Chapter 32 New polymeric extraction materials

Chapter 32

New polymeric extraction materials Abdul Malik

32.1

INTRODUCTION

Sample preparation is an important step in chemical analysis, especially when dealing with traces of target analytes dispersed in complex matrices. Such matrices are commonplace in environmental, petrochemical, and biological samples. Such samples are not generally suitable for direct introduction into analytical instruments for their quantitative or qualitative determination. This incompatibility is related to two factors. First, the complex matrices may have a detrimental effect on the performance of the analytical system, or they may interfere with the analysis of the target analytes. Second, target analyte concentration may be so low in the sample that it may go beyond the detection limit of the analytical instrument. In both cases a sample preparation is necessary to make the sample detectable or compatible with analytical instrumentation. This is achieved through sample clean-up and sample preconcentration. Sample derivatization is also sometimes necessary to facilitate analysis and detection of target compounds. Sample preparation is the most time-consuming and error-prone step in the analytical cycle [1], and has long been a neglected area in analytical chemistry [2]. As a result, sample preparation techniques have not developed at a pace consistent with the rapid growth and sophistication of the analytical techniques [3]. For example, the Soxhlet extraction technique [4], invented more than a hundred years ago, is widely utilized in current analytical practice for the extraction of solid samples. Liquid-liquid extraction is another classical extraction technique that is still extensively used in various formats. In recent years, however, the importance of sample preparation in the overall analytical process is becoming increasingly recognized by analytical chemists and scientists using analytical chemistry in their research [5-9]. Another factor that had a positive impact on further development of extraction techniques is the realization of harmful environmental and health effects of large volumes of organic solvents used in classical sample preparation techniques [9-11]. The positive stand of US Environmental Protection Agency (EPA) and other regulatory agencies worldwide towards reduction in the use of harmful organic solvents has created a favorable environment for the emergence and/or further development of environmentally Comprehensive Analytical ChemistryXXXVII J. Pawliszyn (Ed.) © 2002 Elsevier Science B.V. All rights reserved

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benign sample preparation techniques including solid-phase extraction (SPE) [12], supercritical fluid extraction (SFE) [13-17], accelerated solvent extraction (ASE) [18,19], microwave-assisted extraction (MASE) [20-251, subcritical water extraction [26-28], liquid-liquid microextraction (LLME) [29-34], extraction on a single solvent drop [35-39], and membrane extraction [2,7,40-45]. Recently, much effort has been made to bring sample preparation techniques to the right level of speed and sophistication compatible with those of modern analytical techniques. This is evident from numerous recent developments in the area of sample preparation with emphasis on miniaturization, high throughput, and automation. Especially notable are the explosive growths in the area of SPE [1,12,46-49] and SPME [50-571. Polymers are an important group of materials that often provide superior performance in analytical sample preparation by allowing effective extraction and preconcentration of trace analytes in an environmentally benign way. In solid-phase extraction, polymeric materials provide a number of advantages over their conventional bonded silica counterparts. Among others, these include performance stability over a wide pH range allowing for the extraction of acidic, basic, neutral, and ionic species [1], ease of surface modification through grafting [58] to achieve desired levels of sorbent polarity, better recovery, and reduced dependence of the extracted amount on pre-extraction wetting period. Development of solid-phase microextraction by Pawliszyn and coworkers [59] demonstrated the possibility of complete elimination of the use of organic solvents during extraction and/or preconcentration of target analytes. In this technique a polymeric coating on a fused silica fiber efficiently performs extraction and preconcentration of analytes from various matrices. Besides their use in traditional SPE and SPME formats (i.e., cartridge-, disk-, or packed precolumn-based SPE and fiber-based SPME), polymeric materials are being introduced in new, non-traditional extraction formats including (1) open tubular microextraction (OTME) (also called in-tube SPME) which uses a polymeric coating on the inner surface of a piece of capillary to perform extraction, (2) monolithic capillary microextraction (MCME) which utilizes a porous monolithic bed as the extraction medium, and (3) polymeric solution-mediated extraction. 32.2

SCOPE OF THIS CHAPTER

This chapter focuses primarily on new synthetic polymeric materials in analytical extraction techniques. The text is divided into three major thematic areas where new polymeric materials have been developed and/or utilized as extraction media: (a) SPE, (b) SPME, and (c) polymeric solution-mediated extraction. A brief introduction to the conventional sorbents used in these areas is given to highlight the historical background that dictated the development of new polymeric sorbents. The text also includes brief descriptions of the working principles and specificities of these techniques in various (especially newly 1024

developed) formats. In cases where recent extensive reviews are available for particular types of polymeric extraction materials, the main attention is focused on the most recent developments not covered by existing reviews. Molecularly imprinted polymers are now well-established extraction materials for SPE, and two very recent reviews [47,48] have thoroughly covered the existing literature. However, in the SPME area, MIPs have been introduced only very recently. For this reason, emphasis in this chapter has been placed on molecularly imprinted materials as extraction media for SPME. Extensive reviews have also been published on chemically modified polystyrene-divinylbenzene materials for SPE [1,49,60]. Taking this into consideration, only a brief discussion is provided here of these topics, mainly covering the general principles and latest developments not covered by those recent reviews. Special attention is focused on sol-gel polymers that advantageously combine organic and inorganic components in their structures. A general scheme of sol-gel chemistry is presented that allows for the in situ creation of these polymers in a simple way under mild thermal conditions (often at room temperature) using appropriately designed sol solutions. The sol-gel polymeric coatings and monolithic beds prepared using such sol solutions get chemically bonded to the surface of the substrate and, therefore, possess significantly higher thermal and solvent stabilities-attributes that are important for effective extraction and subsequent release of the analytes from the coatings for further analysis. The organic-inorganic hybrid nature of these polymers provides advanced material properties allowing for the extraction of both polar and nonpolar analytes present in the matrix. A wide variety of functional groups can be incorporated in these polymeric materials by properly designing the sol solution composition. The preparation and extraction performance of sol-gel polymeric coatings and monolithic beds with chemically incorporated polymers, dendrimers, macrocycles, and other organic ligands are presented. Future directions in the development of sol-gel polymers for analytical extraction are pointed out. To our knowledge, sol-gel polymers have not yet been covered by any of the existing reviews on polymeric extraction materials. 32.3

NEW POLYMERIC MATERIALS IN SOLUTION-MEDIATED EXTRACTION

Liquid-liquid extraction (LLE) [61] is a classical extraction technique for effective isolation of dissolved organics from aqueous media. The fact that LLE often employs large volumes of organic solvents to perform extraction makes it hazardous to the environment and to the human health. Because of recent regulatory pressure to reduce the use of hazardous organic solvents, much research work is being devoted to finding an effective alternative to LLE. Aqueous polymeric solutions have the potential to replace hazardous organic solvents used in LLE to perform liquid-phase extraction in an environmentally benign way [62]. Extraction techniques that employ polymeric and related 1025

solutions include (a) cloud-point extraction (CPE) [63,64], (b) micellar extraction (ME) [65,66], (c) extraction based on aqueous biphasic systems (ABS) [67], and (d) extraction using thermoseparating polymer systems (TPS) [68]. Rogers and coworkers [62] have published an extensive review covering the existing literature on all four of these polymer solution mediated extraction techniques up to and including 1999. Here we will focus on new developments (not covered by the above-mentioned review) with emphasis on future trends in this area. 32.3.1

Dendrimers in solution-mediated extraction

A new trend in solution-mediated extraction technology is to use dendrimer solutions to perform the extraction. Goetheer et al. [69] described the use of functionalized poly(propylene imine) (PPI) dendrimers as reactive extractants for anionic species from an aqueous phase to a supercritical carbon dioxide phase. The same dendrimers were found to be efficient phase transfer catalysts. Baars et al. [70] showed that poly(propylene imine) dendrimers with apolar end groups provide highly selective extractants for basic analytes. They used palmitoyl chloride-modified PPI dendrimers (1-5 generations) to extract anionic xanthene dyes from water to an organic phase. The modified dendrimer provided inverted unimolecular dendritic micellar structures with a polar interior of tertiary amines and an apolar periphery. Excellent extraction efficiency was reported: 50 dye molecules per dendrimer molecule (fifth generation). The authors concluded that the extraction of anions is related to the tertiary amine interior of the dendritic micelle, and provides a very useful tool in selective purification of anions. Powell et al. [71] synthesized carbon dioxide-soluble polymers with multiple ligand sites for CO2-based metal extraction. The CO2-soluble polymers were prepared via free-radical polymerization of a fluorinated acrylate and a series of acrylate- and styrene-based monomers functionalized with the desired ligand sites or ligand precursors. They successfully extracted copper and europium using CO2-solutions of the prepared polymers. Based on europium luminescence, the authors established that in this extraction system europium was bound (along with four molecules of water) to the polymer in the inner coordination sphere. Extraction techniques based on CO2-mediated polymeric solutions are appealing due to the environmental friendliness of CO2, the high purity of carbon dioxide as a solvent, and ease of solvent removal from the extracted materials. 32.3.2 Temperature-responsive polymers (TRP) in solution mediated extraction Temperature-responsive (or thermoresponsive) polymers belong to stimuliresponsive polymeric materials-a group of polymers whose molecular conformations and certain physical properties are sensitive to external stimulus

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factors. In the case of temperature-sensitive polymers, the stimulus factor is temperature. Temperature-responsive polymers respond to the temperature change in the environment by a corresponding change in their molecular structure and properties including phase transition, solubility, hydrophobicity, hydrophilicity, etc. [72]. Examples of other types of stimuli-responsive polymers are pH-responsive polymers, photo-responsive polymers, electric field-responsive polymers, and chemical species-responsive polymers. In the latter cases, the external stimuli are pH, light, electric field, and chemical species. Temperature-responsive polymers are soluble in water. However, above a certain temperature (a characteristic value for each TRP), called lower critical solution temperature(LCST), a temperature-responsive polymer becomes insoluble in water due to temperature-induced phase transition. This physical change is reversible, and the polymer goes back into solution when its temperature is lowered below its LCST. Thanks to this property, TRPs have been extensively used in diverse areas of science and technology: as an effective medium for controlled drug delivery [73-77], as controlled permeability membranes [78,79], as bioconjugates [80,81], as nontoxic media for cell culture [82,83], as chromatographic stationary phases [72,84-89], and as environmentally benign extraction media [90-99]. Poly(N-Isopropylacrylamide) (PNIPAAm) is the most widely used and best studied among temperature-responsive polymers known to date. Scheme 32.1 illustrates synthesis of PNIPAAm through radical teleomerization of N-isopropylacrylamide using 3-mercaptopropionic acid as the chain transfer reagent. It also shows the possibility of chemical attachment of PNIPAAm to silica particles that can be used either as a chromatographic stationary phase or as a solid-phase extraction media [72,80,100]. Cole et al. [101] described an alternative synthetic scheme via aqueous redox polymerization of N-isopropylacrylamide. One important factor responsible for the wide use and popularity of poly(Nisopropylacrylamide) as a temperature-responsive polymer is its moderate value for the lower critical solution temperature (-32°C) which, being just above the room temperature, is very convenient for experimentation [97,102]. Below LCST, the polymer chains are hydrated to an expanded, water-soluble form. Above LCST, the polymer chains dehydrate and shrink to a water-insoluble form that separates out of the solution as a gummy precipitate that can be easily isolated. The temperature-responsive phase-transition property of these polymers provides a powerful analytical tool for the preconcentration of trace analytes in aqueous media. When a temperature-responsive polymer is solubilized in an aqueous medium (below LCST) containing trace of organic contaminants, the polymer chains are dispersed throughout the entire volume of the solution, and can bind with individual molecules of the trace contaminant through hydrophobic and/or other forms of molecular level interactions. If, after an appropriate contact period, the system is warmed above LCST, the temperature-responsive 1027

H2C=CH C =O NH

+

-

HOOCCH 2CH 2 SH

CH H3C

HooCCH 2 CH 2 S-CH 2 CH -H C=O NH

AIBN

DMF CH3

H3C

[PAAm

MPA

/

CH CH

Semitlechelic PIPAAmr

0O N-OH _________

O IjN-

-CCH 2 sSCHS

0

DCC

CH-CH

NH CH H3C \CH3 /

Silica

O CHH

2 CHc2NH 2

I Silica SIO CH2 CHCtN-CCH

1,4-dioxane

*- x,._)

I

2CH2SCHCHH

C=O NH

HC

/

CH

\CH3

Scheme 32.1. The synthesis of poly(N-isopropylacrylamide) (PIPAAm) and the coupling to

aminopropyl silica surface. (Reproduced from Ref. [72], with permission of Marcel Dekker).

polymer chains precipitate out together with the contaminant molecules bound to them. This translates into an effective capture of the contaminant molecules (that were originally dispersed throughout the entire volume of the aqueous sample) and their transfer to the precipitated polymeric matrix. Since the volume of the precipitate is exceedingly small compared with the total volume of the solution, the temperature-responsive precipitation results in a remarkable enrichment of the target trace analytes.

The hydration/dehydration phenomena associated with the temperatureresponsive polymers also provide a mechanism to fine tune the selectivity of these polymers based on the temperature to be used. At temperatures below LCST, these polymers are hydrophilic while above LCST they are hydrophobic [72]. This opens up new possibilities for liquid-phase separations in an environmentally benign way by using aqueous mobile phase (no organic modifier) and TRP-bonded stationary phases, and fine tuning the hydrophobicity of the stationary phase by adjusting the temperature. Essentially, this gives the possibility to replace mobile phase composition programming (gradient elution) with temperature-programmed elution. Temperature-responsive polymers have been used for the extraction of hydrophobic and amphiphilic analytes. Figure 32.1 illustrates the preconcentration effect obtained for hydrophobic pesticides in ground water through polymer-mediated extraction using PNIPAAm as the TRP. 1028

(A)

(B)

8 X

c a.

0 0

IL

10min I F i-I Ii

Lni

I

, .

ff II

I -

ch

d

II

I

ii

2 'O -.

.-

0

10

20

30

0

Time (min)

10

20

30

Time (min)

Left: Fig. 32.1. Chromatograms of some pesticides (0.1 gM) in groundwater. Mobile phase, 70% (v/v) acetonitrile; flow rate, 1 ml/min; detection wavelength, 254 nm. (A) without concentration, (B) with polymer-mediated extraction. The polymer phase formed from a 10-ml sample solution containing 0.1% (w/v) PNIPAAm was dissolved by 80 l of acetonitrile. A 20-a1l portion of the resulting solution was injected. (Reproduced from Ref. [97], with permission of American Chemical Society). Right: Fig. 32.2. Chromatograms of river water: (A) without preconcentration, (B) with preconcentration via polymer-mediated extraction. Conditions: 250 x 4.6 mm i.d. Intersil ODS column; 30% (v/v) aqueous acetonitrile mobile phase; 1.0 ml min' flow rate; UV detection at 254 nm. (Reproduced from Ref. [98], with permission of Elsevier.)

An example of temperature-responsive preconcentration of amphiphilic analytes is presented in Fig. 32.2. Here nonionic surfactant, nonylphenol (NP) and its monoethoxylated derivative were pereconcentrated using PNIPAAm as the temperature responsive preconcentration medium. Analyte preconcentration with temperature-responsive polymers does not need the elution step commonly required by other extraction techniques (e.g., SPE). This simplifies the analytical procedure. In temperature responsive extraction, the extracted analytes are dissolved in a very small volume of water or aqueous/organic medium and the solution is directly injected into the HPLC system. Saitoh and coworkers [97,98] explained that since in aqueous solution PNIPAAm is hydrophilic, practically it does not stick to the hydrophobic

reversed-phase HPLC column, neither does it appreciably interfere with the UV detection since it lacks a strong UV-absorbing chromophore. 32.4 32.4.1

NEW POLYMERIC MATERIALS IN SOLID-PHASE EXTRACTION Working principle of solid-phase extraction

Solid-phase extraction (SPE) is based on preferential accumulation of the target analyte(s) from a liquid phase into a solid adsorbent bed (usually within a cylindrical confinement) through which the liquid sample is percolated. This can be materialized in a variety of formats: (a) particle-packed bed in the form of a car1029

tridge or a precolumn, (b) sorbent disk, (c) monolithic bed (a single piece of porous solid within a tubular confinement), and (d) flow-through membranes. The ultimate aim is to obtain an enriched sample of the target analyte(s) and to free it from any interferences that might be present in the original liquid sample. To this end, the solid-phase should be carefully selected so that it provides high selectivity for the target analyte(s). The solid-phase selection should be based on physicochemical and structural characteristics of the analytes to facilitate various types of molecular level interactions with the analytes present in the surrounding environment. In order for SPE to be effective, such interactions between the target analyte(s) and the solid sorbent should dominate over analogous interactions of the target molecules with the original liquid matrix. Once the solid phase has been selected, a number of steps needs to be performed in sequence to obtain a clean, preconcentrated sample of the target analyte(s) free of interferences. These are: (a) activation (conditioning)- pretreatment of the solid phase with an appropriate solvent to create a favorable environment on the solid surface for intimate interaction between the target analyte(s) and the solid sorbent; (b) sample application-passing the liquid sample through the solid bed to provide sorbent/analyte interaction to achieve retention of the target analyte(s); (c) washing-passing an appropriate liquid through the solid bed to selectively remove the interferences (but not the target analyte(s)); and (d) elution-recovery of the extracted and purified target analyte(s) by passing through the sorbent bed a small volume of a solvent that has higher affinity for the target analyte(s) than the affinity of sorbent toward the analyte(s). The practical aspects of performing SPE on a cartridge are illustrated in Fig. 32.3. Solid-phase extraction is a more environmentally benign extraction technique than liquid-liquid extraction (LLE). SPE drastically reduces the consumption of hazardous organic solvents. It is simple and easy to operate. Thanks to its simplicity, reliability, and applicability to a diverse range of samples, SPE has undergone explosive growth over the last few decades, and has now become the most popular sample preconcentration technique [46]. It is evident from the above discussion that the heart of SPE is the solid sorbent used for extraction. Future advances in SPE are strongly dependent on the development of new types of sorbents with advanced sorption characteristics. Novel synthetic polymeric sorbents have great promise for future developments in SPE. Before addressing this central topic of the chapter, a brief discussion highlights the advantages and drawbacks of conventional solid-phase extraction media with emphasis on how polymeric materials can overcome the limitations of conventional extraction media and expand the scope of SPE. 32.4.2

Traditional solid-phase extraction materials

Traditional SPE sorbents include silica-based bonded HPLC packings [46], carbon-based extraction media [104,105], and polystyrene-divinylbenzene 1030

(A) Specimen reservoir Medical-grade polypropylene

Polyethylene Sorbent bed

_

fritted disk (20 p)

?

Fritted disk (20 ) Luer lip

..f~,

Fig. 32.3. (A) Typical design of a solid-phase extraction cartridge. (B) Steps in the operation of a solid-phase extraction experiment. I = compound of

(B)

6

Conditioning

A

Sample

ilution solvent

solvent

(typical manual injection volume)

interest, * = interference. (Re-

produced from R.E. Majors, LC-GC, 4 (1986) 972, by permission of Advanstar Communications).

6

Waste

/ 50 Ipl Sample application

Wash

Elution

(PS-DVB) [1]. The following discussion on traditional SPE sorbents will be brief since the focus of this review is on new polymeric sorbents. 32.4.2.1 Silica-basedextraction materials Although silica is commonly represented by a simple molecular formula (SiO,), in reality, it is an inorganic polymeric material [106]. In silica, silicon and oxygen atoms form tetrahedral structures with the silicon atom at the center, and the four oxygen atoms on four corners of each tetrahedron. In amorphous silica (most commonly used in chromatographic separations and sample preparations), each of these tetrahedra may be chemically attached to up to four neighboring tetrahedra through silicon-oxygen (siloxane) bonds in a random way [107]. Underivatized silica materials are used as an easily available media for normal-phase HPLC separations and sample purifications. Functionalized silica materials are widely used in sample preparation techniques including SPE [108], and as chromatographic stationary phases for HPLC [109,110], SFC [111], capillary electrochromatography [112,113], and GC [114-116]. Silica-based extraction materials are characterized by high mechanical strength that makes them well-suited for high-pressure and high-voltage operations often required in liquid-phase separations and sample preparations. However, silica-based materials inherently possess a narrow pH stability range which excludes their operation under extreme pH conditions. Besides, most silica-based materials (including alkyl-bonded silicas, the most widely used types

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of silica-based stationary phases and extraction media) are characterized by low retention for polar analytes. In solid-phase extraction applications, this results in low breakthrough volumes that make such materials often unsuitable for the preconcentration of polar compounds. Because of these limitations, much research effort is directed towards developing alternative materials that possess a wider range of pH stability. These include the development of polymeric materials [1], zirconia- [117-122] and titania-based materials [123-127], and polymer-encapsulated silica [128-132]. Recently, Kirkland and coworkers [133] reported the possibility of creating silica materials with a wide pH stability range (up to pH 11) via sol-gel technology. 32.4.2.2 Carbon-basedextraction materials Carbon-based sorbents have found useful applications as extraction media [104, 105] and as stationary phases for chromatographic separations [134-136]. The main applications of carbon-based materials in analytical sample preparation involve trapping and preconcentration of volatile organic compounds (VOCs) from air and water. They are also used for the preconcentration of semi-volatile and non-volatile compounds, although to a much lesser extent. Four different types of carbon-based materials have found industrial, environmental, and analytical applications. These are: (a) activated carbon [137, 138], (b) graphitized carbon black [139-145], (c) carbon molecular sieve [146, 147], and (d) porous graphitic carbon [148-155]. (a) Activated carbon: The use of activated carbon can be traced back to the ancient Egyptians who used activated charcoal for medicinal purposes. More recently, activated carbon has found a range of applications thanks to its high surface area and strong sorptive properties. This especially concerns applications involving (a) sampling of volatile compounds with boiling points higher than 0°C (activated charcoal is not a strong enough adsorbent for the retention of gases with boiling points less than 0°C [156]) and (b) isolation and removal of certain undesirable and/or hazardous components from gaseous or liquid media. The first class of applications includes the use of activated carbon as a cheap and efficient sorbent for use in preconcentration tubes for sampling industrial and ambient air. The trapped volatile analytes are then recovered from the sampling cartridge by desorbing with a suitable solvent-most frequently carbon disulfide (for nonpolar analytes) or binary mixture of CS2 with water, methanol, 2-propanol, 2-butanol, or dimethylformamide (for polar analytes) [156,157]-and further analyzed by gas chromatography. The second class of applications include use of activated carbon in water purification, air filtration, manufacture of respirators and protective gas masks, recovery of solvents from process streams, etc. Activated carbon is also a promising candidate for use as an adsorbent in miniaturized traps [158-162]. The strong interaction of activated carbon with adsorbed analytes makes it unsuitable for many analytical applications (especially those involving semivolatile-, low-volatile-, and/or polar compounds) where the extracted analytes

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need to be quantitatively recovered. Because of this strong sorbate/sorbent interaction, it is difficult to desorb such analytes from activated carbon materials resulting in poor recovery. Besides, activated carbon materials are characterized by complicated surface and micropore structures containing various polar groups [163] that are difficult to reproduce, resulting in significant batch-tobatch variations in their properties. The adsorptive properties of activated carbons are greatly affected by the atmospheric humidity. The adsorptive capacity of activated carbon strongly depends on its micropore structure. The huge surface area (-1000 m2 /g) of activated carbon may promote certain catalytic reactions. In such a case, the composition of desorbed analytes may not be representative of those originally adsorbed. (b) Graphitized carbon black (GCB): The term "graphitized carbon black" refers to the forms of carbon prepared by thermal treatment of ordinary carbon black to 2700-3000°C in an inert gas environment, usually in a graphitizing furnace [164,165]. This high-temperature treatment leads to the breakdown of various functional groups of the original carbon black surface, and results in a material with a homogeneous surface that is practically free from unsaturated bonds, ions, free radicals, and lone electron pairs. Graphitized carbons are also free of micropores, and possess a typical surface area of 100 m2 /g [142]. Solute adsorption takes place on the external surface of GCB predominantly due to dispersive interaction between the solute molecules and the nonspecific adsorption sites on the GCB surface containing hexagonal array of carbon atoms. GCB may partially regain surface heterogeneity during cooling of the carbonaceous surface after graphitization [140] and the small amounts of polar adsorption sites present on the surface are capable of offering specific interactions with polar solutes. From a crystallographic point of view, three distinct types of graphitized carbon structures can be differentiated: (a) Bernal perfect threedimensional hexagonal structure of graphite [166]; (b) Lipson-Stokes three-dimensional rhombohedral structure of graphite [167]; and (c) Warren two-dimensional turbostratic structure of graphite [168]. The use of graphitized carbon blacks and carbon-based extraction media in solid-phase extraction have recently been reviewed [104,105]. Recent SPE applications of GCB include the preconcentration of phenols [140,142,144,169], pesticides and herbicides [142,145,169-176], estrogens [177], alkylbenzenesulfonates and metabolites in fish [178], and surfactants in water [142]. Nonpolar nature of graphitized carbon and a wide range of pH stability makes this material attractive for use as an effective sorbent for extraction and preconcentration of analytes under extreme pH conditions not suitable for silica-based extraction media. In this respect, GCB enjoys the advantages inherent in polymeric extraction materials. However, GCB has some significant shortcomings that include excessive solute retention and poor mechanical stability. Attempts have been made to improve mechanical strength of graphitized carbon materials. To this end, a new type of sorbent, porous graphitic carbon (PGC), was developed by immobilizing graphitized carbon on a silica structure.

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(c) Porous graphitic carbon (PGC): Graphitized carbon black is practically nonporous, and has a homogeneous hydrophobic surface. Activated carbon, on the other hand, is porous but suffers from surface heterogeneity. Therefore, carbon-based porous materials with homogeneous hydrophobic surfaces present considerable theoretical and practical interests [148-152,155]. PGCs are such materials; like GCB, they are characterized by a wide pH stability range. They are prepared by impregnating a porous template material (e.g., silica) with organic polymerizing mixture (e.g., phenol-formaldehyde mixture, phenolhexamine mixture, etc.) [148,153,154] and the polymerization reaction is conducted within the pores of the template material. After the polymerization reaction, the in situ created polymer is converted to glassy carbon by heating to -1000°C in an inert environment. At this point, the silica template is removed by treating with an alkali to produce porous graphitic carbon which is further heated to 2000-28000°C in an inert environment. This thermal treatment removes the micropores, improves surface homogeneity, and produces some degree of graphitization determined by the temperature at which the thermal treatment is carried out. The particle and pore size characteristics of the resulting PGC is determined by the chosen template material, while the surface characteristics of the PGC is determined by the final thermal treatment and any other chemical treatment that may follow. A study was undertaken by Puig and Barcelo [144] to compare the performances of PGC and polymeric sorbents in SPE for the preconcentration of phenolic compounds in environmental waters. The results showed that except for one compound, 4-chloro-2-aminophenol, PGC provided inferior performance both in terms of breakthrough volumes and detection limits. (d) Carbonmolecularsieve (CMS): Carbon molecular sieves are carbon-based microporous sorbents with very uniform pores [146,147]. They are characterized by high surface area and strong retention of organic molecules. They are prepared by controlled pyrolysis of suitable organic materials (e.g., polyvinyl chloride, petroleum pitch, etc) at temperatures usually exceeding 400°C. They possess disordered cavity-aperture structure resulting from the cross-linking of crystallites. Important factors that determine the pore structure, pore distribution, and properties of CMS include (a) identity of the pyrolyzed polymer, (b) purity of the polymer, (c) the used method of carbonization, and (d) presence of impurities. Carbon molecular sieves are commercially available (e.g., Carbosieve and Carboxen types from Supelco). 32.4.3

Polymeric extraction materials in SPE

32.4.3.1 Poly(styrene-divinylbenzene)-basedmaterialsfor SPE Polymeric materials have found wide applications in solid-phase extraction technique [1,46-49] as well as chromatographic column packings [179]. Among polymers that have been used as extraction media are (a) polystyrene-divinylbenzene (PS-DVB) [1,46,49] and polysiloxane-based materials [180,181]. Porous

1034

PS-DVB-based sorbents have gained wide popularity among various polymers that have been evaluated as extraction media. This is explained by the fact that PS-DVB-based sorbents are characterized by a number of advantageous properties compared with silica-based surface-bonded extraction media, originally developed for use as stationary phases in HPLC. Although both the silica-based reversed-phase extraction materials and PS-DVB-based sorbents have hydrophobic surfaces, thanks to the presence of aromatic benzene moieties on the surface, PS-DVB-based extraction media allow for selective extraction through n-n: interactions with the analyte molecules. By far the most outstanding advantage of PS-DVB-based polymeric sorbents is their stability over a wide range of pH. This makes them suitable for extraction even under extreme pH conditions that are not compatible silica-based sorbents with surface-bonded organic ligands. A second advantage of the PSDVB-based extraction materials is their ability to extract analytes from aqueous media without requiring prolonged preconditioning with a polar organic solvent. Silica-based surface-bonded extraction media often require such a preconditioning step because of the poor wettability of the bonded hydrophobic ligands with water. Further improvement in the wettability of PS-DVB-based sorbents with water, and hence their ability to extract analytes from aqueous media, was achieved through derivatization of PS-DVB with various polar groups including acetyl [182-184], sulfonate [185], hydroxymethyl [182,183], benzoyl [186], o-carboxybenzoyl [187], 2,4-dicarboxybenzoyl [188], 2-carboxy-3/4-nitrobenzoyl [188], and tetrakis(p-carboxyphenyl)porphyrin [189,190]. The chemical basis of all these derivatization reactions is presented in Scheme 38.2 [49]. PS-DVB has already become a widely used SPE material. Various aspects of synthesis and use of PS-DVB and functionalized PS-DVB have been covered in recent reviews [1,49,188b]. Details on the development of PS-DVB-based extraction materials and their use in analytical extractions can be found in these and other publications [182-190]. 32.4.3.2 Molecularly imprintedpolymers in SPE Recent years have witnessed a rapid growth in the development and utilization of molecularly imprinted polymers [47,48,191-198] especially as highly selective extraction materials for SPE and SPME [47,48,199-208] and as chromatographic stationary phases [192,193,209-212]. The affinity of the sorbent for a particular solute (or class of solutes) can be greatly enhanced by creating binding sites (on the sorbent) that have complementary structures relative to the target analyte(s). The process involved in creating such binding sites is termed "molecular imprinting". To create molecularly imprinted extraction materials, a template (usually the target analyte or a closely related compound) is used during the sorbent preparation. The template molecules are temporarily included into the structure of the polymeric sorbent during its synthesis from monomers. Subsequently, the template molecules are removed from the sorbent leaving behind 1035

AOH N.,

__

% -T

CH3COCI

H

A10 3

Cx/C' 0 \Cb c0t

CO(t

IN(CHAJ

0Scheme 32.2. Scheme of chemical modifications of polystyrene-divinylbenzene (Reproduced from Ref. [49], with permission of Elsevier).

resin.

binding sites on the sorbent with structural features complementary to those of the target solute(s). The principle is illustrated in Fig. 32.4 [191]. The above-illustrated principle is materialized in a versatile way through self-assembly of the template and a functionalized monomers undergoing polymerization and cross-linking reactions [47]. As the polymerization proceeds, the template molecules remain incorporated in the growing polymer. Subsequent removal of the template molecules leaves behind template-selective binding sites in the polymeric structure.

1036

(a)

fsH +-OH

I

+2

Cor 0

synthesise adduct

H

0:)-

I polymerise

(b)

n

mamix

in excess

Fig. 32.4. Schematic representation of the molecular imprinting process for: (a) covalent imprinting of phenyl -D-mannoside using vinylphenylboronic acid; and (b) non-covalent imprinting of Boc-phenylalanine using methacrylic acid. (Reproduced from Ref. [191] by permission of Elsevier). Molecularly imprinted extraction materials show high solute specificity

comparable with or even better than that of immunoaffinity extraction materials [214] prepared through immobilization of antibodies using timeconsuming multiple-step procedures. A significant difference in the extraction

1037

A

B

C

Naproxen

Nap roxen Ibuprofen

Ibuprofcn

I

6

r

5A tob1

Time (min)

Naproxen

20

o

5

lo i Time (min)

2

-o

6

i-

i15

20

Time (min)

Fig. 32.5. HPLC Chromatograms of a standard Ibuprofen sample (A), control plasma sample (B), and control plasma sample spiked with Ibuprofen (C) using column switching techniques. Conditions: 20 gl injection volume; precolumn, RAM-MIP-1 (10 x 4 mm i.d.); analytical column, Cosmosil 5C-18-MS (150 x 4.6 mm i.d.); eluent for pretreatment, 20 mM phosphoric acidacetonitrile (78:22 (v/v), pH 2.24) at 1.0 ml min/min for 5 min; mobile phase, 20 mM sodium phosphate buffer-acetonitrile (75:25 (v/v), pH 7.34) at 1.0 ml/min; detection, UV absorbance at 223 nm; Ibuprofen concentration at 5.0 )g/ml in (A) and (C). (Reproduced from Ref. [215] by permission of American Chemical Society).

properties of the MIPs and immunoaffinity sorbents is that immunoaffinity sorbents generally show high affinity for the solutes in aqueous media while MIPs typically exhibit their optimum solute recognition in organic media [213]. Complete removal of the template molecules from MIPs after their

preparation is of prime importance. However, it may be difficult to achieve complete elimination of the template molecules even after repeated washing of the prepared MIP using various solvents. In such cases, template bleed during analysis (i.e., gradual release of the residual template molecules that could not be removed from the MIP after its preparation and cleanup) may adversely affect the accuracy and precision in the trace-level determination of the target analyte that was used to imprint the polymer. This problem was overcome by using a template, which is structurally related to (but not identical with) the target analyte. For example, to prepare an ibuprofen-specific MIP, Haginaka and Sanbe [215] used naproxen (which is structurally similar to ibuprofen) as the template. Use of this MIP should not affect the trace level determination of ibuprofen even in the event of template bleed, since these two compounds elute wide apart from each other in HPLC as illustrated in Fig. 32.5. Another problem encountered with the use of MIPs for the isolation of target analytes from biological samples is the adsorption of biopolymers (proteins, nucleic acids, etc.) on the surface of the MIP leading to a number of undesirable effects including, blockage of the sorbent pores, reduction of retention and sample capacity, and template recognition ability of the MIP. This also affects the reproducibility of the analytical procedure. A possible solution to this problem is 1038

to develop MIPs with restricted-access material (RAM) characteristics as will be discussed in the next section. 32.4.3.3 Restricted-access media in SPE Restricted-access materials (RAMs) are specially designed extraction media that are widely used for the extraction and purification of small molecules (e.g., drugs and their metabolites) from biological fluids containing proteins, nucleic acids or other biopolymers. Direct introduction of such samples into a chromatographic column is undesirable, since the presence of adsorptive biopolymers in the sample may lead to a number of undesirable effects including retention time shift for the analytes, reduced sample capacity, low analytical reproducibility, column clogging, increased column back pressure, reduction of column lifetime, and column failure. Conventional solid-phase extraction materials are difficult to use for the purification of such biological fluids since proteins and other biopolymers get adsorbed on the surface of the extraction media blocking the pores. RAMs are designed in such a way that the biopolymers cannot enter into their pore structure due to small pore diameters, charge repulsion, and/or other molecular barriers placed on their way to the pores. The internal surface (i.e., surface within the pores) of such a material is derivatized with hydrophobic (and/or ion exchange) ligand(s) while the outer surface is provided with a bonded hydrophilic layer presenting an inert environment for the biopolymers. When a biological sample is passed through a RAM extraction bed, small target molecules are retained by the ligands anchored inside the pores, while proteins and other biopolymers get excluded from the pores and move down the bed with the liquid flow through interparticular channels. Further, biopolymers have little chances for adsorption on the RAM outer surface that presents a hydrophilic inert environment for biopolymers. Historically, restricted-access materials have been differentiated into four major types [216]: (a) internal surface reversed-phase (ISRP) [217], (b) semipermeable surface (SPS) [218,219], (c) shielded hydrophobic phase (SHP) [220], and (d) mixed functional phase (MFP) [221,222]. An ISRP restricted access material consists of high-performance silica material with two types of surface-bonded moieties: (a) hydrophobic group(s) bonded to the internal surface residing within the pore structure, and (b) hydrophilic group(s) bonded to the external surface located outside the pores. This type of RAM can be prepared by using silica containing immobilized polypeptide groups. The hydrophilic external layer is created by selective enzymatic hydrolysis of those polypeptide groups that are located on the RAM surface outside the pores [223-225]. ISRP RAMs have been prepared using silica with various immobilized surface moieties including glycine-L-phenylalanine-L-phenylalanine (GFF) [217,226] and N-octanoylaminopropyl [227,228] groups. ISRP RAMs have been reported to have a typical pore size of 5.2 nm [229] and they exclude proteins with a molecular weight higher than 15,000 dalton [230]. Protein exclusion by such RAMs is assumed to be based on a combination of size exclusion and

1039

charge repulsion mechanisms. In GFF ISRP, for example, negatively charged glycine groups are present at the pore entrance and can take part in the exclusion of negatively charged proteins. A semipermeable surface (SPS) RAM particle normally consists of a core region of functionalized silica and a covalently bonded hydrophilic layer of poly(oxyethylene) on the external surface. Practically, any derivatized silica support (or even non-functionalized, bare silica) can be used to prepare this type of RAM. The external coating of poly(oxyethylene) serves as a hydrophilic barrier to proteins and other biopolymers. This hydrophilic polymeric barrier, effectively prevents macromolecules from entering into the porous structure of the RAM resulting in their exclusion. Small molecules, however, can diffuse through the hydrophilic barrier and interact with the core material resulting in their retention. Compared with ISRP RAMs, SPS restricted-access materials possess greater analytical flexibility since they can be used for the extraction of both polar and nonpolar analytes. This advantage of SPS RAMs stems from the flexibility in using cores with a wide variety of functional groups-polar or nonpolar. It should be mentioned that only nonpolar functional groups are used in ISRP RAMs allowing for the extraction of only nonpolar analytes. Shielded hydrophobicphase (SHP) RAM consists of silica particles containing a bonded hydrophilic polyethylene oxide) network with imbedded hydrophobic phenyl moieties. In this type of RAM the network covers the entire surface of the silica support-both inside and outside the pores. Small molecules can diffuse through the network and interact with the imbedded phenyl groups resulting in their retention. Hydrated macromolecules (e.g., proteins), on the other hand, are excluded because of the hydrophilic shield provided by the poly(ethylene oxide) network. Mixed functionalphase (MFP) RAM consists of silica particles functionalized with a hydrophilic diol group and a hydrophobic group that can be chosen from a wide selection of hydrophobic moieties including phenyl [231,232], butyl [232], octyl [232], and cyclodextrin [221,233]. A general drawback of RAMs is the limited flexibility in manipulating the retention characteristics. This is explained by the fact that to avoid protein precipitation during extraction, the mobile phase organic modifier content needs to be kept at a low value (< 25% for acetonitrile, < 20% for 2-propanol, and < 10% for THF) [223]. Since silica-based RAMs are predominantly used in current analytical practice, a second inherent disadvantage of using RAMs is the narrow pH range over which these materials can be operated. Development of organic polymer-based RAMs could effectively overcome this difficulty. However, to date, there are very few reports on the development and use of totally organic polymer-based restricted-access materials, although polymers have found applications in silica-based RAM technology as a semipermeable hydrophilic barrier 218,2191 and as a hydrophilic shielding network [220]. Haginaka et al. [215,234-235] have recently described a procedure to prepare molecularly imprinted sorbents with restricted-access properties

1040

dibutyl phthalate seed particle

swelling

_

V-65 toluene

(S)-naproxen 4-vinylpyridine EDMA

swelling

swelling hydrophi GMMA GCMA KS2 0,

polymerization

hydrophilization

Scheme 32.3. Synthetic scheme of surface-modified MIP for (S)-naproxen (Reproduced from Ref. [235], with permission of Elsevier).

(RAM-MIPs) using polystyrene-based seed particles. For this, they used a multi-step swelling and polymerization procedure as illustrated in Scheme 32.3. The (s)-naproxen-specific MIP was created on -1 ,um swollen polystyrene particles using 4-vinylpyridine as the functional monomer and ethylene glycol dimethacrylate as the cross-linker. Next, the outer surface of the prepared material was hydrophilized by using a 1:1 mixture of glycerol monomethacrylate (GMMA) and glycerol dimethacrylate (GDMA). The technical details of the whole procedure can be found in Refs. [235,236] Figure 32.6 shows scanning electron micrographs of MIP and RAM-MIP particles prepared by the above-mentioned procedure. RAMs have traditionally been used in the reversed-phase mode. However, normal-phase mode operation is also possible. Recently, Gustavson et al. [237] demonstrated such a possibility. Using a RAM with a bonded polyethylene oxide-based external coating and an underivatized silica core, those authors selectively extracted methyl oleate from a nonpolar hexane environment containing 16 EPA priority PAH pollutants. A second trend is the development of RAM substitutes. In this direction, attempts have been made to use variously functionalized porous polymers as a substitute for RAMs. However, protein adsorption is still a problem for those polymerbased materials. Development of RAMs has had a significant impact on the speed and automa-

tion in biological sample cleanup and analysis. It opened new possibilities for direct sample introduction into a RAM precolumn for sample cleanup followed by on-line transfer of the extracted analytes into the HPLC system using a column switching technique [238-241]. Further developments in the area of restricted-access media are likely to be focused on the development of polymeric materials that have the potential to overcome the shortcomings of silica-based RAMs, particularly the problems associated with their narrow pH stability range. 32.4.3.4 Sol-gel polymeric materialsfor SPE Sol-gel chemistry [106,242] provides an elegant pathway for the synthesis of organic-inorganic hybrid polymers with advanced material properties. A significant advantage of the sol-gel route is that sol-gel reactions take place under 1041

Fig. 32.6. Scanning electron micrographs of the MIP (A, C) and RAM-MIP (B, D) for (S)-naproxen. Magnifications: 2000x (A, B) and 10000x (C, D). (Reproduced from Ref. [215] by permission of American Chemical Society).

extraordinarily mild thermal conditions-often at room temperature. Besides, the organic/inorganic composition and the structure of the synthesized materials (and hence their properties) can be controlled by adjusting the composition of the sol solution. A typical sol solution contains one or more sol-gel precursors (metal alkoxides), an organic ligand with a sol-gel active functional group, a sol-gel catalyst (an acid, a base, or a fluoride), dissolved in a suitable solvent system containing controlled amounts of water. In the presence of water, the sol-gel precursors undergo hydrolysis, and the hydrolyzed products take part in polycondensation reactions in a variety of ways leading to polymeric chain growth through chemical bonding of the hydrolyzed species among themselves and/or with various sol-gel-active species present in the sol solution. Ultimately, these reactions result in the formation of a hybrid organic-inorganic sol-gel polymer. Although the hydrolytic polycondensation route is most frequently used for the synthesis of sol-gel materials, nonhydrolytic pathways for such syntheses have also been reported [243-247]. The use of sol-gel polymers in analytical extractions began relatively recently. Niessner and coworkers [248-252] introduced sol-gel immunosorbents (ISs) to perform immunoaffinity extraction of various polycyclic aromatic hydrocarbons and their derivatives from aqueous media. They prepared stable immunoaffinity sorbents by encapsulating appro-

1042

34 29 -

24

E 19

14 9 4 -1

0

5

10

15

20 Time (min)

25

30

35

Fig. 32.7. HPLC chromatograms of urine samples obtained after (a) C-18 SPE (upper plot) and (b) immunoextraction (lower plot) of hydroxyphenanthrenes (OH-PHEs) and hydroxypyrenes (OH-PYRs). Conditions: 10-ml volumes ofurine sample from the same subject, final volume 3 ml, 100 lp injection volume. Peaks: (1) 2-/3-OH-PHEs, (2) 9-OH-PHE, (3) 1-OH-PHE, (4) 4-OH-PHE, and (5) 1-OH-PYR. (Reproduced from Ref. [252] by permission of American Chemical Society).

priate antibodies in the sol-gel matrix. The sol-gel immunosorbents effectively retained the target analytes and provided sample cleanup by eliminating interferences. Figure 32.7 presents two LC chromatograms obtained after extraction of hydroxyphenanthrenes in a human urine sample using (a) sol-gel immunosorbent (lower trace) and (b) a conventional ODS bonded phase (upper trace). The advantage of the sol-gel immunosorbent over conventional ODS-bonded solid-phase extraction material is evident from a comparison of the two HPLC traces. Compared with the ODS phase, the sol-gel immunosorbent provided a

much cleaner extract by efficiently eliminating the matrix interferences. The flat baseline of the lower HPLC trace (Sol-gel IS) compared with the elevated baseline of the upper trace (ODS phase) makes this point very clear. Seneviratne and Cox [253] prepared dimethylglyoxime (DMG)-doped microporous sol-gel silica material for the solid-phase extraction of nickel (II) from aqueous samples. They also prepared diethylenetriamine (DTA)-doped mesoporous sol-gel silica to extract copper (II). An interesting phenomenon observed by those authors was that in micropores of the sol-gel material, DMG forms 1:1 complex with nickel (II) compared with the characteristic 1:2 ratio observed in free solution. For sol-gel materials with a liquid phase confined in the porous structure, such apparent deviation from "normal" behavior has been observed in others cases too. An example is the unexpected extraction of aromatic hydrocarbons into polar sol-gel silica monolith from hydrophobic solvents [254]. In the confined environment of liquid-filled silica pores, the hydrogen bonding between the silanol groups and other chemical species may play a decisive role in the 1043

TABLE 32.1 Extraction recyclability through four extraction/stripping cycles with 33 mg of sol-gel in 5.0 ml of SrCl 2 solutions. Extraction was conducted were conducted at 23°C. (Reproduced from Ref. [255], with permission of American Chemical Society). Extraction cycle

1 2 3 4

[Sr2 + ] (ppm) Initial

Final

25.0 32.25 24.7 27.6

2.50±0.18 5.2±0.4 0.29±0.02 1.44+0.11

Capacity

Kd

%Sr2+ removed

3.9x10- 2 4.7x10 2 2.8x10 -2 4.2x10 2

1.4x10 3 7.8x10 2 1.3x10 4 2.8x10 3

90.0+0.2 83.8+0.4 98.8+0.2 94.8-0.1

occurrence of unexpected phenomena like extraction of aromatic hydrocarbons by polar silica matrix from a hydrophobic solvent medium. Yost and coworkers [255] described a crown ether-encapsulated sol-gel material for selective extraction of Sr (II) from an aqueous solution containing excess of competing ionic species such as Ca (II). The crown ether ligand, 1,4,10,13tetraoxa-7,16-diazacyclooctadecane-7,16-bis(malonate), is known for high affinity toward Sr(II) ions and was encapsulated in the hydrophilic silica through sol-gel process. The extracted Sr (II) ions were easily recovered from the sol-gel matrix by washing with an acid or ethylenediaminetetraacetic acid (EDTA) disodium salt. This washing also regenerates the sorbent, and extraction procedure can be repeated with the regenerated sorbent. The used crown ether encapsulated sol-gel material was found to remove 91.4 ± 1.3% of the target Sr (II) ions from aqueous solution in presence of excess Ca (II) ions (Table 32.1). The simplicity of the sorbent preparation, extraction, and sorbent regeneration together with high recovery of Sr (II) ions in the presence of excess calcium ions, gives the sol-gel material a competitive edge over traditional sorbents with chemically grafted crown ether ligands. Sol-gel hybrid organic-inorganic materials have tremendous potential for being used as extraction media as well as chromatographic stationary phases. Section 32.6.3.3 will present the applications of sol-gel polymeric materials in SPME, and capillary microextraction using sol-gel surface coatings or monolithic beds. 32.5

NEW POLYMERIC MATERIALS FOR MEMBRANE-BASED EXTRACTION

Membrane-based extraction techniques [256-261] are attracting increasing interest in modern analytical practice. A membrane represents a selective barrier between two phases [262] through which preferential mass transfer can take place from one phase into another. The phase from which the mass transfer takes place is called a donor phase or feeding phase, while the phase on the other

1044

Fig. 32.8. Schematic representation of transport through membranes. (Reproduced from Ref. [263] by permission ofElsevier).

Donor phase

Membrane

Acceptor phase

side of the membrane that receives the transferred mass is called the acceptor phase or permeate phase. Figure 32.8 illustrates the selective mass transfer through a membrane [263]. Membrane-based extraction and separation processes are based on differential transport rates of various chemical species through the interface. In membrane-based systems, the mass transport process can be divided into three categories [263,264]: (a) passive, (b) facilitated, and (c) active. Passive transport is commonly used in applications dealing with analytical extractions or separations. In this mode, the membrane serves merely as a physical barrier. Here, the driving force for the transport process is the electrochemical potential gradient of various molecular or ionic species between the donor and acceptor phases located on the two sides of the membrane. In the facilitated mode of transport, in addition to the electrochemical potential gradient, a carrier is used in the membrane to enhance the permeability during the transport process. Active transport is typical of living cellular membrane where the transport takes place against the electrochemical potential gradient, and is caused by a chemical reaction taking place inside the membrane. Based on the nature of the used membrane and the trapping media (if any), membrane extraction techniques can be differentiated into: (a) supported liquid membrane extraction (SLME) [40,42], (b) microporous membrane liquid-liquid extraction (MMLLE) [42,265], (c) membrane extraction with sorbent interface (MESI) [257,266-271], and (d) polymeric membrane extraction (PME) [272-278]. Here the discussion will be confined primarily to the use of polymeric materials in membranes extraction. Luo et al. [273] have studied the effect of polymer swelling on the mass transfer performance in membrane extraction. Fu et al. have reported the use of potentiometry to study extraction thermodynamics of polyanions into plasticized polymer membrane doped with lipophilic ion exchangers [275]. Melcher and coworkers [276-279] developed and studied membrane extraction systems with silicone rubber membranes and various combinations of aqueous/organic liquids. Recently, Hauser and Popp [261,280] described a silicone rubber hollow fiber-based dynamic membrane extraction device (Fig. 32.9) to enrich and analyze volatile organic compounds from water (or air). Being battery-operated, portable, and easy-to-interface with a portable GC, the device appears to be

1045

Peristaltic pump l

Water sample -

Ambient air

1

Silicone hollow fibre

Activated charcoal

I

-+

Tosample inlet of AirmoBTX

Fig. 32.9. Schematic of a flow cell for dynamic extraction with hollow fibers. (Reproduced from Ref. [280] by permission of Elsevier).

perfectly suitable for field operation. Volatile organic compounds in water are allowed to diffuse through the silicone hollow fiber, and are enriched in the integrated activated charcoal-based sorption tube. Typical enrichment time is 10 min. The enriched analytes are then thermally desorbed and analyzed by GC. A typical example of BTEX analysis in water using dynamic membrane extraction using a silicone hollow tube membrane is presented in Fig. 32.10. Low /zg/l extraction sensitivity was achieved. Li and Zhang [281] developed poly(vinylchloride)-based membranes to extract zinc ions from aqueous media. Mayer et al. [108] compared the performance of octadecylsiloxane-bonded silica- and porous polymer particle-loaded membranes in solid-phase extraction. Recently, Jonsson and Mathiasson published extensive reviews on supported liquid membrane extraction and related techniques [44,282]. Cordero et al. [263] reviewed the interfacing of membrane extraction techniques with GC, HPLC, and CE. Van de Merbel published an elaborate review on on-line coupled membrane-based extraction techniques [2] covering principles, instrumentation, Benzene

Toluene

60000 50000 Ethylbenzene 40000

m-Xylene

30000

o-Xylene

20000 10000 0t

0.00

I,

fWY-lea _PLW s,

50.00

100.00

i

'Y-PL------

150.00

XI

200.00

1, ,

m

]AI ,1

250.00

Fig. 32.10. Gas chromatogram obtained after extraction of 5 .og/l BTEX in water with a 0.3 m silicone hollow fiber 0.7 x 0.9 mm. Temperature, 20°C; extraction time, 10 min. (Reproduced from Ref. [280] by permission of Elsevier).

1046

interfacing (to GC, LC, and CE), and application aspects of dialysis, electrodialysis, and filtration techniques. Bauer [259] reviewed application of membrane extraction in conjunction with mass spectrometry (membrane introduction mass spectrometry, MIMS). One limitation of liquid membrane-based extraction systems is the short membrane lifetime. This can be overcome by using a polymeric membrane instead of liquid membranes. Silicone rubber-based membranes can be a good choice since they possess extended lifetimes. Another significant advantage of polymeric membranes is that they are practically insoluble in aqueous or many organic media, and allow for the use of extraction systems with various combinations of aqueous and organic liquids. On the other hand, the solute diffusion through polymeric membranes is slower than that in the liquid membranes. This results in slower extraction by polymeric membranes. Another disadvantage of using polymeric membranes is that the composition of the membrane remains fixed during the extraction process, which translates into reduced flexibility in fine-tuning the extraction selectivity. Since the silicone rubberbased membranes are hydrophobic, their use as the extraction medium is primarily limited to the extraction of nonpolar analytes, and may not be suitable for the extraction of polar compounds. To overcome this difficulty, hydrophobic polymeric membranes have been surface-modified with a thin layer of some a hydrophilic polymer (e.g., polyacrylate) [283,284]. Polymeric membrane extraction is still an emerging area; an important trend is the flow-through solid-phase extraction using polymeric membranes (membrane SPE). This is consistent with the general trend of miniaturization in separation and extraction technologies. Materialization of membrane extraction in a flow-though mode should make this extraction technology much faster compared with the traditional passive or facilitated modes commonly employed in liquid membrane extraction techniques (SLME or MMLLE). In a recent report, Fritz and Masso [285] described a miniaturized flowthorough membrane extraction device using a miniaturized (1.2 mm in height and 0.7 mm in diameter) sulfonated polystyrene-divinylbenzene resin disk housed inside the removable needle of a 50 Al Hamilton gas-tight syringe as illustrated in Fig. 32.11. The membrane rested on a stainless steel screen. All these parts were assembled in a Teflon ferrule-a standard part of the used Hamilton syringe. Aqueous samples containing eight substituted benzene compounds (at 10 ppb level concentration) were automatically passed through the membrane using a single-syringe infusion pump. It required only 5 ul volume of acetonitrile to elute the extracted compounds. Analyte recovery was higher than 90% with an average relative standard deviation of 4.6%. Another significant advancement in the area of membrane SPE is the recent development of polymeric membranes with a surface-grafted molecularly imprinted layer [272,287,288]. Wang et al. [287] prepared membranes with surface-grafted MIPs. For this, they used a special photoreactive polymer, poly(acrylonitrile-co-(diethylamino)dithiocarbamoylmethylstyrene). Using theo1047

<_

Miniaturized membrane

<_ Teflon ferrule

<(-

55 pm Mesh screen It

Fig. 32.11. Miniaturized-SPE assembly inside removable needle chamber. (Reproduced from Ref. [286] by permission of Elsevier).

phylline as the template, these authors conducted graft copolymerization of acrylic acid and N,N-methylenebis(acrylamide) (MBA) under UV initiation that resulted in a theophylline-specific membrane. However, such a procedure was lengthy and complicated that required a special polymer for membrane formation. The resulting membranes were characterized by low permeability, large macrovoids, and strongly asymmetric pore morphology. The most recent developments in the area of photografted MIP membranes were reported by Ulbricht and coworkers [272,288] who developed a general method for molecular imprinting and surface functionalization of porous synthetic polymeric membranes. Those authors developed a procedure for MIP synthesis in aqueous environment using desmetrin (a triazine herbicide) as the imprint molecule [288]. Their aim was to achieve affinity membrane extraction of the template molecules from aqueous solutions. The authors optimized the synthesis, extraction, and elution conditions. Extraction was performed by filtering the aqueous solution through the MIP membranes at high flow rates, typically 10 ml/min. A 200-fold enrichment and a low nanomolar extraction sensitivity (2 x 10 - 9 M) were achieved. Further development in the area of photografted MIP membranes was reported in ajoint publication by researchers from Germany (Wulff and coworkers) and Ukraine (Sergeyeva and coworkers) [272]. In this joint work, the authors described a surface functionalization procedure to create a thin molecularly imprinted polymeric layer on microfiltration polymer (polyvinylidene fluoride) membranes with a hydrophilic polyacrylate surface layer. The main focus of this work was to create a surface-grafted MIP layer that will minimize

1048

50 MP 40330-

Fig. 32.12. Selectivity of terbumeton20imprinted thin layer MIP composite mem- i

LL~~E

brane to other herbicides of related chemical I10-5 structure. Herbicide concentrations, 10 M in water. (Reproduced from Ref. [272] by o permission of Elsevier). terbumeton

atrazine

desmertyn metribuzine

the nonspecific interaction between the membrane and the analytes (thereby maximize the effect of template-specific interaction), at the same time retaining the high permeability of the membranes without blocking the membrane pores. They photopolymerized a methanolic solution containing a functional monomer (2-acrylamido-2-methyl-l-propane sulfonic acid (AMPS), methacrylic acid (MAA), or acrylic acid (AA)) and a cross-linker (N,N-methylene-bis-acrylamide)

(MBAA) using UV radiation and benzophenone as the photoinitiator. A relatively hydrophobic triazine herbicide, terbumeton, was selected as the template molecule. In contrast to their previous work on the development of a surfacegrafted MIP synthesis procedure using aqueous solutions [288], the focus here was on the synthesis of MIP grafted surface layer using polymerizing solutions prepared in an organic solvent. Such a method should greatly increase the number of templates that can be used for the synthesis. The prepared MIP-grafted membranes showed high specificity for the template molecule (terbumeton), as is evident from Fig. 32.12. It should be noted that flow-through membrane SPE is very attractive from an analytical point of view, and new developments can be expected in this area in the near future. 32.6

NEW POLYMERIC MATERIALS FOR SOLID-PHASE MICROEXTRACTION

32.6.1 Working principle and variants of solid-phase microextraction Solid-phase microextraction (SPME) [50,51] is based on the principle of equilibrium extraction where a stationary phase coating created on the outer surface of a micro fiber (typically 100 m in diameter) directly extracts analytes from the surrounding medium. In the conventional format of SPME, a polymeric coating (typically 7-100 m in thickness) is applied to the SPME fiber covering the outer surface of an approximately 1-cm segment of the fiber at one of its ends. Such a fiber is installed in a specially designed syringe (SPME syringe). For this, the fiber is glued or mechanically mounted into a piece of stainless steel holder connected with the syringe plunger. During handling and/or operation, the outer 1049

2

(A) (B (B)

.1

I

2--'

Left: Fig. 32.13. Modes of SPME operation: (a) direct extraction, (b) headspace SPME. (Reproduced from J. Pawliszyn, B. Pawliszyn and M. Pawliszyn, The Chem. Educator,2 (1997) 1, with permission of Springer). Right: Fig. 32.14. Schematic of a sol-gel fiber (A) and a sol-gel open tubular microextraction capillary (B). 1, sol-gel coating; 2, fused silica fiber (A) or capillary (B).

extraction coating is protected from mechanical damage by retracting the fiber into the needle of the SPME syringe. To carry out extraction, the fiber is brought in contact with the analyte by exposing the coated segment of the fiber into the sample. This is accomplished by simply depressing the plunger of the syringe. Analytes in the surrounding medium are extracted by the stationary phase coating on the external surface of the fiber. Agitation of the sample (e.g., using a stir bar) is often used to assist the extraction process. The working principle of

SPME is briefly illustrated in Fig. 32.13. 32.6.2

Polymeric surface coatings for SPME

Polymeric surface coatings are predominantly used as extraction media in conventional SPME [50-57,289-291] as well as in the newly emerging extraction techniques like in-tube SPME [292-307] or capillary microextraction (CME) [308]. Besides polymeric coatings, in a few instances the use of nonpolymeric porous coatings have also been reported. These include SPME coatings created by using carbon-based materials [143,309,310] or by gluing reversed-phase HPLC particles onto the surface of an SPME fiber [311,312]. Surface coatingbased extraction systems are usually rapid, easy-to-automate, and suitable for on-line operation. They are also compact and suitable for field applications. However, the most fascinating aspect of surface coating-based extraction systems is that they do not require exhaustive extraction of the analytes for quantitation [313]. To create a polymeric coating on a fused silica SPME fiber, the outer protective coating of the fiber is first removed from the designated end-segment, and then a coating of the selected extraction polymer is applied to the bare outer surface of that end-segment. This can be accomplished by simply dipping the bare end segment of the fiber into a liquid containing the coating polymer. In-tube SPME typically uses a polymeric coating on the inner surface of fused 1050

silica capillary. A small segment of a wall-coated GC capillary column is often used for this purpose. Figure 32.14 illustrates the construction of an SPME fiber (Fig. 32.14A) and a wall-coated capillary for in-tube SPME (Fig. 32.14B). Both in conventional and in-tube operation modes of SPME, the polymeric coating is exposed to the environment containing the target analytes, and time is allowed for an equilibrium to establish. Typically, it takes less than an hour to reach the extraction equilibrium. The extraction process is aided either by mechanically agitating the sample with a stir bar (as in conventional SPME) or by flowing the sample over the coated surface (as in in-tube SPME or capillary microextraction). After this, the fiber or the capillary is withdrawn from the sample, and the extracted analytes are rapidly desorbed from the coating for introduction into an analytical instrument-most commonly a GC, HPLC, SFC or a CE system. The analyte desorption can be carried out either thermally by introducing the fiber (with the extracted analytes on its coating) into a heated GC injection port using an SPME syringe specially designed for SPME-GC analysis, or by rinsing the coated capillary with an appropriate solvent as is performed in in-tube SPME/HPLC analysis. SPME has gained wide popularity because of its simplicity, speed, solventless extraction capability, portability, and suitability for automation. A fascinating attribute of SPME techniques is that they simultaneously accomplish a number of important analytical operations in a very simple way. These include sampling, sample preparation, and analyte preconcentration. They also provide a robust mechanism for the introduction of the extracted analytes into an analytical instrument for quantitation. In addition, the fact that the extracted amount of the analyte in SPME is practically independent of the sample volume [291] makes the techniques especially valuable for direct field sampling and analysis. In SPME techniques, a vital analytical role is played by the polymeric coating that serves as the extraction medium, thus making the extraction process environmentally benign by eliminating the need for hazardous organic solvents that are used in large volumes by conventional extraction techniques. Conventional sorbents for SPME include two types of extraction media [314]: (a) homogeneous polymers and (b) composite extraction media consisting of a partially cross-linked polymer embedded with porous particles of a second component. New polymers for SPME coatings include: (1) Nafion, (2) Polypyrrole, (3) molecularly imprinted polymers, and (4) sol-gel hybrid organic-inorganic polymers (sol-gel PDMS, sol-gel PEG, sol-gel-crown ether, sol-gel dendrimer, etc.). From the polarity point of view, conventional SPME coatings can be classified into four categories: (1) nonpolar (e.g., PDMS), (2) moderately polar (e.g., PDMS/DVB composite), (3) polar (e.g., polyacrylate), and (4) highly polar (e.g., Nafion). Since PDMS is nonpolar in nature, it can effectively extract nonpolar analytes from a wide range of matrices. However, PDMS coatings are not efficient for the extraction of polar analytes. More polar SPME coatings are needed for the extraction of polar analytes. Polyacrylate (PA)-based coatings 1051

[317-322] are more suitable for this type of applications. Polyethylene glycol (PEG)-based coatings (e.g., Carbowax 20M and Carbowax/divinylbenze) are also used [323-325]. It should be mentioned that relatively thick coatings are to be used in SPME to provide reasonable sample capacity and extraction sensitivity. 32.6.2.1 Homogeneouspolymeric SPME coatings Polydimethylsiloxanne (PDMS) and polyacrylate (PA) belong to the category of homogeneous SPME coatings. Such coatings are usually applied to the external surface of the solid SPME fiber via an automated coating procedure immediately following the fiber drawing process. The conditions can be optimized to obtain coatings of desired thicknesses. Commercial polyacrylate coated fibers are available with a coating thickness of 85 Am. This coating is only partially cross-linked. PDMS fibers are most widely used in SPME, and are commercially available in three different thicknesses: 7, 30 and 100 Am. The 7-Am thick PDMS coating is cross-linked, while PDMS coatings with the other two thicknesses (30 and 100 Am) are not. It should be pointed out that the degree of cross-linking is difficult to control, and a high degree of cross-linking may significantly change the sorption properties of the polymeric coating. Cross-linking makes the polymeric coatings more rigid, and may change their diffusion and mass transfer characteristics. Besides, it becomes increasingly difficult to stabilize coatings of higher thickness through cross-linking reactions. This might explain, why commercial fibers with PDMS coatings of higher thicknesses (30 and 100 am) are not cross-linked. The smaller coating thickness and the cross-linked structure provide higher thermal and solvent stability to the 7-tm thick PDMS coating. However, because of lower coating thickness it possesses smaller sample capacity. Non-cross-linked and partially cross-linked fibres are characterized by relatively lower thermal stability compared with their cross-linked counterparts. This lower thermal stability is often responsible for the incomplete desorption of analytes (especially the high-boiling ones) that require higher desorption temperature than the allowable upper temperature for such fibers, and leads to the carry-over problem (memory effect) [291,315,316]. Non-cross-linked or partially cross-linked coatings also often show low solvent stability, and SPME fibers with such coatings are not recommended to bring into direct contact with organic solvents that may dissolve the coatings. Because of this solvent stability problem, SPME devices with such coatings may not be suitable for use in hyphenation with liquid-phase separation techniques (e.g., HPLC, CEC) where the extracted sample is often desorbed using liquid mobile phases rich in the organic solvents. 32.6.2.2 Polymeric composite coatings A polymeric coating of this category consists of two components: a porous particulate component, and a nonparticulate component. SPME fibers of this type are prepared by coating the fiber surface with a blend of the two components in which the particulate component is embedded in the partially cross-

1052

linked polymeric component. Unlike the preparation of homogeneous polymeric coatings, such a process is difficult to automate and is usually done manually. Because of the presence of two sorbents in the coating, these coatings provide improved selectivity. In addition, the fiber selectivity can be fine tuned for the analytical problem at hand by proper selection of the two coating components. It should be mentioned that such composite coatings possess lower mechanical stability than the homogeneous polymer coatings. The following commercial SPME coatings belong to this category: (1) polydimethysiloxane/divinylbenzene (PDMS/DVB), (2) polydimethylsiloxane/Carboxen (PDMS/Carboxen), (c) Carbowax/divinylbenzene (CW/DVB), and (d) Carbowax/templated resin (CW/TPR). In written notation of these composite coatings, the non-particulate component is usually put first, followed by a "/" sign that separates it from the particulate component that follows. Small pieces (-2 cm) of these externally coated fibers are then installed on individual fiber assembly to be used for extraction.

32.6.3 Newly emerging polymeric materials for solid-phase microextraction 32.6.3.1 Molecularly imprintedpolymers for SPME Molecularly imprinted polymers are now widely used in solid-phase extraction (SPE), and a wealth of literature (including extensive reviews) is available on the subject [47,48]. However, it is only very recently that MIPs have been introduced in SPME [207,208]. High specificity of MIPs for the template molecules makes them effective media for selective extraction of target analytes. The first report on the use of MIPs as the stationary phase coating for in-tube SPME was made by Pawliszyn and coworkers [208]. They packed a fused silica capillary with particles of an MIP that they prepared by polymerizing a mixture containing methacrylic acid (monomer), ethylene glycol dimethacrylate (cross-linker), and racemic propranolol (template), following a procedure described by Andersson [326]. The particle-packed capillary was used for selective on-line extraction of propranolol and related 5-blockers from biological fluids. For a standard mixture, the authors achieved a detection limit of 0.32 Ag/ml with an RSD value of less than 5%. Koster et al. [207] were the first to report on the use of MIP coatings on SPME fibers. They prepared a molecularly imprinted methacrylate polymer using clenbuterol as a template, and demonstrated the possibility of selective extraction of brombuterol from spiked human urine (Fig. 32.15). Molecularly imprinted polymers have great potential for selective extraction of target analytes from various complex matrices. However, adsorption of biopolymers or humic substances may serve as a barrier to full utilization of this analytical potential. Development of MIPs with restricted-assess characteristics might be one possibility to address this problem. Of course, we need to bear in mind that molecularly imprinted polymeric materials constitute a newly emerging area for SPME, and a lot of research activities can be expected in this area.

1053

100000

80000

9

60000

a)

40000

b)

Fig. 32.15. LC-ECD chromatograms of the extraction (45 min) of human urine with MIPcoated fiber: (a) blank urine, (b) spiked urine (100 ng/ml brombuterol), and washing of the fiber, and (c) blank urine and washing of the fiber. (Reproduced from Ref. [207] by permission of American Chemical Society).

20000

0 Time

(min)

32.6.3.2 Polypyrrole-basedcoatingsfor SPME Although SPME has made remarkable achievements over the last decade in extracting nonpolar, hydrophobic compounds from aqueous media, still there are two major areas where the progress has been only moderate. These two areas are: (a) extraction of polar and ionic analytes, and (b) reliable hyphenation of SPME with liquid-phase separation techniques (e.g., HPLC, CEC, etc.). The limiting factor in the development of both of these areas is the absence of polymeric SPME coatings with desirable sorbent properties.

A closer look at these problems indicates that an effective solution to the first problem will require development of highly polar surface-bonded SPME coatings that will be able to effectively compete with water to extract polar compounds from aqueous media. If the extracted polar compounds need to be analyzed by GC, then the polar polymeric coatings must also withstand high desorption temperature of the GC injection port. To ensure reasonable sample capacity the coatings should also be relatively thick (several tens of micrometer in thickness). Furthermore, if the polar analytes are to be extracted from an organic solvent media, then the coating must also possess high solvent stability. Development of coatings with high solvent stability is also the key to the solution of the second problem mentioned above. One of the main reason for limited progress in the above two areas is that commercially available coatings that have been mainly used to solve those problems often fail to simultaneously satisfy all these requirements. Development of new polymeric coatings with advanced material properties will be a prerequisite not only for the solution of these and other outstanding problems, but also for the further development of SPME as an extraction technique. 1054

To this end, Pawliszyn and coworkers [304-307,327] recently developed poly(pyrrole)-based coatings for SPME. Their main emphasis was to develop polypyrrole (PPY) and poly(N-phenylpyrrole) (PPPY) coatings for reliable hyphenation of in-tube SPME with HPLC for automated on-line extraction and HPLC analysis. Outstanding solvent stability of these polymers made them an appropriate choice for this job, since in such an interface the desorption of the extracted analytes is normally carried out using organic-rich solvents. Another important attribute of polypyrrole-based coatings is their outstanding pH stability. For example, PPY coatings provided stable performance within a wide pH range (1.5-10) [307]. PPY coatings possess a porous structure with an extraction mechanism primarily governed by surface adsorption of the analytes. Table 32.2 compares the SPME performance of PPY coatings with that of commercially available capillaries with the following coatings: (a) polar silica (host), (b) Omegawax (Omeg), (c) SPB-1, and (d) SPB-5. The data clearly demonstrate the superiority of the PPY coating over its commercial counterparts in the extraction of PAHs. The authors explained this enhanced extraction capability of PPY TABLE 32.2 Comparison of the extraction efficiencies for PAHs on different capillary coatings (adapted from Ref. [307], with permission of American Chemical Society) PAH Compound

Detector

Amount of analyte extracted (ng)c or extraction yield

responseb

(%)d

F Naphthalene Acenaphthylene Acenaphthene Fluorene Phenanthrene Anthracene Fluoranthene Pyrene Benza[a]anthracene Chrysene Benzo[b]fluoranthene Benzo[k]fluoranthene Benzo[a]pyrene Dibenz(a.h)anthracene Benzo(ghi)perylene Indeno(1,2,3-cd)pyrene

0.058 0.046 0.027 0.213 0.050 0.024 0.090 0.115 0.082 0.052 0.072 0.107 0.079 0.098 0.099 0.088

Host

Omeg

SPB-1

SPB-5

PPY

0.9 0.6 0.7 1.2 0.8 0.8 1.0 1.1 1.5 1.4 1.9 1.5 1.8 1.4 1.6 19

1.7 1.8 1.8 2.1 2.5 2.6 3.2 3.1 4.8 4.4 6.3 5.8 6.0 4.9 5.2 7.3

1.9 2.5 4.1 3.8 4.4 4.9 5.8 6.1 6.1 5.2 6.4 5.7 6.0 2.9 3.1 6.3

3.0 3.7 4.6 6.3 5.4 7.3 10.3 12.0 9.9 9.2 8.6 8.4 8.8 6.1 6.7 8.8

5.9 7.6 6.7 8.1 11.1 11.8 17.5 18.0 20.7 18.7 23.3 15.7 18.7 10.7 10.8 16.0

aCompound concentrations and experimental conditions (see notes b, c, and d). bF (detector response factor) was obtained by injecting 10 l of 1,g/ml solution (for each PAH 10 ng was injected). CA 1-ml sample (100 ng/ml for each PAH) was analyzed by in-tube SPME, and the amount of analyte extracted (nA) was calculated by equation: nA = FA = (m/Ad)A; the extraction yield (%)= nA x 100/M where is the total amount of analyte in the 1 ml solution (see Ref. [307] for details).

dSelectivity factor was calculated only for 15 cycles on the basis of nA relative to n (naphthalene).

1055

44.0

PPPY Film 19.5

14.5 9.5

Fig. 32.16. SPME-GC analysis of 40 g/ml methanol (1), ethanol (2), and 2-propanol in hexane using PPPY-coated fiber. (Reproduced from Ref. [306] by permission of Elsevier).

4.5 -0.5

6

Time [min]

coatings based on effective T-Ti and hydrophobic interactions between the PPY coatings and PAH solutes. Enhanced extraction by PPY-coated capillaries was also observed for polar compounds, such as phenol and its derivatives. However, PPY-coated capillaries showed reduced extraction efficiency for monocyclic aromatic hydrocarbons compared with commercial capillaries. PPY-coated capillaries also showed enhanced extraction efficiency for organoarsenic compounds and for anionic species [305]. This may be explained based on the multifunctional

nature of the polymer and the positive charge that it carries. Another significant advantage of PPY coatings was found to be their very little affinity for nonpolar solvents like hexane. This fact, combined with high solvent stability of PPY coatings, opens new possibility of extracting polar impurities in nonpolar organic solvents. Figure 32.16 shows a chromatogram, illustrating SPME (followed by GC analysis) of lower aliphatic alcohols from hexane using PPY coating. It should be pointed out that PPY coatings are held on the substrate by physical forces. Their high solvent stability is due to poor solubility of these polymers in a wide range of common organic solvents. 32.6.3.3 Sol-gel polymeric extraction media for solid-phase microextraction In its traditional format, SPME has a number of drawbacks. First, since the stationary phase coating is applied to the outer surface of the fiber, it is more vulnerable to mechanical damage. Second, due to the lack of chemical bonding between the coatings and the substrate, traditional coatings do not possess adequate thermal and solvent stability. Third, free radical cross-linking procedures [328,329] that are commonly used to immobilize thin sub-micrometer coatings in GC columns, do not work very well for thick (several tens of micrometers in thickness) stationary phase coatings needed for SPME. This is especially true for polar stationary phase coatings [330,331]. In a new variant of SPME, so-called in-tube SPME [208,292-308] short segments of commercially available capillary GC columns are used to extract analytes. However, the commercial GC capillary columns have thin stationary phase films (which translates into low sample capacity) as well as low thermal 1056

and solvent stabilities. In most part, this is due to the lack of chemical bonding between the stationary phase coating and the inner walls of the capillary to which these coatings are applied. Sol-gel coating technology [332-336] easily overcomes these problems by providing surface-bonded organic-inorganic hybrid stationary phase coatings. The sol-gel coating technology is applicable for creating stationary phase coatings both on the outer surface of conventional SPME fibers and on the inner walls of the capillary used for in-tube SPME (capillary microextraction). In addition, sol-gel technology can be used for the creation of both thin and thick coatings. Finally, sol-gel technique is very much suitable for in situ creation of porous monolithic beds [337-352] within a fused silica capillary using a properly designed sol solution that solidifies and turns into a single piece of porous structure chemically bonded to the inner walls of the capillary. Monolithic beds are analogs to particle-packed sorption beds. However, because of the facts that the porous sol-gel monolithic bed is created without packing sorbent particles, and that the bed is directly bonded to the inner walls of the capillary, sol-gel monolithic beds are characterized by enhanced mechanical stability. Preparationof sol-gel polymeric coatings for SPME Preparation of a sol-gel coating involves the following operations: (1) preparation of the sol solution, (2) bringing the surface of the substrate (fiber or capillary) in contact with the sol solution, and (3) thermal treatment of the coated surface. Preparationof the sol solution. The preparation of sol solution has been previously described [333,335]. The sol solution consists of the following ingredients: (a) at least one sol-gel precursor, (b) a surface deactivation reagent, (c) an organic polymer (or ligand) with sol-gel-active functional group(s), (d) a sol-gel catalyst, and (d) a suitable solvent system. Typical composition of a sol solution, and roles played by its components in the surface-bonded coating creation process are summarized in Table 32.3. The following is an example of preparing a sol solution that can be used to create a sol-gel PDMS coating, either on the outer surface of an SPME fiber or on the inner surface of a fused silica capillary: 0.1 g of hydroxy-terminated PDMS is dissolved in 100 Al of methylene chloride placed in a micro centrifuge tube (1.5 ml); PMHS (40 Al) and MTMS (100 Al) are added to this solution, and the contents of the centrifuge tube are thoroughly vortexed. Finally, 100 Al of TFA containing 5% water (sol-gel catalyst and source of water) is added to the centrifuge tube and vortexed again. The contents of the centrifuge tube are then centrifuged for 4 min at 13,000 rpm (15,682 G) to separate out any precipitate that might have formed during the mixing process. The clear sol solution from the top part of the centrifuge tube is then transferred to a clean vial for further use. Application of sol-gel coating to the outer surface of SPME fiber. The first report on the preparation of fibers with sol-gel coatings was made by Malik and coworkers [333]. The protective polyimide coating was burnt off from an approx1057

TABLE 32.3 Chemical ingredients of a sol solution designed for the creation of a sol-gel PDMS coating. (Reproduced from Ref. [335], with permission of the Royal Society of Chemistry). Sol-gel solution ingredient

Chemical name

Sol-gel precursor

Methyltrimethoxysilane

Organic polymer with sol-gel-active functional groups Deactivation reagent

Sol-gel catalyst

Function

Serves as a source for the inorganic component of the sol-gel organic-inorganic hybrid coating and delivers it through hydrolytic polycondensation reactions. Also provides active silanol groups on the sol-gel coating to facilitate its chemical bonding to the fiber/capillary surface. 1) Hydroxy-terminated Provides organic component for the sol-gel polydimethylsiloxane organic-inorganic hybrid coating through polycondensation reactions with the hydrolyzed products of the precursor. Poly(methylhydroProvides desired level of high-temperature siloxane) derivatization of silanol groups on the sol-gel coating and thus allows for the control of the organic-inorganic hybrid coating polarity. The derivatization reaction takes place during thermal conditioning of the fiber after completion of the sol-gel coating procedure. Trifluoroacetic acid Plays a dual role int he sol-gel process: (a) as an (TFA) acid-catalyst for the sol-gel process; (b) as a source of water for the hydrolysis of the precursor (the commercial TFA used in this work contained approx. 5% of water). It thus provides a mechanism for the sol-gel process to be conducted in organic solvent-rich environment with controlled amount of water.

imately 1 cm end segment of a fused silica fiber using a cigarette lighter. The bare end of the fiber was then carefully cleaned by using a piece of Kimwipe tissue paper soaked with methanol, making sure that no soot particles remained on the surface. The exposed end of the fiber was further treated with a 0.1 M NaOH solution for 30 min to enhance the concentration of surface silanol groups that would facilitate chemical bonding of the sol-gel coating to the exposed surface. The treated end of the fiber was thoroughly washed with deionized water. At this point, the fiber was ready for coating. To coat, the treated end of the fiber was vertically dipped into the previously prepared sol solution placed in a vial. The fiber was allowed to stay in this position for -20 min. Following this, the fiber was withdrawn from the solution and subjected to a stepwise (or programmed) thermal conditioning in a stream of helium. For a sol-gel PDMScoated fiber, the final conditioning temperature was as high as 360°C. If necessary, the entire process of dipping, withdrawing, and thermal conditioning of the fiber should be repeated to obtain the desired coating thickness. This procedure was used and adapted by Wang et al. [353] to prepare sol-gel polyeth-

1058

Fig. 32.17. Scanning electron micrographs of a sol-gel PDMS-coated SPME fiber: (A) crosssectional view, 342x magnification, and (B) surface view, 3420x magnification. (Reproduced from Ref. [333] by permission of American Chemical Society).

ylene glycol (PEG) coated SPME fibers, and by Zeng et al. [354-356] to prepare SPME fibers with sol-gel crown ether coatings. Scanning electron micrographs in Fig. 32.17 illustrate the cross-sectional view (Fig. 32.17A) and surface view (Fig. 32.17B) of a sol-gel PDMS-coated SPME fiber. They reveal that the sol-gel PDMS coating created on the SPME fiber is remarkably uniform (Fig. 32.17A) and has a porous structure (Fig. 32.17B) that provides enhanced surface area, and facilitates fast mass transfer and improved sample capacity. Application of sol-gel coatings to the innersurface of the capillaryfor in-tube SPME. A general procedure for the creation of sol-gel stationary phase coating on the inner walls of a fused silica capillary was first described by Malik and coworkers [332]. Briefly, to prepare a sol-gel PDMS-coated capillary, a hydrothermally treated fused silica tubing (e.g., 10 m x 250 gm i.d.) was filled with the sol-gel solution using a home-made filling/purging device [334] under 100 psi helium pressure. The solution was allowed to stay inside the capillary for 10-30 min, after which the solution was expelled from the capillary under the same helium pressure. Following this, the capillary was dried by purging with helium for 30 min at room temperature. The capillary was then thermally conditioned under a continuous flow of helium: from 40°C to 350°C at 1°C min', holding the column at the final temperature for 5 h. Finally, the capillary was rinsed with methylene chloride/methanol (50:50 v/v) mixture and dried under helium purge. At this point, the capillary was ready for use in capillary microextraction. The sol-gel coated capillary was cut into small segments (3.5 cm and longer) for use in capillary microextraction. Following the same procedure, longer sol-gel coated

1059

Fig. 32.18. Scanning electron micrograph of a sol-gel PDMS-coated tubing for capillary microextraction. Magnification 10000x. (Reproduced from Ref. [308] by permission of American Chemical Society).

capillaries (typically 10-30 m) can also be prepared for use as a GC separation column. Figure 32.18 is a scanning electron microscopic image of a sol-gel PDMS coating on the inner surface of a 250-,um i.d. fused silica tubing used for capillary microextraction [308]. Scheme 32.4 illustrates sol-gel reactions that take place in the sol solution during in situ creation of a surface-bonded sol-gel PDMS coating, either on the outer surface of an SPME fiber [333] or on the inner surface of a microextraction capillary [308]. Similar reaction schemes were described by Zeng et al. for the preparation of sol-gel crown ether coated SPME fibers [354-356] and by Wang et al. [353] to prepare SPME fibers with sol-gel polyethylene glycol (PEG) coatings. Advantages of sol-gel polymeric coatings in analytical microextraction An outstanding attribute of sol-gel polymeric coatings is that they are chemically bonded to the silica substrates to which they are applied. This gives sol-gel coatings a number of significant advantages over conventional coatings used in microextraction. First, because of chemical bonding to the substrate, sol-gel coatings are characterized by high thermal stability [333]. For example, sol-gel coated PDMS fibers can easily withstand a desorption temperature of 340°C (Fig. 32.19). More recent experimental data showed that sol-gel PDMS coated fibers can be routinely operated using a desorption temperature as high as 360°C. By comparison, for a conventionally coated commercial PDMS fiber, the upper temperature limit is 250-270°C [357] which gives sol-gel coatings about 100°C desorption temperature advantage over their conventional counterparts. This higher temperature stability, in turn, translates into more complete desorption of the extracted analytes and higher peak area precision as illustrated in Table 32.4. Higher thermal stability of sol-gel coatings also provides a reliable solution to sample carry-over problems [291,315,316] caused by incomplete release of the extracted analytes at the desorption temperature. Another significant consequence of higher thermal stability of sol-gel coatings is the possibility of expanding the application range of SPME to higher boiling compounds that will require enhanced thermal stability of the coating for their effective desorption from the fiber.

1060

I. Hydrolysis of tilhesol-gel precursor:

(1

alkyl or alk.xy groups, and R' = alkyl or hydroxy functionalities)

R I C1130-Si-OCII 3

R' Ca(alyst

20 0

+

I HO-Si-O O-Si-

OC113

+ C 3OII

OIl

II. Polycondensation of Hydrolyzed products:

i

I

0

CH3

_O it O-+iiOf o--+

?

? I

-o

P

-O. +

rIO-lit

-O

CH 3 CH3

O-Si O-Si-O-oi-Ot-o i-OH

O-it

O

CH

CH 3

UI

I

0°

0°

ICH 3

ICH 3

CH 3

?

?

CH 3

CH 3

CH 3

CH 3

HO- SiO-ito-i-O ,i_-oiii.

Column Wall

-o.I ?

CHj

ICHJ

VH3

H3

oi-O-rSi-OH

ClH3

O

-OH

CH3

li-OS ---

??

¶H3

O--it O-

1

OS

C113

H3 (i-iC11 3

FH3 ti-1- OH CH 3

Wall-bonded PDMS Coating Scheme 32.4. Chemical reactions involved in sol-gel coating with hydroxy-terminated PDMS stationary phase. (Reproduced from Ref. [332] with permission of American Chemical Society).

Direct chemical bonding of sol-gel coatings to the substrate surface (the outer surface of SPME fibers or the inner surface of the microextraction capillary) provides higher solvent stability to the coatings. This advantage is especially significant in those applications where the coating needs to be exposed to organic solvents. Such applications include analyses done through hyphenation of SPME or CME to liquid- or fluid-phase separation techniques, e.g., HPLC [292-294,302,358-361], SFC [362] or CE [363-365]). Solvent stability of the coating is a prerequisite for these applications since the extracted analytes have to be desorbed from the extraction capillary using an

1061

s

3 I

Fig. 32.19. SPME-GC analysis of PAHs. Conditions: 200 ,am fused silica fiber, sol-gel PDMS coating (10 m thickness); direct 30 min extraction with stirring; desorption temperature, 340°C; carrier gas, helium; split ratio, 100:1; GC column, 10 m x 250 pm i.d., sol-gel PDMS stationary phase; temperature programming, from 40°C (5 min) at 6°C/min; detector, FID (360°C). Peaks: (1) naphthalene, (2) acenaphthylene, (3) fluorene, (4) phenanthrene, (5) fluoranthene. (Reproduced from Ref. [335] by permission of Royal Society of Chemistry).

4

1

__

I

:

0

5

10

15

Y __I t

z

20 25

.

30 35 min

TABLE 32.4 Effects of desorption temperature on the peak area repeatability data for PAHs and dimethylphenols in SPME-GC using sol-gel PDMS-coated fibers conditioned at different temperatures. Conditions: extraction time for each sample (room temperature), 30 min; split-splitless injection (splitless desorption for 5 min), injection temperature is the same as the fiber conditioning temperature; 10 m x 250 pm i.d. sol-gel PDMS capillary GC column; column temperature programmed from 40°C (hold for 5 min) to 230°C at 6°C/min; detector: FID, 350°C. (Adapted from Ref. [333] with permission of American Chemical Society). Compound

(A) Polycyclic aromatic hydrocarbons Naphthalene Acenaphthylene Fluorene Phenanthrene Anthracene Fluoranthene (B) Dimethylphenol isomers 2,5-Dimethylphenol 3,4-Dimethylphenol 2,6-Dimethylphenol 2,3-Dimethylphenol

1062

%RSD (n = 3) Fiber conditioning temperature 250°C

300°C

310°C

320°C

1.67 3.86 9.25 5.93 3.52 2.61

1.04 3.72 3.33 0.65 1.08 1.02

1.01 2.65 3.20 0.52 1.05 0.80

0.98 2.01 3.00 0.50 1.01 0.64

7.14 4,72 5.91 8.73

3.74 3.11 4.84 4.78

0.84 2.84 1.03 2.65

0.58 1.79 1.00 1.23

TABLE 32.5 The extraction precision of the sol-gel PEG coated fibers conditioned at 280°C. The evaluation was made over an extended period of operation consisting of 150 runs. M.P. stands for mean peak area. (Reproduced from Ref. [353] with permission of Elsevier). Compound

Extraction period 10th

Benzene Toluene Ethylbenzene p-Xylene o-Xylene

50th

100th

150th

M.P.

RSD (

M.P.

RSD (%)

M.P.

RSD (%)

14,355 18,301 25,220 26,691 29,732

6.2 5.9 4.7 3.9 4.4

14,210 18,219 25 26,536 29,775

5.3 5.7 341 4.4 4.1

14,307 5.7 18,269 6.1 5.1 25,274 26,598 3.7 29,826 4.7

M.P.

RSD (%)

14,345 18,247 5.3 26,674 29,793

5.8 6.0 25,297 4.1 3.9

organic or aqueous-organic solvent system. Another application area where high solvent stability of the coating is essential, involves extraction of analytes from a nonaqueous environment. It should be mentioned that the surface coatings of most conventionally prepared SPME fibers and/or microextraction capillaries is not chemically bonded to the substrate surface. Immobilization of the coating through cross-linking reactions provides some degree of solvent stability, but direct chemical bonding gives sol-gel coatings a significant solvent-stability advantage over conventional coatings. Because of direct chemical bonding to the substrate, sol-gel coatings possess extended lifetimes as is illustrated by the data presented in Table 32.5. These data illustrate the high degree of run-to-run repeatability of the mean peak area in sol-gel SPME-GC analysis over a course of 150 runs. This, in turn, translates into consistent long-term performance of sol-gel PEG-coated fibers in SPME. Sol-gel coatings have porous structures [333,353] resulting in an enhanced surface area. This facilitates fast mass transfer and rapid establishment of the extraction equilibrium, as is illustrated in Fig. 32.20 showing that the extraction equilibrium on a sol-gel PEG coated fiber was reached within 30 min. This translates into the possibility for speedy analysis using sol-gel coated fibers or capillaries. Jinno et al. [360] compared five different SPME coatings for the extraction of benzodiazepines from human urine. The SPME coatings under investigation were (a) sol-gel C poly(dimethylsiloxane), (b) polyacrylate, (c) Carbowax/ templated resin, (d) methyl-octyl poly(dimethylsiloxane), and (e) poly(dimethylsiloxane). The authors found that sol-gel Cl poly(dimethylsiloxane) coated fiber provided the highest extraction efficiency, as is illustrated in Fig. 32.21. Both PDMS and C,, (undecyl) moieties on the used sol-gel coating are nonpolar in nature, while the extracted benzodiazepines are relatively polar compounds. This higher extraction efficiency of the "nonpolar" sol-gel fiber for benzo-

1063

300 250 200 x

2 150

,-A *-B

1t00

'-C

-D

50

-E 0

Extraction Time (s) Fig. 32.20. Extraction time profile for 10 ng/ml BTEX (constant stirring at room temperature). (A) Benzene; (B) toluene; (C) ethylbenzene; (D)p-xylene; (E) o-xylene. GC conditions: 40°C for 2 min, then programmed at 7°C/min to 110°C, and then 20°C/min to 130°C for 1 min; injection temperature, 280°C; FID, 300°C; desorption time 20 s. (Reproduced from Ref. [353] by permission of Elsevier).

diazepines over other polar coatings (including polyacrylate and Carbowax/

templated resin coatings) may seem paradoxical. However, if one considers that the sol-gel coating also contains polar moieties like silanol groups, this apparent contradiction easily gets resolved. Thus, because of the presence of polar and nonpolar sorption sites, sol-gel coatings are capable of extracting both polar and nonpolar analytes.

Fig. 32.21. Extracted amounts estimated by peak area counts in LC chromatograms with various fiber coatings. Concentration of each analyte 0.1 /g/ml; extraction time, 60 min; extraction temperature, 60°C; desorption time, 30 min (room temperature); desorption solvent, mobile phase (acetonitrile:5 mM phosphate buffer, 40:60). (Reproduced from Ref: I Q. I D- 7 . . . I---i I I LObU] Dy permission or Koyal noclety of Chemistry).

1064

HA

LWIrM

Sol-get

Fiber coatings

UM lU

Us

TABLE 32.6 Names and chemical structures of sol-gel reagents used to prepared a sol-gel ODS monolithic bed in a fused silica capillary. (Reproduced from Ref. [337] with permission of American Chemical Society). Reagent function and reagent name

Reagent structure

1. Sol-gel coprecursor: tetramethoxysilane (TMOS)

OCH3 H3CO-iOCH 3

OCH 3 2. Sol-gel coprecursor: N-octadecyl- dimenthyl [3-(trimethoxy- silyl)propyl] ammonium chloride (C,,-TMS)

C c2

CliCr 9 CH3 CHr2 )3 - SiOC C13]

3. Deactivation reagent: phenyldimethylsilane (PheDMS)

4. Catalyst: trifluoroacetic acid (TFA)

OCH3

i-H

F3C-C-OH

Sol-gel monolithic beds for extraction Preparationof Monolithic beds for Microextraction.A number of procedures has been described for the preparation of sol-gel monolithic beds [337,338,340, 345,351,366-368]. The reported monolithic beds have been exclusively used in separation columns for capillary electrochromatography or HPLC. Perhaps, the simplest of the developed procedures for the preparation of sol-gel ODS monolithic beds, is that described by Hayes and Malik [337]. Various ingredients used to prepare the sol solution for the preparation of sol-gel ODS monolithic columns are presented in Table 32.6. According to this method, a hydrothermally treated fused silica capillary is filled with the sol solution that is allowed to stay inside the capillary for an extended period (a few hours). During this time sol-gel polymerization reactions proceed inside the capillary to transform the sol solution into a porous solid matrix. With both ends sealed, the capillary is then heated (from 30°C to the final temperature which may vary depending on the monolith material) using a program rate of 0.2°C min ', and holding it at the final temperature for 2 h. After this, the monolithic capillary is rinsed with a series of appropriate solvents (e.g., methylene chloride, methanol and water). Before use for extraction, the monolithic capillary is thermally conditioned (e.g., 30-300°C, at 1C/min) under continuous purge with an inert gas. Scanning electron micrographs representing the cross-sectional view of a fused silica capillary with an in situ prepared porous sol-gel monolithic bed and its direct

1065

Fig. 32.22. Scanning electron micrographs of a sol-gel ODS monolithic capillary: (A) cross-sectional view, magnification 1800x; (B) longitudinal view of the wall region showing anchorage of the monolithic structure to the capillary inner surface, magnification 1000x. (Reproduced from Ref. [337] by permission of American Chemical Society).

anchorage to the capillary inner walls are presented in Figs. 32.22A and B, respectively. Scheme 32.5 presents the chemical reactions involved in the preparation of sol-gel ODS monolithic bed that can be used either in capillary electrochromatography (CEC), capillary LC, or in capillary microextraction. Advantages of sol-gel monoliths as extraction media. Like sol-gel coatings, sol-gel monolithic beds are also chemically bonded to the silica substrates to which they are applied (e.g., to the inner surface of a fused silica capillary) (Fig. 32.22B). This chemical bonding gives mechanical stability to the extraction bed. Unlike particle-packed extraction beds (as in SPE cartridges or HPLC columns), in monolithic beds there are no free-to-move particles in the bed. The bed rather represents a single piece of porous solid anchored to the capillary inner walls through chemical bonding. Sol-gel monolithic beds are characterized by enhanced permeability. Sol-gel reaction conditions can be easily optimized to obtain monolithic beds with an order of magnitude (or more) higher permeability than the conventional particle-packed beds. The presence of flow-though channels makes the monolithic bed an effective medium for fast extraction. Like sol-gel polymeric coatings, sol-gel monolithic beds are also characterized by enhanced thermal and solvent stability thanks to their direct chemical bonding to the capillary inner walls. As a result, capillary microextraction with monolithic beds can be easily interfaced, either with a liquid-phase or a gas-phase separation technique. Sol-gel monolithic capillary contains a significantly higher amount of the sol-gel extraction medium compared with that on an SPME fiber or an open tubular microextraction capillary. This translates into higher sample capacity, and consequently, higher sensitivity of sol-gel monolithic microextraction compared with conventional or in-tube SPME (open tubular capillary microextraction). Figure 32.23 presents two gas chromatograms illustrating extraction

1066

Complete Hydrolysis of N-Octadecyldimethyl[3-(trihydroxysilyl)propyl]ammonium CH(CH2 )

.

CH t )Si-oCH (C11 2 CH 3

?cH3

+ 3 H20

3

c

HCH C(CH

2)ir

I

OCH 3

Chloride

o

o

.

(CH) 3 -Si-SI-OH + 3 CH 3OH

CH3

OH

Condensation of Tetrahydroxysilane with IV-Octadecyldimethyl[3-(trihydroxysilyl)propyllammonium Chloride MCCHCH3

OH

H

H3 Ca,

Bonding of the Sol-Gel ODS Coating with the Fused-Silica Surface (~-+

i? _..- cH .HOi-(CHz). i (Hzt)fCI 2 OH CH3

Si-OH +

Si--

O

I 0si-o-si-

CH,

OH

0

0

Deactivation of the Sol-Gel Mediated Fused-Silica Coated Surface with Phenyldimethylsilane (PheDMS)

A.

-.

,

OH

oOH

H

HJ 3 C--CH3

CH3I

Si-(CH2)N-(CH)fCH O C113

a

+

i-(CH 2"N(CH2).CH

OH

H 3 H 3 C-I,-CH3

B. -2 TH &

Hs$H +

O-SI-O-Si--(CH2 ) 0 OH

N(CH2 ),7-CH3 CH3

- 2°Si-O-S-0-Si(CH~t:)yCH

OH

O

+ lc CH3

.,d $-d

)TCHa

l13

Scheme 32.5. Chemical reactions involved in the preparation of sol-gel ODS stationary phase. (Adapted from Ref. [336] with permission of American Chemical Society).

sensitivities of two sol-gel microextraction capillaries with (A) a sol-gel ODS monolithic bed and (B) a sol-gel ODS coating. An aqueous sample containing a PAH (fluorene) and an alcohol (hexa-

decanol) was extracted under identical conditions using laries. These results suggest that the sample capacity sensitivity) of the used sol-gel monolithic capillary is magnitude higher than that of the capillary with the coating.

these two sol-gel capil(and hence extraction at least two orders of surface-bonded sol-gel

32.6.3.4 Sol-gel dendrimer-basedextraction media for SPME Dendrimers constitute a unique class of molecular species that possess highly branched, tree-like architecture [369]. Thanks to their branched architecture, dendrimers have outstanding material properties which have been exploited in a wide range of scientific disciplines [370-374]. Because of this unique molecular

1067

1nnn

(B) 900

900

800

800 -

700

700 -

600

600 -

M. 500

500 -

o

E

Open Tubular

400

400 -

300

300 -

200

200 -

100 0

o O x I

: LL

100-

0-

I

0

2I

10

20

I 30 min

Fig. 32.23. Comparison of the extraction sensitivity in capillary microextraction using: (A) a sol-gel ODS monolithic capillary (50 Am i.d.) and (B) a sol-gel ODS-coated fused silica capillary (250 Am i.d., -1 gm coating thickness). (From Ref. [368]).

architecture, dendrimer-based materials also have great potential for being used in analytical chemistry (e.g., as effective extraction media, as selective stationary phases or mobile phase additives for various chromatographic and electromigration separation techniques). However, only on rare occasions have these analytical potentials of dendrimers been utilized [70,375-380]. Recently, Newkome et al. [381] described a method for the synthesis of phenyl-terminated dendrimers with a trimethylsilyl derivatized sol-gel-active root (Fig. 32.24). Using sol-gel coating technology [232], the authors chemically bonded these dendritic moieties to the fused silica capillary inner surface in the form of a stationary phase coating. These sol-gel dendrimer-coated capillaries were further used as GC columns that showed unique selectivity. In an ongoing project, these authors have made use of these sol-gel dendrimer-coated capillaries to perform microextraction [382]. Figure 32.25 shows a gas chromatogram of polycyclic aromatic hydrocarbons that were extracted from an aqueous matrix using a sol-gel dendrimer-coated capillary. In principle, a wide variety of dendrimers can be prepared and easily bonded to silica particles for use as solid-phase extraction media or HPLC stationary phase. It should also be possible to prepared sol-gel monolithic beds with dendritic ligands. Such monolithic beds can also be used as extraction media or

1068

-

f)

(EtO) 3SiCH 2CH 2CH 2NH 1

(EtO) 3SiCH 2 CH 2CH 2N 3

Fig. 32.24. Chemical structures of phenyl-terminated dendrimers with a triethoxy-derivatized sol-gel active root. (A) First generation dendrimer; (B) second generation dendrimer; and (C) third generation dendrimer. (Adapted from Ref. [381] with permission of Elsevier).

as stationary phases for liquid- or gas-phase separation techniques. Because of extraordinary selectivity and advanced material properties offered by dendrimers, a significant growth in research activities directed towards analytical applications of these uniquely designed molecules can be expected in the near future.

32.7

CONCLUSION

The use of polymeric extraction materials with advanced material properties is a growing trend in analytical extraction, sample cleanup, and preconcentration. Polymeric materials are being used in a wide variety of extraction formats, including solution-mediated extraction, solid-phase extraction, and solid-phase microextraction. A wide range of pH stability is an attribute of many polymers, and this inherent property of polymeric materials is being exploited to solve analytical extraction problems that involve operations under extreme pH conditions where silica-based extraction materials fail to provide stable performance. Sig1069

Fig. 32.25. CME-GC analysis of PAHs using a sol-gel dendrimer-coated fused silica extraction capillary. Extraction conditions: gravity-fed extraction, 30 min; post extraction purge with helium, 2 min. GC conditions: sol-gel PDMS capillary column (10 m x 0.25 mm i.d.); desorption temperature, from 30 to 300°C; column temperature programming, from 30°C (5 min) at 15°C/ min; detector, FID (300°C). (From Ref. [382]).

4 I

'IAs' 5

10

15 20 Minutes

25

30

nificant attention is being paid to the development of molecularly imprinted polymers, temperature-responsive materials, restricted-access materials, dendritic materials, conductive polymers, and sol-gel polymeric materials. Although molecularly imprinted polymers are currently finding a wide range of applications in solid-phase extraction, they have just been introduced in the solid-phase microextraction area. Molecularly imprinted polymers with restricted-access properties show great promise in selective preconcentration, cleanup, and automated analysis of biological samples. Sol-gel polymeric materials constitute another group of advanced material systems that have great interest for analytical extraction technology. Thanks to the organic-inorganic hybrid nature of sol-gel materials, they often provide selectivity that is difficult to achieve by using either purely inorganic or purely organic materials. Moreover, this selectivity can be easily fine-tuned by changing the proportions of the organic/inorganic ingredients of the sol solution used to create these materials. Sol-gel reactions can take place under extraordinarily mild thermal conditions (normally at room temperature) that allow their application on various substrates without requiring the use of high temperature. Sol-gel polymeric materials get chemically bonded to the silica substrates they are applied to. This chemical bonding is responsible for high thermal and solvent stability of sol-gel materials that can be effectively utilized in analytical extractions. It can be assumed with confidence that future progress in analytical extraction technology will be closely related to the development of newer polymeric materials with advanced material properties.

1070

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