Operationally defined fractions

However, for the past 30 years fractional separation has been the basis for most asphalt composition analysis (Fig. 10). The separation methods that have been used divide asphalt into operationally defined fractions. Four types of asphalt separation procedures are now in use ( /) chemical precipitation in which / -pentane separation of asphaltenes is foUowed by chemical precipitation of other fractions with sulfuric acid of increasing concentration (ASTM D2006) (2) solvent fractionation separation of an "asphaltene" fraction by the use of 1-butanol foUowed by dissolution of the 1-butanol solubles in... 

Table 22.1 Operationally Defined Fractions of Organic Matter. ...

Marine HA and FA are operationally defined fractions, and their composition may be variable (values are from Nis-senbaum, 1979). Phosphate associated with HA and FA may be organically bound. Alternatively, it may be inorganic orthophosphate linked to HA and/or FA through metal bridges (Laarkamp, 2000). 

As with the bulk POM and DOM, the operationally defined fractions of UDOM and humic substances are quantified by elemental analysis and via broad molecular-class detection. Other strategies involve measurement of the natural isotopic composition, both stable and radioactive, of the various fractions. Efforts are underway to develop more sophisticated techniques, such as solid-state NMR and high-resolution mass spectrometry, far identification of specific bonds and functional groups. 

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. 

A similar scheme [126] using the same operationally defined fractions determines 15 elements Be, Ca, Co, Cr, Cu, Fe, K, Li, Mn, Ni, P, Pb, Ti, V, and Zn, with recoveries of 83 to 110%. The extractants used for each fraction are shown in Table 5.8. The Measurements and Testing Programme of the European Commission (EC) has a recommended method for sequential extractions. The method distinguishes four fractions, as shown in Table 5.9. 

In soil research, the term speciation is often applied to operationally defined fractionation of heavy metals into five or more components.25 Typically, water soluble, exchangeable, organically bound (which includes what is in biomass), amorphous oxide bound, crystalline oxide bound, and residual fractions are measured.26 Sometimes residual fractions are further subdivided according to particle size distributions to give amounts in sand, silt, and clay fractions. Similar fractionation procedures are often applied to aquatic sediments.27 In arid regions, often the calcium carbonate bound fractions of heavy metals are also measured.28 Because of the constraints of detection limits, generally only cadmium, copper, iron, manganese, and zinc are usually monitored by flame spectrometry in such heavy metal speciation studies.28... 

Methods of Speciation and Fractionation. It is apparent that in order to understand the mobility of arsenic and its availability for reactions, methods of speciation and fractionation must be applied to sediment samples in field and laboratory studies. In this paper speciation refers to the separation and quantitative determination of inorganic arsenic, methanearsonic acid, and cacodylic acid. Compartmentalization involves identifying the major compartments for arsenic in a heterogeneous system (e.g. aqueous, adsorbed, occluded,...) and determining the amounts of arsenic in each compartment. Fractionation involves the extraction of arsenic from operationally defined fractions of the solid phase of an aquatic system (e.g. sediment). 

Arsenic was distributed among the operationally defined fractions of the sediment solids of a sediment core collected in September, 1977 as shown in Figure 3. Generally, the order of abundance of As in the fractions was OH" (Fe and Al) > oxalate (amorphous or occluded) Cl" (exchangeable) ... 

TABLE 12.1. Operationally Defined Fractions and Corresponding Extracted Components... 

Operationally Defined Fraction Extracted components (Possible Forms of Binding TEs) Physicochemical Mobility Potential Bioavailability Common Extractants... 

The so-called Driscoll methods are all based around ion-exchange separation of the monomeric inorganic aluminium from the monomeric organic aluminium. The column packing is Amberlite IR-120 cation exchange resin [159]. In essence, the Driscoll method directly measures three operationally defined fractions ... 

The phosphorus concentration in each operationally defined fraction is typically detected using the phosphomolybdate-blue colorimetric reaction and spectrophoto-metric detection (American Public Health Association, 1995), with or without pre-treatment and/or filtration. The term total phosphorus applies to samples subjected to hydrolysis and/or oxidation pretreatment intended to convert all forms of phosphorus to phosphate (Maher and Woo, 1998 Monaghan and Ruttenberg, 1999). Phosphorus that reacts with molybdate in untreated samples is referred to as reactive phosphorus . Samples may be unfiltered or filtered. When samples are unfiltered, the operationally defined fractions are total... 

Many protocols for collecting rhizosphere soil have been developed, although the lack of a precise delimitation within a continuum means that subdivision has been performed as a function of the research aims. Soil scientists of different disciplines may use different procedures, and the rhizosphere soil so obtained has to be considered as an operationally defined fraction. According to Lynch (1990), this may indicate different things to different researchers. The procedure followed to obtain rhizosphere and bulk soils obviously depends on the type of plants investigated and whether they are cultivated (in containers or in the field) or are growing naturally. 

Numerous speciation procedures have been developed in order to describe heavy metal forms in soils (Himer 1992 Rauret 1993 Zeien and Bruemmer 1989). In this study, the sequential extraction method according to Zeien and Bruemmer (1989,1991) was applied to soil samples collected from sanitary protection zones. Both surface (0-20 cm) and subsurface (30-40 cm) soil samples were examined. The method involved the sequential extraction of 7 operationally defined fractions ... 

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