Binding equilibrium of I? to serum albumin with resonance Rayleigh scattering

The binding equilibrium between l⁻ and human serum albumin (HSA) or bovine serum albumin (BSA) has been studied by means of the resonance Rayleigh scattering (RRS) and equilibrium dialysis. It has been found for the first time that RRS and multiple frequency scattering (MFS) are enhanced as the l⁻ binding to the HSA and BSA, but fluorescence quenches. The equilibrium dialysis results suggest that the binding of l⁻ to HSA and BSA fits a phase-distribution model other than Scatchard model, and that the order of magnitude of its phase-distribution constant was found to be 10⁴. It is most probable that Cl⁻ or other anion ions influence the binding of l⁻ by changing the ionic strength in the solution. The dialysis at different pH indicates that the binding mechanism is due to the electrostatic forces between the l⁻ and protonated basic amino-acid residues.     

FIA-potentiometry in the sub-Nernstian response region for rapid and direct chloride assays in milk and in coconut water

A simple and reliable FIA-potentiometric system for rapid assays of chloride in certain food samples is described and evaluated. The system is constituted by an aquarium air pump to propel the carrier solution, a manually operated injector, a homemade dialysis flow cell, a solid-state chloride detector (Ag/AgCl), a reference electrode and a multimeter connected to a microcomputer for data acquisition. The dialysis unit enables direct analysis of liquid food samples without any other previous treatment. The principal novelties are the precision (R.S.D. of 1.2% for whole milk) and rapidity (90 determinations/h) of FIA measurements near and below the lower end of the linear (Nernstian) response region of the chloride ion-selective electrode (ISE), with an estimated detection limit (3 s) of 0.4 mg L⁻¹ Cl⁻ in the sample injected in donor stream. Data of peak potential versus sample chloride concentration (donor stream) was accurately fitted with a quadratic polynomial over the range between 4 and 1000 mg L⁻¹ (r² = 0.9999) and used as a calibration curve. The method was applied to the determination of chloride in milk and in coconut water samples. The validation of the results was done by comparison with a NIST reference material (milk) or by capillary electrophoresis (coconut water). For all analysis, no significant difference at a 95% confidence level was observed.     

Efficient Computation of Optimal Controls for the Exchange Processes During the Dialysis Therapy

In order to reduce the frequency of acute complications during the dialysis therapy the exchange processes of water and different solutes within the patient as well as across the dialyzer membrane shall be optimally controlled. With regard to a clinical application, this task requires the efficient treatment of a large-scale control problem, formulated in terms of a dynamical optimization problem. Equality and inequality conditions are given by the system describing the exchange processes and by the consideration of technical and medical constraints, respectively. Above all the complexity of the describing system prevents the application of standard optimization techniques as well as the construction of closed loop control laws and implies the construction of a control procedure which is specially adapted to the problem. The presented optimization method—denoted as controller PSEUDYGALG—represents a numerical iterative descent procedure, based on the approach of admissible direction. The procedure assumes an appropriate parameterization of the control problem as well as the availability of information about the input-output structure of the underlying describing system. In order to achieve the required efficiency, adaptive penalization strategies for the performance criterion and update modules for the descent information of each iterative step are presented. The controller allows both the treatment of badly and well conditioned control problems which are characterized by the occurrence and the absence of contradictional requirements for the performance criterion, respectively. PSEUDYGALG represents an off-line control method, but due to the achieved efficiency an on-line deployment by receding horizon approaches is in principle possible. Even though the controller has been developed for the dialysis problem it can be applied to a wide range of comparable control problems if the two assumptions—appropriate parameterization and knowledge about the input-output structure of the underlying system—are met.     

Ultrathin self-assembled polyelectrolyte multilayer membranes

The paper is concerned with ultrathin membranes prepared upon alternating layer-by-layer adsorption of cationic and anionic polyelectrolytes on a porous substructure. The formation of the polyelectrolyte multilayer membranes is characterised and the transport of gases, liquid mixtures and ions across the membranes is studied. In particular, the use of the membranes for alcohol/water separation under pervaporation conditions, and for the separation of mono- and divalent ions is described. It is demonstrated that upon a suitable choice of polyelectrolytes and substructures, and a careful optimisation of preparation and operation conditions, membranes can be tailored exhibiting an excellent separation capability. Received 4 September 2000     

Methylated arsenic species in estuarine porewaters

Inorganic arsenic, monomethylarsenic and dimethylarsenic species have been observed in samples of sediment porewater collected from the Tamar Estuary in South-West England. Porewater samples were collected using in situ dialysis. The arsenic species were separated by hydride generation and concentrated by liquid nitrogen trapping, prior to analysis by directly coupled gas chromatography-atomic absorption spectroscopy. The predominant dissolved arsenic species present was inorganic arsenic (5-62 m?g dm^?3). However, this is the first time significant concentrations of methylated arsenic species have been quantified in estuarine porewaters (0.04–0.70 m?g dm^?3), accounting for between 1 and 4% of the total dissolved arsenic. The presence of methylated arsenic compounds in porewaters is attributed to in situ environmental methylation, although the possibility of methylated arsenic species being derived from biological debris cannot be excluded.     

Charge mosaic membranes prepared from laminated structures of PVA-based charged layers: 1. Preparation and transport properties of charged mosaic membranes

A charge mosaic (CM) membrane has high permselectivity for electrolytes. While there are many reports of attempts to prepare such membranes, it is difficult to make CM membranes for practical applications. We report the preparation of CM membranes from laminated structures of charged-poly(vinyl alcohol) (PVA) membranes. The membranes were prepared by alternately stacking negatively charged base membranes and positively charged base membranes and by cutting the stack of charged layers. Permeation experiments were performed in a dialysis system consisting of the membrane and mixed solutions of KCl and sucrose. Although the salt flux through the membrane was about 30 times less than that through the charge mosaic membrane Desalton^® (Tosoh Co. Ltd.), which was prepared using microphase separation, the permselectivity for salt of our membrane is more than 30 times higher than that of Desalton^®.     

Permeate flux inflection due to concentration polarization in crossflow membrane filtration: A novel analytic approach

A convection-diffusion equation for membrane filtration is analytically solved assuming fast crossflow velocity of a simple shear flow tangential to the membrane surface. In the direction normal to the membrane surface, solute concentration varies in a partially exponential and partially power-wise manner. The permeate flux in an asymptotic limit is proportional to the inverse square root of the distance from the inlet of the membrane channel. Osmotic pressure due to retained solutes on the membrane surface controls the profile of the permeate flux, which undergoes an inflection along the tangential direction if applied pressure is more than four times the feed osmotic pressure.     

In-vivo and in-vitro testing to assess the bioaccessibility and the bioavailability of arsenic, selenium and mercury species in food samples

In-vivo and in-vitro gastrointestinal (GI) extractions, also known as oral bioaccessibility and bioavailability, are important approaches to assess chemical risk to humans. We give an overview of in-vivo and in-vitro bioaccessibility and bioavailability assays for testing arsenic, selenium and mercury (As, Se and Hg) species from food samples. We critically evaluate the parameters affecting in-vivo and in-vitro processes. In addition, we consider the effect of cooking food on bioaccessibility and bioavailability, and stability and transformation, of species during in-vivo or in-vitro processes. The bioaccessibility and bioavailability of As, Se and Hg species are affected by the sample matrix, cooking food and the experimental conditions applied (gastric and intestinal pH, incubation temperature and residence time). Regarding species degradation and transformation during in-vitro procedures, good stability has been observed for most As species, except for certain arsenosugars. Important transformations during in-vitro processes have been reported for Se species [e.g., conversion of γ-glu-Se-MeSeCys to Se-MeSeCys, and organic Se species (MeSeCys, SeCys2 and SeMet) degradation to inorganic Se]. Finally, we summarize speciation and detection conditions for As, Se and Hg speciation, and quality control to assure reliable measurements.     

Continuous Determination of Free Cyanide by Means of Membrane Diffusion of Gaseous HCN and An Electrode Indicator Technique

Abstract

A continuous measurement system for free cyanide has been developed based on the principle of diffusion across a gas-permeable membrane to affect the separation of hydrogen cyanide from the acidified sample solution. The cyanide absorbed in the alkaline indicator solution is subsequently analyzed by an indirect technique using a silver ion-selective electrode. In the concentration range of 30 to 400 μg CN^?/L, the accuracy and precision of this method is approximately two percent. The detection limit of this system is approximately 0.5 μg CN^?/L.     

Isolation of Lipophilic Persistent Organic Pollutants From Human Breast Milk

Background: Lipid removal from biological samples can be achieved by addition of concentrated sulfuric acid. However, certain persistent organic pollutants (POPs) such as chlorophenols are decomposed by sulfuric acid treatment and, thus, a more gentle lipid reduction method is needed for extraction of many environmental contaminants from biological samples. Membrane dialysis extraction (MDE) is a non-disruptive method to extract POPs from biological matrices.

Methods: Human breast milk samples were spiked with radiolabelled p,p′-dichlorodiphenyl trichloroethane ([C-14]-DDT) as a POP proxy and extracted using solid phase extraction (SPE). The extracts obtained were dialyzed by MDE in low-density polyethylene tubings containing a mixture of n-hexane and dichloromethane for 24 h, 48 h, or 72 h.

Results: The lipid content was reduced by 86.2% after one dialysis cycle of 24 h using MDE, and 87.1% recovery of the [C-14]-DDT standard was obtained. The DDT recovery could be further increased up to 96.3% and 98.1% by repeating the dialyses for one or two more cycles, respectively. However, the increased [C-14]-DDT recovery includes a concomitant increase in lipid carryover from 13.8% with one dialysis cycle to 22.1% with three cycles.

Conclusion: An SPE procedure for extracting POPs from breast milk and dialytic conditions for isolation of the extracted POP with minimal lipid carryover was established. The method is nondestructive and acceptable recoveries can be obtained within a single solvent shift as demonstrated by spiking standards. The lipid carryover was minimized, and the method may be considered for lipid removal before HPLC or GC analysis of environmental contaminants.