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//-->In-Tube Solid-Phase Microextractionand On-Line Coupling withHigh-Resolution GCHanwen Wang, Wenmin Liu and Yafeng Guan,Dalian Institute of Chemical Physics, Dalian, China.The guest authors validated an in-tube solid-phase microextraction device, which was designed for on-linecoupling with a capillary gas chromatography system, for the trace analysis of organic contaminants in water.They used a 5 m 0.53 mm, 1.2 µmdfpoly(dimethylsiloxane) phase capillary column as the in-tubeextractor. The dynamic extraction technique used a high sampling flow-rate, thermal desorption and valveswitching in a novel system design. Compared with classic SPME, the on-line in-tube SPME systemincreased enrichment factors dramatically, and, because of on-line operation, improved the precision ofquantification. The cost per sample was the same as that of classic fibre SPME, and might even be lower inlong-term use because of the use of an ordinary switching valve and conventional GC column extractor.Belardi and Pawliszyn1first described theconcept of solid-phase microextraction(SPME) based upon the principle ofpartitioning of analytes between theextracting phase and the matrix (air, waterand so forth). Many have developedmodifications of this extractive approachand associated devices to improve theavailability and application of this samplepreparation technique.2Among thoseapproaches, SPME has been implementedpractically by placing the fibre in amicro-syringe. This approach has beencommercialized by Supelco (Bellefonte,Pennsylvania, USA).To date, this manual SPME syringe deviceis commonly used for off-line samplepreparation.3–5However, samplepreparation techniques that cannot beautomated are used less often for routineanalysis, even if they offer other attractivefeatures, such as high selectivity orsensitivity. Thus, by modifying a commercialgas chromatography (GC) autosampler,R. Eisert and colleagues6,7realized theinterfacing of fibre SPME–GC in a quasi-automated mode.In-tube SPME has been used in HPLC asan efficient and simple preparationmethod, and it offers several advantagesover the fibre SPME syringe–LC approach.8In-tube SPME is similar to fibre SPME, butthe extraction device has a piece of fused-2silica GC capillary column in place of afibre. Conceptually, in-tube SPME shouldpreserve the advantages of SPME andcould offer improved enrichment efficiency,quantification and automation through theon-line coupling with a chromatograph. Byusing a piece of bonded-phase capillary GCcolumn for sorption, a larger amount ofstationary phase and a more robust film areobtained, relative to outside-coated filmsof conventional SPME fibres. Thesedifferences result in higher enrichmentfactors and longer extractor life. Becausemany capillary GC stationary phases arecommercially available, in-tube SPMEenables easy changing of theextraction-phase polarity, which extendsthe application range of the method.The analysis of aqueous samples usingin-tube SPME–GC has been reported in theliterature.9–11For the extraction step, thesample solution was pushed or pulledthrough the capillary extractor at areasonable flow-rate. The adsorbedanalytes were then desorbed with aminimum amount of stripping solvent foroff-line collection before chromatographicanalysis.9In addition, the capillary extractorcould be assembled manually in front ofthe GC analytical column with a press-fitconnector and a piece of precolumn. Atemperature-programmed GC runcompleted the procedure of both thermaldesorption and analysis.10,11Using thoseapproaches, researchers obtainedpromising results and avoided the problemsthat are observed with fibre SPME such asthe bleed from the ultra thick film and theappearance of ghost peaks.On-line extraction-capillary GC is anattractive method for the analysis ofaqueous samples. Several methods foron-line extraction GC have been reportedin the literature, including membraneextraction,12liquid–liquid extraction13andsolid-phase extraction with small packedcartridges.14In contrast to theseconventional methods, the completeremoval of water can be achieved easily byusing an open-tubular capillary. Mol andco-workers15developed a method usingopen-tubular trapping columns for on-lineextraction–capillary GC in the analysis ofaqueous samples. In that study, they usedtwo switching valves and organic solventfor desorption.In this instalment of “Sample PreparationPerspectives,” we will present a noveldevice for coupling on-line in-tube SPMEwith capillary GC. We will demonstrate thismethod’s application for the analysis ofcontaminants in water.QuantificationAs shown in Equation 1, it is common foranalysts to calculate the theoreticalLC•GC Europe, 17(3),144–151 (2004)Sample Preparation PerspectivesIn-tube SPME has been used in HPLC as an efficient andsimple preparation method, and it offers severaladvantages over the fibre SPME syringe–LC approach…Conceptually, it should preserve the advantages of SPMEand could offer improved enrichment efficiency,quantification and automation through the on-linecoupling with a chromatograph.recovery or to evaluate quantificationbased upon equilibrium theory:16ExperimentalOn-line in-tube SPME instrumentset-up and procedure:Figure 1 is aschematic of the on-line in-tube SPMEdevice developed in our study. The systemconsists of a six-port valve and three gasflow controllers (both from Fuli Corp.,Wenling, China), a homemade stainlesssteel micro tee piece, a homemade 5 m0.53 mm, 1.2 µmdf conventional cross-linked OV-1 (poly-[dimethylsiloxane])capillary column used as extractor, and amini water-circulating pump (TengdaCorp., Tianjing, China). A homemade ovencapable of heating at a rate of 290 °C/minto temperatures greater than 320 °Cprovided fast and uniform heating for thecapillary extractor. A deactivated 1 m100 µm fused-silica capillary (Ruifeng,Yongnian, China) in close contact with apiece of heating resistor wire was used asthe analyte transfer line from the in-tubeSPME system to the GC system. Anadiabatic sleeve covered the transfer lineto maintain heat.During the extraction period (the solid-line position of the valve in Figure 1), thesample solution was forced through thecapillary extractor by the push of auxiliary1(KDVsVowherenis the amount of extractedanalyte, Cthe initial concentration of theanalyte in the matrix,KDthe distributionconstant of the analyte,Vsthe volume ofthe stationary phase andVthe volume ofsample.A concept of negligible depletionextraction was recently proposed for easierquantification and higher enrichmentfactors.17For in-tube SPME, the extractionprocess will not influence the freeconcentration of the analyte in the matrixwhen a sufficient amount of samplesolution (quasi-infinite relative to extractingphase volume) passes through theextractor. In this instance,KDVSV,and the absolute amount of extractedanalyte can be easily obtained fromEquation 2:nKDVsCFigure 1:Schematic diagram of the on-line in-tube SPME system coupled with thehigh-resolution GC system. 1 six-port valve, 2 flow controller forsampling, 3 flow controller for desorption gas, 4 flow controller for auxiliarygas, 5 sample vial, 6 mini water-circulating pump, 7 micro tee piece,8 capillary transfer, 9 capillary extractor, 10 precolumn, 11 press-fit ormicro union, 12 analytical column, 13 on-column injector.867PAuxiliarygas A25N1Auxiliarygas BAuxiliarygas C4310Oven9111213www.lcgceurope.com([2]nCKDVs[1]GC ovengas and the suction force of the miniwater-circulating pump. A negativepressure at the N point of the tee piecewas generated because of the suction ofthe mini water-circulating pump. The headpressure of the GC column forced thecarrier gas through the transfer line to thetee piece (as shown on the Figure 1).Because the pressure at point P is alwayshigher than that at point N, we avoidedthe problem of direct influx of the aqueoussolution into the GC system during theextraction process.After the aqueous sample was drainedcompletely from the extractor, the six-portvalve was switched to the dotted-lineposition for desorption. To achieve lowerdetection limits in high-resolution GC andto obtain a sharp desorption band, theextractor should heat up as fast as possible(at a rate as high as 290 °C/min). Thedesorption of analytes from the capillaryextractor occurs very fast because they arepurged by the auxiliary gas through thecapillary. The thermal desorption time isapproximately 2–6 min for the 5 m0.53 mm capillary column under a6 mL/min purage-gas flow. Another pathof auxiliary gas at a flow-rate ofapproximately 1–2 mL/min is used asmake-up gas through the tee piece toprevent any back diffusion of analytes. Itmust be noted that the desorbedanalytes are introduced directly to thehigh-resolution GC system after theswitching valve, instead of through thevalve, to eliminate the possibility of anycarryover or dead volume along the sampletransfer line. However, using the high-temperature switching valve is unnecessaryin this system design. The more expensivevalve is not required for this system design,which makes the device less expensive.Finally, the desorbed analytes weretransferred to the homemade coldretention gap in the model 6890N gaschromatograph (Agilent Technologies,Beijing, China), with an initial oventemperature of 30 °C, through the hotcapillary transfer line and were refocusedon the head of the analytical column bythe retention gap.18We used a sequentialtemperature-programmed high-resolutionGC run to accomplish the separation anddetection of analytes of interest. Thus, thetotal process of analysing organiccompounds in aqueous samples, includingthe on-line extraction, thermal desorptionand sampling to high-resolution GC, wasautomated using the above-mentionedin-tube SPME device.3Sample Preparation PerspectivesResults and DiscussionsEvaluation of on-line in-tube SPMEcoupled with high-resolution GC:Weused aqueous samples containing a seriesof alkanes (C10–C19) at the microgram-per-litre level to evaluate the performanceof our apparatus. As Table 1 shows, thein-tube SPME capillary extractor has anincreased amount of solid stationary phaseand a much larger exchanging surface,when compared with the fibre SPME system,which result in drastic increases of theextraction efficiency and enrichment factor.In the experiments, a 5 m 0.53 mm,1.2 µmdfpoly(dimethylsiloxane) phasecapillary extractor provides approximately10 µL of solid phase for extraction, which isroughly 10-fold more solid phase than thatof an SPME fibre ( 1 µL). Figures 2(a) and2(b) show that the extraction of the same15 mL aqueous sample with a 10 µg/Lconcentration of each alkane to obtain anapproximately 30-fold concentration of theanalytes of interest requires 2.5 min forin-tube SPME versus 35 min for fibre SPME.When the extraction time was limited to5 min, we were able to extract only fewanalytes of interest by fibre SPME [see Figure2(d)]. However, with a 40-min extractiontime, the peaks obtained by in-tube SPMEare roughly 50-fold higher than that of fibreSPME (extraction for 35 min), even forcomponents with concentrations as low as2 µg/L [Figure 2(c)]. In addition, the baselinesof chromatograms from in-tube SPME arevery smooth because the capillary extractorshave thinner and stronger film of bondedphase, and ghost peaks normally appearwith fibre SPME.We performed six replicate experimentsof on-line extraction, desorption anddetection for each concentration of samplesto examine the method’s reproducibility.The precision of quantification, obtainedusing a 5 m 0.53 mm, 1.2 µmdfpoly(dimethylsiloxane) capillary, varies from5% to 15% relative standard deviation(RSD) (forn6), depending upon theTable 1:Comparison of a typical apolar capillary extractor (5 mExtractor TypeCapillary extractor100 µm apolar SPME fibre*V†A‡V§A0.53 mm, 1.2µmdf) and a 100 mm apolar SPME fibre.Character of ExtractantsBonded on the inner wallCoated outside the fibrePhase Volume(µL)9.9852*0.9734‡Exchanging Surface ofExtraction (mm2)8321†9.7§Type of MixingDuring ExtractionTurbulence of flow convectionAgitation by stirrerDidfL,whereDi is the inner diameter of capillary tube,df is the film thickness of solid phase andLis the length of capillary column.DiL.DodfL,whereDo is the outer diameter of fibre core ( 110 µm),dfis the coating film thickness of solid phase andLis the length of fibre (normally 10 mm).DoL.Figure 2:Comparison of chromatograms of spiked aqueous samples by in-tube SPME and fibre SPME: (a) extraction of 15 mLsamples within 2.5 min with alkanes of 10 µg/L level by on-line in-tube SPME; (b) extraction of 15 mL samples within 35 min withalkanes of 10 µg/L level by fibre SPME; (c) extraction of 300 mL samples within 40 min with alkanes of 2 µg/L level by on-linein-tube SPME; (d) extraction of 15 mL samples within 3 min with alkanes of 10 µg/L level by fibre SPME. Column: 30 m 0.53 mm,0.6 µmdfMXT-1 (Restek Corp., Bellefonte, Pennsylvania, USA) with a 5 m retention gap; carrier gas: hydrogen at 8 mL/min; ovenprogramme: 30 °C for 0.5 min, 30–110 °C at 40 °C/min, 110 °C for 1 min, 110–250 °C at 10 °C/min, 250 °C for 10 min; fibredesorption time in (b): 4 min. Peaks: 1n-C12, 2n-C13, 3n-C14, 4n-C15, 5n-C16, 6n-C17, 7n-C18, 8n-C19.(a)700600Response (pA)5004003002001002(b)90Response (pA)831256 712345678(c)9008007006005004003002001002(d)45 67 8Response (pA)321468101246810129080Response (pA)8070605040244706050403068101224681012Time (min)Time (min)4LC•GC Europe, 17(3),144–151 (2004)Sample Preparation Perspectivesalkanes and concentration of samplesstudied. Table 2 shows the precision andlinearity for all compounds investigatedunder different concentration conditions.The average RSD was 8.0% for the 0.5 µg/Lconcentration level and 5.3% for the 20µg/L concentration level.We determined linearity by extractingspiked aqueous samples withconcentrations ranging from 0.1 µg/L toTable 2:Precision, linearity and sensitivity of on-line in-tube SPME–high-resolution GC.100 µg/L. The method was linearthroughout at least three orders ofmagnitude. The coefficient of correlationachieved was better than 0.99 (Table 2). Inaddition, we found no carryover ormemory effects with this on-line coupledin-tube SPME–high-resolution GC system.Alkanes Repeatability RSD* Repeatability RSD* Correlation Detection(%) for 0.5 µg/L(%) for 20 µg/LCoefficient† Limit‡(S/N 3 µg/L)n-C12n-C13n-C14n-C15n-C16n-C17n-C18n-C19*n6.14.89.27.27.46.66.27.05.811.36.45.35.24.35.55.24.70.996§0.990.9960.9960.9980.9960.9980.9930.3\0.070.050.0300.0240.0170.012/3#0.01Applications of On-LineIn-Tube SPME Coupled withHigh-Resolution GCDetermination of PAHs with on-linein-tube SPME, high-resolution GC andflame ionization detection:Polycyclicaromatic hydrocarbons (PAHs) are animportant class of environmental pollutantsthat represent a risk for living organismsand human health. Among varioustechniques of PAH determination, SPME19and stir-bar sorptive extraction20haverecently gained wide acceptance. Forsub-parts-per-billion level measurement,the SPME device has a limited enrichmentfactor; however, the stir-bar sorptiveextraction device, which has a two-orders-higher enrichment factor than the SPMEsystem, needs several hours to reachextraction equilibrium, especially forfour-ring and larger polynuclear compounds.In addition, a number of manual steps arenecessary to successfully use the stir-barsorptive extraction technique.In our experiment, we examined 16 PAHstandards spiked in water at microgram-per-litre or sub-microgram-per-litreconcentrations. We applied two types ofclean water as the sample matrices:Wahaha purified drinking water (WahahaCorp., Hangzhou, China) [Figure 3(a)] andtap water from the laboratory faucet[Figure 3(b)]. The resulting chromatogram[Figure (3)] shows that the 16 PAHs werewell separated without any tailing peaks.The lowest detection limit for most PAHswas estimated to be less than 0.01 mg/L,much lower than that estimated by thefibre SPME technique. In addition, theoverall extraction time was within 40 minfor sample volume of 400 mL, quite shortcompared with stir-bar sorptive extractionor fibre SPME. It was interesting to notethat trace amounts of plasticizers werefound in the purified water, but no peak ofbutylbenzyl phthalate was found in the tapwater [Figure (3)]. However, many morevolatile compounds were in the tap water,which we suspected were halogenatedhydrocarbons that resulted from thechlorination process.Determination of chlorinatedpesticides with on-line in-tube SPME,high-resolution GC, and electron-5† Concentration range from 0.1 µg/L to 100 µg/L/L.‡ Extraction flowrate: 10 mL/min for 300 mL samples of the lowest concentration of 0.1 µg/L.§ Concentration range from 1 µg/L to 100 µg/L/L.\Detection with the lowest concentration of 1 µg/L/L.# The lowest detection limit with the use of 100 µm poly(dimethylsiloxane) fibre.Figure 3:On-line in-tube SPME–GC–flame ionization detection chromatogram of(a) Wahaha purified water and (b) tap water spiked with polyaromatic hydrocarbons.Column: 30 m 0.53 mm, 0.6 µmdfDB-1 (Agilent Technologies, Wilmington,Delaware, USA) with a 5 m retention gap; carrier gas: hydrogen at 8 mL/min; ovenprogramme: 30 °C for 0.5 min, 30–110 °C at 40 °C/min, 110 °C for 1 min, 110–300 °Cat 8 °C/min, 300 °C for 10 min. Peaks: 1 naphthalene (2 µg/L), 2 acenaphthylene(2 µg/L), 3 acenaphthene (2 µg/L), 4 fluorine (0.2 µg/L), 5 phenathrene(0.2 µg/L), 6 anthracene (0.2 µg/L), 7 fluoranthene (0.2 µg/L), 8 pyrene(0.2 µg/L), 9 benzo[a]anthracene (0.2 µg/L), 10 chrysene (0.2 µg/L),11 benzo[b]fluoranthene (0.2 µg/L), 12 benzo[k]fluoranthene (0.2 µg/L),13 benzo[a]pyrene (0.2 µg/L), 14 indeno[1,2,3-cd]pyrene (0.2 µg/L),15 dibenzo[a,h]anthracene (0.2 µg/L), 16 benzo[ghi]perylene (0.2 µg/L),17 hexachlorocyclopentadiene, 18 diethyl phthalate,19N-nitrosodiphenylamine,20 di-n-butylphthalate, 21 butylbenzyl phthalate.(a)2252001751501251007550117 218564 192037891021111412131516Response (pA)5(b)22520017515012510075502551015202530Response (pA)No Peak101520Time (min)2530www.lcgceurope.comSample Preparation Perspectivescapture detection:To determinechlorinated pesticides in aqueous samples,we coupled our in-tube SPME deviceon-line with a model 3800 capillary gaschromatograph (Varian Inc., Palo Alto,California, USA), which was equipped withan electron-capture detector. We dividedstandard mixtures of 26 total pesticidesinto three groups, and each pesticidecompound was spiked directly into a silexvial at the 0.25 µg/L level using amicrosyringe. Figure 4 depicts the resultingchromatogram. It is noteworthy to pointout that the detectability of the peaks ofFigure 4:On-line in-tube SPME–GC–ECD chromatograms of aqueous samples spikedwith chlorinated pesticides. Column: 30 m 0.53 mm, 0.6µmdfDB-5 (AgilentTechnologies) with a 5 m retention gap; carrier gas: nitrogen at 8 mL/min; ovenprogramme: 30 °C for 0.5 min, 30–150 °C at 40 °C/min, 150 °C for 1 min, 150–280 °C at7 °C/min, 280 °C for 10 min; detection: electron capture. Peaks: 1-BHC, 2-BHC,3-BHC, 4op-DDE,5pp-DDE, 6op-DDD,7pp-DDD, 8pp-DDT,9 iprodione, 10 lidane, 11 pentachloronitrobenzene, 12 vinclozolin,13 keithane, 14op-DDT,15 cyhalothrin lambda, 16 dicloran,17 fenpropathrin, 18cis-permethrin,19trans-permethrin,20cis-fenvalerate,21trans-fenvalerate,22cis-deltamethrin,23trans-deltamethrin.14005Response (mV)300200100pp -dichlorodiphenyltrichloro-ethane(pp -DDT) and iprodione, which are barelydetectable using GC systems that displaypoor inertness, was rather good. The lowestdetection limits of some components suchas benzene hexachloride (BHC) wereestimated at the sub-nanogram-per-litrelevel, which was obtained in only 5 minwith extraction of a 30 mL aqueous sample.Actually, lower detection limits could bereached by using a larger sample volume.Determination of phosphorus-containing pesticides with on-linein-tube SPME, high-resolution GC, andpulsed-flamephotometric detection:We coupled the same apparatus on-linewith the capillary gas chromatograph, thistime equipped with a pulsed-flamephotometric detector (operated in thephosphorus mode) to determine thephosphorus-containing pesticides. Wespiked 16 standard pesticides at 0.5 µg/Leach in a 30 mL aqueous sample beforeextraction, and the dilution of compoundswas performed in a silex vial using amicrosyringe. The chromatogram obtainedis illustrated in Figure 5. The lowestdetection limit observed was 0.05 µg/L formost of the analytes with an extractiontime of only 5 min.5101520ConclusionWe performed in-tube SPME on-linecoupling with high-resolution GC by usinga simple device for the trace analysis oforganic compounds in aqueous samples.The in-tube SPME–high-resolution GCmethods demonstrated in our studysuccessfully performed the on-lineextraction, desorption and sampling ofvarious contaminants in water, followed byanalysis of high-resolution GC withdifferent detectors. The novel in-tubeSPME–high-resolution GC device presentsthe following advantages over manual fibreSPME–high-resolution GC:• It enables much higher enrichmentfactors than those of fibre SPME becauseof its 10-fold greater volume ofextracting phase compared with fibreSPME. In our experiments, theenrichment factors by means of in-tubeSPME were at least 50-fold that of fibreSPME, if both experiments wereperformed under optimum conditions,even though the ratio of extractingphase volume was approximately 10.• It provides faster extractions than thoseof fibre SPME because of the drasticallylarger exchanging surface.• It is performed as a fully on-lineoperation; therefore, it provides high-LC•GC Europe, 17(3),144–151 (2004)150Response (mV)100505101520300Response (mV)20010051015Time (min)2025306 [ Pobierz całość w formacie PDF ]