A systematic determination of diffusion coefficients of trace elements in open and restricted diffusive layers used by the diffusive gradients in a thin film technique

A systematic comparison of the diffusion coefficients of cations (Al, Cd, Co, Cu, Mn, Ni, Pb, Zn) and oxyanions (Al, As, Mo, Sb, V, W) in open (ODL) and restricted (RDL) diffusive layers used by the DGT technique was undertaken. Diffusion coefficients were measured using both the diffusion cell (D_cell) method at pH 4.00 and the DGT time-series (D_DGT) method at pH 4.01 and 7.04 (pH 8.30 was used instead of 7.04 for Al) using the Chelex-Metsorb mixed binding layer. The performance of Chelex-Metsorb as a new DGT binding layer for Al uptake was also evaluated for the first time. Reasonable agreement was observed between D_cell and D_DGT measurements for both ODL and RDL, except for V and W. The ratios of D_cell/D_DGT for V of 0.44 and 0.39, and for W of 0.66 and 0.63 with ODL and RDL respectively, were much lower due to the formation of a high proportion of polyoxometalate species at the higher concentrations required with the D_cell measurements. This is the first time that D values have been reported for several oxyanions using RDL. Except for Al at pH 8.30 with ODL, all D_DGT measurements were retarded relative to diffusion coefficients in water (D_W) for both diffusive hydrogels. Diffusion in RDL was further retarded compared with ODL, for all elements (0.66–0.78) with both methods. However, the degree of retardation observed changed for cations and anions at each pH. At pH 7.04 cations had a slightly higher D_DGT and oxyanions had a slightly lower D_DGT than at pH 4.01 for both ODL and RDL. It is proposed that this is due to partial formation of acrylic acid functional groups (pK_a ≈4.5), which would be fully deprotonated at pH 7.04 (negative) and mostly protonated at pH 4.01 (neutral). As Al changes from being cationic at pH 4.01 to anionic at pH 8.30 the results were more complex.     

Crispin St J. A. Nash-Williams (1932-2001)

Crispin St John Alvah Nash-Williams was born in Cardiff on 19December 1932. His father was Keeper of Archaeology at the NationalMuseum of Wales and also Senior Lecturer in Archaeology at UniversityCollege, Cardiff. War broke out when Crispin was six; his fatherjoined the army and his consequent absence changed the family'slife.     

Terahertz-pulse emission through laser excitation of surface plasmons in a metal grating

The second-order processes of optical-rectification and photoconduction are well known and widely used to produce ultrafast electromagnetic pulses in the terahertz frequency domain. We present a new form of rectification that relies on the excitation of surface plasmons in metal films deposited on a shallow grating. Multiphoton ionization and ponderomotive acceleration of electrons in the enhanced evanescent field of the surface plasmons results in a femtosecond current surge and emission of terahertz electromagnetic radiation. Using gold, this rectification process is third or higher-order in the incident field.     

Initial value adjusting method and graph theoretical analysis for the solution of nonlinear multipoint boundary value problems with varying system dimensions

The initial value adjusting method for the solution of nonlinear multipoint boundary value problems in which the system dimensions vary over subintervals is proposed. To reduce the computer storage requirements and the excessive amount of computer time, an algorithm based on a digraph and its associated Boolean matrices is also proposed. The effectiveness of these algorithms is illustrated by an application to a five compartment model from pharmacokinetics.     

The scaling method for free interface problems in ordinary differential equations

A method is proposed for the solution of boundary value problems in which the locations of the interfaces are unknown. Numerical results are obtained for the steady state solution of a shafttype furnace.