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Imran Ali1 Vinod.K. Gupta2 Hassan Hassan Y. Aboul-E Aboul-Enein nein3 1 Afzal Afzal Hussain Hussain 1
Department of Chemistry, Jamia Millia Islamia (A Central University) New Delhi, India 2 Department of Chemistry, Indian Institute of Technology Roorkee, India 3 Pharmaceutical and Medicinal Chemistry Department, Pharmaceutical and Drug Industries Research Division, National Research Centre, Dokki, Cairo, Egypt
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Review Hyphe Hyphenat nation ion in sample sample prepar preparati ation: on: Advanc Advanceme ement nt from from the the micr micro o to the the nan nano wor world Analysis at trace levels, an ideal area of application for hyphenated techniques, is steadily gaining importance. Many sample pre-concentration and clean-up methods have been hyphenated with core analytical techniques to accomplish the task of low level detection. detection. The present present article describes describes the state of the art of hyphenation hyphenation of various various techniques techniques such as solid phase extraction, extraction, micro-solid micro-solid phase phase extraction, extraction, dialysis, and chromatographic modalities etc. with liquid chromatography, gas chromatogr matograph aphy, y, capil capillar lary y electr electroph ophore oresis sis,, and spect spectros roscop copic ic method methods. s. Beside Besides, s, attempts have been made to address the hyphenation approach in microfluidic devices. Keywords: Capillary electrophoresis / Gas chromatography / Liquid chromatography / Micro-fluidic devices devices / Soli Solid d phase phase extracti extraction on /
Recei Received ved:: March March 4, 200 2008; 8; revis revised:March10, ed:March10, 200 2008; 8; accept accepted:March17, ed:March17, 200 2008 8 DOI 10.1002/jssc.200800123 10.1002/jssc.200800123
1 Introduction Normally, many analytes in biological and environmental samples are present at very low concentrations in the nano nano or level level ranges ranges,, which which are beyond beyond the reach of detect detection ion by conven convention tional al analyt analytica icall instru instrument ments. s. Besides, thousands of impurities also present in biological and environmental environmental matrices matrices disturb analyses analyses and, hence, sample preparation preparation of biological biological and environenvironmental matrices is essential prior to introduction onto analytical machines. One of the most important trends Aboul-Enein, PharmaceutiPharmaceutiCorrespondence: Professor Hassan Y. Aboul-Enein, cal and Medicina Medicinall Chemistr Chemistry y Departme Department, nt, National National Research Research Centre, Dokki, Cairo 12311, Egypt E-mail:
[email protected] Fax: +20-2-33370931 atomic absorpti absorption on spectrom spectrometry etry;; AES, Abbreviatio Abbreviations: ns: AAS, atomic atomic emission spectrometry; spectrometry; DMAE, dynamic microwave-assisted extraction; DSAE, dynamic sonication-assisted sonication-assisted extraction; electron capture capture detector; detector; EF, enrichment enrichment factor; FID, ECD, electron flame ionization detection; GF, gel filtration; HCH, hexachlorocyclohexane; HGAAS, hydride hydride generati generation on atomic atomic absorpti absorption on spectroscopy; ICP, inductively inductively coupled plasma spectrometry; spectrometry; IC, ion chromatography; chromatography; IDA, iminodiacetic acid; IMAC, immobilized metal affinity chromatography; IPLC, ion pair liquid chromatography; ISP-CGC, immunoaffinity immunoaffinity sample pretreatmentpretreatmentcapillary gas chromatography chromatography system; LLE, liquid– liquid extraction; MMLLE, microporous membrane membrane liquid – liquid extraction; extraction; NP, normal-phase; OTT, open-tubular trapping; OPPs, organophosphorus pesticides; PAHs, polycycli polycyclicc aromatic aromatic hydrocarhydrocarbons; PHWE, pressurized hot water extraction; PCB, polychlorinated biphenyl; lRPLC, micro reverse phase liquid chromatography; SFE, supercritical fluid extraction; SLM, stratum lacunosum-moleculare i 2008 WI WILEY-VCH Ve Verlag Gm GmbH & Co. KG KGaA, We Weinheim
to simplify these complications is the generation of simple, rapid, and reliable procedures for sample preparation. Method development and setup requires the use of material of known compositions, e.g. certified reference materials. Therefore, spiking experiments have to be performed formed for method method qualit quality y control control.. Integr Integrati ation on and autoautomation of all the steps between sample preparation and detect detection ion signif significa icantly ntly reduce reduce the time time of analys analysis, is, increasing increasing both reproduci reproducibility bility and accuracy accuracy.. In 1995, the seventh symposium on handling environmental and biological samples in chromatography was held on May 7–10, at Lund, Sweden. This symposium was in continuation of the series started by the late Dr. Roland Frei, one of the early visionaries in sample preparation technologies technologies in analytical analytical sciences. sciences. A survey survey of the papers presented at this symposium indicates that five points have been been highlighted highlighted and considered considered as essential essential during sample preparation. First, the need for a continuous search for new technologies was realized, so that the high cost due to chemicals and experimental labor may be reduc reduced. ed. Second Secondly, ly, the need need was recogn recognize ized d for increasing sensitivity with better and more selective concentration centration techniques, techniques, which has driven driven scientists scientists to examine affinity and immuno-affinity supports that can selectively remove compound classes for further investigation. Thirdly, the development of multidimensional chromatogra chromatographic phic techniques techniques allowing allowing on-line on-line sample sample clean-up, clean-up, which provides provides several several advantages advantages including automation, automation, better reproduci reproducibility bility,, and closed closed system system capability was advocated. The fourth point considered was the development of better sample preparation techniques enabling enabling more effective effective use of biosensors biosensors and www.jss-journal.com
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other sensors because exposure to raw matrices can foul many sensors. The last and the fifth point, which requires considerable attention from the scientist, was the quality movement which has found its way into sample handling. Therefore, extraction, purification, and pre-concentration of the natural samples are very important and essential operations in separation science, but, of course, involve the use of costly chemicals and time. Moreover, these techniques are not able to prepare samples containing analytes at nano levels. In view of this, a new trend of hyphenationis emerging in which a sample preparation unit is coupled with an analytical instrument. This hyphenation technology is the latest development and future of sample preparation in the present century. In view of these developments, the present article discusses state-of-the-art sample preparation through hyphenation.
2 Sample preparation techniques Basically, sample preparation is a complex and sensitive step in analysis, which requires considerable expertise, especially when dealing with samples containing analytes at micro or nano level concentrations. Many off-line methods have been used for sample preparation, including solvent extraction (23%), solid phase extraction (48%), supercritical fluid extraction (11%), immunoaffinity extraction (5%), matrix solid phase dispersion (2%), automated solid phase extraction (SPE 2%), dialysis (5%), solid phase micro extraction (3%), and mole mass filtration (1%). To provide a quick impression and to permit comparison, these percentages are shown in Fig. 1. Solvent extraction (liquid –liquid extraction) is a classical method of sample preparation exploiting unequal distribution of solutes in two immiscible liquid phases. It has been used for the extraction of many compounds of biological and environmental importance [1 – 2]. Extraction from liquid and solid samples is carriedout by using a variety of solvents such as hexane, acetone, acetic acid, benzene, toluene, methanol, acetonitrile, petroleum ether, ethyl ether, iso-octane, pentane, dichloromethane, etc. This method has certain drawbacks such as high consumption of costly solvents and time. Besides, the disposal of used solvents (environmental hazards) and emulsion formation are other problems associated with this technique. Of course, solid phase extraction (SPE) is a quite practical method involving the use of reversed phase (C8, C18, etc. silica gel) adsorption phases in the form of discs and cartridges, but it also has certain drawbacks [3]. SPE requires multiple steps and costly solvents, and is a time-consuming process since the solvent concentration should be protected from evaporation. Sometimes, clotting, channelling, and percolation create problems in sample preparation in this sort of sample i 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 1. Percentage contribution of different sample preparation techniques (source Toxline and Current Contents ; years: 1997–1998–1999).
handling modality. Chromatographic techniques are also important and effective methods of sample preparation and include HPLC, GPC, and SFC. However, these are not affordable in all laboratories due to their costly instrumentation and running costs [4, 5]. Besides, membrane filtration and dialysis have also been used for sample preparation but they are also limited to certain applications [4, 5]. Due to advances in separation science in the new millennium the demand for analyses is increasing at nano or lower level detection limits and scientists in academia and industry as well as government agencies require data at such low limits all over the world [6]. Under such circumstances, the role of sample preparation becomes crucial, and creates a need for greater focus on miniaturization and non-exposure of samples during their preparation. Moreover, rapidity, efficiency, selectivity, reproducibility, low cost, and low limits of detection are demanded by today's separation science. A literature search and our experience indicate that these demands can be satisfied by hyphenation.
3 Hyphenation technology Basically, the above-cited methods are used for sample preparation in biological and environmental matrices. However, certain drawbacks make these methods less than ideal since they are not effective in the case of low amounts of samples and consume costly solvents and time. Besides, contamination and poor recoveries may also occur during experiments. The factors underscored the need for hyphenated techniques. Hyphenation is nothing but the coupling of a sample preparation unit with the core analytical instrument. It has been found more effective than conventional methods in terms of efficiency, effectiveness, selectivity, high recoveries, suitability for small samples, and for samples containing components that are difficult to analyze and give rise to procedural problems. Therefore, some papers have been published dealing specifically with biological and environmental samples. The use of hyphenation has been www.jss-journal.com
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classified on the basis of the core analytical technique, as discussed in the following sections.
3.1 Hyphenation in liquid chromatography Basically, liquid chromatography represents a landmark in the history of separation science and sample preparation is a key issue in this area. Many workers have attempted to hyphenate sample preparation units with liquid chromatographs and some important research work is discussed herein. Johansen et al. [7] described the hyphenation of Automated Sequential Trace Enrichment of Dialysates (ASTED) system with HPLC for analysis of antidepressant drugs in plasma. In this system the protein and particles were purified through a semi-permeable membrane and collection of the drug molecules on a trace enrichment column (TEC) was followed by HPLC analyses on a Supelcosil column (150 64.6 mm). The ASTED system consisted of a cellulose acetate dialysis membrane and interactions of analytes with the cellulose acetate membrane have been reported for basic drugs such as the opiate derivative pholcodine, benzodiazepines, and the neuroleptic drug clozapine [8 – 10]. Most of the antidepressant drugs showed ionic and hydrophobic interactions with the membrane and were selected as model substances to investigate more closely the ability of cationic surfactants to inhibit analyte-membrane interactions. The author optimized ASTED– HPLC conditions to achieve maximum recoveries by adding cationic surfactants to the donor solution in the dialyser, by the effect of chain length and concentration of cationic surfactants, pH of the donor solution, and the volume of the acceptor solution. Furthermore, the authors used a chemometric approach via factorial design and response surface modeling for optimization strategies. The developed unit was applied successfully for monitoring mianserine, imipramine, desimipramine, amitriptyline, and nortriptyline drugs in human plasma. Dialysis was performed for 12.8 minutes after which the six port valve was switched to the injection position and the enriched analytes were loaded onto TEC with HPLC mobile phase (acetonitrile – methanol– 0.005 M ammonium phosphate buffer (pH 7.0) (70:15:15 v/v/v)) at a flow rate of 1.5 mL/ min The limits of detection of the reported drugs in human plasma were in the range of 17 – 39 nM/L with UV detection. Cheng et al. [11] studied the biotransformation of Daspartic acid into L-aspartic acid with the help of SPE– HPLC hyphenation. The column used in HPLC was of ligand exchange type allowing the chiral separation of Dand L-aspartic acid. The mobile phase used for SPE was 5 mM sodium-1-octanesufonate (pH 2.2) at 0.1 mL/min flow rate while HPLC eluent was 0.25 mM CuSO 4 (pH 3.6) at a flow rate of 0.5 mL/min. Kema et al. [12] developed an automated on-line SPE– HPLC method for profiling of i 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 2. Schematic representation of on-line SPE system coupled to HPLC with fluorescence detection, on top SPE with conditioning of the cartridge and sample injection, the middle panel represents the system during backward-flush elution of the cartridge and the bottom panel shows the system during regeneration of the cartridge [12].
plasma indoles tryptophan, 5-hydoxytryptophan (5-HTP), serotonin, and 5-hydroxyindoleacetic acid (5-HIAA) compounds for diagnosing of carcinoid tumors in patients. The SPE cartridge consisted of hydrophobic polystyrene resin and the analytes were enriched on SPE due to various interactions such as hydrogen bonding, van der Waals forces, steric effects etc. The fluorometric detector permitted detection of several metabolically related indole derivatives. HPLC conditions were Inertsil column (25063 mm) with mobile phase of different ratio of 50 mM potassium dihydrogen phosphate adjusted to pH 3.3 with phosphoric acid with acetonitrile as eluent. This set-up is shown in Fig. 2, indicating a coupling of SPE and HPLC along with its working mechanism. The SPE cartridge is pre-conditioned with acetonitrile, dipotassium EDTA in water (5g/L), and water at 3 mL/min flow rate; followed by autosampling of the enriched ingredients onto the HPLC column. By the time chromatographic separation on the analytical column is complete, the SPE unit has been made ready for the next sample preparation and injection. The authors advocated this hyphenation as an emerging technique due to its direct injection procedure in combination with column switching that offered the possibility of combining sample pre-purification, concentration, and analysis simultaneously. Hasselstrom et al. [13] described a fully automated online SPE–HPLC–UV system for quantification of quetiapine, an antipsychotic drug, in human serum. SPE was conducted on a C2 packing and the mobile phase used for HPLC was MeOH–20 mM NH 4CH3COO (pH 5.0) (99:1, v/v) at a flow rate of 1.0 mL/min with 257 nm. Similarly, Mandrioli et al. [14] also studied quetiapine by SPE– HPLC www.jss-journal.com
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hyphenated separation. Kato et al. [15] also described an on-line solid-phase extraction method, coupled with HPLC –MS-MS for the determination of 16 phthalate metabolites in human urine. The method employed a conventional analytical column for the chromatographic separations of these analytes and the mobile phase used was A (0.1% acetic acid in water) and B (0.1% acetic acid in acetonitrile) at a flow rate of 0.35 mL/min. The limits of detection ranged from 0.11 to 0.90 ng/mL. A similar set-up was described by Kuklenyik et al. [16] for the extraction and measurement of perfluorinated organic acids and amine in human serum and milk. A 12lL volume of the reconstituted serum or milk extract was auto-injected on to HPLC at a 300- lL/min flow rate with 20 mM ammonium acetate (pH 4) in water and methanol as mobile phase. The HPLC gradient program (14 min) was started at 60% methanol in the mobile phase followed by an increase in organic content to 80% in 0.5 min, which was kept for 9 min. Later on, the mobile phase organic content was decreased in 0.5 min to 60% methanol, where it was kept for 3 min to equilibrate the column. Furthermore, the same group [17] described an automated on-line hyphenation of SPE with HPLC– MS for the extraction and measurement of isoflavones and lignans in urine. The mobile phases used were 10 mM ammonium acetate (pH 6.5) and methanol–acetonitrile (50:50 v/v) at a flow rate of 0.8 mL/min, respectively. The detection limits were in the range of 0.2– 0.7 ng/mL. These authors described these hyphenations as an innovation in separation science. Koster et al. [18] reported the analysis of lidocaine in urine by an on-line SPME – LC method. A polydimethylsiloxane (PDMS) coated fiber was directly immersed into buffered urine with optimized contact time, pH, ionic strength, and temperature. The extraction yields were 22% in about 45 min with a reproducibility of a 5% expressed as relative standard deviation. The detection limits were 25 – 1000 ng/mL. Volmer et al. [19] studied the eleven corticosteroid and two steroid conjugations in a urine sample by SPME–LC–MS. Several SPME optimization factors such as polarity of fibres, extraction time and effect of ionic strength, were investigated, and their impact on the SPME/LC/MS technique was studied. The method was sensitive with detection limits between 4 and 300 ng/mL and precision between 4.9 and 11.1% RSD. Kim et al. [20]. developed sol-gel titania-based coating capillary micro extraction (CME) coupled with HPLC for the extraction and analyses of polycyclic aromatic hydrocarbons, ketones, and alkylbenzenes at high pH. To perform CME–HPLC, a so-gel TiO2 –PDMS capillary was installed in an HPLC injection port as an external sampling loop. HPLC conditions were ODS column (25064.6 mm) with acetonitrile– water (80:20v/v) as mobile phase. The target analytes were extracted on-line by passing the aqueous sample through this sampling i 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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loop. The sol-gel titania–PDMS coated capillaries were used for on-line extraction and HPLC analysis of polycyclic aromatic hydrocarbons, ketones, alkylbenzenes, and a wide range of other less volatile or thermally labile compounds [21] that are not amenable to GC separation. Hashi et al. [22] described the determination of polycyclic aromatic hydrocarbons (PAH) in the atmospheric particulates by using on-line enrichment coupled with fast high-performance liquid chromatography with fluorescence detection. The limits of detection of PAH were in the range of 0.02–0.23 ng/mL with recoveries between 87 and 12% for spiked atmospheric particulate sample. The mobile phase used was acetonitrile–water (72:28, v/v) at a flow rate of 1.0 mL/min. Altun et al. [23] developed and validated a method for local anesthetics in human plasma through on-line MEPS by using a cationexchanger with a flow rate of 0.20 mL/min. Abdel-Rehim [24] developed and validated a new sensitive, selective, and accurate on-line micro-extraction in packed syringe (MEPS) technique hyphenated with HPLC for the determination of lidocaine, prilocaine, ropivacaine, and mepivacaine in human plasma. The extraction recoveries were in the range of 60–90%. Veuthey et al. [25] described on-line solid phase extraction to achieve nano analysis of drugs in biological samples. In this hyphenation technique, the single column performs two functions, i.e. extraction and separation. The column was connected to a detection system via a switching valve. The sample was directly injected on to the extraction support with and after the extraction, the valve was switched, and analytes were transferred to the detector with the eluting mobile phase followed by extraction support re-equilibration. According to the authors, the method was simple and several applications have been published for the direct analysis of biofluids. Quintana et al. [26] described an automated on-line hyphenation of SPE– HPLC incorporating multi syringe flow injection analysis (MSFIA), bead injection, and lab-on-valve (BI– LOV) prior to HPLC. The potential of the novel MSFI–BI– LOV hyphenation for on-line handling of complex environmental and biological samples prior to reversedphase chromatographic separations was assessed for the expeditious determination of five acidic pharmaceutical residues viz. ketoprofen, naproxen, bezafibrate, diclofenac, and ibuprofen along with one metabolite, i.e. salicylic acid, in surface water, urban wastewater, and urine. The column used was an Xterra RP-18 (3.96150 mm) with the mobile phases A: MeOH–water (20:80, v/v) and B: MeOH– water (95/5, v/v), both containing 0.1% (v/v) formic acid at flow rates of 1.0 mL/min. The detection limit was 0.02– 0.67 ng/mL. Clarkson et al. [27] described hyphenation of solidphase extraction with liquid chromatography and nuclear magnetic resonance: application for identification of flavonol glycosides (kaempferol 3-O-(6-O-a-L-rhamwww.jss-journal.com
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nopyranosyl)-b-D-glucopyranoside; kaempferol 3-O-(2,6-diO-a-L-rhamnopyranosyl)-b-D-glucopyranoside; quercetin 3-O-(2,6-di-O-a-l-rhamnopyranosyl)-b-D-glucopyranoside (rutin); and isorhamnetin, 3-O-(6-O-a-L-rhamnopyranosyl)b-D-glucopyranoside) and three 5-a-cardenolides (coroglaucigenin 3-O-6-deoxy-b-D-allopyranoside; coroglaucigenin 3-O-(4-O-b-D-glucopyranosyl)-6-deoxy-b-D-glucopyranoside; 39-O-acetyl-39-epiafroside) were identified. Zhang et al. [28] described an automated on-line hyphenation of SFC–2-D–HPLC–MS for sample preparation, separation, detection, and identification of the fruiting bodies of Ganoderma lucidum, and at least 73 components in the extract were resolved with a calculated peak capacity of up to 1643. The SFE and 2-D HPLC systems were fitted with a Hypersil-CN (5 lm, 1200A, 15064.6 mm id) and Chromolith Flash columns, respectively. In the first dimensional separation, the binary mobile phase was composed of A (water) and B (methanol) with a flow rate of 0.1 mL/min In the second dimensional separation, the mobile phase was composed of C (water) and D (acetonitrile) with a flow rate of 4 mL/min. The same group reported a simple SFE–HPLC system for comprehensive analyses of traditional Chinese medicines [29]. However, for complex samples, it is impossible to separate all components by one-dimensional chromatography. Therefore, two-dimensional HPLC has been developed and was regarded as a powerful technique for the separation of proteins, peptides, polymers, natural products, and other complex mixtures by different workers [30–43]. Taylor et al. [44] established an on-line SFE–HPLC–UV/ ESI-MS technique for the quantitative analysis of Hyperforin pertoratum. Ritter et al. [45] described the hyphenation of an electrolytic on-line eluent generation device with high performance anion-exchange chromatography coupled with UV detection for the determination of a wide range of intracellular metabolites from mammalian cells. The detection wavelength of the UV detector was switched from 220 to 260 nm and the detection limits were in the range of mM. Two Dionex AS11 analytical columns (25062 mm id) were used with 0.35 mL/min as mobile phase flow rate. Tuytten et al. [46] described an on-line automated SPE– HPLC– ESI-MS method for targeted metabolomic analysis of urinary modified nucleoside levels. The unit comprised a boronate affinity column as a trapping device, a hydrophilic interaction chromatography (HILIC) separation, and information-dependent MS detection modes. The system was applied to biological samples, detecting a number of modified nucleosides. Clarkson et al. [47] described HPLC–SPE–NMR hyphenation for structural elucidation of some natural products. Lambert et al. [48] described the identification of natural products by using SPE–HPLC coupling with Cap-NMR. This coupling was used for identification of sesquiterpene lactones and esterified phenylpropanoids present i 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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in an essentially crude plant extract (toluene fraction of an ethanolic extract of Thapsia garganica fruits). Lin et al. [49] described hyphenation of in-tube solidphase micro-extraction (SPME) and pressure-assisted CEC (p-CEC) by installing a poly(methacrylic acid-co-ethylene glycol dimethacrylate) monolithic capillary at a six-port valve in a CEC system. Theobromine, theophylline, and caffeine were chosen as model drugs to facilitate comparison with the results obtained by in-tube SPME–HPLC. The detection limits of these three analytes were improved more than 100 times when compared with direct analysis by l-HPLC. Besides the above-cited methods of sample preparation, some modalities of liquid chromatography have also been used as sample preparation methods and hyphenated with HPLC. Only one article on size exclusion chromatography coupled with HPLC is cited. Pomazal et al. [50] described analyses of copper, iron, manganese, and zinc in blood samples by exploiting the hyphenation of SEC with an HPLC–ICPAES unit. Besides, this device was also used to monitor metalloproteins in erythrocytes and blood plasma samples. Optimization was achieved via parameters like pH, flow rate, and salt concentration. For optimizing experiments, blood samples from one female subject were used and the direct determination of the elements was performed by ICP-AES on blood fractions of ten different subjects to obtain the average concentration ranges.
3.2 Gas chromatography Gas chromatography is considered the best choice for analysis of volatile compounds, including several agricultural, industrial, and other chemical compounds. Of course, many xenobiotics are present at trace concentrations and cannot be analyzed directly and this circumstance compels scientists to perform sample preparation, i.e. to adopt pre-concentration and hyphenation approaches. Many sample preparation methods have been coupled with GC for analyses of various species and these include LLE, SPE, membrane, etc. Membrane extraction is considered to be one of the best extraction techniques because it has the important advantage that the sample and the extractant can continuously be kept in contact without physical mixing, thus providing the basis for a continuous, real-time process permitting automation and on-line connection to instruments [51]. Consequently, membrane techniques have advanced during few decades to a stage permitting the solution of numerous analytical problems. These techniques allow the simultaneous extraction and enrichment of analytes and typically facilitate selective extraction at trace levels while consuming small amounts of solvents. Automated on-line liquid – liquid membrane extraction (LLME) has also been reported for determination of PCB [52] and of anesthetics in blood [53]. For PCB www.jss-journal.com
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determinations, Barri et al. [52] designed a miniaturized membrane extraction card (referred to as the ESy card) connected to a GC injector via an electromechanical installation which controls pre-treatment and triggers the GC instrument. The EF (enrichment factor) exhibited by this hyphenation was between 33 and 40 for PCBs in river water. Shen et al. [53] used a sample processor system consisting of an auto-sampling injector, dilutors, and a six-port valve connected to a GC injector loop for achieving EF up to 50 for some local anesthetics in blood plasma. Abdel-Rehim [24] developed and validated a sensitive, selective, and accurate on-line sample preparation technique for the determination of lidocaine, prilocaine, ropivacaine, and mepivacaine in human plasma. The online micro-extraction unit was a packed syringe (MEPS; silica gel C 2) coupled with GC–MS. The plasma samples (50–1000 lL) were drawn through the syringe by an auto-sampler and passed through the solid support, resulting in their adsorption onto the solid phase. The solid phase was then washed once with water (50 lL) to remove proteins and other interfering material. The MEPS technique differed from commercial solid-phase extraction (SPE) in the way in which the packing was inserted directly into the syringe, and not into a separate column. MEPS was capable of handling sample amounts from10 to 1000 lL of plasma, urine, or water in GC applications. MEPS took only about one minute for each sample with greater robustness than the SPME technique, and gave recoveries between 60 and 90%. GC experimentation conditions were 908C column temperature for 3 min followed by an increase up to 280 8C at a rate of 508C per min; with helium as carrier gas at 2.0 mL/min flow rate. Li et al. [54] described an hyphenation of SPE with programmable temperature vaporizers– large volume injection/gas chromatography/mass spectrometry (PTV– LVI/ GC/MS) for on-line sample preparation and separation of semi-volatile organic compounds (pesticides and herbicides) in a variety of water samples. The authors utilized this unit for real life samples of chlorinated tap water, well water, and river water. Furthermore, optimization achieved minimum limit of detection (0.1 lg/L) with relative recoveries in the range of 70–120% and a relative standard deviation of less than 15%. The schematic representation of an SPE Twin–PAL PTV–LVI/GC/MS system is shown in Fig. 3. Pawliszyn et al. [55–59] developed solid phase micro extraction (SPME) methods for non-volatile compounds. A fused silica rod with a polymeric coating on the surface was employed as the extraction medium for the adsorption of volatile analytes from aqueous sample solutions. The SPME fiber was inserted into the GC injector port for desorption and analyses of the analytes. Brossa et al. [60] described an automated on-line SPE– GC–MS set-up for the determination of a group of endoi 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 3. Schematic representation of an SPE Twin–PALPTV– LVI/GC/MS system. S1: upper PAL head and syringe, S2: lower PAL head and syringe, C1: upper PAL control unit, C2: lower PAL control unit, FC: sample flow cell, W1: wash station for upper PAL syringe, W2: wash station for lower PAL syringe, SR: solvent reservoir, SS: standard station, SV: on-line sample spiking vial, ET: sample extraction tray, CT: eluate collection tray, ST: standard tray, WS: water sample source and WW: water waste container [54].
crine disruptors in water samples. The chromatographic column used was HP-5 MS (28 m 6250 lm id) and the limits of detection of the method were between 0.001 and 0.036 lg/L.
3.3 Hyphenation in capillary electrophoresis Nowadays, capillary electrophoresis (CE) is valued as a versatile technique exhibiting high speed, high sensitivity, lower limits of detection, and low running costs, and represents a major trend in analytical science; the number of publications on this technique has increased exponentially [61 – 64]. CE is suitable for samples that may be difficult to separate by liquid chromatography and gas chromatography, or at least complements these techniques since the principles of separation are different. The lower detection limits of CE lead to the possibility of separating and characterizing very small quantities of materials, which normally require pre-concentration and sample preparation strategies, especially in unknown matrices. Therefore, some on-line methods have been reported from time to time to achieve the goal of micro level separation and detection in CE. Su et al. [65] studied the analysis of riboflavin in beer by CE – LED coupled with stacking micellar electrokinetic chromatography (MEKC) as pre-concentration techniques. The detection limit reported was 1.0 ng/mL with 38000 theoretical plates per meter. Hsieh et al. [66] hyphenated sweeping-micellar electrokinetic chromatography with CE to analyze trans-resveratrol in red wine. The CE buffer was methanol–water solution (25:75, v/v) and the system was operated at 77 kV with detection at a wavelength of 369 nm; the detection limit was 5 ppb. Fang et al. [67] described an on-line centrifuge microwww.jss-journal.com
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reported an on-line dialysis/FIA– CE sample clean-up procedure for metal ion analysis; with coupling via a specially designed interface (Fig. 4). Samples were continuously pumped into a dialysis unit and the outgoing acceptor stream containing the analytes was allowed to fill a rotary injector in the FIA part of the system. Multiple sample injections were possible in one electrophoretic run, and the entire analytical procedure could easily be mechanized. The repeatability of the unit was in the range of 1.6–3.3% (n = 7). This unit was applied in a wide range of real samples with complicated matrices like milk, juice, slurry, and liquors from the pulp and paper industry.
3.4 Hyphenation in spectroscopy
Figure 4. Schematic representation of FIC– CE coupling, (X): cross-sectional view of the FIA–CE interface and (Y): schematic diagram of the FIA–CE system used for on-line sample dialysis, (S): sample, (A): acceptor stream, (E): electrolyte, (M): dialysis membrane, (D): UV detector, (V1): injection valve in filling position, (V2): injection valve in inject position, (W): waste, (C): capillary, (Pt): platinum electrodes, and (HV): high-voltage supply [70].
extraction back-extraction field-amplified sample injection capillary electrophoresis system (CME–OLBE–FASICE) for determining trace ephedrine derivatives in urine and serum. CME and OLBE–FASI were two separate concentration units. The detection limits of this set-up were between 0.15 to 0.25 ng/mL on using photodiode array UV detection at 192 nm. The separations were achieved on an uncoated fused-silica capillary (50.2 cm650 lm id). Zhang et al. [68] developed hyphenation of immobilized metal affinity chromatography with capillary electrophoresis (IMAC– CE) for on-line concentration and analysis of peptides and proteins. The polymer monolithic immobilized metal affinity chromatography (IMAC) materials were prepared by an iminodiacetic acid (IDA) type adsorbent covalently bonded with monolithic rods of macroporous poly(glycidyl methacrylate-co-ethylene dimethacrylate). Cu(II) was subsequently introduced into the support via interaction with IDA. Liu et al. [69] described a microdialysis hollow fiber as a macromolecule trap for on-line coupling of solid phase micro-extraction and capillary electrophoresis for analysis of protein samples. The detection limit was 3.0 610 – 7 M with UV absorbance detection. Kuban and Karlberg [70] have i 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
As in case of chromatography and capillary electrophoresis, pre-concentration and sample preparation techniques were also hyphenated with spectroscopic instruments leading their capabilities to analyze samples of low volume or having poor ingredients. Many modalities of spectroscopy have been reported in the literature of identification of various inorganic and organic species. The most important spectroscopic techniques are atomic absorption spectrometry (AAS), inductively coupled plasma spectrometry (ICP), nuclear magnetic resonance spectrometry (NMR), atomic emission spectroscopy (AES), mass spectrometry (MS), infrared (IR), atomic fluorescence spectrometry (AFS) etc. Normally, the detection limits of these techniques ranged from mg to lg and if applied in biological and environmental samples having low concentrations of ingredients, the analytical results become inadequate. Sample pre-concentration and preparation are the tools used by analytical scientist to overcome such challenging problems. In view of these facts, some papers have addressed on-line hyphenation of sample pre-concentration and preparation techniques hyphenated with spectroscopic techniques. Sometimes, chelation of metal ions with suitable reagents enhanced the detection [71– 73]. Danesi [74] described a simplified model for the carrier-facilitated transport of metal ions through hollow fiber supported liquid membranes. Yang et al. [75] reported the clean-up, extraction, and enrichment of numerous metals including Pb, Cu, Cr, La, Ce, Zn, and Co by HF-SLME and coupled it with AAS and ICP-MS as shown in Fig. 5. Katarina et al. [76] described an on-line hyphenation of sample preparation method by using a computer controlled pre-treatment system (Auto-Pret AES) coupled with ICP-AES for the sample pretreatment and determination of trace metals in water samples. This system enabled determination of trace metals at the ppt level. Fan et al. [77] synthesized diphenylcarbazone-functionalized silica gel for SPE able to withstand 1– 6 mol/L HCL or H2SO4 as well as common organic solvents. This SPE was www.jss-journal.com
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Figure 5. Schematic of hollow fiber column pre-concentration unit with ICP-AES [75].
used for the extraction of Hg(II) selectively from eight metal ions with similar characteristics such as Cd(II), Ni(II), Co(II), Mn(II), Pb(II), Zn(II), Cu(II), and Fe(III). A micro-column packed with diphenylcarbazone-functionalized silica gel was coupled with flow-injection (FI) spectrophotometry for the selective separation, pre-concentration, and determination of Hg(II) in six different cigarette samples with detection limit of 0.90 ng/mL. Motomizu et al. [78] described an on-line flow injection inductively coupled plasma atomic emission spectrometer (FI– ICP-AES) system using anion- and cation-exchange resin disks for the speciation of chromium species in fresh water. Two kinds of ion exchange resin disks packed in line-filters were fixed and serially connected on the loop of each six-way valve. Five milliliters of a sample solution (pH 4.5) was introduced into the system. Anionic chromate ion, Cr(VI), was collected on the anionexchange resin disk while cationic chromium ion, Cr(III), was collected on the cation-exchange resin disk. The collected species were then sequentially eluted by 2 M nitric acid and nebulized to the plasma of ICP-AES. The detection limit of Cr(VI) and Cr(III) were 0.04 and 0.02 lg/L respectively. This method was applied to the speciation of Cr(III) and Cr(VI) in fresh water samples. Similarly, Jitmanee et al. [79} reported arsenic speciation in fresh water by using inductively coupled plasma-atomic emission spectrometry coupled with pre-concentration system containing solid phase anion exchange resin. Two miniaturized columns with a solid phase anion exchange resin, placed on two 6-way valves were used for
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the solid-phase collection/concentration of arsenic(III) and arsenic(V), respectively. The limit of detection for both As(III) and As(V) were 0.1 lg/L. In the same year, Sumida et al. [80] described on-line pre-concentration speciation of Cr(III) and Cr(VI) by using dual mini-columns coupled with plasma-atomic emission spectrometry in water samples. Cr(III) was collected on the first column packed with iminodiacetate resin. Cr(VI) in the effluent from the first column was reduced to Cr(III), which was collected on the second column packed with iminodiacetate resin.
3.5 Hyphenation in microfluidic devices Microfluidic devices are an innovation in separation science as they can be used to analyze samples of low volume and having low-concentration ingredients. Among various methods using microfluidic devices, nano-liquid chromatography (NLC) and nano-capillary electrophoresis (NCE) are the two most important techniques, and are used to achieve separations at nano levels. During a literature survey we found few papers dealing with on-line chip-based sample preparation methods in NLC and NCE. Attempts have been made to discuss these in the following paragraphs. Huynh et al. [81] described the first hyphenation of micro-dialysis NCE system for monitoring the hydrolysis of fluorescein mono-b-D-galactopyranoside (FMG) by b-Dgalactosidase. The layout of the microdialysis/microchip CE device shown in Fig. 6 indicates channel lengths, voltage scheme, perfusate (20 mM sodium phosphate buffer, pH 7.4). Furthermore, the same authors [82] presented an on-line microdialysis sampling unit coupled with NCE. The authors used this set-up for amino acid and peptide analyses. Wilson et al. [83] reported an on-line desalting of macromolecule (betaine-type amphoteric or zwitterionic surfactant solutions) using a two-layered laminar flow system due to differential diffusion of analytes. Wheeler et al. [84] developed an on-line sample preparation method for MALDI-MS, which depended on an electro-wetting-on-dielectric-based technique. Ramsey et al.
Figure 6. Schematic representation of on-line hyphenation of micro-dialysis sample preparation unit with NCE [81].
i 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 7. Schematic representation of chip-based solid phase extraction-MEKC device. (a): Layout of the entire device and (b): expanded view of the extraction region of the device. The dotted lines represent the direction of fluid flow during extraction; the solid line signifies flow during elution/ injection. (Narrow channels are ca . 55 lm wide, the column chamber is ca . 210 lm wide with all channels ca . 15 lm deep.) [85].
[85] coupled SPE to micellar electrokinetic chromatography to give a system permitting completely automated extraction, elution, injection, separation, and detection steps, respectively and separately. The authors reported fast analysis of rhodamine B yielding pre-concentration factors of more than 200 in less than 5 min with a 60 femtomolar detection limit. A schematic representation of this hyphenation is shown in Fig. 7, clearly indicating sample preparation and separation components. Legendre et al. [86] described a chip-based on-line solidphase extraction (SPE) for DNA and polymerase chain reaction in NLC. The amount injected was 600 nL of blood sample. Xiao et al. [87] presented a sample preparation method on a PDMS/glass chip coupled to gas chromatography. The authors tested this assembly for analysis of ephedrine from aqueous solution and reported good reproducibilities of extraction and analysis. Sample stacking has recently come to be regarded as the best pre-concentration technique in capillary electrophoresis, and has been tested in the NCE format [88]. Many modifications have been made to sample stacking, which include field-amplified sample stacking (FASS) [89– 91], large-volume sample stacking (LVSS) [92, 93], pHmediated stacking [94, 95], and micellar electrokinetic chromatography (MEKC) stacking [96 – 98]. Some reviews have been published on sample stacking techniques for a wide variety of compounds [88, 99–108]. Terabe et al. i 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 8. Schematic representation of (a): channel design of the multi-T microfluidic chip and (b): NCE with negative pressure large volume sample injection. SP: Syringe pump, V: 3way valve, HV: high-voltage power supply, T: T-shape connector [115].
[109– 111] developed cation- and anion-selective exhaustive injection-sweeping-micellar electrokinetic chromatography (CSEI-, ASEI-sweeping-MEKC) methods for increased sensitivity and detection. Britz-McKibbin et al. [112, 113] designed an on-line focusing method based on different mobilities of cationic analytes between background electrolyte (BGE) and sample matrix, which is called velocity difference-induced focusing (V-DIF). Cong et al. [114] reported on-line sample pre-concentration using field amplified stacking injection in NCE. According to the authors, pressure-driven flows into or from the branch channels, due to bulk velocity, can be used for liquid transportation in the channels. The detection sensitivity was improved 94-, 108-, and 160-fold for fluorescein-5-isothiocyanate, fluorescein disodium, and 5-carboxyfluorescein, respectively, relative to a traditional method. Similarly, Zhang and Yin [115] developed multiT microchip integrated field amplified sample stacking (FASS) coupled with NCE. According to the authors, a volumetrically defined large sample plug was formed in one step within 5 s by negative pressure in the headspace of two sealed sample waste reservoirs. The authors reported precisions in migration time and RSD as 3.3% and 1.3% for rhodamine123 (Rh123) and fluorescein sodium salt, respectively. Schematic representation of the channel design of a multi-T microfluidic chip and NCE system with negative pressure large volume sample injection is shown in Fig. 8. Jung et al. [116] designed, fabricated, and characterized a novel field-amplified sample stacking (FASS)-NCE chip having photo-initiated porous polymer for the analyses of fluorescein and bodipy. Furthermore, the authors www.jss-journal.com
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Table 1. Applications of hyphenation in sample preparation of different compounds in various matrices. Compounds
Matrices
Hyphenation modalities
LOD
Refs.
Biological matrices Trazodone Benzodiazepines Benzodiazepines Biogenic amines
Plasma Plasma Plasma Ce lls and culture medium
3 lg/L 5 – 25 lg/L 0.5 – 2 lg/L 10 fmol/lL
[118] [119] [120] [121]
Biological samples Beverages a nd f eed s tuff Fortified t issue Oils and fats Plasma and serum samples
SPE – GC – FID SPE – GC – NPD ISP – CGC Dialysis – R PLC – f luorescence – ED Dialysis –RPLC –fluorescence Dialysis – RPLC – fluorescence Dialysis – RPLC – fluorescence LLE – GC – FID Dialysis– RPLC– fluorescence
0.1 nM 1 – 278 lg/L 2.5 – 5 ng/g 0.04 – 0.08 ng/100 g 10 lL
[122] [123] [124] [125] [126]
Human plasma
Dialysis –RPLC –UV
0.050 –0.055 lmol/L [10]
Human p lasma Plasma Biological samples Plasma
Dialysis – R PLC – fluorescence SLM – GC Dialysis –RPLC –fluorescence Dialysis – RPLC – UV
5 lg/L 1 lg/L 0.1 nM 0.1 – 0.8 lg/L
[127] [128] [122] [129]
RAM – MIP –RPLC –fluoresence Dialysis – IC – HGAAS Dialysis – lRPLC – MS SLM – IPLC – UV SLM – LC – biosensor Dialysis – lRPLC – UV SLM – lRPLC – UV SPE – RPLC SFE–NPLC–UV
a10 ng/mL 1.0 – 2.18 lg/L 0.1 nM 2 – 18 nM 50 lg/L 0.1 mg/L 80 nM 0.0 5ng/mL a0.3 – 7.4 ng/g
[130] [131] [132] [133] [134] [135] [136] [137] [138]
PHWE – PRLC – fluorescence SPE – LC/MS/MS RAM – RPLC – MS/MS Dialysis – RPLC – RI Dialysis –RPLC –UV
1 lg/mL 0.05 ng/mL 300 pg/mL 0.6 – 0.41 lg/g 0.10 mg/L
[139] [140] [141] [142] [143]
Bismuth, cadmium & lead Cadmium
Human plasma Urine Plasma Urine Plasma Rat bile Plasma Urine Biological samples Blood, milk, tissue Food Human plasma and urine Human plasma Foods and beverages Blood and dermal rat microdialysates Urine Biological RM
SPE – GF – AAS SPE – ICP-AES
0.002 – 0.013 ng/mL 0.05 ng/mL
[144] [145]
Environmental matrices Endocrine disruptors HCH & ethers Organic pollutants Phenols Phenols OPPs Micro contaminants Micro contaminants Pesticides Pesticides Pesticides Pesticides Endocrine disruptors Triazines & OPPs
Water Water Water Water Water Water Water Water Water Water Water Water Water Wastewater
SPE – GC – MS LLE – GC – ECD or FID SPE – GC – ECD SPE – GC – FID SPE – GC – FID SPE – GC – AED SPE – GC – FTIR SPE – GC – MS LLE – GC – AED SPE – GC – MS SPE – GC – MS SPE – RPLC – MS – MS SPE – GC – MS SPE – GC – NPD or MS
[146] [147] [148] [149] [150] [151, 152] [153] [154, 155] [156] [157] [158] [159] [142] [160]
Alkylthio-s-triazineherbicides Vinclozolin Cationic surfactants Drugs Wine aroma compounds Organic compounds Organic acids
River water Water Aqueous samples Water Wine Aerosol particles Aerosol particles
SLM –RPLC –UV MMLLE – NPLC – UV MMLLE –NPLC –UV SFE – RPLC – UV – MS SFE – GC – FID SFE – NPLC – GC – MS SFE – NPLC – GC – MS
0.1 – 20 ng/L 20 – 480 pg/L 4.1 – 6.3 ng/L 1 – 27 ng/L 0.3 – 2 lg/L 2 – 5 ng/L 100 – 1000 ng/L 0.2 – 20 ng/L 1 – 5 lg/L 2 – 20 ng/L a1 lg/L 0.4 – 13 ng/L 0.1 – 20 ng/L 15 – 25 ng/L (NPD) 1.5 ng/L (MS) 0.03 lg/L 1 lg/L 0.7 –5 lg/L 200 ppb 0.8 – 3.4 lg 0.02 – 0.04 ng/m3 0.4 ng/m 3
Ciprofloxacin Amino a cids Fluoroquinolones Sterols Methylenedioxylated amphetamines Clozapine & N -desmethylclozapine Verapamil & Norverapamil Local anesthetics Ciprofloxacin Phenytoin, Carbamazepine & Phenobarbitone Tramadol Arsenic species Ropivacaine & metabolite Ropivacaine Phenols Meropenem Bambuterol Atrazine mercapturate & planar PCBs
N -Methylcarbamates Piritramide Cyproterone acetate Sugars and organic acids Fluconazole
i 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
[161] [162] [163] [164] [165] [166] [167]
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Table 1. Continued Compounds
Matrices
Hyphenation modalities
LOD
Refs.
PAHs PAHs PAHs PAHs PAHs PAHs Brominated flame Retardants Organophosphorus e sters Organophosphorus Esters Sulfonamide antibiotics & pesticides Hexavalent chromium Explosives Selenium Cadmium Cr(III) & Cr(VI) As(III) & As(V) Cr(III) & Cr(VI)
Aerosol particles Soil and sediment Soil and sediment Sediment Sediment Soil Sediment
SFE – NPLC – GC – MS PHWE – MMLLE – GC – FID PHWE – MMLLE – GC – FID PHWE – NPLC – GC – FID SFE – GC – MS SFE – RPLC – UV PHWE – NPLC – GC – FID
0.02 – 0.04 ng/m3 0.65 – 1.66 lg/g 0.11 – 1.22 lg/g 0.01 lg/g a0.2 lg/g 0.2 – 4 ng 0.70 – 1.41 ng/g
[168] [169] [170] [171] [172] [173] [174]
Air p articulates Air particulates
DMAE – SPE – GC – NPD DSAE –SPE –GC –NPD
90.9 – 186.2 pg/m3 0.1 ng/m 3
[175] [176]
Natural water
SPE – RPLC – MS/MS
0.5 – 5 ng/L
[177]
Colourants Filters Cu alloys, Ni sponge SRM river water Fresh water Fresh water River water Tap water Wastewater Seawater Seawater and river water
Dialysis – IC – UV SFE – RPLC – UV SPE – AAS SPE – GF – AAS SPE – ICP-AES SPE – ICP-AES SPE – ICP-AES
5 lg/L 9.5 – 56.8 ng/filter 0.2 ng/mL 2 610 – 4 ng/mL 0.02 – 0.04 ng/mL 0.1 ng/mL 0.08 – 0.15 ng/mL
[178] [179] [180] [181] [78] [79] [80]
SPE – ICP-AES SPE –ICP-AES
0.05 ng/mL 0.001 –0.2 ng/mL
[146] [182]
Cadmium Trace metal
described a 1000-fold signal increase during detection. This polymer material provided a region of high flow resistance, which allowed electromigration of sample ions resulting in pre-concentration. Lichtenberg et al. [117] developed a microchip device for field amplification stacking (FAS), which allowed the formation of comparatively long, volumetrically defined sample plugs with a minimal NCE bias. The authors studied fluidic effects; arising from solutions with mismatched ionic strengths, in chip-based electrokinetically. Furthermore, these authors developed a new chip layout for full column stacking with subsequent sample matrix removal by polarity switching. Some important hyphenation examples are given in Table 1.
4 Concluding remarks In the present scenario of separation science advancement, hyphenation is continuously gaining importance for the determination of analytes at micro or lower concentration levels. This technique is more useful in biological samples where the volumes of matrices are low, e.g. blood of infants, cerebrospinal fluid, DNA, and other hormone and enzyme samples. It has been observed that the development of hyphenation techniques is not yet complete and is still progressing. Briefly, the art of hyphenation in separation science will be in great demand during the present century. i 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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