**2. Fundamental principles of sorbent extraction**

Sorption can be defined as a process by which a substance (*sorbate*) is sorbed (*adsorbed* or *absorbed*) on or into another substance (*sorbent*) [5]. In the sample preparation context, the term sorbent refers to the solid extracting phase, including solid-supported liquid phases upon which an analyte is retained. Schwarzenbach and co-workers [6] made a distinction between absorption meaning into a three dimensional matrix, and adsorption as meaning onto two dimensional surface. Figure.1. shows a schematic representation of analyte adsorption (analyte accumulation onto the sorbent surface) and absorption (analyte accumulation into the bulk of the sorbent) type extraction mechanisms.

**Figure 1.** Schematic representation of (a) absorptive and (b) adsorptive extraction processes [7].

Although sorbent based extraction techniques could be classified on the basis of either adsorption or absorption, the two processes are not completely separable as they occur concurrently. In principle, what differs is the extent of contribution of the predominant extraction mechanism as that is solely dependent on the nature of the extraction phase. Consequently, it may be difficult to distinguish between the two processes experimentally [8]. thus the general term *sorption* is often used to refer to a combination of these processes. Despite the uncertainty of the extent of contribution of adsorption or absorption mechanisms in extraction, fundamentally, all sorbent based extraction techniques are guided by the thermo‐ dynamic partition or distribution coefficient *K*, which is usually expressed as the ratio of analyte concentration in the sorbent phase, *Csorbent* to that in the sample phase, *Csample*;

$$K = \frac{C\_{\text{solvent}}}{C\_{\text{sample}}} \tag{1}$$

For adsorption, surface structure (porosity and surface area governing the available sites for analyte retention) is the more important sorbent characteristic compared to chemical compo‐ sition. While the chemical composition (governing the diffusion coefficient of the analyte into the sorbent) of the liquid phase is the more important for absorption.

Sorption from the sample phase is essentially a dynamic process in a heterogeneous system in which transport of the analytes between the sorbent and the sample phase is achieved. The process proceeds by a decrease in free energy until it reaches the minimum value (that is equilibrium). The mechanism of analyte adsorption or absorption is governed by the charac‐ teristics of interactions between the analyte and active sites of the sorbent. Therefore, sorbent selection is based on the binding mechanisms between the sorbent and analyte of interest. Table.1. shows different interaction mechanisms with their corresponding energies.


**Table 1.** Energies of interaction mechanisms [9].

The objective of the chapter is to equip the readers with sufficient knowledge that would enable them to fabricate and employ electrospun nanofibers as sorbents not only for SPE but for a wide range of applications. A discussion of the fundamental principles of SPE, the challenges associated with further research efforts and the unlimited potential that lies in electrospinning to address SPE will be presented. The use of electrospun nanofibers for SPE is an area that is still at its infancy (Scifinder scholar search of the words *"Electrospinning*"*and*"*Solid Phase Extraction"* showed 34 hits which consisted of 30 papers, 2 review articles, 2 patents and 0 books/book chapters) thus the chapter will be presented mostly in a postulative manner on the basis of the experiments conducted in the authors' research lab as well as literature reports.

Sorption can be defined as a process by which a substance (*sorbate*) is sorbed (*adsorbed* or *absorbed*) on or into another substance (*sorbent*) [5]. In the sample preparation context, the term sorbent refers to the solid extracting phase, including solid-supported liquid phases upon which an analyte is retained. Schwarzenbach and co-workers [6] made a distinction between absorption meaning into a three dimensional matrix, and adsorption as meaning onto two dimensional surface. Figure.1. shows a schematic representation of analyte adsorption (analyte accumulation onto the sorbent surface) and absorption (analyte accumulation into the bulk of

**Figure 1.** Schematic representation of (a) absorptive and (b) adsorptive extraction processes [7].

Although sorbent based extraction techniques could be classified on the basis of either adsorption or absorption, the two processes are not completely separable as they occur concurrently. In principle, what differs is the extent of contribution of the predominant extraction mechanism as that is solely dependent on the nature of the extraction phase. Consequently, it may be difficult to distinguish between the two processes experimentally [8].

**2. Fundamental principles of sorbent extraction**

the sorbent) type extraction mechanisms.

2 Advances in Nanofibers

The process of analyte sorption can be assumed to consist of multiple steps. Any of the steps may become rate limiting in controlling sorption of an analyte. The analyte may interact with a sorbent in at least four ways:


or other solvent molecules by the analyte. In the adsorption process, analytes may compete for sites; therefore, adsorbents have limited capacity. Three steps occur during the adsorption process on porous sorbents; (a) *film diffusion* (when the analyte passes through a surface film to the solid phase surface), (b) *pore diffusion* (when the analyte passes through the pores of the solid phase), and (c) *adsorptive reaction* (when the analyte *binds*, *associates* or *interacts* with the sorbent surface) [11].

**2.1. Solid phase extraction process**

desorption [18].

functional groups.

Classically, batch mode liquid solid extractions were used in which the liquid sample was placed in contact with the bulk free flowing solid extracting phase. Equilibrium between the two phases was allowed to occur, followed by physical separation (decanting or filtering). Advancements of liquid solid extractions could be said to have taken two approaches; the first being solid phase microextraction (SPME) which consists of a two-step process in which the sorbent or solid supported sorbent is allowed to reach equilibrium before analyte desorption [14-17]. Figure.3. shows the most widely used version of SPME which is based on an organic polymer coated fused silica fiber operated in a syringe format. The syringe is designed to move the fiber in and out of the needle, which allows exposure of the fiber during extraction and

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**Figure 3.** Schematic representation of the components of a commercially available SPME device [18].

*Conditioning*: functions to activate or "wet" the sorbent to prepare for its interaction with the analyte. This is especially necessary for hydrophobic sorbents that would not be activated by an aqueous sample. If the sorbent is not adequately conditioned, poor analyte retention may be achieved. If pH is critical for retention, then the conditioning solvent has to be matched to

*Loading*: when the liquid sample is added to the sorbent, sufficient residence time should be allowed for maximum analyte-sorbent interaction. This is especially critical when employing ion exchange to provide for adequate residence time of the sample solution in the sorbent since the analyte has to achieve an appropriate orientation for electrostatic retention with the sorbent

that of the sample to prepare for maximum analyte retention during the loading step.

While packed sorbent format SPE generally involves four steps;


For porous sorbents, most of the surface area is inside the nanopores of the sorbent (see Fig. 2.). Nanopores of the sorbent are classified into three as; *micropores* (diameters smaller than 2 nm), *mesopores* (2 to 50 nm), and *macropores* (greater than 50 nm) [12]. Most of the surface area is derived from the small diameter micropores and the medium diameter mesopores. Porous sorbents vary in pore size, shape, tortuosity and are characterized by properties such as particle diameter, pore diameter, pore volume, specific surface area and particle distribution.

**Figure 2.** Schematic representation of porous regions of a sorbent [13].

#### **2.1. Solid phase extraction process**

or other solvent molecules by the analyte. In the adsorption process, analytes may compete for sites; therefore, adsorbents have limited capacity. Three steps occur during the adsorption process on porous sorbents; (a) *film diffusion* (when the analyte passes through a surface film to the solid phase surface), (b) *pore diffusion* (when the analyte passes through the pores of the solid phase), and (c) *adsorptive reaction* (when the analyte *binds*, *associates*

**3.** If the analyte is ionisable in aqueous solution, there may be an electrostatic attraction between the analyte and the charged sites on the sorbent surface. Sorbents specifically designed to exploit these types of ionic interactions are referred to as *ion-exchange* (either

**4.** It is possible that the analyte and the sorbent may be chemically reactive toward each other such that the analyte becomes covalently bonded to the sorbent. This type of sorption is generally detrimental to analytical recovery and may lead to slow or reduced

For porous sorbents, most of the surface area is inside the nanopores of the sorbent (see Fig. 2.). Nanopores of the sorbent are classified into three as; *micropores* (diameters smaller than 2 nm), *mesopores* (2 to 50 nm), and *macropores* (greater than 50 nm) [12]. Most of the surface area is derived from the small diameter micropores and the medium diameter mesopores. Porous sorbents vary in pore size, shape, tortuosity and are characterized by properties such as particle

diameter, pore diameter, pore volume, specific surface area and particle distribution.

or *interacts* with the sorbent surface) [11].

**Figure 2.** Schematic representation of porous regions of a sorbent [13].

anion or cation exchange).

recovery [6, 11].

4 Advances in Nanofibers

Classically, batch mode liquid solid extractions were used in which the liquid sample was placed in contact with the bulk free flowing solid extracting phase. Equilibrium between the two phases was allowed to occur, followed by physical separation (decanting or filtering). Advancements of liquid solid extractions could be said to have taken two approaches; the first being solid phase microextraction (SPME) which consists of a two-step process in which the sorbent or solid supported sorbent is allowed to reach equilibrium before analyte desorption [14-17]. Figure.3. shows the most widely used version of SPME which is based on an organic polymer coated fused silica fiber operated in a syringe format. The syringe is designed to move the fiber in and out of the needle, which allows exposure of the fiber during extraction and desorption [18].

**Figure 3.** Schematic representation of the components of a commercially available SPME device [18].

While packed sorbent format SPE generally involves four steps;

*Conditioning*: functions to activate or "wet" the sorbent to prepare for its interaction with the analyte. This is especially necessary for hydrophobic sorbents that would not be activated by an aqueous sample. If the sorbent is not adequately conditioned, poor analyte retention may be achieved. If pH is critical for retention, then the conditioning solvent has to be matched to that of the sample to prepare for maximum analyte retention during the loading step.

*Loading*: when the liquid sample is added to the sorbent, sufficient residence time should be allowed for maximum analyte-sorbent interaction. This is especially critical when employing ion exchange to provide for adequate residence time of the sample solution in the sorbent since the analyte has to achieve an appropriate orientation for electrostatic retention with the sorbent functional groups.

*Washing*: this serves to remove interferences retained on the sorbent leaving behind the analytes of interest. In some cases, it could be used to wash-off the analyte of interest while retaining the interferences. In mechanisms employing ion exchange, the pH of the wash solvent should be sufficient enough to disrupt the charged sites of interferences but not affect the analyte.

**3. Nanofiber based sorbents**

preferred.

polymer based.

SPE formats.

Sorbent selection for SPE is normally guided by the ability of a material to retain analytes selectively as well as facilitate rapid and complete elution. The sorption process must be reversible [19, 20]. In addition to reversible sorption, SPE sorbents should possess a large specific surface area as well as exhibit stability in the sample matrix and elution solvents. Lastly, a sorbent material with a good surface contact with the sample solution would be most

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Poole and co-workers [21] categorized SPE sorbents into three as; (i) general purpose, (ii) class specific and (iii) analyte specific. The most common retention mechanisms in SPE are based on van der Waals forces, π-π interactions, hydrogen bonding, dipole-dipole interactions and ion exchange interactions. As a result, sorbents can be classified on the basis of retention mechanisms as; (i) reversed phase (ii) normal phase and (iii) ion exchange. From the materials perspective, sorbents are classified into three as; (i) carbon based (ii) inorganic based and (iii)

The SPE sorbent fabrication technique of choice could be viewed as one that produces a material that exhibits chemical and morphological properties that can be easily modified. Similarly, an optimal SPE sorbent material combines the following benefits: (i) small diameter, (ii) large specific surface area, (iii) simplified fabrication/synthesis, (iv) ability to be modified in order to incorporate all sorbent chemistries/functionalities, (v) ability to be modified in order to incorporate all sorbent morphologies (vi) a material that can be packed in the lower (less than 10) mg range without presenting a backpressure limitation or low analyte recoveries.

The advent of nanotechnology has been a major leap forward in the research area of sorbent based sample preparation techniques as it opened up possibilities for a new class of materials that could be used in SPE applications. The main benefit of nanostructured materials is their large specific surface area that facilitates the miniaturization of SPE allowing for the use of a

Although nanoparticles offer improved performance as sorbent material [22], they inherently exhibit some limitations. One of the main challenges is associated with their handling in packed

Given the fact that nanoparticles have shown excellent properties as sorbent material, it would seem prudent to focus developmental efforts on nanostructured material with the aim to address the challenges of nanoparticles. This brings about a need for an alternative fabrication approach that carries with it the benefits of nanoparticles at the same time addressing some (if not all) of their limitations. From our research group's perspective, the use of electrospun nanofibers as sorbent material is seen as a possible way of carrying along the benefits of nanoparticles, whilst addressing their limitations and increasing the possibilities for modifying sorbent morphology and functionality. It is upon this background that the section has been compiled to present and discuss relevant examples that demonstrate the potential of electro‐

reduced sorbent bed mass that achieves high extraction efficiency.

spinning as an alternative fabrication technique.

*Elution*: the elution solvent should be strong enough to disrupt all analyte-sorbent interactions in order to obtain the highest recoveries. However, there is a limit to the strength as harsh solvents would not only desorb analytes from the sorbent, but also strip strongly retained interferences.

Figure.4. shows schematic representations of the four approaches typically taken in practical SPE applications. Figure.4. (a) shows a three step SPE process in which; (1) the sorbent is conditioned after which the sample is loaded and finally (2) the analyte is eluted with the interferences being retained (clean up) or the analyte is simply eluted into a smaller sample volume (preconcentration).

Figure.4. (b) shows a four step SPE process in which; (1) the sorbent is conditioned after which the sample is loaded, (2) the interferences eluted and finally (3) the analyte is eluted (clean up) or the analyte is simply eluted into a smaller sample volume (preconcentration).

Figure.4. (c) shows a five step SPE process in which; (1) the sorbent is conditioned after which the sample is loaded, (2) the interferences eluted, (3) a fraction of analyte is selectively eluted and finally (4) the second fraction of analytes is eluted.

**Figure 4.** Schematic representation of practical SPE operations
