Synchronous fluorescence for simultaneous determination of hydroquinone and resorcinol in air samples

Several phenolic compounds are present in tobacco smoke. They are formed from the pyrolysis of tobacco during the smoking process and all of them are toxic. Therefore, the determination of these compounds in air samples is important. A rapid, simple, and sensitive method using a synchronous spectrofluorimetry technique was developed to quantify hydroquinone and resorcinol simultaneously. One of the advantages of this method is the simple and rapid sampling technique, which uses water as the absorption solution of the analytes in the air sample. The precision of the method (%RSD) was 1.8% and the detection limits were 0.125 mg m^–3 and 0.292 mg m^–3 for hydroquinone and resorcinol, respectively.     

Molecular dynamics study of polymeric chemical gels: Effect of persistence length and crosslinker density

Using molecular dynamics, we study the formation of chemical gels from an initial solution of reactive polymers that undergo a crosslinking reaction. We study the effect of the polymer persistence length and different densities of crosslinkers along the chains. As the reaction progresses, different structural features are identified in the system leading to the development of a percolated cluster. These features are (a) single strands, (b) double strands, and (c) bridges. We found that the total numbers of these three kinds of features are roughly independent of the persistence length; however, the average lengths of single and double strands grow with this variable. The average length of double strands strongly increases with increasing crosslinker density and the amount of single strands sharply falls as crosslinker density grows. We also found that general structural features of polymer networks are highly dependent on chain persistence length and crosslinker density. Fully flexible chains with high density of crosslinkers result in inhomogeneous network structures with large voids. In contrast, precursor chains with high rigidity and scarce number of crosslinkers result in homogeneous networks having small cavities. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1343–1350     

Changes in the spectroelectrochemical properties of copper(II) hexacyanoferrate(III) during electrochemical insertion of alkaline ions

Journal of Solid State Electrochemistry - In this work, the influence of the electrochemical insertion of different alkali ions, e.g., lithium, sodium, and potassium, on the electrochromic...     

Nuclear, chemical, and physical characterization of nuclear materials

The goal of nuclear forensics is to establish an unambiguous link between illicitly trafficked nuclear material and its origin. The Los Alamos National Laboratory (LANL) Nuclear Materials Signatures Program has implemented a graded “conduct of operations” type analysis flow path approach for determining the key nuclear, chemical, and physical signatures needed to identify the manufacturing process, intended use, and origin of interdicted nuclear material. This analysis flow path includes both destructive and non-destructive characterization techniques and has been exercized against different nuclear materials from LANL’s special nuclear materials archive. Results obtained from the case study will be presented to highlight analytical techniques that offer the critical attribution information.     

Mesh selection for discrete solution of boundary problems in ordinary differential equations

Numerische Mathematik - In order to use finite difference approximations with non-uniform meshes in boundary value problems, it is necessary to develop procedures for mesh selection. In this paper...     

A Note on a Maximal Function Over Arbitrary Sets of Directions

Let M_S be the universal maximal operator over unit vectors ofarbitrary directions. This operator is not bounded in L²(R²).We consider a sequence of operators over sets of finite equidistributeddirections converging to M_S. We provide a new proof of N. Katz'sbound for such operators. As a corollary, we deduce that M_Sis bounded from some subsets of L² to L². These subsets arecomposed of positive functions whose Fourier transforms havea logarithmic decay or which are supported on a disc. 1991 MathematicsSubject Classification 42B25.     

Spin-one-Ising model for (CO)1−x (N2) x mixtures: A finite size scaling study of random-field-type critical phenomena

A qualitative model for solid mixtures of diatomic molecules, where one species (called CO, to be specific) carries both a dipole moment and a quadrupole moment, while the other species (calledN ₂) has only a quadrupole moment, is studied by Monte Carlo methods. We use spinsS _i =±1 to represent the orientations of the CO electric dipole moment, if the lattice sitei is taken by a CO molecule, whileS _i =0 if the site is taken by anN ₂ molecule. Assuming nearest-neighbor antiferroelectric interactions between CO molecules, and a bilinear dipole-quadrupole coupling between CO andN ₂, the randomly quenchedN ₂ molecules act like random fields do in the random field Ising model. In previous work it was already shown that this crude model is in very good agreement with experimental data in two dimensions (adsorbed layers), where the random fields induces a rounding of the transition. Here Monte Carlo simulations of the three-dimensional version of this model are presented and analyzed with finite size scaling concepts. As expected from the theory, a behaviour qualitatively different from the two-dimensional case is detected. The Monte Carlo data provide qualitative evidence that the random field induces crossover to an universality class with critical exponents distinct from the pure Ising model, but it is not feasible to us to study large enough systems that would allow a reliable estimation of these exponents. But the results show that dilution without dipole-quadrupole coupling has much less drastic effects on the critical behavior, and that in the presence of this coupling very small impurity concentrations do indeed change the critical behavior.This paper is dedicated to Professor Herbert Wagner on the occasion of his 60th birthday     

Random-matrix model for dissipative two-level systems

Assuming a random-matrix model for the system-bath interaction and using a series expansion for the evolution operator, we studied the relaxation of a nondegenerate two-level system. For times larger than the duration of a collision and smaller than the Poincaré recurrence time, we calculate the survival probability of still finding the system, at timet, in the same state in which it was prepared att=0. For a fixed initial state of the bath, we obtain an exponential transition rate, but when we average over initial states we may get a mixture of decay constants that could distroy the exponential behavior of the transition rate.     

The effect of layer’s thickness on the optical response of superlattices

We apply the Theory of Finite Periodic Systems to study the photoluminescent behavior of superlattices. Using this formalism, where intra-subband energy levels are neatly identified, we study the optical response for different barrier and well thicknesses, and show that this quantity is very sensitive to small changes in the superlattice parameters. Varying the barrier thickness (for fixed well-width), or the well-width (for constant barrier thickness), the intensities of the relevant PL-peaks change drastically. We make clear also that this highly accurate formalism can be used to design optimal superlattice-optoelectronic devices.