Optical fiber coupled light emitting diode based absorbance detector with a reflective flow cell

We present a versatile, optical fiber coupled light emitting diode (LED) light source based flow-through optical absorbance detector. The LED source is readily changeable. Optical fibers are used to carry light from the electronics/display unit to a reflective flow-through cell and back. The cell can thus be located remotely from the electronics unit and the umbilical connection is not susceptible to electrical noise. The noise level of this detector with LEDs of different emission maxima were observed to be in the range of 3-20 muAU under actual use conditions, with a maximum short term drift of 4 muAU/min after the initial warm-up period. When the analyte absorbance is well matched with the source emission characteristics, the detector response is linear with concentration over at least two orders of magnitude. The liquid flow path through the cell is linear with a large exit aperture such that bubbles are not trapped in the optical path. The optical arrangement is such that the incident light crosses the liquid flow orthogonally and is reflected back by a rear mirror to the receiver fiber. This arrangement reduces the refractive index sensitivity by an order of magnitude relative to conventional Z-path flow cells.     

Geometric constraints at the surfactant headgroup: effect on lipase activity in cationic reverse micelles

The primary objective of the present article is to understand how the geometric constraints at the surfactant head affect the lipase activity in the reverse micellar interface. To resolve this issue, surfactants were designed and synthesized, and activity was measured in /water/isooctane/n-hexanol reverse micellar systems at z ([alcohol]/[surfactant])=5.6, pH 6.0 (20 mM phosphate), 25 degrees C across a varying range of W0 ([water]/[surfactant]) using p-nitrophenylalkanoates as the substrate. It was observed that lipase activity increases from surfactants to with the increment in surface area per molecule (Amin) because of the substitution by the bulky tert-butyl group at the polar head. However, the activity was found to be similar for despite an enhancement in the hydrophilic moieties at the interface. This unchanged lipase activity is presumably due to the comparable surface area of to originating from the rigidity at the surfactant head. Noticeably, the enzyme activity improved from with the simultaneous increment of both the hydroxyl group and the flexibility of the headgroup whereas that for increased exclusively with the flexibility of the headgroup. The common parameter in both groups of surfactants and is the flexibility of the headgroup, which possibly enhance Amin and consequently the lipase activity. Thus, the geometric constraints at the surfactant headgroup play a crucial role in modulating the lipase activity profile probably because of the variation in interfacial area.     

Computational elimination of interelemental interference in the analysis of copper, cobalt and nickel in polymetallic sea bed nodules by flame atomic absorption spectrometry

A method has been described for the estimation of Cu, Co and Ni in polymetallic sea bed nodule samples by flame atomic absorption spectrometry using single component calibration standards. Interelemental interference plays a crucial role in the estimation of Cu, Co and Ni by flame AAS. Calibration by multicomponent matrix matching standards often fails because of wide variations in the compositions of actual samples. Cu, Co and Ni values are depressed in the presence of appreciable amounts of Fe and Mn present in the sea nodule matrix. Observed interferences have been eliminated computationally using a two parameter theoretical model. The method has been successfully applied for the estimation of Cu, Co and Ni in real life sea nodule samples by flame AAS using single component calibration standards.     

Molecular dynamics for very large systems on massively parallel computers: The MPSim program

We describe the implementation of the cell multipole method (CMM) in a complete molecular dynamics (MD) simulation program (MPSim) for massively parallel supercomputers. Tests are made of how the program scales with size (linearly) and with number of CPUs (nearly linearly) in applications involving up to 10⁷ particles and up to 500 CPUs. Applications include estimating the surface tension of Ar and calculating the structure of rhinovirus 14 without requiring icosahedral symmetry. © 1997 by John Wiley & Sons, Inc.     

Understanding the Influence of Donor-Acceptor Diazo Compounds on the Catalyst Efficiency of B(C6F5)3 Towards Carbene Formation

Diazo compounds have been largely used as carbene precursors for carbene transfer reactions in a variety of functionalization reactions. However, the ease of carbene generation from the corresponding diazo compounds depends upon the electron donating/withdrawing substituents either side of the diazo functionality. These groups strongly impact the ease of N₂ release. Recently, tris(pentafluorophenyl)borane [B(C₆F₅)₃] has been shown to be an alternative transition metal-free catalyst for carbene transfer reactions. Herein, a density functional theory (DFT) study on the generation of carbene species from α-aryl α-diazocarbonyl compounds using catalytic amounts of B(C₆F₅)₃ is reported. The significant finding is that the efficiency of the catalyst depends directly on the nature of the substituents on both the aryl ring and the carbonyl group of the substrate. In some cases, the boron catalyst has negligible effect on the ease of the carbene formation, while in other cases there is a dramatic reduction in the activation energy of the reaction. This direct dependence is not commonly observed in catalysis and this finding opens the way for intelligent design of this and other similar catalytic reactions.     

Multilayer chitosan-based open tubular capillary anion exchange column with integrated monolithic capillary suppressor

We describe a multilayered open tubular anion exchange column fabricated by alternately pumping solutions of chitosan and glutaraldehyde. The column is terminated in an integrally bonded monolithic suppressor cast around a mandrel of a tungsten wire, composed of an acrylic acid (AA)-ethylene dimethacrylate (EDMA) monolith that is made with sufficient porogen for the monolith to function as a membrane. For a 4.5m long 75 μm bore column coated with 24 successive layers of the condensation polymer (estimated to contain ~72 molecular layers) and coupled to 1cm length of a suppressor fabricated with 55-60% AA, effective separation of several common anions (F(-), Cl(-), NO(2)(-), Br(-), NO(3)(-), average number of theoretical plates ~12,000) and adequate suppression of 1 mM KOH used as eluent was observed at a flow rate of 800 nL min(-1) to obtain sub-picomol detection limits at an operating pressure of ~1 bar. The separation is not time efficient but the system can be meritorious in unique niche applications where a small form factor is desired and liquid volume and power consumption are more important than separation speed.     

Central limit theorems for a class of irreducible multicolor urn models

We take a unified approach to central limit theorems for a class of irreducible multicolor urn models with constant replacement matrix. Depending on the eigenvalue, we consider appropriate linear combinations of the number of balls of different colors. Then under appropriate norming the multivariate distribution of the weak limits of these linear combinations is obtained and independence and dependence issues are investigated. Our approach consists of looking at the problem from the viewpoint of recursive equations.     

Double Coil Resonance Experiment and Partial Reduction of Wave Packet in Quantum Mechanics

Interference and measurement aspects for the double coil resonance experiment are reanalyzed. The resulting situation is analogous to partial reduction of wave packet in quantum mechanics. Using convergence results of relative frequencies, magnitudes of the intensity are calculated when prior probabilities are assigned to the coefficients associated with the states.