Electrical field-induced extraction and separation techniques: promising trends in analytical chemistry--a review.

Analytica Chimica Acta 814 (2014) 1–22

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Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca

Review

Electrical field-induced extraction and separation techniques: Promising trends in analytical chemistry – A review Yadollah Yamini a,∗ , Shahram Seidi b , Maryam Rezazadeh a a b

Department of Chemistry, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Iran Department of Analytical Chemistry, Faculty of Chemistry, K.N. Toosi University of Technology, Tehran, Iran

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Sample preparation is an important issue in analytical chemistry.

• Application of electrical potential reduces time and enhances selectivity in sample preparation. • Review provides an overview of principles and applications of electrical fields in sample preparation. • Advantages, disadvantages and point to the corresponding limitations of these techniques are discussed. • Review is interested for readers that are appreciated to field of electrochemically modulated extractions.

a r t i c l e

i n f o

Article history: Received 20 July 2013 Received in revised form 7 December 2013 Accepted 16 December 2013 Available online 24 December 2013 Keywords: Sample preparation Electroextraction Voltage Electrical driving force Microextraction Membrane-based extraction

a b s t r a c t Sample preparation is an important issue in analytical chemistry, and is often a bottleneck in chemical analysis. So, the major incentive for the recent research has been to attain faster, simpler, less expensive, and more environmentally friendly sample preparation methods. The use of auxiliary energies, such as heat, ultrasound, and microwave, is one of the strategies that have been employed in sample preparation to reach the above purposes. Application of electrical driving force is the current state-of-the-art, which presents new possibilities for simplifying and shortening the sample preparation process as well as enhancing its selectivity. The electrical driving force has scarcely been utilized in comparison with other auxiliary energies. In this review, the different roles of electrical driving force (as a powerful auxiliary energy) in various extraction techniques, including liquid-, solid-, and membrane-based methods, have been taken into consideration. Also, the references have been made available, relevant to the developments in separation techniques and Lab-on-a-Chip (LOC) systems. All aspects of electrical driving force in extraction and separation methods are too specific to be treated in this contribution. However, the main

Abbreviations: CEC, capillary electrochromatography; CE, capillary electrophoresis; CPs, chlorophenols; DEHP, di-(2-ethylhexyl) phosphate; DPV, differential pulse voltammetry; ELMME, electrochemical liquid membrane microextraction; EMLC, electrochemically modulated liquid chromatography; ED, electrodialysis; EF, electrofiltration; EME, electrokinetic membrane extraction; EMIS, electromembrane ion source; EM-SPME, electromembrane surrounded solid phase microextraction; EMF, electro-microfiltration; EO, electro-osmosis; FID, flame ionization detector; HIS, histidine; HF-LPME, hollow fiber-based liquid-phase microextraction; ITIES, interface between two immiscible electrolyte solutions; IC, ion chromatography; LOC, Lab-on-a-Chip; LLE, liquid–liquid extraction; LDS-USAEME, low-density solvent based ultrasoundassisted emulsification microextraction; LPME, liquid-phase microextraction; ␮TAS, Micro Total Analysis Systems; MEMS, microelectromechanical systems; ME, microchip electrophoresis; MEC, microchip electrochromatography; MWCO, molecular weight cut-off; NPPE, 2-nitrophenyl pentyl ether; NPOE, 2-nitrophenyl octyl ether; N.E., nonelectrically; PHE, phenylalanine; PEME, pulsed electromembrane extraction; SPE, solid-phase extraction; SLM, supported liquid membrane; TDP, tridecyl phosphate; TEHP, tris(2-ethylhexyl) phosphate; TRY, tryptophan; VALC, voltage-assisted liquid chromatography. ∗ Corresponding author. Tel.: +98 21 82883417; fax: +98 21 88006544. E-mail addresses: [email protected], [email protected] (Y. Yamini). 0003-2670/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.aca.2013.12.019

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Y. Yamini et al. / Analytica Chimica Acta 814 (2014) 1–22

aim of this review is to provide a brief knowledge about the different fields of analytical chemistry, with an emphasis on the latest efforts put into the electrically assisted membrane-based sample preparation systems. The advantages and disadvantages of these approaches as well as the new achievements in these areas have been discussed, which might be helpful for further progress in the future. © 2013 Elsevier B.V. All rights reserved.

Contents 1. 2. 3.

4. 5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrochemical techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrically assisted extraction techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Liquid-based extraction techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Solid-based extraction techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Membrane-based extraction techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1. Electromembrane extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The other roles of electrical driving force in analytical techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concluding remarks and future trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Yadollah Yamini was born in 1968 in Dizajkhalil (Shabestar, Iran), and obtained his B.Sc. from Mazandaran University (Babolsar, Iran, 1990), his M.Sc. from Tarbiat Modares University (Tehran, Iran, 1992) under the supervision of Dr. M. Ashraf Khorassani, and his Ph.D. from Tarbiat Modares University (1996) under the supervision of Prof. M. Shamsipur. He has been a faculty member of Tarbiat Modares University since 1997, and was promoted to Professor in 2006. Professor Yamini’s research involves the development of sample preparation methods based on the elimination or reduction of organic solvents, such as solid-phase extraction, supercritical fluid extraction, solvent microextration, solid-phase microextraction, electromembrane microextraction, and supramolecular solvent microextraction. He is the referee of manuscripts for more than 45 different ISI journals. He has published more than 245 scientific articles in the ISI journals. Also, he wrote a chapter, entitled “Environmental applications of cloud-point extraction”, from Comprehensive Sampling and Sample Preparation, which was published by Elsevier in 2012.

Maryam Rezazadeh is currently a Ph.D. student in analytical chemistry at the Department of Chemistry, Tarbiat Modares University (TMU), under the direction of Prof. Yadollah Yamini. She received her bachelor’s degree (B.Sc.) in pure chemistry at the National University of Iran (Shahid Beheshti University, SBU, Tehran, Iran) in 2009. She got her master’s degree (M.Sc.) under the direction of Prof. Yadollah Yamini at TMU in 2011. Her field of interest is the development of new microextraction technologies, with an emphasis on electrically induced sample preparation methods and separation techniques.

1. Introduction Analytes of environmental or biological origin, which usually occur in complex matrices, are able to disturb the separation and data analysis stages. With regard to these concerns, a series of steps are required to remove the interfering substances, preconcentrate the analytes, and increase the sensitivity. As a consequence, the development of new analyte extraction methods and technologies has received major attention from the analytical research communities around the world.

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Shahram Seidi is currently an assistant professor at the Department of Analytical Chemistry, K.N. Toosi University of Technology (Tehran, Iran). He got his bachelor‘s degree (B.Sc.) in applied chemistry from Arak University (Arak, Iran) in 2006. He received his master’s degree (M.Sc.) at the National University of Iran (Shahid Beheshti University, SBU, Tehran, Iran), under the direction of Prof. Nahid Mashkouri Najafi and Prof. Alireza Ghasempour in 2008. In 2012, he obtained his Ph.D. in analytical chemistry from Tarbiat Modares University (TMU), under the direction of Prof. Yadollah Yamini. He attended a Postdoctoral Fellowship at TMU in 2012. His research focuses mainly on the development of miniaturized sample preparation methods, especially electrically driven ones, as well as separation techniques.

These innovative techniques are derived from traditional sample treatment methods, namely liquid–liquid extraction (LLE) and solid-phase extraction (SPE). The benefits, which motivated the research and development of sample preparation techniques, are analytical improvement, scaling down the sample sizes, overcoming the sample matrix effects, automation, speed of response, and lower costs. In recent years, a number of general reviews on sample preparation have been documented, covering different aspects of various methods [1–8]. Electric field-induced extraction is a topic that is less investigated. Fig. 1A–D illustrates the number of publications related to some of the main electrically assisted extraction techniques with the keywords of “electromembrane extraction”, “electrochemically solid-phase extraction”, “electroextraction”, and “electrochemically modulated liquid–liquid extraction” that are limited to the titles and keywords of articles incorporated in the Scopus database [9]. Fig. 1E shows the total number of publications in each year, obtained by summation of publications associated with the mentioned techniques in different years. Although this graph is not an exact criterion for all publications concerning the electrically assisted extraction methods, it can provide useful insight into these techniques. Regarding Fig. 1E and the number of citations of electrically assisted sample preparation techniques accessible at online databases, it can be concluded that these methods are gradually becoming quite commonplace in analytical chemistry. The starting point for electrically induced extraction techniques dates back to the beginning of the 19th century, when the initial studies on the migration capability of particles in solutions under


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Fig. 1. Evolution of the number of publications related to some of the main electrically assisted extraction techniques; (A) electromembrane extraction, (B) electrochemically solid phase extraction, (C) electroextraction, (D) electrochemically modulated liquid extraction and (F) total number of publications in each year related to the mentioned techniques in different years (Scopus database, Tuesday 18th December 2012).

an electric field and the extension of this phenomenon to the separation methods were carried out [10–12]. The advantages of using the electric field in separation techniques led to more comprehensive works by various research groups to investigate the potential of this new driving force in sample pretreatment methods. Electrically driven sample pretreatment was first exploited for preparative purposes, with the innovation of a U-tube apparatus by Tiselius [13]. Then, electrodialysis (ED) was extensively employed due to the selective transportation capacity of its membranes; so that the ED today is considered as a routine sample pretreatment technique with various areas of application like industries, food, and bioanalysis [14–18]. In subsequent years, the electric field was utilized for sample pretreatment purposes in liquid-phase extraction methods [19–23]. With the emergence of modern trends in analytical chemistry, in terms of simplification and miniaturization of analytical procedures, and minimization of organic solvent consumption in sample preparation, during the last decade, and noticeable improvements

made through the use of electric field in sample preparation and separation techniques, different new electrically enhanced microextraction methods have been introduced in recent years [24–29]. According to the following equation, the relative distribution of an ion (KD ) between two phases can be altered via the application of different voltages [30]: ln kD =

ZF (E − E 0 ) RT

(1)

where E is the equilibrium ion transfer potential (the Galvani potential difference between the phases), and E0 is the standard potential difference of transfer that is related to the standard Gibbs energy of transfer. R, T, F, and Z are the universal gas constant, temperature, Faraday’s constant, and the charge of the ion distributed across the interface, respectively. In this way, it becomes possible to control different properties of an extraction system, such as selectivity, clean-up rate, and

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Application of Electrical Field in Analytical Chemistry

On/In-line Non-Electrically assisted

Hyphenation with sample preparation methods Off-line Electrically assisted

VALC

Micro iltration

Electrically driven separation techniques EMLC

Ultra iltration

CE

CEC

Nano iltration

Liquid membranes

Molecular sieving

Size exclusion (iltration)

Membrane based methods Pervaporation

Solubility/diffusivity

Charge (electrical assisted) Gas separation

Reverse osmosis

SPME Electrokinetic membrane extraction

Electrokinetic trapping

SPE Electro-microiltration

Electroiltration

Electro-osmosis

LLE

ITIES

ELMME Electrodialysis

Separation techniques

Solid based methods

Liquid based methods

Extraction techniques

Electrodeposition

Combination with sample preparation methods

Electrosynthesis of membranes, SPE and SPME sorbents

Electrochemical reactions

Electrochemical techniques

Fig. 2. Different applications of electrical driving force in analytical chemistry (the gray boxes are related to non-electrically based techniques to give a more comprehensive insight).

efficiency, with the aid of a variable potential difference imposed across the liquid–liquid or solid–liquid phase boundaries. Various applications of electrical driving force to analytical chemistry can be generalized to extraction, separation, and electrochemical techniques. More details on each method are represented in Fig. 2, and will be discussed in the next sections. One of the clear benefits of electrically assisted extraction techniques is their easy integration and implementation with the chip-based devices; this subject is an interesting area of investigation nowadays [31–40]. In this paper, the focus is on electrically asisted membranebased extraction techniques, to give a general ideaof the impact of electric field on sample treatment processes and also to evaluate the actual potential of these methods. A conceptual clustering method is exploited, as a powerful and inventive way, to well scrutinize the different features of various electrically assisted techniques as well as to cause the information to be kept in mind for a longer period of time. In addition, a concise description of the theoretical principles of these methods is presented, for a better understanding of them. To this end, a comprehensive survey of the previous

and current novelties and applications of electrically assisted sample pretreatment techniques, and the review papers represented by some specialist researchers is conducted [30,41–45]. The advantages, disadvantages, and limitations of these techniques are discussed, which can help the future scientists to overcome the existing problems and improve the electrically induced sample preparation methods. Finally, the outlook for the developments of these techniques is given. This review offers an overview of the most recent advances in the area of electrochemically modulated extraction techniques, which might be of interest to more experienced users, especially the practitioners who are experts in environmental and bioanalysis methods. 2. Electrochemical techniques Electrical driving forces have been widely employed in those sample pretreatment techniques that are followed by electrochemical processes. Among these techniques, electrodeposition is


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known as a proper method for separation and preconcentration of extremely low amounts of analytes in bulk samples, which is instrumentally very simple. Speciation [46], and increasing of selectivity [47] and sensitivity of spectrometry techniques [48] are the other remarkable achievements of electrodeposition. Furthermore, electrochemical techniques have been utilized extensively in combination with sample preparation methods with the electric field applied throughout extraction, as well as those without electrical driving force [25,49–51]. For example, Saraji et al. used in situ differential pulse voltammetry coupled with hollow fiber-based liquid-phase microextraction for electrochemical analysis of desipramine in plasma and urine samples [50]. The results indicated that both the selectivity and sensitivity of the method increased owing to the elimination of interferences. One of the very attractive uses of electric fields, which is rarely pointed out in the literature, is in synthesizing and preparing the membranes [52,53] and solid-phase sorbents for SPE or solid-phase microextraction (SPME) [54–57]. Electrochemical reactions, which take place as a result of electron transfer between chemical compounds and electrode surfaces, are another outcome of applying the electric fields. Electrochemical reactions have been widely explored, as drastic tools to find different chemical information, individually or together with other analytical techniques (e.g., mass spectrometry or spectroscopic methods) [58]. Nevertheless, it should be noted that these reactions are not desirable phenomena in the cases of extraction, purification, and separation processes; since (as will be shown later) they cause serious problems during some extraction procedures, and are one of the main restricting agents.

procedure rather than an electrochemical detection technique. The system works on the principle that an aqueous phase (donor phase) flows over a stationary jellified organic phase (acceptor phase). The electrochemical basis of the ITIES is that the ions are dissolved in each electrolyte solution and then transferred across the interface according to their Gibbs energies, which induces an electric potential difference. In all published works for the ITIES extractions, the analytes are determined electrochemically, and no reports can be found on determination by means of chromatography systems. However, Arrigan et al. predicted the combination of ITIES extraction method with chromatographic techniques in the future. Electrochemical liquid membrane microextraction (ELMME) is a new sample preparation strategy based on a three-phase system consisting of a solid support, an organic liquid, and an aqueous liquid, which combines liquid membrane extraction with electrochemical detection. This technique was established by Zhu et al. in 2009 [25]. In the ELMME, the target substance initially undergoes an electrochemical reaction that changes its redox states, causing it to have more affinity with the organic liquid phase in the absence of extra redox reagents. After the extraction, the extracted species near the electrode surface could be directly determined through an electrochemical method with no other additional steps. The ELMME is a green sample preparation process and a typical technique of integrating the separation and analysis procedures. The excellent enrichment performance, simplicity, stability, trouble-free operation, low cost, and less consumption of organic solvents make this method worthy to be further scrutinized.

3. Electrically assisted extraction techniques

Electrochemical methods have found amazing applications in SPE and SPME techniques, comprising synthesis of sorbents and enhancement of extraction efficiency [28,64–69]. In the case of synthesis of sorbents, the electrochemical methods afford tailoring of key characteristics including thickness, conductivity, degree of oxidation, color, and morphology [70]. This fact has propelled various research groups toward the synthesis of new coatings [55,71–73]. Among these coatings, conducting polymers have attracted a lot of interest due to their multifunctional properties [55]. The concept of utilizing electric fields in SPE is derived from a technique termed electrochemically modulated liquid chromatography (EMLC) [30]. The EMLC was originally proposed by Fujinaga et al. in 1963 [74], then by Blaedel and Strohl [75], and independently by Roe in 1964 [76]. Usually, electric fields have two different effects on the solid-based extraction methods; first is a direct influence on the surface of solid sorbent that allows the manipulation of extraction [64–69], and second is an indirect influence, which either permits electrokinetic migration of charged analytes toward the solid sorbent [77] or facilitates elution stage [28]. Electrically assisted solid-phase extraction/microextraction and electrokinetic trapping are the instances of direct and indirect effects of electric fields on solid-based extraction techniques, respectively. According to the literature, the increasing interest in using electric fields in solid-based extraction methods can be attributed to several reasons:

The use of electrochemistry in manipulating the extractions of ionic and neutral analytes for sample preparation has been studied via solid–liquid, liquid–liquid, and membrane extraction methods. Utilization of electrical driving force, as the auxiliary energy, leads to substantial improvements. 3.1. Liquid-based extraction techniques The LLE is one of the oldest sample preparation methods, which has been exploited so far because of its simplicity and acceptable extraction efficiency. Nonetheless, this technique has some drawbacks; so that several attempts have been made to compensate them. Application of auxiliary energies, such as the electric field, is one of these efforts, which has received much attention. The idea of liquid–liquid electroextraction was first proposed in 1992 by Stichlmair et al. for extraction of fuchsine acid [23]. Generally, the functions of electric field in LLE can be classified into two groups. The first is the dispersion of one liquid phase in another to enhance mass transfer and extraction efficiency [59], and the second is the transport of charged species across a sharp phase boundary that is formed between the two immiscible liquid phases, namely liquid–liquid electroextraction [60,61]. Liquid–liquid electroextraction can be carried out off-line or on-line in tandem with some analytical instruments like capillary electrophoresis (CE) or high-performance liquid chromatography (HPLC) [60,62]. Both the direct current (d.c.) and alternating current (a.c.) electric fields can be employed in liquid–liquid electroextraction, even though most reports are associated with the d.c. one [63]. Electrochemically modulated liquid–liquid extraction technique, which is also called the interface between two immiscible electrolyte solutions (ITIES), is another electrically driven extraction method that was introduced by Arrigan et al. in 2005 [24]. The authors suggested the ITIES system as a sample preparation

3.2. Solid-based extraction techniques

1. The features of conducting polymers can be modified by varying the conditions throughout electropolymerization step to enable the extraction of analytes with different sizes and charges. 2. Compared to conventional solid-based extraction, in which a material with a fixed number of exchange sites is employed, electrically assisted extraction offers higher flexibility; since, in this method, the characteristics of the material and thus the number of its exchange sites can be externally controlled via electrochemically controlling the charge of the material [78,79].

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Membrane Structures

Symmetrical aspects

Asymmetrical

Material aspects

Microsieves

Symmetrical

Nanocomposite Membranes

Porous skin

Dense skin

Porous layer

Dense layer

Ceramic Membranes Track-etched Membranes Anodized Alumina Membranes

Template Leaching Streching of a Polymer Film Phase Inversion Coating of Membranes Interfacial Polymerisation

Fabrication Processes

Polymer Membranes

Fig. 3. A general classification of membrane structures based on symmetrical and material aspects. The dashed rectangle indicates different fabrication process of polymeric membrane.

3. The use of polymer-based fiber films in solid-based extraction techniques can be extended to the analysis of neutral, electro-inactive analytes by taking advantage of electrochemically controlled hydrophobic/hydrophilic ‘switching’ [80]. 4. Electrically assisted solid-based extraction methods can be exploited for the extraction of ions and analytes that normally need to be derivatized prior to traditional solid-based extraction [81,82]. 5. In electrically assisted solid-based extraction techniques, the extraction and desorption stages are merely performed through altering the potential of conducting polymer-coated electrode. Therefore, in these methods, it is not necessary to change the solvent to allow desorption of compounds [31]. 6. Desorption of electrostatically held analytes, by varying the electrochemical potential of polymer, may also be more rapid relative to the desorption techniques commonly utilized in conventional solid-based extractions [32]. This makes the above method particularly interesting for use in conjunction with miniaturized analytical systems. 3.3. Membrane-based extraction techniques Since initial membrane experiments on osmosis were carried out in the 18th century using biological membranes, and a functional synthetic reverse osmosis membrane was produced from cellulose acetate polymer by Sidney Loeb [83], membranes have found applications in various parts of modern sample preparation and separation methods implemented in industrial societies worldwide. From the 18th century until now, a big revolution has happened in development of various membranes for different applications ranging from nanofiltration (partial desalination) to ultrafiltration (virus removal) and microfiltration (elimination of suspended

solids) [84]. A general classification of membrane structures in terms of symmetry and composition is depicted in Fig. 3. Because of a wide variety of polymeric materials, more research attempts have been made to develop polymeric membranes in comparison with other membranes; the different fabrication processes for synthesis of this type of membranes are represented in Fig. 3. For a detailed description, interested readers are referred to Ref. [84]. Fig. 2 illustrates some membrane-based separation procedures arranged according to the mechanism of separation. Discussion about sample preparation methods with no electrical driving forces is beyond the scope of this review; perfect information on these techniques is available in the literature [84,85]. Procedures, in which electric fields are used, include: ED, electro-osmosis (EO), electrofiltration (EF), electro-microfiltration (EMF), and electrokinetic membrane extraction (EME). The ED and EO have similar principles; so that in both methods an electric field is applied between two electrodes that are located on both sides of a membrane or fiber [42,81]. Nevertheless, in the ED, charged solutes are transported across the membrane, which is normally an ion exchange or a molecular weight cut-off (MWCO) membrane, whereas in the EO it is the solvent that passes through the membrane. The generated molecular flux in the ED is owing to diffusion and impact of potential difference, and, in this technique, separation is achieved based on the charges of solutes and their molecular masses. Filtration is a term that has been established to indicate a specific domain of membrane technology in between ultrafiltration and reverse osmosis [42,84]. The EF is a kind of filtration process carried out in the presence of an electric field. Various filtration procedures are categorized in terms of membrane pore size; so that membranes with pore sizes in the range of 50 nm to 5 ␮m are exploited for microfiltration [42,84].


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Fig. 4. (A) The primary equipment for electromembrane extraction, (B) effects of container volume and stirring rate on the extraction recovery.

Application of voltage improves the efficiencies of all electrically assisted membrane-based processes, like ED, EO, and EF, compared to non-electrically (N.E.) ones; nonetheless, bubble formation and thermal degradation of membrane are important impediments to increasing of voltage in the former processes [86]. Extensive studies on the ED, EO, and EF techniques have been comprehensively reviewed in the literature by different specialists [42,84,86]. Meanwhile, it is worth mentioning that utilization of membranes and electrical driving forces is not restricted to sample treatment methods in analytical chemistry. For instance, electromembrane ion source (EMIS) technique is considered as a potential approach for direct mass spectrometric (MS) sampling under ambient conditions [87]. The EMIS affords both the on-line sampling and MS characterization of inorganic ions. It even seems to be a fairly suitable method for examining by far the largest organic entities such as biomolecules. In various EMIS applications, a dense network of moderately long (10–20 ␮m), yet very narrow (submicrometer), channels in track membrane governs the transport of charged species as well as the field structure [87]. After a brief discussion about these techniques, now we will focus on the uses of a more recent electrically assisted membranebased procedure, EME, for a variety of sample types, which have drawn special attention in the area of analytical sample pretreatment. The concept of EME was initially suggested by Pedersen-Bjergaard and Rasmussen in 2006 [29]. Several reviews are available, in which the EME has been partially or completely described by the originators and also some experts [30,41–45,88]. This review intends to view the EME from a different perspective and present recent developments, novelties, and innovations relevant to this method. 3.3.1. Electromembrane extraction The basis of EME is the migration of charged species in an electric field. The applied voltage in the EME system causes ionizable compounds to be transported from an aqueous sample solution across a supported liquid membrane (SLM) into an acceptor phase. The primary EME setup is displayed in Fig. 4A; the features of this setup are similar to hollow fiber-based liquid-phase microextraction (HFLPME), except that the former employs two platinum electrodes that are placed in the sample solution and acceptor phase and thus supply the electric field. In the EME, a d.c. potential, typically in the range of 1–300 V, is applied to make the ionized forms of target

analytes migrate across a 200 ␮m-thick SLM, located in the pores of a porous hollow fiber, into an aqueous acceptor solution inside the lumen of hollow fiber. Theoretical studies were carried out to examine the obvious mechanism and the extraction kinetics of EME, and the influences of various parameters on the EME efficiency. The primary EME setup was exploited for extraction of several classes of analytes, comprising acidic, basic, and zwitterionic compounds, and inorganic cationic/anionic species, and coupled to various analytical instruments, like HPLC, CE, ion chromatograph (IC), and gas chromatograph (GC) with different detectors, for analysis of acceptor phase composition. Although the EME markedly reduces the extraction time and provides more efficient extraction and sample clean-up, especially in comparison with HF-LPME, it suffers from some weaknesses. Hence, scientists have tried to develop new extraction setups and novel techniques to overcome these difficulties. 3.3.1.1. Theoretical studies on EME. In order to figure out the influential variables in EME, this extraction method has been explored theoretically. Commonly, two different sets of parameters affect the extraction efficiency of EME. The first group incorporates setuprelated parameters and the other consists of parameters that are connected with extraction procedure and mechanism. Here is a brief explanation of each class of variables. 3.3.1.1.1. Setup-related parameters. Electrode materials: Since electrolysis may occur at the surface of electrodes, they should be made of inert materials such as platinum, which is a good choice for this aim. Electrode distance: The distance between electrodes is a significant parameter for successful EME. As it is known, the strength of an electric field (E) is gained from the equation below: E=

V d

(2)

wherein d is the inter-electrode distance and V is the applied voltage. It is apparent that if electrical charges cannot cross an organic membrane, the behavior of electrodes and phase interfaces will be analogous to that of a capacitor. Under these circumstances, the polarized SLM can act as a dielectric capacitor [89]. Eq. (2) is a well-known equation, which defines the breakdown voltage of a capacitor and the electric current that passes through the dielectric

8


of a capacitor or the organic liquid membrane in an electromembrane system [90]. The electric current level should be controlled in an electromembrane process to enable phase transfer and electrokinetic migration of analytes, and avoid any electrochemical reactions. The breakdown voltage of a capacitor fully determines the critical voltage of an electromembrane system, which depends on the inter-electrode distance. As seen from Eq. (2), increasing of the distance between electrodes diminishes the strength of electric field and the current level, and therefore limits the phase transfer of analytes. On the other hand, decreasing of inter-electrode distance raises the electric current and lowers the breakdown voltage, thereby making the dielectric conductive; in these conditions, sparking may happen, which can even cause breaking of electrodes. Moreover, placing the electrodes very close to each other leads to the enhanced migration of ions through the SLM and Joule heating. This phenomenon may result in system instability and punctuation of the SLM due to a rise in temperature [91–93]. The differences in the optimum inter-electrode distances in various publications may be attributed to the fact that this parameter is affected by the configuration of the EME setup. Accordingly, the optimized value of inter-electrode distance in an EME setup cannot be employed for another setup with a different configuration. Even though inter-electrode distance has been considered as an effective parameter in the majority of EME publications, its impact has only been explained descriptively and no experimental investigations have been documented for it in the literature. The influence of inter-electrode distance can be straightforwardly anticipated and described by means of theoretical rules (Eq. (2)). It should be taken into account that there are some limitations on increasing the inter-electrode distance, owing to the restricted internal diameter of sample container. This subject must be considered for preparation of a setup for the EME; so that the EME setup must possess the ability to maintain the inter-electrode distance constant. Thickness of electrodes: The breakdown voltage of a capacitor is directly dependent on the surfaces of electrodes [94]. For this reason, the electric current passing through the phase interfaces is raised by increasing the surfaces of electrodes or enhancing their thicknesses. Hence, the thickness of electrodes should be considered; so that the electric current of a system must be adequate to facilitate phase transfer of analytes, but it must be controlled to prevent from electrochemical reactions, instability problems, and (in some cases) sparking. The thickness of electrodes is affected by the inner diameter of hollow fiber, and a significant amount of acceptor phase is lost using thick electrodes. So, the electrodes must have suitable diameters and be firm enough to possess physical stability and have constant inter-electrode distance. Consequently, this parameter should be tested to reach the most favorable extraction conditions. Hollow fiber characteristics: Hollow fibers, which act as a liquid membrane supporter, are commercially accessible with given characteristics. They have internal diameters of 150, 300, 600, and 1200 ␮m. When the hollow fiber inner diameter is known, the collected volume of acceptor phase (Vcylinder ) can be calculated theoretically: Vcylinder = r 2 h

(3)

where r and h are hollow fiber internal diameter and height, respectively. Using thicker electrodes becomes practically feasible by enhancing the inner diameter of hollow fiber. In this way, repeatability and reproducibility may increase; as the inter-electrode distance is more likely to remain unchanged during an extraction procedure. Besides, increasing of the hollow fiber length (and thus reduction of its internal diameter) also results in the enhancement of available surface of organic phase, which may lead to the

improvement of extraction efficiency. In the meantime, to impregnate a non-polar organic solvent, the hollow fiber must possess an appropriate hydrophobicity, and the pore size of hollow fiber can affect the capillary forces, which helps retention of organic solvent and control of sample cleanup. Container characteristics: Mass transfer in an EME system is accomplished by convection and electrokinetic migration. In small compartments, the effect of electric field on the migration of analytes is greater in comparison with large compartments [95]. The influences of volume of container and stirring rate on the extraction recoveries of a group of model analytes were scrutinized simultaneously; the obtained results are depicted in Fig. 4B. Variation of the average response demonstrated that the impact of electric field declined and the importance of stirring increased by increasing the container volume. Also, stirring was effective even with small compartments. 3.3.1.1.2. Parameters related to extraction procedure and mechanism. The flux of analytes, driven by an electrical potential over a SLM, in EME has been described by means of a mathematical model based on the Nernst–Planck equation [96]. It was assumed that the EME process is analogous to the ionthophoretic transportation of drugs through the skin, and the modified theoretical expression of the latter [97–102] was employed to describe the flux of analytes in EME. The results from the mathematical model were verified experimentally, and it was proved that the following equation could be exploited to determine the flux of analytes across the SLM: Ji =

Di h

1+

ln x

x−1 x − exp(−)

(Cih − Cio exp(−))

(4)

wherein Di is the diffusion coefficient of analyte, h is the thickness of membrane, Cih is the analyte concentration at the SLM/sample interface, and Cio is the analyte concentration at the acceptor/SLM interface. Also, is a function of electrical potential [96], and is the ratio of the total ionic concentration in the sample solution to that in the acceptor solution, which is defined as the ion balance. Therefore, the flux of analytes (or the extraction efficiency) is affected by the nature of the SLM that determines the diffusion coefficients of target substances (Di ), the thickness of SLM, and the ionic strengths of the donor and acceptor phases (), which are principally determined via the pHs of both phases and the applied voltage. Moreover, time is another influential parameter in the extraction efficiency of EME. Composition of organic liquid membrane: Choosing the composition of organic liquid membrane is one of the most essential steps in the EME technique. The extractability of analytes is highly influenced by the nature of the SLM, and it has been revealed that the extraction recovery improves as the viscosity of the organic solvent decreases [103]. The SLM should be water-immiscible and have a polarity similar to that of polypropylene fiber so that it can be easily immobilized within the pores of the fiber. The SLM should also have an appropriate electrical resistance to keep the electric current of the system in its lowest possible level, even after applying high voltages, and possess specific chemical properties to permit phase transfer and electrokinetic migration of model analytes [104]. Meanwhile, charged analytes should have proper solubility in the SLM, in order to be transported across it. The organic solvents, which are utilized as the SLM for extraction of different kinds of analytes, are tabulated in Table 1 [104–131]. In summary, the lipophilicities of the organic liquid membrane and the analyte should be consistent with each other, and the SLMs with high values of log P are suitable for hydrophobic compounds, while hydrophilic analytes prefer the more hydrophilic organic solvents [133]. Although 2-nitrophenyl octyl ether (NPOE) is an excellent organic solvent for extraction of non-polar basic substances, it is not efficient for extraction of polar compounds with small values of log P (the log P value for the non-charged form of

Table 1 The organic solvents used as SLM in EME, and their properties. SLM

Analyte

Typea

Log Pb

NPOE

5.35

H-acceptorb

H-donorb

4

0

Molar volumeb (cm3 ) 240.825

Polar surface areab (Å) 55.05

Number of heteroatoms

Basic

>2.5

1, 2 3, 4 5, 6 7, 8 1, 2 3, 4 5, 6 7, 8 1, 2 3, 4 1, 2 3, 4 7, 8 3, 4 3, 4 5, 6 7, 8 9, 10

35–90 16–95 0–76 0–24 0–79 0–48 69 83 0–70 55 0–33 0 0 0 0 0–55 0 0

[107,125,130] [29,104,107,125] [29,104,107,119] [104,107] [29,104,107] [107] [104] [110] [29,104] [123] [104,107] [104,107] [107] [107] [107] [104,107,128] [104,107] [107]

3, 4 5, 6 1, 2 1, 2

0 0 0 0

[29] [29] [29] [29]

3, 4 5, 6 1, 2 1, 2 5, 6 – 1, 2 3, 4 5, 6 3, 4 3, 4 5, 6

3 3–95 7 4 46–77 90–107c 14 16, 25 95 18–20 22 39–61

[29] [29,124] [29] [29] [128] [113] [115] [115] [124] [115] [115] [128]

3, 4 5, 6 1, 2 1, 2 5, 6 5, 6 5, 6 7, 8

– – – – 20.9 24.2 21.8 11.4

[29] [29] [29] [29] [131] [131] [131] [131]

3, 4 5, 6 1, 2 1, 2

0 0 0 0

[29] [29] [29] [29]

1.5–1.0

1.0–0.5 0.5>

1

0

232.979

9.23

Basic

>2.5 2.5–2.0 2.0–1.5

1-Octanol

2.88

1

1

158.095

20.23

Basic

>2.5

Metal Acidic

2.5–2.0 2.0–1.5 0.5> 0.5> >2.5

2.5–2.0 0.5> 1-Octanone

Dodecyl acetate

2.95

5.88

1

2

0

0

157.908

263.112

17.07

26.3

Basic

>2.5

Acidic

2.5–2.0 2.0–1.5 1.5–1.0 1.0–0.5 0.5>

Basic

>2.5 2.5–2.0 2.0–1.5


Log Pb

2.0–1.5

5.12

Ref

Type

2.5–2.0

Dihexyl ether

ERa %

9

10

Table 1 (Continued) SLM

Analyte

Typea

Log Pb

H-acceptorb

H-donorb

NPOE + DEHPd

5.35

4

0




Basic

>2.5

3, 4 5, 6 7, 8 1, 2 5, 6 1, 2 5, 6 1, 2 3, 4 3, 4 5, 6 7, 8

0–75 0–4 10 5–31 0 13–30 54–62 2–37 59 0–10.8 0.6–37 13

[29,104,118] [29,104] [104] [29,104] [104] [29,104] [118] [104,127] [104] [104,114] [104,114,121] [104]

3, 4 5, 6 1, 2 1, 2

0 0 0 0

[29] [29] [29] [29]

3, 4 5, 6 1, 2 1, 2

0 0 0 0

[29] [29] [29] [29]

2.5–2.0 2.0–1.5

3, 4 5, 6 1, 2 1, 2

0 0 0 0

[29] [29] [29] [29]

>2.5

3, 4

13

[29]

2.5–2.0 2.0–1.5

5, 6 1, 2 1, 2

78–79 73 73

[29] [29] [29]

3, 4 5, 6 7, 8 1, 2 3, 4 5, 6 1, 2 3, 4 1, 2 3, 4 3, 4 5, 6 7, 8

0–78 53–83 31 53–91 63–66 70 17–57 70 78 0 0 0 0

[104,122] [104] [104] [104,120] [120] [104] [104,120] [120] [104] [104] [104] [104] [104]

1.5–1.0 0.5>

0

0

164.634–419.399

0

Basic

>2.5 2.5–2.0 2.0–1.5

Silicone oil AS 4

–

1

0

–

17.07

Basic

>2.5 2.5–2.0 2.0–1.5

Soybean oil

Peppermint oil (menthone)

NPOE + TEHPd

–

2.76

5.35

–

1

4

–

0

0

–

175.053

240.825

–

17.07

55.05

Basic

Basic

Basic

>2.5

>2.5

2.5–2.0

2.0–1.5 1.5–1.0 0.5>


Log Pb

2.0–1.5

3.76–10.76

Ref

Type

2.5–2.0

Kerosene

ERa %


Analyte

Typea

Log Pb

H-acceptorb

H-donorb

NPOE + TEHP + DEHPd

5.35

4

0


Polar surface areab (Å)

Type

55.05

>2.5

3, 4 5, 6 7, 8 1, 2 5, 6 1, 2 1, 2 3, 4 3, 4 5, 6 7, 8

0–61 0–5 – 39–64 – 69 7 44 6 10–53 23

[104] [104] [104] [104] [104] [104] [104] [104] [104] [104,126] [104]

61 11–86 74–91 8 100 34 95.9–106.7c

[105] [105] [105] [105] [105] [105] [111]

0.5>

1-Ethyl-2nitrobenzene

1-Isopropyl-4nitrobenzene

1-Nitropropane

2.94

3.43

0.94

1

3

3

3

1

0

0

0

141.589

134.092

151.348

90.835

20.23

45.82

45.82

45.82

Acidic

>2.5

Inorganic anion

2.5–2.0 1.0–0.5 0.5> 0.5>

1, 2 3, 4 5, 6 5, 6 3, 4 5, 6 5, 6

Basic

>2.5

1, 2

38–70

[108,130]

2.5–2.0 2.0–1.5

3, 4 5, 6 1, 2 7, 8 1, 2

63–95 54–80 80 70 78

[106,108] [106,108] [106] [110] [106]

>2.5

3, 4

59

[106]

2.5–2.0 2.0–1.5

5, 6 1, 2 1, 2

88–91 93 86

[106] [106] [106]

3, 4 5, 6 1, 2 1, 2

0 0 0 0

[106] [106] [106] [106]

3, 4 5, 6 1, 2 1, 2

7 3–5 11 7

[106] [106] [106] [106]

1, 2 3, 4 5, 6 1, 2 7, 8 1, 2

8–10 14 7–11 14 14 8

[130] [106] [106] [106] [110] [106]

Basic

Basic

>2.5 2.5–2.0 2.0–1.5

1-Nitropentane

1.96

3

0

151.348

45.82

Basic

>2.5 2.5–2.0 2.0–1.5

Nitrobenzene

1.92

3

0

101.275

45.82

Basic

>2.5

2.5–2.0 2.0–1.5



2.0–1.5 1.5–1.0

2.37

Ref

Log Pb

2.5–2.0

1-Heptanol

ERa %

11

12


Analyte

Typea

Log Pb

H-acceptorb

H-donorb

Dodecyl-2nitrophenylether

7.58

4

0

2-Nitrophenyl pentyl ether (NPPE)

3.82

4

0


191.304


55.05


Basic

>2.5

3, 4

7

[106]

2.5–2.0 2.0–1.5

5, 6 1, 2 1, 2

5, 3 11 7

[106] [106] [106]

>2.5

5, 6

60–75

[107]

7, 8 1, 2 3, 4 1, 2 3, 4

83–85 47 55 2 37

[107] [107] [107] [107] [107]

1, 2 3, 4 1, 2 3, 4

43 70 60 36

[107] [107] [107] [107]

Basic

Basic

2.5–2.0 2.0–1.5

4-Nitro-m-xylene + diisobutyl ketoned

2.97

3

0

133.825

45.82

1-Octanol + DEHP + 4nitro-m-xylene + diisobutyl ketone

Basic

1.0–0.5 0.5>

3, 4 3, 4 5, 6

6–19 2 6

[107] [107] [107]

Basic

0.5>

>10

0

[109]

Basic

0.5>

>10

61–78

[109]

1-Octanol + DEHP + 4nitro-m-xylened

2.88

1

1

158.095

20.23

Basic

0.5>

>10

32–49

[109]

1-Octanol + DEHP + diisobutyl ketoned

2.88

1

1

158.095

20.23

Basic

0.5>

>10

46–61

[109]

4-Nitro-mxylene + DEHP + diisobutyl ketoned

2.97

3

0

133.825

45.82

Basic

0.5>

>10

52–73

[109]

Basic

0.5>

>10

43–59

[109]

Basic

1.0–0.5

Metal

0.5> 0.5>

>10 >10 >10 –

0–42 2–53 34–57 6.6–11.1

[103] [103] [109] [112]

Acidic

0.5>

1, 2 3, 4 5, 6

87–106c 87–106c 87–106c

[92] [92] [92]

1Octanol + DEHP + NPOE 1-Octanol + DEHPd

Toluene

2.88

2.72

1

0

1

0

158.095

105.709

20.23

0


Log Pb

2.0–1.5

NPPE + DEHPd

Ref

Type

2.5–2.0

NPPE + TEHPd

ERa %

Table 1 (Continued) SLM Typea

Analyte Log Pb

H-acceptorb

H-donorb

Molar volumeb (cm3 )


Type

Log Pb


ERa %

Ref

−0.37

2

1

98.33

29.46

Basic

0.5>

>10

Current

[116]

Tetrahydrofurfuryl alcohol + DEHPd

−0.37

2

1

98.33

29.46

Basic

0.5>

>10

0

[116]

2-Phenylethanol

1.5

1

1

119.757

20.23

Basic

0.5>

>10

Current

[116]

2Phenylethanol + DEHPd

1.5

1

1

119.757

20.23

Basic

0.5>

>10

0

[116]

Isophorone + DEHPd

1.5

1

0

152.644

17.07

Basic

0.5>

>10

Current

[116]

Basic

0.5>

>10

0

[116]

Isophorone Furfuryl alcohol

0.21

2

1

86.044

33.37

Basic

0.5>

>10

Current

[116]

Furfuryl alcohol + DEHPd

0.21

2

1

86.044

33.37

Basic

0.5>

>10

0

[116]

Santolina alcohol

3.08

1

1

180.043

49.46

Basic

0.5>

>10

Current

[116]

Santolina alcohol + DEHPd

3.08

1

1

180.043

49.46

Basic

0.5>

>10

Acceptable

[116]

1-Chloropinacolone

0.99

1

0

136.129

17.07

Basic

0.5>

>10

0

[116]

1Chloropinacolone + DEHPd

0.99

1

0

136.129

17.07

Basic

0.5>

>10

Current

[116]

Acetophenone (and +DEHP)

1.67

1

0

120.964

17.07

Basic

0.5>

>10

Current

[116]

Cyclohexanone (and +DEHP)

0.82

1

0

102.952

17.07

Basic

0.5>

>10

Current

[116]

Benzaldehyde (and +DEHP)

1.45

1

0

101.099

17.07

Basic

0.5>

>10

Current

[116]

1,2,4-Trifluoro-5nitrobenzene (and +DEHP)

1.8

3

0

113.904

45.82

Basic

0.5>

>10

0

[116]

2-Octyl-1-dodecano (and +DEHP)

8.83

1

1

356.553

Basic

0.5>

>10

0

[116]

2-Nitrophenethyl alcohol (and +DEHP)

1.59

4

1

131.598

66.05

Basic

0.5>

>10

0

[116]

2-Hexyl-1-decanol (and +DEHP)

6.80

1

1

290.526

20.23

Basic

0.5>

>10

0

[116]

2-Nonanone

3.02

1

0

174.216

17.07

Basic

0.5>

>10

0

[116]

2-Nonanone + DEHPd

3.02

1

0

174.216

17.07

Basic

>10

Acceptable

[116]

2023


Tetrahydrofurfuryl alcohol

13

14


Analyte Log Pb

H-acceptorb

H-donorb

Methylacetophenone

2.18

1

0

137.471

Methylacetophenone + DEHPd

2.18

1

0

137.471

3-Nitrostyrene (and +DEHP)

2.41

3

0

3.08

1

Perillyl alcohol + DEHP

3.08

1-Nonanol

3.39

1-Nonanol + DEHPd

Ref

Type

Log Pb


17.07

Basic

0.5>

>10

0

[116]

17.07

Basic

0.5>

>10

Acceptable

[116]

127.223

45.82

Basic

0.5>

>10

0

[116]

1

161.843

20.23

Basic

0.5>

>10

0

[116]

1

1

161.843

20.23

Basic

>10

Acceptable

[116]

1

1

174.602

20.23

Basic

0.5>

>10

0

[116]

3.39

1

1

174.602

20.23

Basic

0.5>

>10

0–32

[116]

1-Nonanol + tridecyl phosphate (TDP)d

3.39

1

1

174.602

20.23

Basic

0.5>

>10

0–24

[116]

2-Octanone + DEHPd

2.95

1

0

157.908

17.07

Basic

0.5>

>10

0–40

[116]

2-Octanone + TDP

2.95

1

0

157.908

17.07

Basic

0.5>

>10

3–58

[116]

Eugenol

2.4

2

1

156.283

29.46

Basic

0.5>

>10

Acceptable

[117]

−0.69

1

1

42.548

20.23

Anion

0.5>

1, 2 3, 4 5, 6

7.2 8.4–32 72.7

[129] [129] [129]

DEHP

2.83

4

1

327.775

65.57

TEHP

9.48

4

0

469.062

54.57

TDP

4.78

4

2

Perillyl alcohol d

d

Methanol

a b c d

Molar volumeb (cm3 )

–


82.23

DEHP: di-(2-ethylhexyl) phosphate, ER%: extraction recovery percent, NPPE: 2-nitrophenyl pentyl ether, NPOE: 2-nitrophenyl octyl ether, TDP: tridecyl phosphate, TEHP: tris-(2-ethylhexyl) phosphate. The log P values are related to non-charged compounds (reference: www.chemspider.org). Relative recoveries were reported. Properties were reported for constituent solvent.


Typea

ERa %


compound). Pedersen–Bjergaard and his research team figured out how to modify the NPOE for extraction of polar drugs with the aid of some additives like di-(2-ethylhexyl) phosphate (DEHP) and tris(2ethylhexyl) phosphate (TEHP) [105]. Studies also confirm that high content of heteroatoms diminishes the solubility of target analytes in the SLM [103], and that the recoveries decline as the number of heteroatoms in the chemical structures of analytes increases (Table 1). Nevertheless, the reduction of solubility as a result of high content of heteroatoms may be compensated via the addition of some carriers [119]. It was demonstrated that singly charged basic drugs with log P > 2 could be effectively extracted by means of nitro-aromatic compounds as the SLM [103]. However, for medium polar analytes (1 < log P < 2), the addition of TEHP to the SLM is needed to acquire a good liquid membrane composition, and for extremely polar drugs (log P < 1), the addition of DEHP to the SLM is necessary to attain admissible extraction recoveries [103]. Furthermore, a mixture of NPOE, DEHP, and TEHP could be beneficial to extraction of most hydrophilic compounds [105,126]. As it is observed from the data in Table 1, another parameter, which should be considered for selection of SLM, is the number of H-acceptor or H-donor functionalities in an organic solvent. According to Table 1, organic solvents, containing several H-acceptor groups, are appropriate SLMs for extraction of basic analytes. Hence, nitro-aromatic compounds with a higher number of H-acceptor (4) and a lower number of H-donor groups (0) are excellent solvents for basic drugs. In contrast, a proper liquid membrane for extraction of acidic analytes is the one that possesses a relatively high number of H-donor functionalities. Accordingly, long-chain alcohols are desirable alternatives for this aim [106]. Because water-immiscible solvents with a high number of H-donor and a low number of Hacceptor groups have not still been discovered, the EME faces some difficulties in the extraction of acidic drugs. Alcohols have the same number of H-donor and H-acceptor functionalities (1), and thus could be employed for extraction of both the acidic [92,106,129] and basic [108,110,113,114,117,118,124,128,129] analytes. Gjelstad and coworkers examined the role of SLM composition in selective extraction of basic drugs under a moderately low electrical potential difference [133]. They proved that the mass transfer of protonated basic drugs across the SLM was largely dependent on structure. Since basic drug substances with log P values below 2.3 were too polar to penetrate into the SLM under low-voltage conditions, almost no mass transfer was observed for them using NPOE as the SLM [132]. The number of basic functionalities was another effective parameter, and almost no mass transfer was detected for more hydrophobic dibasic drugs (log P ≥ 2.3), whereas substantial mass transfer was observed for monobasic drugs. In addition, differences in mass transfer were detected for hydrophobic and monobasic drugs, which appeared to be partly correlated with the polar surface areas of the drugs. Investigations by Nojavan et al. verified that the type of electrical charge has no effect on the extraction efficiencies of analytes, and that the charge position might be an influential parameter [128]. Balchen et al. explored various analyte–SLM interactions affecting the extraction efficiency [118]. They stated that pure eugenol, which is capable of establishing solvent interactions, could be exploited for extraction of hydrophobic peptides. Nonetheless, the addition of DEHP into 1-octanol led to the formation of a suitable solvent for both the hydrophobic and hydrophilic peptides, which caused the extraction to proceed based on ionic interactions between the peptides and DEHP (as an ion-pairing reagent). Besides, complexation of the peptides with the crown ether present in the SLM composition was crucial for improving the selectivity for the most hydrophobic peptides [118]. In a recent work by Yamini et al., SLM engineering was employed for highly selective extraction of drugs [121]. They proposed a method for selective extraction of atenolol (hydrophilic ␤-blocker) in the

15

presence of propranolol and betaxolol (hydrophobic ␤-blockers). They considered the fact that a liquid membrane of pure NPOE is efficient for extraction of non-polar drugs (the addition of DEHP to NPOE has a negative impact on their extractability), while the addition of DEHP to the liquid membrane is required for extraction of polar substances [121]. All represented works confirm that the composition of SLM is of vital importance to the EME efficiency, and affects the extraction recovery, extraction speed, system stability, range of applied voltage, and selectivity. Ion balance (): It is deduced from Eq. (4) that the decreasing of ion balance results in the enhancement of flux. So, high recoveries may be acquired, when the ion concentration in the acceptor solution is high compared to the donor solution [96]. Ionic concentrations of the phases are chiefly determined by phase pHs. For this reason, a pH gradient is essential for an excellent extraction. To extract the typical basic analytes, the sample solution should have an adequate pH value, in order to convert the target analytes into their ionized forms and make them able to migrate through the electric field. If the pH value of the acceptor phase is low (below the pKa values of the basic analytes), the analytes are straightforwardly released into the acceptor solution. The pH gradient is more important, when the SLM contains an ion-pairing reagent such as DEHP; because a fairly high concentration of H+ is needed in the acceptor phase to establish an interaction with the anionic carrier (DEHP) and release the cationic analytes. It was revealed that the pH variation in the acceptor phase noticeably influences the extraction efficiency, but the change in the pH of donor solution has no significant effect on the extraction recovery [104,123]. Yamini et al. designed some experiments to thoroughly scrutinize the impact of ion balance [123]. They showed that the flux of analytes is proportional to (based on the theoretical model (Eq.4)); not only the ratio of the total ionic concentration of the donor phase to that of the acceptor phase affects the flux, but also the exact ionic concentrations (or pHs) of the donor and acceptor phases influence the extraction recovery independently. Nevertheless, the role of acceptor phase pH is more important. For example, = 10 could be obtained, when there are 1 mM HCl in the acceptor phase (Ca ) and 10 mM HCl in the donor phase (Cd ), or when Ca = 10 and Cd = 100, or when Ca = 100 and Cd = 1000. All of these cases have identical values of ion balance, and the first has the lowest recovery and the latter the greatest. Since it has been demonstrated that the variation in the pH of donor solution has no substantial effect on the extraction recovery [123], the results could be explained by the role of acceptor phase pH; so that the extraction recovery improves as the concentrations of ionic species in the acceptor phase increase. However, the donor phase pH should not be overlooked. In fact, the optimal pH of donor solution depends on the nature of analytes. In these procedures, proper conditions must be applied so that all of the analytes exist in their ionized forms, while has its minimum value. The impact of ion balance was tested for extraction of doubly charged drugs with medium polarity (1 < log P < 2) [103]. It was figured out that the ion balance could enhance the extraction efficiencies of these analytes. The extractability of doubly charged basic drugs improved as the pH of donor solution was raised owing to charge reduction (only one ionizable group in the chemical structure of each drug could be ionized, making the drug singly charged) [103]. Gjelstad et al. proved that the EME of basic drugs from the sample solution could also be done at higher pH values (pH 10–11) [133]. This phenomenon was probably due to the extraction of the analytes via a mixed-mode mechanism at high pH values [133]. Since the analytes were partially ionized, the charged analytes were extracted by electrokinetic migration and the neutral analytes could be extracted through a passive diffusion mechanism. Nonetheless, the results indicated that the mixed-mode

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Fig. 5. Condenser effect and current behavior of an EME system by applying voltage. Current vs. time reproduced with permission from Elsevier-modified Ref. [105].

mechanism did not affect the extraction kinetics, and that the EME system reached equilibrium after 5 min [133]. Hence, the EME of the charged analyte molecules was the principal mechanism, even at high pH values. Other possible reasons for this phenomenon (i.e., the extraction of basic drugs at high pH values) were the establishment of equilibrium at each pH value for the analytes, and the efficient extraction of the drugs, which partially existed in their cationic forms, toward the cathode, even at pHs greater than their pKa values. Applied voltage and extraction time: The major driving force for the migration of analytes across the liquid membrane is produced by the electric field. The strength of an electric field is dependent on the applied voltage, and thus the applied voltage influences the flux of analytes through the membrane [96]. Although it is anticipated that the extraction recovery will improve as the applied voltage is enhanced, there are some limitations on increasing of voltage. In some cases, the enhancement of voltage may lead to a decline in recovery because of mass transfer resistance, which is due to the build-up of boundary layers of ions at the interfaces on both sides of SLM or the saturation of analytes in the acceptor phase. Also, when the pH is slightly raised in the acceptor solution (in the case of extraction of basic analytes) owing to electrolysis, the analytes are back-extracted into the donor phase, causing the extraction efficiency to diminish. Consequently, the extractability may decrease by increasing the strength of electric field [108,119,120,124,127,134,135]. Kjelsen et al. revealed that by improving the sample-to-SLM distribution ratio, the EME could be performed at potential differences (5–10 V) obtainable by common batteries [107]. They also figured out that the range of applied voltage is determined by the nature of SLM, and the range may become broader as the electrical resistance of the liquid membrane is enhanced. Thus, for some organic liquid membranes, such as long-chain alcohols, the applied voltage is limited to 100 V, whereas NPOE can tolerate up to 300 V. It is worthy to note that a rise in the applied voltage makes the system instable due to an increase in the current level. If the resistance of the system is assumed constant (for a typical SLM), one can prove, by using Ohm’s law, that the current will be raised by enhancing the applied voltage. 3.3.1.1.3. The nature of observed current in EME. By applying a potential difference over the liquid membrane in EME, a short peak current is initially observed, followed by a slow and gradual decline in current until reaching a stabilized level in the system

(Fig. 5). The amounts of peak current and final stabilized current depend on the polarities of the SLM and analytes, the applied potential, and the total concentrations of ions in both the donor and acceptor phases. For instance, typical stabilized currents of 5 ␮A and 200 ␮A were reported, when a voltage of 300 V was applied across two different SLMs consisting of pure NPOE and NPOE with 25% (w w−1 ) DEHP, respectively [105]. This phenomenon could be ascribed to the increasing of liquid membrane polarity by addition of DEHP, which led to the decreasing of the liquid membrane electrical resistance [105]. The observed current in EME may be attributed to two different sources: ion exchange current (iex ) and electrolysis reaction (ie ). The ion exchange current is produced by the migration of cations and anions in opposite directions through the SLM under an electric field, and the exchange of ions between the donor and acceptor phases (iex ). Ion transportation across the SLM has been experimentally confirmed in a recent paper by Dryfe et al. [136]. Ie is probably generated via the electrolysis reactions on the surfaces of electrodes. Therefore, the total current in EME (it ) can be calculated by the following equation: it = iex + ie

(5)

Suppose that no electrolysis reactions take place on the surfaces of electrodes (particularly at the beginning of the extraction procedure), then: it = iex

(6)

As it is known, iex is not measurable; so, what is the origin of the observed current at the beginning of extraction operation? The origin of this current comes back to different electromobility of various anions and cations that cross the membrane in opposite directions (Fig. 5). If the migration rate of both anions and cations to be equal through the SLM, the ion exchange current cannot be measured. Therefore, with ignoring of electrolysis reactions (an ideal supposition) no electrical current should be observed in EME. Actually, current behavior in EME is like a capacitance; at the beginning of extraction by applying voltage a double layer is suddenly created in both sides around the SLM and a condenser effect is created (Fig. 5). The initial peak current is due to this effect, which charged the sample and acceptor solutions [105]. This process is expected to be crucial for electrokinetic migration in the system


[105]. After this initial peak current, the current was principally determined by the flow of ions across the liquid membrane. 3.3.1.2. Applications. Several works have been published using primary EME setup for extraction of different analytes. EME is still a new microextraction technique. Therefore, application of the method for extraction of different compound from different matrices should be investigated. Since the first application of EME method was reported for extraction of basic drugs [29], this technique has been wildly used for extraction of this class of analytes [93–96,103,105,107,109,111,119–123,125–127,132,133,137,138]. The efficiency of EME has been compared with conventional HF-LPME [95,120,127] and it was shown that using electrical field impressively improves extraction recovery and reduces extraction time. However, by reversing the direction of electrical field, EME could effectively perform for extraction of acidic compounds [92,106,116,124,139,140] and organic anions [129]. EME has been used for extraction of metal ions [113,114,141] and inorganic anions [112,142] as well as peptides [104,108,110,117,118,143,144] and amino acids [115,145]. This method is capable of coupling with different analysis techniques such as HPLC [92,93,104,108,110,116–124,126–128,133,140,144], CE [29,95,96,103–107,109–115,130,132,137–139,141,143], and IC [129,142]. Coupling of EME to CE has been fully reviewed by Gjelstad et al. [88]. More recently, Fakhari et al. coupled EME with differential pulse voltammetry using modified screen-printed electrode for the determination of sufentanil in urine and plasma samples [91]. This new analytical approach presents advantages of both EME and electrochemical determination such as good selectivity and cleanup, low cost, high sensitivity and easy operation. 3.3.1.3. EME setup development. Despite the high potential of EME method for efficient extraction of ionizable compounds in relatively short time, it associated with some drawbacks. The main trouble of EME is that electrical potential differences above 300 V were found to be inappropriate (especially in analysis of real samples containing large amounts of ionic components) due to the system suffering from bubble formation at the electrodes, instability problems, punctuation of the SLM and sparking [104,108,110,111,113,119,120]. It was claimed that the punctuation of SLM (particularly when analyzing the drugs in human plasma samples), which leads to decreased resistance between the electrodes as well as system instability as a result of an increase in the current level, may be due to the emulsifying capacity of plasma [110]. Therefore, Gjelstad et al. demonstrated that adding microliter amounts of the SLM organic solvent to the bottom of the hollow fiber can improve the system stability [109]. However, it seems that the emulsifying capacity of plasma is not the only reason for this event, and the presence of high concentration levels of ionic species may be the main cause. Thus, controlling of the level of electrical current seems to be more beneficial to solving this problem. Two different methods were suggested to overcome the instability problems of EME including the application of stabilized constant direct electrical current [146] and pulsed voltage instead of constant d.c. voltage [134]. One solution for the difficulties of analysis of real samples and the instability of the extraction system was presented by Yamini et al. [134]. By applying suitable voltage, all ionic species in the solution select appropriate orientations under the electrical field and the ambulations of ions form two sheets of charges on both sides of SLM, which have opposite polarities and are separated by the organic phase (Fig. 6A). This boundary layer leads to mass transfer resistance and decreases the extraction efficiency. On the

17

other hand, by accumulation of the ions, the charge density of the double layer increases over time (Fig. 6B) and results in an increase in Joule heating and a decrease in the electrical resistance of the SLM. Therefore, even by applying a relatively low voltage in a long time, the resistance of the system is gradually reduced. Thus, the current level increases with time if the applied voltage is constant. Also, such a structure may act as a parallel-plate capacitor, which has a known charge capacity. So, a limited amount of charge can be accumulated on the double layer and with the increase in the extraction time some sparking may be observed. Therefore, pulsed electromembrane extraction (PEME) was introduced as an efficient alternative using a simple and inexpensive electronic device which creates pulsed voltage in combination with common d.c. constant power supplies [134]. In this method, the duration of the pulse is long enough for the analytes to migrate from the sample solution, through the SLM, and into the acceptor phase; but it is so short that the thickness of the boundary layer is minimized (Fig. 6C). Hence, PEME could increase stability by decreasing the thickness of the double layer at the interfaces and improve extractability by eliminating this mass transfer barrier and PEME could be introduced as an efficient microextraction technique with higher preconcentration factors, relatively short extraction time, higher precision, and a more stable system (even by applying high voltages) in comparison with conventional EME [134]. In another work, PEME followed by HPLC–ultraviolet detection (HPLC–UV) was employed for simultaneous extraction of histidine (HIS), phenylalanine (PHE) and tryptophan (TRY) [145]. It was observed that PEME offers more efficient extraction and stability in comparison with conventional EME for extraction and quantification of amino acids in foods and biological samples which can attribute to minimization of thickness of double layer at the interfaces. Also, taking into account that zwitterions have no mobility in an electrical field at their isoelectric point, two-way PEME was performed as a novel approach for highly selective extraction of TRY as a model analyte using the isoelectric pH of TRY for acceptor phase [145]. To this end the outage period was substituted by backward step at which the direction of electrical field was reversed. Therefore, at the backward step PHE and HIS are transferred again into donor phase while TRY remains in the acceptor phase. Although, selectivity is one of the advantages of EME method, there is some limitation in extraction of different classes of analytes. According to the direction of electrical field, EME could be performed for extraction of acidic or basic compounds. For simultaneous analysis of acidic and basic drugs, Basheer et al. designed an interesting EME setup using four sheets of porous polypropylene membrane which were combined and heat-sealed at three edges [147]. Their EME setup is shown in Fig. 7A. In this work, analytes were extracted across the SLM and into an aqueous phase and then transferred to an organic acceptor phase. The first step (from sample solution to aqueous phase) is based on electrokinetic migration and the transport mechanism for the analytes into organic acceptor phase is passive diffusion and the flux is basically controlled by distribution ratios. Finally, the organic acceptor phase was analyzed by gas chromatography followed by mass spectrometry detection (GC–MS) [147]. Yamini et al. introduced a simple and different setup utilizing two pieces of hollow fiber for simultaneous extraction of acidic and basic analytes (Fig. 7B) [135]. The details have been explained in a recent review [88]. In comparison with sheet membranes, the volume of acceptor phase is more repeatable in their setup, when hollow fibers were employed as acceptor phase containers, and the precision of the proposed method may be increased. Since the most effective transport mechanism for the analytes is electrokinetic migration during the extraction, the extraction time may be decreased in comparison with Basheer’s setup [135]. Fakhari et al.

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Fig. 6. Schematic diagram of PEME setup at (A) beginning of pulse duration, (B) end of pulse duration, and (C) end of outage period. Reproduced with permission from Elsevier-modified Ref. [134].

utilized this setup followed by CE to simultaneous determination of ibuprofen (an acidic drug) and thebaine (a basic drug) from plasma and urine samples [148]. Another disadvantage of EME is its incompatibility with GC instrument. The GC is a simple and fast analysis technique which can be easily coupled with different types of sensitive detectors such as flame ionization detector (FID) and mass spectrometer. The acceptor phase of EME is an aqueous solution and direct injection of water may cause some problems for GC. However, there is an EME report for direct injection of aqueous acceptor phase into GC injection port [125]; the main challenge in the coupling of EME with GC is to substitute the acceptor phase with an organic solvent. Recently, Guo et al. reported the EME followed by low-density solvent based ultrasound-assisted emulsification microextraction (EME-LDS-USAEME) combined with derivatization for determining chlorophenols (CPs) in water samples and analysis by GC–MS [149]. However, this method has been applied to the analysis of CPs in simple matrices, for which good results could only be obtained by ultrasound-assisted emulsification microextraction (USAEME). Therefore, Yamani and his research group presented the first coupling of EME with dispersive liquid–liquid microextraction (DLLME) to develop a new pretreatment method for extraction of tricyclic antidepressants from biological samples [150]. To this end (as it is shown in Fig. 7C), after the EME step, the acceptor solution was collected and injected into an alkaline solution (pH = 12) for converting extracted analytes to their neutral forms. Following that, the DLLME procedure was performed on this solution. Two-phase EME was another proposed method for making EME compatible with GC. Nevertheless, the first successful attempt to EME coupling with GC was published by Basheer et al. [147]. As it was discussed above, the aim of their work was simultaneous extraction of acidic and basic drugs. To achieve this goal, a combination of EME and conventional HF-LPME was performed and the analytes were finally transferred into organic acceptor phase [147]. Hence, the GC–MS was employed for analysis of final acceptor solution [147].

More recently, electromembrane surrounded solid phase microextraction (EM-SPME) coupled with GC followed by FID (GCFID) was for the first time introduced by Yamini and coworkers [151]. Their setup was consisted of a pencil lead which was used as a cathode and located into the lumen of the hollow fiber. Substantial selectivity and cleanup, simplicity, compatibility with GC, inexpensive and rapid extraction are the main advantages of the presented method. Moreover, possibility to apply high voltages as well as analysis of complicated matrices could be provided by this method. The results showed that EM-SPME may have a great potential as a microextraction technique, and opens a new era to this field of analytical chemistry. The other EME drawback is that it is a non-exhaustive extraction method and recoveries were limited due to the system reach equilibrium. To overcome this limitation, Eibak et al. introduced a new EME setup for exhaustive extraction of basic drugs from human plasma sample (Fig. 7D) [152]. The basis of their work was to increase the ratio between organic- and aqueous donor phase by optimization of the surface available for electrokinetic migration. Therefore, three pieces of hollow fibers were used to increase the volume of acceptor phase and the contact area between SLM and the donor phase [152]. In another work this EME setup was used to study the properties of biopolymers (alginate and chitosan) as sampling media for dried blood spots [153]. Also, basic drugs were extracted under stagnant conditions demonstrating that admissible detection limits are obtainable by extraction from a few dozen microliters of biological fluids [152,154,155]. Also, the application of carbon nanotubes was considered for enhancing the absorption capacity of the SLM and increasing the extraction recoveries [156,157]. Since 2006, the EME has been associated with rapidly progressing. This is due to its advantages such as selectivity and sample cleanup which eliminated interferences to the high extent in complicated matrices, preconcentration, fast kinetics with exact control of the beginning, and termination of the extraction. It seems that the EME is opening new doors for microextraction techniques and it may followed by some extra developments in the future.


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Fig. 7. Equipment for (A and B) simultaneous extraction of acidic and basic analytes, (C) EME coupled with DLLME, and (D) exhaustive electromembrane extraction. Reproduced with permission from Elsevier-modified Refs. [135,147,150,152].

4. The other roles of electrical driving force in analytical techniques Electrical driving force has been created substantial improvements in separation techniques by different ways (Fig. 2). The roles of this auxiliary energy in separation techniques can be classified as the following: 1- The separation techniques that are based on the electrical driving forces, such as CE, capillary electrochromatography (CEC), voltage-assisted liquid chromatography (VALC) and EMLC, microchip electrophoresis (ME), and microchip electrochromatography (MEC). 2- The separation techniques that are coupled, off-line or on/inline, with an electrically assisted extraction method as a sample preparation process. 3- The separation techniques, in which an electrically assisted extraction method is used for detection. The efficiency of separation processes can usually be assured either by an energy input or by external forces (Fig. 2). Huge changes were obtained in separation techniques by invention of electrically assisted ones. A comprehensive discussion about the CE, CEC, ME, MEC, EMLC, and VALC, including the theoretical

understanding, instrumental, advantages, disadvantages, and recent developments, is available in the literature [158–176]. Another way that electrical driving forces affect the separation techniques is the combination of electrically assisted sample preparation methods with these techniques. As shown in Fig. 2, the electrically assisted sample preparation techniques can be coupled off-line or on/in-line with different separation techniques. Up to now, the most reported works have been used an off-line coupling and a few publications can be found for on-line combination with separation techniques in the literature. It is worth mentioning that electrically assisted extraction techniques not only can be used as sample preparation methods but also they can be utilized as detector in separation techniques. Application of a new type of amperometric detector for nonredox ions based on ion-transfer reactions at the microinterface between water and NPOE-poly(vinyl chloride) gel [177,178], as well as polypyrrole (PPy) and overoxidized sulfonated polypyrrole (OSPPy) electrodes as a potentiometric detector in IC to determine some anions and cations in water samples [64] are the instances that can be enumerated. In recent years, miniaturized designs termed Lab-on-a-Chip (LOC) has attracted the attentions to perform downscaled laboratory analyses and for high throughput and highly specific analysis in chemistry, biology and medicine [179]. LOCs are a subset of

20


microelectromechanical systems (MEMS) which also indicated by “Micro Total Analysis Systems” (␮TAS) [38,179–182]. With ␮TAS technology, chemical analysis can be performed on very small sample sizes and with a very low consumption of chemicals and reagents due to the small dimensions of the devices. The small dimensions and short diffusion distances also give the potential for very rapid analysis [38]. One of the first applications of LOCs in analytical chemistry was their benefits in separation techniques. Due to smaller separation lengths of electrophoretic techniques which is following by increase in electrical fields, these techniques are perfectly suited for downscaling. Several review articles have been published that provide printed guides to the best improvements and advances related to different chip based chromatography techniques [168,183–187]. Typical processes such as filtration [188–190] and dialysis [191,192] had been initially implemented upon microchips. Recently, several reports have been emerged on implementation of some electrically assisted liquid and solid based extraction methods as well as membrane-based approaches [31–40]. Regarding the chip based sample preparation methods which explained in the literature, it can be concluded that miniaturization of extraction techniques is one of the more attractive challenging todays and a bright future as well as enormous revolution is predicted in this field of analytical chemistry.

Undoubtedly, the use of electrical driving force, in the course of recent years, has received renewed attentions in different fields of science. On the other hand, what can be stated about the impacts that these concepts have had on the “real world”? In fact, the answers to these questions will be so different depending on the considered application field. In the case of analytical chemistry, although more studies are required to really establish the fundamental roles and the effects of the electric fields but according to what was discussed here, glorious future is expected for electrically assisted sample preparation techniques so that they will create enormous developments in this concept. Fabrication of new membranes and coating films, development and improvement of new electrically assisted sample preparation methodologies, more efforts toward ␮TAS technology, trying to overcome limitations and drawbacks of existing electrically induced extraction methods, endeavor in order to on-line coupling with different analysis instruments such as chromatographic and spectrometric ones and investigation of applicability of electrically assisted extraction methods as detector in separation technique are definitely interesting directions in the future.

5. Concluding remarks and future trends

References

Different roles of electrical driving force as a powerful auxiliary energy in various extraction techniques including liquid, solid and membrane based methods is taken into consideration in this overview. Also, the recent achievements as well as advantages and disadvantages in each technique were investigated in more details. Some novel ideas were introduced during different sections of the review regarding the investigated literature which may be helpful to further development in the future. As explained in this review, an extensive range of electrically assisted extraction and sample treatment techniques, such as electrically assisted solid phase extraction, electrically assisted solid phase microextraction, and EME, has been emerged after the first report related to application of electrical driving forces in LLE. Although some of these techniques such as liquid–liquid electroextraction and EA-SPE have not been found remarkable attention in analytical chemistry but they have opened a new perspective to development and emerging of subsequent sample preparation methods. Also, some of them like membrane based techniques have found a worldwide impact in different parts of today’s modern industrial society. The substantial supremacy of electrical driving force in extraction techniques comparing with other auxiliary energies is possibility to manipulate relative distribution of the analytes (KD ) between the two phases. This advantage makes it possible to control different properties of an extraction system such as selectivity, clean-up, rate and efficiency by the imposition of a variable potential difference across the liquid–liquid or solid–liquid phase boundaries. On the other hand, remarkable potential of electrical driving force is only not restricted to extraction and sample treatment techniques but also, they have shown noticeable efficiency to synthesize of various types of membranes and coating films. The other advantage resulted by application of electrical driving force in extraction and sample treatment techniques is easy integration of the used setups for these methods to implement in microchip based devices for LOC so that this subject has been converted to an interesting research field todays.

Acknowledgements The support, provided by the Grant Agency of the National Elite Foundation (Shahid Chamran’s Scientific Prize, Grant No: 15/37651, Tehran, Iran), is highly appreciated.

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