the effects of dodecyl-azone on DGAVP-iontophoresis. Pretreatment with dodecyl-azone did not significantly change steady state fluxes and lag times of DGAVP-iontophoresis across human stratum corneum, but resulted in a significant 3-fold lag time decrease and a 3-fold flux increase of DGAVP-iontophoresis across snake skin. CONCLUSIONS: The results of these in vitro studies emphasize the importance of the appendageal pathway for iontophoretic peptide transport across human stratum corneum.

Iontophoresis of a model peptide across human skin in vitro: effects of iontophoresis protocol, pH, and ionic strength on peptide flux and skin impedance.

This study deals with effects of electrical (current density, frequency and duty cycle) and chemical (buffer pH and ionic strength) conditions on the flux of the octapeptide, 9-desglycinamide, 8-arginine-vasopressin (DGAVP), through dermatomed human skin. A pulsed constant current was applied during iontophoresis. The anode faced the anatomical surface of the skin samples inside the diffusion cells. The resistive and capacitative components of the equivalent electrical circuit of human skin could be calculated by fitting the voltage response to a bi-exponential equation. The skin resistance prior to iontophoresis varied between 20 and 60 k omega.cm2. During iontophoresis a decrease of skin resistance and an increase of the series capacitances was observed, which were most pronounced during the first hour of iontophoresis; thereafter both quantities gradually levelled off to an apparent steady state value. The reduction of the resistance during iontophoresis increased non-linearly with increasing current density between 0.013-0.64 mA.cm-2. The steady state resistance and capacitances did not vary significantly with frequency and duty cycle of the current pulse. There was no pH dependence of skin resistance at steady state. Between pH 4 and 10, the steady state peptide flux had a bell-shaped pH-dependence with a maximum of 0.17 nmol.cm-2.h-1 at pH 7.4, which is close to the I.E.P. of the peptide. Lowering the ionic strength from 0.15 to 0.015 M NaCl increased the steady state flux at pH 5 and pH 8 by a factor 5 to 0.28 +/- 0.21 and 0.48 +/- 0.37 nmol.cm-2.h-1, respectively. Together these observations suggested that DGAVP is transported predominantly by volume flow.(ABSTRACT TRUNCATED AT 250 WORDS)

Direct measurement of antigens in serum by time-resolved fluoroimmunoassay.

A long-lived fluorescence label (Tb3+) has been attached to the antigen of interest by using a bifunctional chelating agent 1-(p-benzenediazonium)-EDTA. A nonequilibrium competitive-binding immunoassay protocol, in conjunction with time-resolved detection of the long-lived fluorescence label, allows the antigen to be analyzed directly in samples containing diluted human serum. Results obtained for immunoglobulin G with this simple and rapid procedure correlated well (r = 0.93) with those by a commercially available fluorescence immunoassay method.

Enzyme-linked sandwich immunoassay for insulin using laser fluorimetric detection.

Human serum samples are assayed for insulin by an enzyme-linked sandwich immunoassay. Horseradish peroxidase is used as an enzyme label for antibody, and enzyme activity is measured by means of the fluorogenic substrate, p-hydroxyphenylacetic acid. The product is detected by excitation of fluorescence with the 325-nm line of a continuous-wave helium/cadmium ion laser on line with reverse-phase high-pressure liquid chromatography. The incubation period is 90 min and the limit of detection of insulin is 30 pM, corresponding to 5 microunits/ml. This method correlates highly with radioimmunoassay, with coefficient of correlation r = 0.95.