Dialysis membranes high-efficiency

Dialysis units provided highly efficient means for increasing selectivity in a dynamic system by placement in front of a lithium-selective electrode constructed by incorporating 14-crown-4 ether 3-dodecyl-3 -methyl-1,5,8,12-tetraoxacyclotetradecane into a PVC membrane that was in turn positioned in a microconduit circuit by deposition on platinum, silver or copper wires. The circuit was used to analyse undiluted blood serum samples by flow injection analysis with the aid of an on-line coupled dialysis membrane. For this purpose, a volume of 200 pL of sample was injected into a de-ionized water carrier (donor) stream and a 7 mM tetraborate buffer of pH 9.2 was... 

Hemodialysis with high-efficiency dialysis membranes resulted in a removal of plasma vancomycin of about 60% (calculated half-life 2 hours) in two children with initial plasma vancomycin concentrations of 238 gg/ml and 182 gg/ml (114). 

Bunchman TE, Valentini RP, Gardner J, Modes T, Kudelka T, Maxvold NJ. Treatment of vancomycin overdose using high-efficiency dialysis membranes. Pediatr Nephrol 1999 13(9) 773. ... 

Dialysis membranes are classified as conventional (standard), high-efficiency, and high-flux. Conventional dialyzers, mostly made of cuprophane, have small pores that limit clearance to relatively smaU molecules such as urea and creatinine. High-efficiency membranes have large surface areas and thus have a greater ability to remove water, urea, and other small molecules from the blood. High-flux... 

The improved PEP/pyruvate system developed by Kim and Swartz [7] yielded 350 pg mL of chloramphenicol acetyl-transferase (CAT) in the first hour of an E. coli batch reaction. The same reaction system yielded 750 pg mL CAT in 3 hours when periodically fed with amino acids, PEP, and magnesium acetate [6]. This is remarkable efficiency for a system lacking a dialysis membrane, and is a feature that weU suits robotic handling and high-throughput screening (HTS) strategies. 

The dialysis membrane is a critical factor contributing to the efficiency of the dialysis separation system. In addition to their specificity on allowing the passage of a certain species of analyte while obstructing interfering components, which is a common requirement for all dialysis systems, on-line systems demand better mechanical and kinetic properties. Dialysis membranes of FI systems are expected to allow the achievement of high transfer factors within very short contact times of usually less than 30 seconds, while the membranes should be able to withstand hundreds of analytical cycles without significant deterioration in their performance. 

The first hemodialysis devices utilized natural cellulose (cuprophan) membranes, which possessed predominantly small pores. These membranes permitted the removal of excess fluid, ions, and small molecules, but prohibited the removal of substances above approximately 1200 Da in size. Larger molecules, such as P2-microglogulin (P2M, ll.SkDa), accumulated in the blood and were thought to contribute to many of the additional health problems and high mortality of patients on dialysis. This idea, coined the middle molecule hypothesis by Bapp et al. [342], led to the development of new synthetic polysulfone or polyacrylonitrile dialysis membranes that possessed larger pores and, in combination with equipment to control transmembrane pressure, permitted more efficient elimination of middle molecules. 

In laboratory experiments, selective membranes were already applied years ago for the pH-control during the electro-dialysis of pH-sensitive colloids. In the three-compartment cells used, the electrode chambers were rinsed with distilled water. On account of the high mobility of the H+ ions, the desalting cell was inclined to become acid. To oppose this effect, anode- and cathode membranes with different polarity were sought for. At the beginning the influence of these membranes on the current efficiency — i.e. the amount of salt removed per unit of charge flown through — was mentioned only sporadically (5,23). 

With two-phase liquid membrane extraction as with classical LLE, the extraction efficiency is limited by the partition coefficient. If this is very high, it is possible to work with a stagnant acceptor and stUl obtain a considerable enrichment into a small extract volume. With smaller partition coefficients, it might be necessary to arrange the acceptor phase to move with a slow flow rate to successively remove the extracted analyte and maintain the diffusion through the membrane. This will then lead to a lower degree of enrichment. The situation is similar to that for dialysis, and various focusing approaches can be applied to improve it, such as an SPE column or a retention gap. 

There are a number of different membrane techniques which have been suggested as alternatives to the SPE and LLE techniques. It is necessary to distinguish between porous and nonporous membranes, as they have different characteristics and fields of application. In porous membrane techniques, the liquids on each side of the membrane are physically connected through the pores. These membranes are used in Donnan dialysis to separate low-molecular-mass analytes from high-molecular-mass matrix components, leading to an efficient cleanup, but no discrimination between different small molecules. No enrichment of the small molecules is possible instead, the mass transfer process is a simple concentration difference over the membrane. Nonporous membranes are used for extraction techniques. 

Smith and co-workers designed a microfabricated dialysis device for sample cleanup before ESI MS. A microdialysis membrane sandwiched between two chips having micromachined serpentine channels provided efficient desalting for both DNA and protein samples before subsequent ESI ion trap MS. In a continuation study, they used a fabricated dual microdialysis membrane for removing both high- and low-molecular-weight species that... 

Hydrophobic membranes, e.g., PTFE, permit the efficient removal of volatile analytes from the sample matrix by diffusion though the micropores [257]. As these membranes have a high diffusion efficiency for many gaseous species, selectivity is usually low. For hydrophilic porous membranes, mass transference usually relies on dialysis, provided differences in donor and acceptor stream pressures are low [258] the chemical species originally in the donor stream migrate through the solvent in the interstitial volume of the membrane. Ionic species are therefore efficiently separated from the macromolecules in the sample matrix. Increasing the difference in pressures of both streams favours the micro-filtration process therefore, filtration and dialysis may occur simultaneously [259,260]. 

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