Affinity sorbent, partitioning

Figure 2. Partitioning in two-phase system with the use of an affinity sorbent. Reproduced with permission from Ref. 2. Copyright 1986, Wiley-Interscience.

The principle of SPE is selective retention of the analytes on the sorbent by nonpolar, polar, and ionic interactions, and affinity and partition between the analytes, the sorbent, and/or the solvent. 

The working capacity of a sorbent depends on fluid concentrations and temperatures. Graphical depiction of soration equilibrium for single component adsorption or binary ion exchange (monovariance) is usually in the form of isotherms [n = /i,(cd or at constant T] or isosteres = pi(T) at constant /ij. Representative forms are shown in Fig. I6-I. An important dimensionless group dependent on adsorption equihbrium is the partition ratio (see Eq. 16-125), which is a measure of the relative affinities of the sorbea and fluid phases for solute. 

Whereas several specific soil attributes are advocated as being responsible for DOC sorption in the mineral soil (Table V), it appears that the greater the clay or aluminum and iron oxide content of a soil, the greater its adsorptive capacity for DOC. For example, there is a positive correlation between m (the measure of the affinity of a substance for the sorbent or the partition coefficient) and soil clay content, dithionite extractable iron (Fej), and oxalate extractable aluminum (Al0) (Moore et al., 1992 Nelson et al., 1993 Kaiser and Zech, 1998). Direct measurements of the surface area of soil particles also correlate very well with DOC adsorption capacity (Nelson et al., 1993). Furthermore, Nelson et al. (1993) report that riverine DOC concentrations are negatively correlated to the clay content of watershed... 

Adsorption isotherms are used to quantitatively describe adsorption at the solid/ liquid interface (Hinz, 2001). They represent the distribution of the solute species between the liquid solvent phase and solid sorbent phase at a constant temperature under equilibrium conditions. While adsorbed amounts as a function of equilibrium solute concentration quantify the process, the shape of the isotherm can provide qualitative information on the nature of solute-surface interactions. Giles et al. (1974) distinguished four types of isotherms high affinity (H), Langmuir (L), constant partition (C), and sigmoidal-shaped (S) they are represented schematically in Figure 3.3. 

The principle of SPE involves a partitioning of analytes to be extracted between two phases a solid phase, the sorbent, and a liquid phase, the matrix, which contains possible interferences. Analytes must have a greater affinity for the solid phase than for the sample matrix (retention or adsorption step), and they are... 

In order to predict or estimate the affinity that a new analyte may have for a reversed-phase sorbent, the capacity factor may be useful. The capacity factor of an analyte is a function of the distribution coefficient of the analyte between the sorbent and water (mobile phase). The distribution coefficient is similar in concept to the log octanol-water partition coefficient except that the solid phase replaces the octanol phase. The distribution coefficient of the solute for the sorbent, for the reversed phase is given by the following equation ... 

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