Fractional dialysis, defined

In operationally defined speciation the physical or chemical fractionation procedure applied to the sample defines the fraction isolated for measurement. For example, selective sequential extraction procedures are used to isolate metals associated with the water/acid soluble , exchangeable , reducible , oxidisable and residual fractions in a sediment. The reducible, oxidisable and residual fractions, for example, are often equated with the metals associated, bound or adsorbed in the iron/manganese oxyhydroxide, organic matter/sulfide and silicate phases, respectively. While this is often a convenient concept it must be emphasised that these associations are nominal and can be misleading. It is, therefore, sounder to regard the isolated fractions as defined by the operational procedure. Physical procedures such as the division of a solid sample into particle-size fractions or the isolation of a soil solution by filtration, centrifugation or dialysis are also examples of operational speciation. Indeed even the distinction between soluble and insoluble species in aquatic systems can be considered as operational speciation as it is based on the somewhat arbitrary definition of soluble as the ability to pass a 0.45/Am filter. 

Davies, P.H. 1976. Use of dialysis tubing in defining the toxic fractions of heavy metals in natural waters. Pages 110-117 in R.W. Andrew, P.V. Hodson, and D.E. Konasewich (eds.). Toxicity to Biota of Metal Forms in Natural Water. Proc. Workshop, Duluth, MN, October 7-8, 1975, Great Lakes Res. Advis. Bd., Int. Joint Comm. 

Techniques can be classified into two main categories those that detect total metal concentrations and those that detect some operationally defined fraction of the total. Methods which detect total concentrations such as inductively coupled plasma spectrometry, neutron activation analysis, atomic absorption spectrometry and atomic emission spectrometry have no inherent speciation capabilities and must be combined with some other separation technique(s) to allow different species to be detected (approach A in Fig. 8.2). Such separation methods normally fractionate a sample on the basis of size, e.g. filtration/ultrafiltration, gel filtration, or a combination of size and charge, e.g. dialysis, ion exchange and solvent extraction (De Vitre et al., 1987 Badey, 1989b Berggren, 1989 1990 Buffle et al., 1992). In all instances the complexes studied must be relatively inert so that their concentrations are not appreciably modified during the fractionation procedure. 

The electrophoretic pattern of mucoproteose fraction prepared by dialysis and lyophilization showed one fairly well-defined peak with mobility of —0.6 X 10 , and another poorly defined peak with more negative mobility. 

Donnan dialysis of conjugate bases of weak acids presents an additional factor for control, namely pH. Cox and Cheng (34) investigated preconcentration of a number of anions in this category. Sample pH was adjusted to ensure that a major fraction of the weak acid system was in anionic form. Maximum enrichment factor, defined as ratio of receiver concentration to sample concentration after some given, fixed time (usually 30 minutes), was obtained when pH of the receiver solution was much less than the... 

A number of fractionation procedures have been developed over the last 15 years in order to distinguish between the various aqueous forms of aluminium. These methods include dialysis, ion exchange (both batch and column), HPLC, F ion-selective electrode, NMR, species specific extractions, filtration and computational techniques. All of these procedures usually measure operationally defined aluminium fractions i.e. groups of aluminium species are measured, rather than a single species, and the values obtained depend upon the precise procedure used for the analysis). Therefore, present speciation procedures may measure slightly different forms of aluminium depending on the conditions, and thus they should accordingly provide different results. 

All the fractions were subjected to dialysis to remove salts and low molecular weight hydrolysis products. This procedure was performed for 3 days against running deionised water. The nominal molecular weight cut-off of the tubing was given as 12-14 thousand Daltons by the manufacturer (Spectra/Por 5). All fractions were freeze dried. Yield is defined as the amount of total extracted material based on lOOg of treated wood chips. The amount of extracted material comprised both solubles in the extraction liquids and solids in pellets. 

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