Separation and detection of VX and its methylphosphonic acid degradation products on a microchip using indirect laser-induced fluorescence

The application of indirect LIF (IDLIF) technique for on-chip electrophoretic separation and detection of the nerve agent O-ethyl S-[2-(diisopropylamino)ethyl] methylphosphonothiolate (VX) and its major phosphonic degradation products, ethyl methylphosphonic acid (EMPA) and methylphosphonic acid (MPA) was demonstrated. Separation and detection of MPA degradation products of VX and the nerve agent isopropyl methylphosphonofluoridate (GB) are presented. The negatively charged dye eosin was found to be a good fluorescent marker for both the negatively charged phosphonic acids and the positively charged VX, and was chosen as the IDLIF visualization fluorescent dye. Separation and detection of VX, EMPA, and MPA in a simple-cross microchip were completed within less than a minute, and consumed only a 50 pL sample volume. A characteristic system peak that appeared in all IDLIF electropherograms served as an internal standard that increased the reliability of peak identification. The negative peak of both VX and the MPAs is in agreement with indirect detection theory and with previous reports in the literature. The LOD of VX and EMPA by IDLIF was 30 and 37 microM, respectively. Despite the fact that the detection sensitivity is relatively low, the rapid simultaneous on-chip analysis of both VX and its degradation products as well as the separation and detection of the MPA degradation products of both VX and GB, increases detection reliability and may present a choice when sensitivity is not critical compared with speed and simplicity of the assay.     

Nonlinear problem of an electrical discharge in a flow

The nonlinear intial boundary value problem describing the relaxation of a quasi-neutral discharge in a gas flow with coplanar, heated and nonheated electrodes of finite extension is formulated in the diffusion approximation. Relaxation occurs from an initial breakdown to a steady-state or zero discharge in a weak electric field. A nonlinear transformation is applied to get an equivalent nonlinear problem, where nonlinearity is treated as a small perturbation. An analytic solution is obtained and criterions for existence and sustainment of a steady-state discharge against plasma losses due to convection, diffusion and recombination is discussed. Some numerical results are exhibited.     

The role of the cation in the oxygen isotopic exchange in crystalline sulfate salt hydrates

The oxygen isotopic exchange during dehydration and decomposition of five sulfate salt hydrates (CoSO₄·6H₂O, NiSO₄·7H₂O, ZnSO₄·7H₂O, CaSO₄·2H₂O, Li₂SO₄·H₂O) was studied in detail by temperature programmed desorption mass spectrometry (TPD-MS) in a supersonic molecular beam (SMB) inlet mode. Crystals of the ¹⁸O-enriched salts were grown and the detailed desorption steps of the various gaseous products released during dehydration and decomposition of these compounds were recorded. The desorption patterns confirmed the known characteristic stepwise dehydration of these salts, where regardless of the crystalline structure and composition, in all the salts (excluding the Li and Ca sulfates) a major group of n ? 1 loosely bounded water of crystallization molecules (out of total of n molecules in the fully hydrated form) are released at adjacent temperatures in a typical low temperature range (<200 °C), while the last, most strongly bounded water molecule, consistently desorbs at relatively higher temperatures (240 < T < 440 °C). Interestingly, it is established that the oxygen isotopic exchange occurs exclusively between that latter, most strongly bound water molecule, and the salt anion. Remarkably, the results point out that the exchange process is mostly of solid-solid nature. Finally, the results point out that the probability of the isotopic exchange increases with the increment in the desorption temperature of the last dehydration step, i.e. with the bond strength in the monohydrate, between the last water molecule of crystallization and the cation.     

Time-dependent viscous flow in a straight channel

An infinite straight channel, filled with an incompressible viscous fluid, is closed at one end by a piston. This is set in motion with finite acceleration and then maintained at constant velocity until the flow pattern in the fluid reaches a steady state. The development of velocity profiles, stream lines, and streak lines is investigated by direct numerical solution of the complete Navier Stokes equations. It is found that the nonconvex velocity profiles found in previous work on steady-state problems appear from the beginning, and their development is studied. In the downstream region alternative methods can be used which serve as a check on the accuracy of the numerical procedures. The asymptotic behaviour downstream is studied in some detail.Nomenclature a acceleration of piston - f(t) nondimensional distance travelled by piston up to time t - 2l width of channel - p pressure (in units of u ₀ ² ) - R Reynolds number, lu ₀/ - t ₀ time during which piston is accelerated - u ₀ final velocity of piston - (u, v) (x, y) components of fluid velocity (relative to piston, in units of u ₀) - x distance measured downstream from piston (in units of l) - y distance from central axis of channel (in units of l) - vorticity - density of fluid - kinematic viscosity - stream function     

Equilibrium arrival times to a queue with order penalties

Suppose customers need to choose when to arrive to a congested queue with some desired service at the end, provided by a single server that operates only during a certain time interval. We study a model where the customers incur not only congestion (waiting) costs but also penalties for their index of arrival. Arriving before other customers is desirable when the value of service decreases with every admitted customer. This may be the case for example when arriving at a concert or a bus with unmarked seats or going to lunch in a busy cafeteria. We provide game theoretic analysis of such queueing systems with a given number of customers, specifically we characterize the arrival process which constitutes a symmetric Nash equilibrium.     

Surface-enhanced Raman scattering detection of cholinesterase inhibitors

A new sensitive surface-enhanced Raman scattering (SERS) assay for detection of cholinesterase inhibitors such as organophosphorous pesticides using silver colloidal nanoparticles was developed and optimized. Acetylcholinesterase (AChE) mediated the hydrolysis of acetylthiocholine to produce thiocholine, which interacted with the silver nanoparticles to give a specific SERS spectrum. Variation in enzyme activity due to inhibition was measured from changes in intensity of a characteristic peak (772 cm⁻¹) of the SERS spectrum that was directly correlated with the concentration of produced thiocholine. The method was demonstrated for the detection of paraoxon as reference AChE inhibitor. Limit of detection of paraoxon for 5 min incubation at 25 °C was 1.8 × 10⁻⁸ M. This assay can be utilized for the detection of trace amounts of any AChE inhibitor.     

Dormant versus Evolving Aminopalladated Intermediates: Toward a Unified Mechanistic Scenario in PdII‐Catalyzed Aminations

Pd^II‐catalyzed alkene aminopalladation and allylic C?H activation are two competing reaction sequences sharing the same reaction conditions. This study aimed at understanding the factors that bias one or the other path in the intramolecular oxidative cyclization of two types of N‐tosyl amidoalkenes. The results obtained are in accord with the initial generation of a high‐energy cyclic (5‐ or 6‐membered) aminopalladated intermediate. However, this latter species can evolve only if the following specific conditions are met: the availability of distocyclic β‐H elimination pathway, the presence of a strong terminal oxidant, or the availability of a carbopalladation pathway. Conversely, the cyclic alkylpalladium complex is only a latent species in equilibrium with the initial substrate and cannot evolve. Such a reactivity hurdle leaves the way open for alternative reactivities such as allylic C?H activation of the olefinic substrate to generate a η³‐allyl complex followed by its interception by the nitrogen nucleophile, [3,3]‐sigmatropic rearrangement, or decomposition. This study proposes a unifying mechanistic picture that connects these competing mechanisms.