《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_fibrous molithic-45

Analytica Chimica Acta 655(2009)8-29 Contents lists available at Science Direct MYTCA Analytica Chimica Acta ELSEVIER journalhomepagewww.elsevier.com/locate/aca Review article Developments and applications of capillary microextraction techniques: A review Hiroyuki Kataoka*, Atsushi Ishizaki, Yuko Nonaka, Keita Saito School of pharmacy, Shujitsu University, 1-6-1, Nishigawara, Okayama 703-8516, Japan ARTICLE INFO A BSTRACT Sample preparation is important for isolating desired components from complex matrices and greatly eceived 14 August 2009 influences their reliable and accurate analysis. Recent trends in sample preparation include miniaturiza- tion, automation, high-throughput performance, and reduction in solvent consumption and operation ccepted 22 September 2009 time. This review focuses on novel microextraction techniques using capillaries for off-line and on-line Available online 26 September 2009 mple preparation Open-tubular trapping (OTT). in-tube solid-phase microextraction(SPME),wire- l-tube SPME, fiber-in-tube solid-phase extraction(SPE). sorbent-packed capillary in-tube SPME and monolithic capillary in-tube SPMe are critically evaluated and applications of these techniques in bio logical, pharmaceutical, environmental and food analyses are summarized. Capillary microextraction o 2009 Elsevier B V. All rights reserved. On-line analysis Automated analysis Contents 1. Introduction 8 2. Capillary microextraction techniques en-tubular d with gas chi 2.1.1. On-line OTr-GC system 2. 1.2. Off-line OrT-GC systems 2. 2. In-tube solid-phase microextraction 2. 2.2. Optimization of parameters.... 2. 2.3. Capillary coatings oo1122345 2.3. Packed capillary in-tube SPME 2.3.1. Wire-in-tube SPMe and fiber-in-tube Spe 5 2.3. 2. Sorbent-packed capillary in-tube SPME 4. Monolithic capillary in-tube SPME 3. Applications of capillary microextraction techniques 3.1. Recent applications to biological and pharmaceutical samples 3.3. Recent applications to food samples 4. Conclusions and future perspectives the complex matrices such as biological, environmental and food samples yet. Among the analytical processes such as sampling, sam- In recent years, sensitivity and specificity of analytical instru- ple preparation, separation, detection and data analysis ments have been achieved, but most of them cannot directly handle preparation is important for isolating desired components from complex matrices and greatly influences their reliable and accurate re, over 80% of analysis time is generally Corresponding author. Tel. +81 86 8342: fax: +81 86 8342. spent on the sampling and sample preparation steps including E-mail address: kataoka@shujitsuac jp(H Kataoka). extraction, concentration, fractionation and isolation of analytes 0003-2670/s-see front matter o 2009 Elsevier B v. All rights reserved. doi:10.1016aca2009.09032

Analytica Chimica Acta 655 (2009) 8–29 Contents lists available at ScienceDirect Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca Review article Developments and applications of capillary microextraction techniques: A review Hiroyuki Kataoka∗, Atsushi Ishizaki, Yuko Nonaka, Keita Saito School of Pharmacy, Shujitsu University, 1-6-1, Nishigawara, Okayama 703-8516, Japan article info Article history: Received 14 August 2009 Received in revised form 19 September 2009 Accepted 22 September 2009 Available online 26 September 2009 Keywords: Sample preparation Capillary microextraction Open-tubular trapping In-tube solid-phase microextraction On-line analysis Automated analysis abstract Sample preparation is important for isolating desired components from complex matrices and greatly influences their reliable and accurate analysis. Recent trends in sample preparation include miniaturiza￾tion, automation, high-throughput performance, and reduction in solvent consumption and operation time. This review focuses on novel microextraction techniques using capillaries for off-line and on-line sample preparation. Open-tubular trapping (OTT), in-tube solid-phase microextraction (SPME), wire￾in-tube SPME, fiber-in-tube solid-phase extraction (SPE), sorbent-packed capillary in-tube SPME and monolithic capillary in-tube SPME are critically evaluated and applications of these techniques in bio￾logical, pharmaceutical, environmental and food analyses are summarized. © 2009 Elsevier B.V. All rights reserved. Contents 1. Introduction ............................................................................................................................... ........... 8 2. Capillary microextraction techniques ............................................................................................................... 10 2.1. Open-tubular trapping coupled with gas chromatography ................................................................................. 10 2.1.1. On-line OTT-GC systems............................................................................................................ 11 2.1.2. Off-line OTT-GC systems ........................................................................................................... 11 2.2. In-tube solid-phase microextraction......................................................................................................... 12 2.2.1. Operation system ................................................................................................................... 12 2.2.2. Optimization of parameters ........................................................................................................ 13 2.2.3. Capillary coatings ................................................................................................................... 14 2.3. Packed capillary in-tube SPME ............................................................................................................... 15 2.3.1. Wire-in-tube SPME and fiber-in-tube SPE ......................................................................................... 15 2.3.2. Sorbent-packed capillary in-tube SPME............................................................................................ 15 2.4. Monolithic capillary in-tube SPME ........................................................................................................... 15 3. Applications of capillary microextraction techniques ............................................................................................... 16 3.1. Recent applications to biological and pharmaceutical samples ............................................................................. 17 3.2. Recent applications to environmental samples .............................................................................................. 17 3.3. Recent applications to food samples ......................................................................................................... 26 4. Conclusions and future perspectives ................................................................................................................ 26 References ............................................................................................................................... ............ 27 1. Introduction In recent years, sensitivity and specificity of analytical instru￾ments have been achieved, but most of them cannot directly handle ∗ Corresponding author. Tel.: +81 86 271 8342; fax: +81 86 271 8342. E-mail address: hkataoka@shujitsu.ac.jp (H. Kataoka). the complex matrices such as biological, environmental and food samples yet. Among the analytical processes such as sampling, sam￾ple preparation, separation, detection and data analysis, sample preparation is important for isolating desired components from complex matrices and greatly influences their reliable and accurate analysis [1]. Furthermore, over 80% of analysis time is generally spent on the sampling and sample preparation steps including extraction, concentration, fractionation and isolation of analytes 0003-2670/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2009.09.032

H Kataoka et aL/ Analytica Chimica Acta 655 (2009 )8-29 [2]. Therefore, sample preparation has been recognized as the have the above advantages over traditional lle and conventional main bottleneck of the analytical process, especially for analysis SPE LPME including single drop microextraction( SDME)is newly of trace components. Efficient sample ation requires that developed sample preparation technique using minimal amounts sample loss be kept to a minimum, so that the analyte can be of solvent Fiber SPMe and SBSe are widely used microextraction covered in good yield; that coexisting components be removed techniques that use a fiber and a stir-bar-coated polymeric station- efficiently; that problems do not occur in chromatography and ary phase, respectively, as extraction devices. These techniques are electrophoresis systems: that the procedure be performed con- usually performed in vessel, and absorption or adsorption of ana- veniently and quickly and that the cost of analysis be kept to a lytes occurs at the outer surface of the extraction device. In contrast, minimum. Previous sample preparation techniques, however, have in-tube in-needle and in-tip SPMe are unique sample preparation been associated with various problems, such as complicated and techniques that use a capillary tube a microsyringe and a pipette time-consuming operations, the requirement for large amounts of tip respectively, as extraction devices. Absorption or adsorption of sample and organic solvents, and difficult automation. Forexample, analytes occurs at the inner, polymer-coated surface and the outer if sample preparation is time-consuming, the number of samples is surface of packed sorbent. limited and multi-step procedures are prone to loss of analytes.Fur Microextraction techniques using a capillary tube have several thermore, use of harmful chemicals and large amounts of solvent advantages over other microextraction techniques, such as minia cause environmental pollution and health hazards for operators, turization, automation, high-throughput performance, on-line d extra-operational costs for waste treatment. coupling with analytical instruments and no solvent consumption. Traditional liquid-liquid extraction(LLE) and conventional A capillary microextraction technique using an open-tubular fused- lid-phase extraction(SPE) have been widely used for the prepa- silica capillary column as extraction device was first developed ration of biological, environmental and food samples 3-8]. Recent in 1986 as"open-tubular trapping(OTT)"[48. In OTT, a gaseous trends in sample preparation have focused on miniaturization, or liquid sample was passed through the open capillary and the automation, high-throughput performance, on-line coupling with volatile organic compounds trapped on the capillary coating were analytical instruments and low-cost operations through extremely analyzed by thermal desorption and gas chromatography (gc) low or no solvent consumption. Minimizing sample preparati Subsequently, OTT was applied to the analysis of polycyclic aro- steps is effective, not only in reducing sources of error but in reduc- matic hydrocarbons in aqueous samples, coupled with GC [49]. steps is also particularly advantageous for measuring trace do n In a similar capillary microextraction technique, in-tube SPME, g time and cost. Using a minimum number of sample preparati Itra-trace analytes in complex matrices. Microextraction tech- the extracted analytes on the capillary were an nalyzed niques, such as liquid-phase microextraction(LPME)(4,5,9-13], by solvent desorption, coupled on-line with high-performance Ind solid-phase microextraction(SPME)3-47, including in-tube liquid chromatography(HPLC)[50]. Although commercial GC cap in-needle, and in-tip SPME and stir-bar sorptive extraction(SBSE), illary columns are usually used for both OTT and in-tube SPMe, Fused silica Polymer coating capillar Fused silica capillary GC capillary segment Fused silica Sorbent packing Fused silica Fig. 1. Devices for capillary microextraction (A)Open-tubular capillary for OTT and in-tube SPMe, (B)fber-packed capillary, (C) sorbent-packed capillary, (D)monolithic

H. Kataoka et al. / Analytica Chimica Acta 655 (2009) 8–29 9 [2]. Therefore, sample preparation has been recognized as the main bottleneck of the analytical process, especially for analysis of trace components. Efficient sample preparation requires that sample loss be kept to a minimum, so that the analyte can be recovered in good yield; that coexisting components be removed efficiently; that problems do not occur in chromatography and electrophoresis systems; that the procedure be performed con￾veniently and quickly; and that the cost of analysis be kept to a minimum. Previous sample preparation techniques, however, have been associated with various problems, such as complicated and time-consuming operations, the requirement for large amounts of sample and organic solvents, and difficult automation. For example, if sample preparation is time-consuming, the number of samples is limited andmulti-step procedures are prone to loss of analytes. Fur￾thermore, use of harmful chemicals and large amounts of solvent cause environmental pollution and health hazards for operators, and extra-operational costs for waste treatment. Traditional liquid–liquid extraction (LLE) and conventional solid-phase extraction (SPE) have been widely used for the prepa￾ration of biological, environmental and food samples [3–8]. Recent trends in sample preparation have focused on miniaturization, automation, high-throughput performance, on-line coupling with analytical instruments and low-cost operations through extremely low or no solvent consumption. Minimizing sample preparation steps is effective, not only in reducing sources of error but in reduc￾ing time and cost. Using a minimum number of sample preparation steps is also particularly advantageous for measuring trace and ultra-trace analytes in complex matrices. Microextraction tech￾niques, such as liquid-phase microextraction (LPME) [4,5,9–13], and solid-phase microextraction (SPME) [3–47], including in-tube, in-needle, and in-tip SPME and stir-bar sorptive extraction (SBSE), have the above advantages over traditional LLE and conventional SPE. LPME including single drop microextraction (SDME) is newly developed sample preparation technique using minimal amounts of solvent. Fiber SPME and SBSE are widely used microextraction techniques that use a fiber and a stir-bar-coated polymeric station￾ary phase, respectively, as extraction devices. These techniques are usually performed in vessel, and absorption or adsorption of ana￾lytes occurs at the outer surface of the extraction device. In contrast, in-tube, in-needle and in-tip SPME are unique sample preparation techniques that use a capillary tube, a microsyringe and a pipette tip, respectively, as extraction devices. Absorption or adsorption of analytes occurs at the inner, polymer-coated surface and the outer surface of packed sorbent. Microextraction techniques using a capillary tube have several advantages over other microextraction techniques, such as minia￾turization, automation, high-throughput performance, on-line coupling with analytical instruments and no solvent consumption. A capillary microextraction technique using an open-tubular fused￾silica capillary column as extraction device was first developed in 1986 as “open-tubular trapping (OTT)” [48]. In OTT, a gaseous or liquid sample was passed through the open capillary and the volatile organic compounds trapped on the capillary coating were analyzed by thermal desorption and gas chromatography (GC). Subsequently, OTT was applied to the analysis of polycyclic aro￾matic hydrocarbons in aqueous samples, coupled with GC [49]. In a similar capillary microextraction technique, in-tube SPME, an aqueous sample was passed through an open capillary and the extracted analytes on the capillary coating were analyzed by solvent desorption, coupled on-line with high-performance liquid chromatography (HPLC) [50]. Although commercial GC cap￾illary columns are usually used for both OTT and in-tube SPME, Fig. 1. Devices for capillary microextraction. (A) Open-tubular capillary for OTT and in-tube SPME, (B) fiber-packed capillary, (C) sorbent-packed capillary, (D) monolithic capillary

H Kataoka et aL/ Analytica Chimica Acta 655 (2009)8-29 new capillaries were recently devised as microextraction devices a vital consideration in the use of an OTT column, whereas equilib- [7, 15,20, 33]. In addition to open-tubular capillaries, fiber-packed, rium extraction necessitates that a portion of the analyte remains sorbent-packed and rod-type monolith capillaries were developed in the sample after passing through the sorbent. OTT and in-tube to improve extraction efficiency and specificity( Fig. 1)[15, 20, 23. SPME are usually used in combination with GC and LC, respe This review focuses on novel capillary microextraction tech- tively. Therefore, for convenience, coupling with GC is called OTT niques for off-line and on-line sample preparation. OTT, in-tube and coupling with LC is called in-tube SPME Fiber-packed capillary PMe, wire-in-tube SPMe, fiber-in-tube SPE, sorbent-packed cap- microextraction, called"fiber-in-tube SPE", is a modified method ry in-tube SPMe and monolithic capillary in-tube SPMe are using capillary tubes(Fig 1B) packed with fibrous rigid-rod hetero ically evaluated, and their applications to biomedical, pharma- cyclic polymers, which increase extraction efficiency by decreasing ceutical, environmental and food analyses are summarized and capillary volume or by increasing the extracting surface. In con- discussed The details of capillary microextraction techniques for trast, pieces of micro-LC capillary columns packed with extracting sample preparation have also been described in books 1, 2, and phase can also be used for sorbent-packed(Fig 1C)and rod-type well-documented reviews 3, 5-7, 13-25]. monolith( Fig. 1D)capillary microextraction. In these techniques, analytes are absorbed or adsorbed at the outer surface of the packed sorbent Open-tubular and packed capillary microextraction tech- 2. Capillary microextraction techniques niques can be applied to the analysis of particulate-free gas and clean water samples, and analytes can be highly enriched by passing Several types of microextraction devices using capillaries have the sample through the capillary. These capillary microextraction been developed. Pieces of open-tubular capillary GC columns techniques are considered advantageous for the pretreatment of (Fig 1A)are used for OTT and in-tube SPME. In these techniques, complex sample matrices prior to chromatographic and capillary analytes are absorbed or adsorbed at the innersurface-coated inter- electrophoretic processes because they enable on-line analysis at nally with a thin film of the extraction phase. Two methods have low operating costs and with no environmental pollution. In this been developed In the first, for complete trapping of analytes, the section, we describe the characteristics and optimization of these volume of sample should not exceed the breakthrough volume In capillary microextraction techniques. the second, sorption is carried out as an equilibrium process. The sample is passed through the capillary column, but the amount of the analyte retained by the stationary phase at equilibrium is 2. 1. Open-tubular trapping coupled with gas chromatography directly related to its concentration in the sample solution. In both the exhaustive and non-exhaustive(equilibrium) methods, ana In OTT, ambient air, solution, or solution headspace is sampled lytes are desorbed with an appropriate solvent and transferred by passing a gas or liquid through the open capillary. The analytes desorbed thermally. Although the design of in-tube SPME appears Retention of the analytes is based on partitioning into the stationary similar to that of oTT for on-line sample preparation, equilibrium phase. The analytes are subsequently desorbed, either with a small versus exhaustive extraction remains a fundamental difference amount of solvent or by thermal desorption. The sample is forced to between the two techniques. The elimination of breakthrough flow through the capillary and analytes reach the trapping medium (A)Extraction mode Helium(carrier gas OTT sample drying Helium(carier gas) N OTT drying Fig. 2. Schematic diagram of on-line open-tubular trapping (A)Extraction mode(trapping). (B)injection mode(desorption).(1)Three-way flow selection valve, (2)six-port witching valve, (3)on/off valve and P is a pressure gauge

10 H. Kataoka et al. / Analytica Chimica Acta 655 (2009) 8–29 new capillaries were recently devised as microextraction devices [7,15,20,33]. In addition to open-tubular capillaries, fiber-packed, sorbent-packed and rod-type monolith capillaries were developed to improve extraction efficiency and specificity (Fig. 1) [15,20,23]. This review focuses on novel capillary microextraction tech￾niques for off-line and on-line sample preparation. OTT, in-tube SPME, wire-in-tube SPME, fiber-in-tube SPE, sorbent-packed cap￾illary in-tube SPME and monolithic capillary in-tube SPME are critically evaluated, and their applications to biomedical, pharma￾ceutical, environmental and food analyses are summarized and discussed. The details of capillary microextraction techniques for sample preparation have also been described in books [1,2,51] and well-documented reviews [3,5–7,13–25]. 2. Capillary microextraction techniques Several types of microextraction devices using capillaries have been developed. Pieces of open-tubular capillary GC columns (Fig. 1A) are used for OTT and in-tube SPME. In these techniques, analytes are absorbed or adsorbed at the inner surface-coated inter￾nally with a thin film of the extraction phase. Two methods have been developed. In the first, for complete trapping of analytes, the volume of sample should not exceed the breakthrough volume. In the second, sorption is carried out as an equilibrium process. The sample is passed through the capillary column, but the amount of the analyte retained by the stationary phase at equilibrium is directly related to its concentration in the sample solution. In both the exhaustive and non-exhaustive (equilibrium) methods, ana￾lytes are desorbed with an appropriate solvent and transferred off-line or on-line to a GC or HPLC column. Analytes can also be desorbed thermally. Although the design of in-tube SPME appears similar to that of OTT for on-line sample preparation, equilibrium versus exhaustive extraction remains a fundamental difference between the two techniques. The elimination of breakthrough is a vital consideration in the use of an OTT column, whereas equilib￾rium extraction necessitates that a portion of the analyte remains in the sample after passing through the sorbent. OTT and in-tube SPME are usually used in combination with GC and LC, respec￾tively. Therefore, for convenience, coupling with GC is called OTT and coupling with LC is called in-tube SPME. Fiber-packed capillary microextraction, called “fiber-in-tube SPE”, is a modified method using capillary tubes (Fig. 1B) packed with fibrous rigid-rod hetero￾cyclic polymers, which increase extraction efficiency by decreasing capillary volume or by increasing the extracting surface. In con￾trast, pieces of micro-LC capillary columns packed with extracting phase can also be used for sorbent-packed (Fig. 1C) and rod-type monolith (Fig. 1D) capillary microextraction. In these techniques, analytes are absorbed or adsorbed at the outer surface of the packed sorbent. Open-tubular and packed capillary microextraction tech￾niques can be applied to the analysis of particulate-free gas and clean water samples, and analytes can be highly enriched by passing the sample through the capillary. These capillary microextraction techniques are considered advantageous for the pretreatment of complex sample matrices prior to chromatographic and capillary electrophoretic processes because they enable on-line analysis at low operating costs and with no environmental pollution. In this section, we describe the characteristics and optimization of these capillary microextraction techniques. 2.1. Open-tubular trapping coupled with gas chromatography In OTT, ambient air, solution, or solution headspace is sampled by passing a gas or liquid through the open capillary. The analytes are trapped on the coating of a short piece of a capillary GC column. Retention of the analytes is based on partitioning into the stationary phase. The analytes are subsequently desorbed, either with a small amount of solvent or by thermal desorption. The sample is forced to flow through the capillary and analytes reach the trapping medium Fig. 2. Schematic diagram of on-line open-tubular trapping. (A) Extraction mode (trapping), (B) injection mode (desorption). (1) Three-way flow selection valve, (2) six-port switching valve, (3) on/off valve and P is a pressure gauge

H Kataoka et aL/ Analytica Chimica Acta 655 (2009 )8-29 coated onto the walls by diffusion. The thermal stability of GC sta- smoothly evaporate during analyte transfer For example, the on- tionary phases allows collected analytes to be thermally desorbed line OTt/GC technique, using a commercial GC capillary coated with from a trap after sampling. These analytes can be desorbed directly polydimethylsiloxane(PDMs), has been used for the isolation and onto a GC column for analysis, avoiding dilution of the sample with enrichment of organic pollutants in air matrices [58]. solvent. The likelihood of sample cross-contamination and possible degradation are minimized because intermediate sample handling 2.1.2. Of-line OTT-GC eps are eliminated. Another advantage is that even very volatile The off-line oTt/GC method is used for the sampling and enrich- compounds can be enriched at ambient temperature, omitting the tof volatile compounds in air and water samples. These include need for a cryogenic refocusing step. Although the OTT approach methods for the microextraction of volatile organic compounds overcomes some mechanical stability problems inherent to con-(VOCs)using an inside needle capillary adsorption trap device 54 ventional SPME fibers, it involves complex instrumental setup and and a new high-performance cryofocusing system for the capil- unfavorable sampling conditions, for example high pressure drops lary microextraction of VOCs in aqueous matrices and headspace from long traps and limited flow-rates. An additional disadvan- GC applications [61. In the latter device, the compounds tage is the need for dry purging with nitrogen after extraction, to centrated in a fused-silica transfer capillary with the aid of liqui completely remove the water from the capillary walls. Several OTT nitrogen. A glass tube liner(ca 25 cm x 1. 5 mm i.. ) is inserted into approaches, involving off-line or on-line coupling with GC, have the heated (200'C)injector of the gas chromatograph in place been described 49, 52-71 of the standard glass liner, and also extends further through a liq- uid nitrogen container made with styroform-like material Inside 1. 1. On-line OTT-GC syste his glass tube, the fused-silica transfer line passing through the schematic diagram of an on-line OTT GC system 56] is shown oven door is connected like a pre-column to the analytical high in Fig. 2. In this system, water samples are pumped through an resolution GC column. It can move fast between the heated and OTT capillary(2 m x 0.32 mm i.d. )coated with a 5-um thick sta- cooled zones; when this movement starts, cryofocused analytes tionary phase by an HPLC pump at a flow-rate of 1.5 mLmin-. The are injected"at once"resulting in symmetrical and sharp injection sampling time required to reach equilibrium is determined exper- bands with"zero"carryover. This cryofocuser resulted in repeat imentally. After sampling, the OTT capillary is dried by purging for able and accurate injections, which helped GC separations of VOcs 5 min with 1 mL min nitrogen to completely remove the water. coming from solid, gaseous or aqueous matrices. The high repro- he retained analytes are thermally desorbed by heating the cap- ducibility of the system depends on the high temperature stability illary under stop flow conditions. The analytes released from the of the heated cooled zones and on the negligible thermal mass per stationary phase of the capillary are transferred into the void vol- unit length of fused-silica. ume of the column, the six-port valve is switched to the injection Sol-gel capillary microextraction methods have been developed position, and the analytes are transferred to the analytical column for the solventless preconcentration of trace analytes 57, 64-70 by the flow of a carrier gas. The amount injected can be varied by The capillary with the extracted analytes can then be connected varying the time during which the six-port valve is left in the injec- to the inlet end of the gc column using a two-way press-fit tion position. Finally, the analytes are separated on the gC column. fused-silica connector housed inside the gC injection port. the The on-line OTT/GC system is also applied to the concentration and analytes can be desorbed from the extraction capillary by rapid analysis of gaseous samples [55] A second on-line OTT/GC system uses phase-switching port. The desorbed analytes are transported down the system by 49, 52,53], in which the analytes are adsorbed or absorbed from the flow of helium and further focused at the inlet end of the a methanol-water mobile phase to the stationary phase of an Ott GC column maintained at 30C. Several sol-gel coatings, includ capillary. Subsequently, the aqueous phase is removed by purg ing poly(dimethylsiloxane)(PDMS), poly(ethylene glycol)(PEG). ing the trap with nitrogen and the analytes are desorbed with polytetrahydrofuran, cyano-PDMS, and organic-inorganic hybrid organic solvent and transferred to the GC system using a pro- materials, have been developed for the extraction of non-polar, grammed temperature vaporizing(PTv)injector as interface. The moderately polar and polar compounds. Sol-gel technology can main advantage of analyte enrichment based on capillary column is fine tune the selectivity of a sol-gel coating simply by chang the ability to completely and reliably remove water after sampling ing the relative proportions of organic and inorganic components by purging a short plug of gas through the capillary In principle of the sol solution For extraction, aqueous samples are prepared the otr capillary has two disadvantages compared with packed by further diluting to ng/mL concentrations. a thermally condi columns: (1)its retention power is generally weaker and (2)sam- tioned capillary(10-40 cm x 0.25 mm i.d. )can then be vertically ling flow-rate is limited due to the slow diffusion of analytes in connected to the lower end of the gravity-fed sample dispenser. a water. The retention power of an OTT capillary (i. e a 2 m piece of 50 mL aliquot of aqueous sample is then poured into the dispenser the gC column)can be greatly enhanced by swelling the station- from its top end, and allowed to flow through the microextrac ary phase with an organic solvent prior to sampling 52] allowing tion capillary under gravity. While passing the sample through the practical use of OTT for on-line extraction-GC of aqueous sam- the capillary, the analytes can be sorbed by the sol-gel coating time becon about 2.5 mL For large sample volumes, the sampling on the inner wall of the capillary. Extraction requires 30-40 min time becomes unacceptably long due to the requirement for low for establishment of equilibrium. The capillary is then detached sampling flow-rate(ca 0.1 mLmin ) Obviously, flow-rate should from the dispenser and the residual sample droplets are removed not exceed a certain threshold, ca 0.2 mLmin-l, or breakthrough by touching one end of each microextraction capillary tube with willoccur immediately Alternatively, a thick-film stationary phas a tissue. The capillary can then be installed in the gC injection will increase the breakthrough volume. For example, much higher port, with 3 cm of its lower end protruding into the gC oven. sample flow-rates, i. e. up to 4 mLmin, are allowed for coiled or This end is then interfaced with the inlet of a GC capillary col- stitched columns, because deformed capillaries induce a secondary umn using a deactivated two-way press- fit quart connector. The flow, enhancing radical dispersion[53]. water should not dissolve analytes are thermally desorbed from the capillary by rapidly rais- in the organic solvent, or should the organic solvent dissolve in ing the temperature of the injector. Although low detection limits water. In the former case, water would be injected into the GC sys- were achieved for the analysis of phenols, alcohols, and amines tem, while in the latter case the swollen stationary phase would water [ 57] the procedure could not be automated and, therefore, lose(part of)the swelling agent. Finally, the organic solvent should was highly time-consuming. Recently, a high pH-resistant surface-

H. Kataoka et al. / Analytica Chimica Acta 655 (2009) 8–29 11 coated onto the walls by diffusion. The thermal stability of GC sta￾tionary phases allows collected analytes to be thermally desorbed from a trap after sampling. These analytes can be desorbed directly onto a GC column for analysis, avoiding dilution of the sample with solvent. The likelihood of sample cross-contamination and possible degradation are minimized because intermediate sample handling steps are eliminated. Another advantage is that even very volatile compounds can be enriched at ambient temperature, omitting the need for a cryogenic refocusing step. Although the OTT approach overcomes some mechanical stability problems inherent to con￾ventional SPME fibers, it involves complex instrumental setup and unfavorable sampling conditions, for example high pressure drops from long traps and limited flow-rates. An additional disadvan￾tage is the need for dry purging with nitrogen after extraction, to completely remove the water from the capillary walls. Several OTT approaches, involving off-line or on-line coupling with GC, have been described [49,52–71]. 2.1.1. On-line OTT-GC systems A schematic diagram of an on-line OTT/GC system [56] is shown in Fig. 2. In this system, water samples are pumped through an OTT capillary (2 m × 0.32 mm i.d.) coated with a 5-m thick sta￾tionary phase by an HPLC pump at a flow-rate of 1.5 mL min−1. The sampling time required to reach equilibrium is determined exper￾imentally. After sampling, the OTT capillary is dried by purging for 5 min with 1 mL min−1 nitrogen to completely remove the water. The retained analytes are thermally desorbed by heating the cap￾illary under stop flow conditions. The analytes released from the stationary phase of the capillary are transferred into the void vol￾ume of the column, the six-port valve is switched to the injection position, and the analytes are transferred to the analytical column by the flow of a carrier gas. The amount injected can be varied by varying the time during which the six-port valve is left in the injec￾tion position. Finally, the analytes are separated on the GC column. The on-line OTT/GC system is also applied to the concentration and analysis of gaseous samples [55]. A second on-line OTT/GC system uses phase-switching [49,52,53], in which the analytes are adsorbed or absorbed from a methanol–water mobile phase to the stationary phase of an OTT capillary. Subsequently, the aqueous phase is removed by purg￾ing the trap with nitrogen and the analytes are desorbed with organic solvent and transferred to the GC system using a pro￾grammed temperature vaporizing (PTV) injector as interface. The main advantage of analyte enrichment based on capillary column is the ability to completely and reliably remove water after sampling by purging a short plug of gas through the capillary. In principle, the OTT capillary has two disadvantages compared with packed columns: (1) its retention power is generally weaker and (2) sam￾pling flow-rate is limited due to the slow diffusion of analytes in water. The retention power of an OTT capillary (i.e. a 2 m piece of the GC column) can be greatly enhanced by swelling the station￾ary phase with an organic solvent prior to sampling [52], allowing the practical use of OTT for on-line extraction-GC of aqueous sam￾ples, up to about 2.5 mL. For large sample volumes, the sampling time becomes unacceptably long due to the requirement for low sampling flow-rate (ca. 0.1 mL min−1). Obviously, flow-rate should not exceed a certain threshold, ca. 0.2 mL min−1, or breakthrough will occur immediately. Alternatively, a thick-film stationary phase will increase the breakthrough volume. For example, much higher sample flow-rates, i.e. up to 4 mL min−1, are allowed for coiled or stitched columns, because deformed capillaries induce a secondary flow, enhancing radical dispersion [53]. Water should not dissolve in the organic solvent, or should the organic solvent dissolve in water. In the former case, water would be injected into the GC sys￾tem, while in the latter case the swollen stationary phase would lose (part of) the swelling agent. Finally, the organic solvent should smoothly evaporate during analyte transfer. For example, the on￾line OTT/GC technique, using a commercial GC capillary coated with polydimethylsiloxane (PDMS), has been used for the isolation and enrichment of organic pollutants in air matrices [58]. 2.1.2. Off-line OTT-GC systems The off-line OTT/GC method is used for the sampling and enrich￾ment of volatile compounds in air and water samples. These include methods for the microextraction of volatile organic compounds (VOCs) using an inside needle capillary adsorption trap device [54] and a new high-performance cryofocusing system for the capil￾lary microextraction of VOCs in aqueous matrices and headspace GC applications [61]. In the latter device, the compounds are con￾centrated in a fused-silica transfer capillary with the aid of liquid nitrogen. A glass tube liner (ca. 25 cm × 1.5 mm i.d.) is inserted into the heated (∼200 ◦C) injector of the gas chromatograph in place of the standard glass liner, and also extends further through a liq￾uid nitrogen container made with styroform-like material. Inside this glass tube, the fused-silica transfer line passing through the oven door is connected like a pre-column to the analytical high￾resolution GC column. It can move fast between the heated and cooled zones; when this movement starts, cryofocused analytes are injected “at once” resulting in symmetrical and sharp injection bands with “zero” carryover. This cryofocuser resulted in repeat￾able and accurate injections, which helped GC separations of VOCs coming from solid, gaseous or aqueous matrices. The high repro￾ducibility of the system depends on the high temperature stability of the heated/cooled zones and on the negligible thermal mass per unit length of fused-silica. Sol–gel capillary microextraction methods have been developed for the solventless preconcentration of trace analytes [57,64–70]. The capillary with the extracted analytes can then be connected to the inlet end of the GC column using a two-way press-fit fused-silica connector housed inside the GC injection port. The analytes can be desorbed from the extraction capillary by rapid temperature programming of 100 ◦C min−1 of the GC injection port. The desorbed analytes are transported down the system by the flow of helium and further focused at the inlet end of the GC column maintained at 30 ◦C. Several sol–gel coatings, includ￾ing poly(dimethylsiloxane) (PDMS), poly(ethylene glycol) (PEG), polytetrahydrofuran, cyano-PDMS, and organic–inorganic hybrid materials, have been developed for the extraction of non-polar, moderately polar and polar compounds. Sol–gel technology can fine tune the selectivity of a sol–gel coating simply by chang￾ing the relative proportions of organic and inorganic components of the sol solution. For extraction, aqueous samples are prepared by further diluting to ng/mL concentrations. A thermally condi￾tioned capillary (10–40 cm × 0.25 mm i.d.) can then be vertically connected to the lower end of the gravity-fed sample dispenser. A 50 mL aliquot of aqueous sample is then poured into the dispenser from its top end, and allowed to flow through the microextrac￾tion capillary under gravity. While passing the sample through the capillary, the analytes can be sorbed by the sol–gel coating on the inner wall of the capillary. Extraction requires 30–40 min for establishment of equilibrium. The capillary is then detached from the dispenser and the residual sample droplets are removed by touching one end of each microextraction capillary tube with a tissue. The capillary can then be installed in the GC injection port, with ∼3 cm of its lower end protruding into the GC oven. This end is then interfaced with the inlet of a GC capillary col￾umn using a deactivated two-way press-fit quart connector. The analytes are thermally desorbed from the capillary by rapidly rais￾ing the temperature of the injector. Although low detection limits were achieved for the analysis of phenols, alcohols, and amines in water [57], the procedure could not be automated and, therefore, was highly time-consuming. Recently, a high pH-resistant surface-

H Kataoka et aL/ Analytica Chimica Acta 655 (2009)8-29 bonded organic-inorganic hybrid zirconia coating was developed the capillary can be easily blocked. Therefore, to prevent plugging to analyze polycyclic aromatic hydrocarbons(PAHs), ketones, and of the capillary column and flow lines, it is necessary to filter or Idehydes in aqueous samples 66 centrifuge sample solutions before extraction. Although yields are generally low, these compounds may be extracted reproducibly using an autosampler, and all extracts may be introduced into an 2. 2. In-tube solid-phase microextraction LC column after in-tube spme In-tube SPME [ 3, 15, 24, 50 is an effective sar technique that uses an open-tubular capillary column as an SPME 2.2.1. Operation system device and can be coupled on-line with HPLC or LC-MS. It was In in-tube SPMe, organic compounds can be directly extracted developed to overcome some problems related to the use of con- and concentrated in the stationary phase of the column by introduc- entional fiber SPMe, such as fragility low sorption capacity, and ing the sample, using a programmed autosampler, untilequilibrium bleeding from thick-film coatings of fiber. Unlike fiber SPMe, where is achieved or until extraction is sufficient. After desorption, the a sorbent coating on the outer surface of a small-diameter solid analytes can be directly transferred to an LC column. For static rod serves as the extraction medium, in-tube SPme typically uses desorption, a solvent is drawn into the capillary and the desorbed a piece of fused-silica capillary with a stationary phase coating analytes are sent into the injection loop of the valve. If desorp- its inner surface(e. g, a short piece of GC column) for extraction. tion is efficient in the initial mobile phase, the mobile phase is tube SPMe has also been termed "coated capillary microextrac- directly passed through the capillary to the column for dynamic tion". This method can directly extract target analytes in aqueous desorption. The procedures of in-tube SPMe, including extraction, matrices and concentrate the analytes into the internally coated concentration, desorption, and injection, can be easily automated tationary phase of a capillary. the analytes can then be desorbed using a conventional autosampler. In-tube SPme operation sys by introducing a stream of mobile phase, or by using a static des- tems can be categorized as flow through extraction systems(coated orption solvent when the analytes are more strongly adsorbed to capillary microextraction)[76. in which solutions are passed con- the capillary coating. The desorbed compounds can subsequently tinuously in one direction through an extraction capillary column; be injected into the LC column for analysis. One alternative to or as draw/eject extraction systems (in-tube SPME)[0]. in which coated fiber is an internally coated capillary, through which the sample solution is repeatedly aspirated into and dispensed from this technique is that it enables automation of the SPME-HPlc diagrams of on-line in-tube SPME systems. Although their designs process, allowing extraction, desorption and injection to be per- appear similar to those of on-line OTT systems using a switching ormed continuously using a standard autosampler. In addition, it technique, equilibrium versus exhaustive extraction remains a fun- has lower detection limits compared with fiber SPME-HPLC sys- damental difference between the two methods. The elimination of tems. Automated sample handling procedures not only shorten breakthrough is a vital consideration in the use of an OTT column. be used with all GC commercial columns, thus increasing the A flow through capillary microextraction system [76, 77 is number of stationary phases, allowing a wide field of applica- shown in Fig. 3. The complete analytical system consists of an tion. Although these stationary phases are unsuitable for extraction automatic six-port valve, two pumps (sample pump and wash of polar compounds, the problem can be improved by deriva- pump) and an LC system. The capillary column is installed in tization of compounds [72-75. The automated on-line in-tube the switching six-port valve. The enrichment procedure is divided SPME-assisted derivatization technique has been developed for into 4 steps: conditioning, extraction, washing and desorption the analysis of dimethylamine by extraction/derivatization with The extraction capillary column is rinsed and conditioned with derivatizing agent previously coated on capillary [72]. In-tube Mili-Q water(wash pump, 1 min, 1.5mLmin-l) During extraction privatization techniques improve detectability through increasing( Fig 3A), the six-port valve is switched to the""LORD"position, selectivity and sensitivity, and enhance the separation of analytes and the aqueous sample is pumped through the column(sample with poor chromatographic behavior. The main disadvantage of pump, 5 min, 4.0 mLmin-1) This is followed by a washing step the tech is the requirement for very clean samples, because (wash pump), in which the capillary column is rinsed for 1 min (B) Inject position(desorption) sⅸ port valve Capillary column Sample Wash LC column pump pump LC column pump pump Detector Fig. 3. Schematic diagram of automated on-line coated capillary microextraction(flow through extraction system).(A)Load position(extraction),(B)inject positio

12 H. Kataoka et al. / Analytica Chimica Acta 655 (2009) 8–29 bonded organic–inorganic hybrid zirconia coating was developed to analyze polycyclic aromatic hydrocarbons (PAHs), ketones, and aldehydes in aqueous samples [66]. 2.2. In-tube solid-phase microextraction In-tube SPME [3,15,24,50] is an effective sample preparation technique that uses an open-tubular capillary column as an SPME device and can be coupled on-line with HPLC or LC–MS. It was developed to overcome some problems related to the use of con￾ventional fiber SPME, such as fragility, low sorption capacity, and bleeding from thick-film coatings of fiber. Unlike fiber SPME, where a sorbent coating on the outer surface of a small-diameter solid rod serves as the extraction medium, in-tube SPME typically uses a piece of fused-silica capillary with a stationary phase coating on its inner surface (e.g., a short piece of GC column) for extraction. In-tube SPME has also been termed “coated capillary microextrac￾tion”. This method can directly extract target analytes in aqueous matrices and concentrate the analytes into the internally coated stationary phase of a capillary. The analytes can then be desorbed by introducing a stream of mobile phase, or by using a static des￾orption solvent when the analytes are more strongly adsorbed to the capillary coating. The desorbed compounds can subsequently be injected into the LC column for analysis. One alternative to a coated fiber is an internally coated capillary, through which the sample flows or is drawn repeatedly. The main advantage of this technique is that it enables automation of the SPME-HPLC process, allowing extraction, desorption and injection to be per￾formed continuously using a standard autosampler. In addition, it has lower detection limits compared with fiber SPME-HPLC sys￾tems. Automated sample handling procedures not only shorten the total analysis time, but are more accurate and precise than manual techniques. Another important advantage is that it can be used with all GC commercial columns, thus increasing the number of stationary phases, allowing a wide field of applica￾tion. Although these stationary phases are unsuitable for extraction of polar compounds, the problem can be improved by deriva￾tization of compounds [72–75]. The automated on-line in-tube SPME-assisted derivatization technique has been developed for the analysis of dimethylamine by extraction/derivatization with derivatizing agent previously coated on capillary [72]. In-tube derivatization techniques improve detectability through increasing selectivity and sensitivity, and enhance the separation of analytes with poor chromatographic behavior. The main disadvantage of the technique is the requirement for very clean samples, because the capillary can be easily blocked. Therefore, to prevent plugging of the capillary column and flow lines, it is necessary to filter or centrifuge sample solutions before extraction. Although yields are generally low, these compounds may be extracted reproducibly using an autosampler, and all extracts may be introduced into an LC column after in-tube SPME. 2.2.1. Operation system In in-tube SPME, organic compounds can be directly extracted and concentrated in the stationary phase of the column by introduc￾ing the sample, using a programmed autosampler, until equilibrium is achieved or until extraction is sufficient. After desorption, the analytes can be directly transferred to an LC column. For static desorption, a solvent is drawn into the capillary and the desorbed analytes are sent into the injection loop of the valve. If desorp￾tion is efficient in the initial mobile phase, the mobile phase is directly passed through the capillary to the column for dynamic desorption. The procedures of in-tube SPME, including extraction, concentration, desorption, and injection, can be easily automated using a conventional autosampler. In-tube SPME operation sys￾tems can be categorized as flow through extraction systems (coated capillary microextraction) [76], in which solutions are passed con￾tinuously in one direction through an extraction capillary column; or as draw/eject extraction systems (in-tube SPME) [50], in which the sample solution is repeatedly aspirated into and dispensed from an extraction capillary column. Figs. 3 and 4 illustrate schematic diagrams of on-line in-tube SPME systems. Although their designs appear similar to those of on-line OTT systems using a switching technique, equilibrium versus exhaustive extraction remains a fun￾damental difference between the two methods. The elimination of breakthrough is a vital consideration in the use of an OTT column, whereas equilibrium extraction requires a portion of the analyte to remain in the sample after passing through the sorbent. A flow through capillary microextraction system [76,77] is shown in Fig. 3. The complete analytical system consists of an automatic six-port valve, two pumps (sample pump and wash pump) and an LC system. The capillary column is installed in the switching six-port valve. The enrichment procedure is divided into 4 steps: conditioning, extraction, washing and desorption. The extraction capillary column is rinsed and conditioned with Mili-Q water (wash pump, 1 min, 1.5 mL min−1). During extraction (Fig. 3A), the six-port valve is switched to the “LORD” position, and the aqueous sample is pumped through the column (sample pump, 5 min, 4.0 mL min−1). This is followed by a washing step (wash pump), in which the capillary column is rinsed for 1 min Fig. 3. Schematic diagram of automated on-line coated capillary microextraction (flow through extraction system). (A) Load position (extraction), (B) inject position (desorption)