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Analytica Chimica Acta 638 (2009) 162–168

Contents lists available at ScienceDirect

Analytica Chimica Acta

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reparation of alumina-coated magnetite nanoparticle for extraction ofrimethoprim from environmental water samples based on mixed hemimicellesolid-phase extraction

ei Suna, Chuanzhou Zhangb, Ligang Chena, Jun Liua, Haiyan Jina, Haoyan Xua, Lan Dinga,∗

College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, ChinaJilin Province Product Quality Supervision Test Institute, 20 Weixing Road, Changchun 130022, China

r t i c l e i n f o

rticle history:eceived 27 October 2008eceived in revised form 23 February 2009ccepted 23 February 2009vailable online 5 March 2009

eywords:lumina-coated magnetite nanoparticlesixed hemimicelles

olid-phase extractionrimethoprim

a b s t r a c t

In this study, a new type of alumina-coated magnetite nanoparticles (Fe3O4/Al2O3 NPs) modified by thesurfactant sodium dodecyl sulfate (SDS) has been successfully synthesized and applied for extractionof trimethoprim (TMP) from environmental water samples based on mixed hemimicelles solid-phaseextraction (MHSPE). The coating of alumina on Fe3O4 NPs not only avoids the dissolving of Fe3O4 NPs inacidic solution, but also extends their application without sacrificing their unique magnetization charac-teristics. Due to the high surface area of these new sorbents and the excellent adsorption capacity aftersurface modification by SDS, satisfactory concentration factor and extraction recoveries can be producedwith only 0.1 g Fe3O4/Al2O3 NPs. Main factors affecting the adsolubilization of TMP such as the amount ofSDS, pH value, standing time, desorption solvent and maximal extraction volume were optimized. Underthe selected conditions, TMP could be quantitatively extracted. The recoveries of TMP by analyzing the

nvironmental water samples four spiked water samples were between 67 and 86%, and the relative standard deviation (RSD) rangedfrom 2 to 6%. Detection and quantification limits of the proposed method were 0.09 and 0.24 �g L−1,respectively. Concentration factor of 1000 was achieved using this method to extract 500 mL of differentenvironmental water samples. Compared with conventional SPE methods, the advantages of this newFe3O4/Al2O3 NPs MHSPE method still include easy preparation and regeneration of sorbents, short timesof sample pretreatment, high extraction yields, and high breakthrough volumes. It shows great analytical

tion o

potential in preconcentra

. Introduction

Since magnetic carrier technology (MCT) was first reported byobinson et al. in 1973 [1], the synthesis of micro (or nano) mag-etic carriers has been attracting intense interest due to their wideromising applications such as protein and enzyme immobiliza-ion, immunoassay, RNA and DNA purification, cell isolation, andarget drug [2–6]. A distinct advantage of this technology is that

agnetic materials can be readily isolated from sample solutionsy the application of an external magnetic field. Furthermore, mag-etic nanoparticles possess large surface areas and have uniqueagnetic properties. When certain special functional ligands with

ffinities for target molecules are bound onto these magneticanoparticles, selective removal of toxic target compounds fromomplex environmental matrices can be obtained.

∗ Corresponding author. Tel.: +86 431 85168399; fax: +86 431 85112355.E-mail address: dinglan@jlu.edu.cn (L. Ding).

003-2670/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.aca.2009.02.039

f organic compounds from large volume water samples.© 2009 Elsevier B.V. All rights reserved.

Recently, a new solid-phase extraction (SPE) method based onmixed hemimicelles (hemimicelles and admicelles) (MHSPE) hasbeen proposed for the preconcentration of a variety of organicpollutants from complex environmental matrices [7–20]. In thismethod, the sorbents used are produced by adsorbing ionic sur-factants onto the surface of metal oxides, such as sodium dodecylsulfate (SDS)-coated alumina [7,9,10,12–15,17] or cetyltrimethy-lammonium bromide (CTAB)-coated silica [8,11,20]. Hemimicellesconsist of monolayers of surfactants adsorbing head down on anoppositely charged mineral oxide surface. Admicelles are surfac-tant bilayers formed from hemimicelles, under addition of moresurfactant, by interaction of surfactant hydrocarbon chains. The useof mixed hemimicelles assemblies in SPE has a number of advan-tages, such as high extraction yield, easy elution of analytes and highbreakthrough volume. Furthermore, the sorbents of this MHSPE

technique are easy to regenerate. However, because of a relativelysmall surface area of the micro-particle sorbents used, the reportedMHSPE method may lead to a relatively low extraction capability inaddition to being time-consuming when large volume samples areloaded [8,9,11,14,16].

L. Sun et al. / Analytica Chimica

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Fig. 1. Structure of TMP.

Considering those mentioned above, a new MHSPE technology isroposed that can combine the advantages of MHSPE and magneticanoparticles to fabricate magnetic nanosized MHSPE adsorbentsith high surface area, high chemical stability and rapid magnetic

eparability. It can be assumed that its application in analyticalhemistry can improve the adsorption capacity of analytes, avoidhe time-consuming enrichment process of loading large volumeamples in conventional SPE method. In the literatures, there haveeen only three applications to report about this technology soar. Zhao et al. studied MHSPE based on cetyltrimethylammoniumromide-coated magnetic nanoparticles for the preconcentrationf phenolic compounds from environmental water samples [18],nd later synthesized silica-coated magnetite nanoparticles toeplace pure magnetic nanoparticles for the same determina-ion in a similar method [20]. Li et al. also successfully appliedetyltrimethylammonium bromide-coated nano-magnets Fe3O4or the determination of chlorophenols in environmental wateramples based on MHSPE [19].

Trimethoprim (TMP), a dihydropteroate synthesase inhibitor,s commonly used in combination with sulfonamides for broad-pectrum antimicrobial therapy (Fig. 1) [21,22]. It blocks the foliccid metabolism, and thus produces a synergistic antibacterialctivity. Several analytical methods have been developed for deter-ining TMP by high performance liquid chromatography (HPLC)

oupled with various detectors, such as ultraviolet (UV) [21,23–25]r mass spectroscopy (MS) [22–24,26–31]. Because of the veryow concentration level of TMP and the complex matrix of manynvironmental samples, a sample preconcentration becomes nec-ssary for a reliable determination of this compound. SPE based onifferent types of sorbents such as Oasis HLB [21,27,30,31], molecu-

arly imprinted polymers [26], anion-exchange [22] and restrictedccess media (RAM) [25] is the most widely used. There are somether preconcentration techniques, such as solid-phase microex-raction (SPME) [28] or pressurized liquid extraction (PLE) [29].owever, the majority of these methods used are labor intensive,

ince they usually employ complex pretreatment steps and requireuch longer analysis time.In this study, alumina-coated magnetite nanoparticles

Fe3O4/Al2O3 NPs) were successfully synthesized and modi-ed by SDS in acidic media to form mixed hemimicelles for thextraction of TMP from environmental water samples. Comparedith the nonmagnetic micro-absorbents, the alumina-coatedagnetite nanoparticles can meet the need of rapid and effective

xtraction of large volume samples with an additional magnet.DS, adsorbed onto the surface of Fe3O4/Al2O3 NPs, is required foromplete adsorption of TMP. Experimental factors affecting thextraction efficiency were studied.

. Experimental

.1. Chemicals and water samples

Trimethoprim was obtained from National Institute for the Con-rol of Pharmaceutical and Biological Products (Beijing, China).

standard stock solution (1000 �g mL−1) was prepared by dis-

Acta 638 (2009) 162–168 163

solving an appropriate amount of TMP in methanol and storedunder dark conditions at 4 ◦C. Working solutions were obtaineddaily by appropriately diluting the stock solution with pure water.Chromatographic-grade acetonitrile (ACN) and glacial acetic acid(HAc) were purchased from Fisher Corporation (Pittsburgh, PA,USA). Ferrous chloride (FeCl2·4H2O), ferric chloride (FeCl3·6H2O),sodium dodecyl sulfate, triethylamine (TEA) were supplied byGuangfu Fine Chemical Research Institute (Tianjin, China). Alu-minum isopropoxide was obtained from Sinopharm ChemicalReagent Co. (Shanghai, China). The pure water was prepared byMilli-Q water purification system (Millipore, Bedford, MA, USA). Allother reagents were of analytical grade.

The primary and final sewage effluent samples were taken fromhospital of Jilin University (Changchun, China). The lake water sam-ple was collected from Nanhu (Changchun, China). The surfacewater sample was obtained from Mudanyuan (Changchun, China).The tap water sample came from our laboratory. All water sam-ples were collected randomly and stored at 4 ◦C. The spiked watersamples were made by adding certain amounts of TMP standardsolution to the real water samples of fixed volume and stored atroom temperature for at least half a month.

2.2. Preparation of alumina-coated magnetite nanoparticle

The Fe3O4 nanoparticles (Fe3O4 NPs) were prepared by chemicalcoprecipitation method [32]. Ferrous chloride (2.0 g), ferric chloride(5.2 g), and hydrochloric acid (12 mol L−1) (0.85 mL) were dissolvedin 25 mL pure water. The mixture was added dropwise into 250 mLNaOH solution (1.5 mol L−1) under vigorous stirring with nitrogengas passing continuously through the solution during the reaction.After the reaction, the obtained Fe3O4 NPs precipitate was sep-arated from the reaction medium under the magnetic field, andrinsed with 200 mL pure water four times. Then, the product wasoven dried at 80 ◦C.

The Fe3O4/Al2O3 nanoparticles (Fe3O4/Al2O3 NPs) were pre-pared according to Li et al. [4] with minor modification. Aluminumisopropoxide (1.0 g) was dissolved in ethanol (60 mL) to form a clearsolution. Fe3O4 NPs (0.1 g) were then dispersed in the freshly pre-pared solution for 5 min with the aid of ultrasonic. A mixture ofwater and ethanol (1:5, v/v) was added dropwise to the suspensionof Fe3O4 NPs with vigorous stirring. The mixture was stirred forhalf an hour after the addition. Subsequently, the suspension wasstanding for one hour before separating and washing with ethanol.After five cycles of separation/washing/redispersion with ethanol,the powder obtained was oven dried and calcined at 500 ◦C for threehours.

The magnetic property of Fe3O4 NPs and Fe3O4/Al2O3 NPs wasanalyzed using a JDM-13 vibrating sample magnetometer (VSM,Jilin University, Changchun, China) at room temperature. A scanningelectron microscopy (SEM) image was obtained using a JSM-6500FSEM instrument (JEOL, Tokyo, Japan).

2.3. MHSPE procedures

The MHSPE procedure was carried out as follows. Firstly, 0.1 gFe3O4/Al2O3 NPs and 80 mg SDS was added into 500 mL watersample and the pH was adjusted to about 2.0 with nitric acid(10.8 mol L−1) to form mixed hemimicelles assemblies. The mixturewas stirred for 15 min. Subsequently, the Fe3O4/Al2O3 NPs were iso-lated by placing a strong magnet and the supernatant was pouredaway. The preconcentrated target analyte absorbed on SDS-coated

Fe3O4/Al2O3 NPs was eluted with methanol (3 × 1 mL). The eluateswere combined. The solution was dried with a stream of nitrogenat 40 ◦C and dissolved in 0.5 mL methanol. A 10 �L of this solutionwas injected into the HPLC system for analysis. Illustration of thewhole procedure of the preparation of SDS-coated Fe3O4/Al2O3 NPs

164 L. Sun et al. / Analytica Chimica Acta 638 (2009) 162–168

Ft

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2

wMoai2

ig. 2. Schematic illustration of the preparation of SDS-coated Fe3O4/Al2O3 NPs andheir application for preconcentration of the analyte based on MHSPE.

nd their application as SPE sorbents for enriching the analyte washown in Fig. 2.

.4. Batch experiments

Adsorption isotherm and predominant factors optimizationere measured with batch experiments following the similarHSPE procedure. Fe3O4/Al2O3 NPs (0.1 g) were added to a series

f 250 mL aqueous solutions. For SDS adsorption isotherm, variablemounts of SDS (0–180 mg) were added to the aqueous suspensionn the absence of TMP and the pH of the solution was adjusted to.0 with nitric acid (10.8 mol L−1). After vigorous stirring for about

Fig. 3. VSM magnetization curves of Fe3O4 NPs (a) and Fe3O4/Al2O3 NPs (b).

Fig. 4. SEM image of Fe3O4/Al2O3 NPs.

15 min, the Fe3O4/Al2O3 NPs were isolated by a magnet and theconcentrations of SDS in the supernatants were determined by anABI Q-Trap mass spectrometer (Applied Biosystems Sciex, FosterCity, USA). The instruments are equipped with electrospray ion-ization (ESI) source and interfaced to a computer running AppliedBiosystems Analyst version 1.4 software. The quantification wasperformed by using the selected ion monitoring mode (SIM) innegative mode.

To examine the effect of amount of SDS added to the solutionon the adsorption of TMP, variable amounts of SDS (0–180 mg)were added to the aqueous suspension in the presence of TMP(4 �g L−1) and the pH of the solution was adjusted to 2.0. After vig-orous stirring for about 15 min, the Fe3O4/Al2O3 NPs were isolatedby a magnet and the adsorbed analyte was eluted and quantifiedusing HPLC–UV. When investigating the effect of the pH value onthe adsorption of TMP, pH was varied from 1.0 to 7.0 while the addedSDS amount was kept at 80 mg following the same determinationprocedures.

2.5. HPLC–UV analysis

The TMP was separated and quantified using a liquidchromatography–ultraviolet (HPLC–UV) system. The HPLC analysiswas carried out on an waters system (Milford, MA, USA) equippedwith a 1525EF binary HPLC pump, a 2487 dual wavelength detector,a heated column compartment, and a G1397A degasser (Agilent,Palo Alto, CA, USA). The analytical column was a Pinnacle 11 C18column (250 mm × 4.6 mm i.d., 5 �m) (Restek, Bellefonte, PA, USA).The injection volume was 10 �L. The mobile phase was the mixtureof acetonitrile and 0.1% triethylamine water solution (adjusting pHto 5.9 with glacial acetic acid) (20:80; v/v) and the flow-rate was setat 1.0 mL min−1. The UV detection of TMP was performed at 271 nm.

3. Results and discussion

3.1. Characterization of Fe3O4 NPs and Fe3O4/Al2O3 NPs

The magnetization curves show that both Fe3O4 NPs andFe3O4/Al2O3 NPs exhibit typical superparamagnetic behavior dueto no hysteresis (Fig. 3). There is no remanence and coercivity, sug-gesting that such NPs are superparamagnetic. The large saturation

magnetization, is 54.05 emu g−1 for Fe3O4 NPs and 8.15 emu g−1

for Fe3O4/Al2O3 NPs, which is sufficient for magnetic separationwith a conventional magnet. Apparently, the nonmagnetic Al2O3coating on the Fe3O4 NPs results in the decrease of Fe3O4/Al2O3NPs in the magnetic strength. Fig. 4 displays the SEM image of

L. Sun et al. / Analytica Chimica Acta 638 (2009) 162–168 165

FF

Fo

3

3

mMt(a

2Nt

FN

ig. 5. Typical structure of the surfactant aggregates formed on the surface ofe3O4/Al2O3 NPs.

e3O4/Al2O3 NPs, which illustrates the uniform size distributionf the nanospheres.

.2. Mixed hemimicelles solid-phase extraction (MHSPE)

.2.1. Adsorption isotherm of SDS on Fe3O4/Al2O3 NPsThe adsorption isotherms were useful for understanding the

echanisms of adsolubilization of analytes and optimizing theHSPE conditions. Generally, the adsorption of ionic surfactant on

he surface of mineral oxides can be divided into three regionshemimicelles, mixed hemimicelles, and admicelles (below andbove the critical micelles concentration, cmc)) (Fig. 5) [19].

Fig. 6 shows the experimental adsorption isotherm of SDS at pH.0. The amounts of SDS absorbed on the surface of Fe3O4/Al2O3Ps were obtained from the total amount SDS added to the solu-

ion subtracting the remaining in the solution. Seen from Fig. 6,

ig. 6. Adsorption isotherm of SDS on the surface of Fe3O4/Al2O3 NPs. Fe3O4/Al2O3

Ps amount, 0.1 g; pH, 2.0; solution volume, 250 mL.

Fig. 7. Effect of amount of SDS added on the adsorption of TMP. Fe3O4/Al2O3 NPsamount, 0.1 g; pH, 2.0; TMP concentration, 4 �g L−1; solution volume, 250 mL.

SDS was nearly all absorbed on the surface of Fe3O4/Al2O3 NPsbefore 30 mg. This region could be considered as hemimicelles,where SDS molecules were adsorbed on the oppositely chargedFe3O4/Al2O3 NPs surface to form single layer coverage throughcoulombic attraction. Mixed hemimicelles was the region from30 to 120 mg. In this region, the adsorption amount of SDS onFe3O4/Al2O3 NPs increased gradually with the increase of SDSamount added to the solution, which resulted from the fractionalformation of bilayers (admicelles) by hydrophobic interactions.When the amount of added SDS was 120 mg, the amount of SDSadsorbed achieved the highest and then kept constant, which sug-gested that the surface of Fe3O4/Al2O3 NPs had been saturatedby SDS and began to form micelles in the solution when theamount of SDS continued increasing, which was the so-called admi-celles.

3.2.2. Effect of the amount of SDS on adsorption of TMPThe outer surface of hemimicelles is hydrophobic whereas that

of admicelles is ionic, which provides different mechanisms forretention of organic compounds and both are suitable for the SPEmethod. In mixed hemimicelles phase, the adsorption is driven byboth hydrophobic interactions and electrostatic attraction becauseof the formation of hemimicelles and admicelles on the surface ofmineral oxides [20]. From Fig. 7, we can see that in the absence ofSDS, the TMP was hardly adsorbed onto the surface of Fe3O4/Al2O3NPs. The adsorption amount of TMP increased remarkably withthe increasing amount of SDS. Maximum adsorption was obtainedwhen SDS amounts were between 70 and 140 mg. When SDSamount was above 140 mg, the adsorption of the analyte decreasedgradually, which may be attributed that the SDS molecules began toform micelles in the bulk aqueous solution and the micelles causedthe TMP to redistribute into the solution again. Given these find-ings, 80 mg was selected as the final addition amount of SDS in thenext studies.

3.2.3. Effect of solution pHpH is one of the factors influencing the adsorption behavior of

mixed hemimicelles system due to the change of the charge den-sity on the Fe3O4/Al2O3 NPs surface. As shown in Fig. 8, maximumadsorption of TMP was obtained when pH was 2.0. The adsorp-tion amount decreased when the pH increased from 2.0 to 7.0. This

can be attributed to the fact that the positively charged surface ofFe3O4/Al2O3 NPs was favorable for the adsorption of anionic sur-factants. When pH value was higher, the positive charge densityof Fe3O4/Al2O3 NPs surface was lower. The electrostatic attractionbetween negative charges of SDS and positive charges of the surface

166 L. Sun et al. / Analytica Chimica Acta 638 (2009) 162–168

Fa

octbpf

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aaffioNtiac

3

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F02

ig. 8. Effect of pH on the adsorption of TMP. Fe3O4/Al2O3 NPs amount, 0.1 g; SDSmount, 80 mg; TMP concentration, 4 �g L−1; solution volume, 250 mL.

f Fe3O4/Al2O3 NPs was not strong enough to produce hemimi-elles, which made against the great adsorption of TMP. However,he TMP adsorption decreased slightly when pH was 1.0, which maye explained by the reduced adsorption of SDS ion, probably due torotonation of their sulfate groups [15]. The pH of 2.0 was selectedor the next studies.

.2.4. Standing and magnetic separating timeThe experimental results indicated that the standing time had

n obvious effect on the target analyte adsorption (Fig. 9). Thebsorption process of TMP from water samples must equilibrateor enough time to obtain satisfactory recoveries. 15 min was suf-cient to achieve satisfactory adsorption with the TMP recoveryf 81.6%. Meanwhile, in the experiment, SDS-coated Fe3O4/Al2O3Ps possessed superparamagnetism properties and large satura-

ion magnetization, which enabled them to be completely isolatedn a short amount of time (less than 1 min) by a strong magnet. Inword, rapid separation rate and escape from the time-consumingolume passing shortened the analysis time greatly.

.2.5. Desorption conditions

Organic solvents are known to cause disruption of surfactant

ggregates [9]. The desorption of TMP from the SDS mixed hemim-celles on surface of Fe3O4/Al2O3 NPs was studied using differentinds of organic solvents (acetonitrile, ethanol, methanol). Quan-

ig. 9. Effect of standing time on the adsorption of TMP. Fe3O4/Al2O3 NPs amount,.1 g; SDS amount, 80 mg; pH, 2.0; TMP concentration, 4 �g L−1; solution volume,50 mL.

Fig. 10. Effect of water sample volume on the adsorption of TMP. Fe3O4/Al2O3 NPsamount, 0.1 g; SDS amount, 80 mg; pH, 2.0; TMP amount, 2 �g.

titative recovery (above 80%) of TMP was obtained using 3 mL(3 × 1 mL) methanol while a higher volume acetonitrile and ethanolwas required for the same desorption of TMP. Thus, 3 mL (3 × 1 mL)methanol was recommended for desorption.

3.2.6. Maximal extraction volumeThe maximal enrichment volume of water sample for TMP was

determined using a series of different volume aqueous solutions(250–700 mL) spiked with fixed 2 �g of TMP in the optimal condi-tions. The effect of sample volumes on the enrichment of the analytewas shown in Fig. 10. The recoveries of TMP (75–82%) were satis-factory with the water sample volume ranging from 250 to 500 mL.When above 500 mL, recoveries decreased quickly and insufficientrecovery was considered to occur. Thus, 500 mL was considered tobe the maximal enrichment volume for water samples. By dryingthe eluent (3 mL of methanol) with a stream of nitrogen at 40 ◦C anddissolving in 0.5 mL methanol, preconcentration factor of 1000 canbe easily achieved. The result proved that Fe3O4/Al2O3 NPs MHSPEmethod showed great analytical potential in preconcentration largevolume water samples.

3.3. Analytical performance

Quantitative parameters such as linear range, correlation coeffi-cient, detection and quantification limits were evaluated. A series ofworking standard solutions was prepared by diluting the stock solu-tion of TMP with pure water. The linear range of calibration curve forTMP was 0.24–30 �g mL−1. The calibration equation for TMP wasy = 11961x + 386.25 (R2 = 0.9993). The detection and quantificationlimits were determined by analysing four blank water samples fol-lowing the similar procedure as in Section 2.3, and were calculatedbased on their signal-to-noise ratio of 3 and 10. They were 0.09and 0.24 �g L−1, respectively. TMP exhibited good linearity and lowdetection and quantification limit.

3.4. Analysis of environmental water samples

In order to validate the developed method, it was applied tosome real environmental water samples. Table 1 lists the concentra-tions and recoveries found for the target compound TMP, expressed

as the mean value and the corresponding standard deviation. Acertain amount of TMP which had been identified by comparingretention time in liquid chromatograms and [M+H]+ ion (m/z 291.2)and fragment ions (m/z 95.2, 133.1, 161.1, and 261.2) of TMP in massspectroscopy with the standard TMP containing SDS, was found in

L. Sun et al. / Analytica Chimica

Table 1Results of recoveries of real water samples spiked with TMP, mean ± S.D. (n = 5).

Water samples Spiked (�g L−1) Detected (ug L−1) Recovery (%)

Tap water 0.00 n.d. –1.00 0.79 ± 0.02 79 ± 52.00 1.68 ± 0.02 84 ± 2

10.00 8.06 ± 0.10 86 ± 2

Nanhu 0.00 n.d. –1.00 0.70 ± 0.03 70 ± 62.00 1.34 ± 0.03 67 ± 4

10.00 8.00 ± 0.08 80 ± 2

Mudanyuan 0.00 n.d. –1.00 0.80 ± 0.02 80 ± 52.00 1.46 ± 0.03 73 ± 3

10.00 8.20 ± 0.16 82 ± 4

Hospital primarysewage effluent

0.00 0.25 –1.00 0.84 ± 0.09 69 ± 52.00 1.72 ± 0.13 76 ± 4

10.00 8.32 ± 0.31 81 ± 4

Hospital finals

0.00 n.d. –

hmwwphot

Fe1

ewage effluent 1.00 0.71 ± 0.04 71 ± 62.00 1.54 ± 0.07 77 ± 4

10.00 8.31 ± 0.15 83 ± 2

ospital primary sewage effluent. For the case of the other environ-ental water samples, no TMP was found. The recoveries of TMPere studied by adding a certain amount of TMP standard solutionith three concentrations (1.0, 2.0, and 10.0 �g L−1) into water sam-

les. The spiked samples were stored at room temperature at leastalf a month, and analyzed by the proposed method. The recoveriesf TMP were between 67 and 86% with the relative standard devia-ion of recoveries ranging from 2 to 6%, demonstrating satisfactory

ig. 11. The chromatograms of 1 �g L−1 standard of TMP (a), hospital primary sewageffluent sample (b), and hospital primary sewage effluent sample spiked with�g L−1 TMP (c) after preconcentration.

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Acta 638 (2009) 162–168 167

recovery and good precision of this method. Fig. 11 shows liquidchromatograms of 1 �g L−1 standard TMP, hospital primary sewageeffluent sample and spiked hospital primary sewage effluent sam-ple after preconcentration. It is found that SDS does not interferewith the separation and determination of TMP in the blank samplefrom Fig. 11(a). In Fig. 11(b and c), there are many impurities whichresult from the complicated matrix of water sample. Therefore, SDSin the proposed method has no influence on the determination.

4. Conclusions

In this research, Fe3O4/Al2O3 NPs were successfully synthesizedand a new type of Fe3O4/Al2O3 NPs mixed hemimicells solid-phase extraction method was proposed. This method combinesthe advantages of MHSPE and magnetic nanoparticles. Comparedwith the traditional SPE method, in this new Fe3O4/Al2O3 NPsMHSPE method, the magnetic separation greatly improved the sep-aration rate and SDS-coated Fe3O4/Al2O3 NPs MHSPE possesseshigh extraction efficiency and capacity for the target analyte. Easyregeneration is another property of Fe3O4/Al2O3 NPs, and the exper-iments have proved that these Fe3O4/Al2O3 NPs can be reusedat least 20 times on average without the obvious decrease ofrecovery after wash/calcine procedures. Furthermore, it avoids thetime-consuming column passing (about 1 h in conventional MHSPEmethod) [8] and filtration operation, and no clean-up steps wererequired. This new Fe3O4/Al2O3 NPs MHSPE was applied for theextraction of TMP from environmental water samples. The resultsshowed that this method was effective for the preconcentration oftrace TMP in large volume environmental water samples. In addi-tion, in the proposed method SDS has no influence on the TMPdetermination by HPLC because SDS has no absorption in ultravioletregion. It is anticipated that the proposed method has great analyt-ical potential in preconcentration of organic compounds from largevolume real water samples.

Acknowledgement

This work was supported by the National Natural Science Foun-dation of China. (Grant number: 20875037).

References

[1] P.J. Robinson, P. Dunnill, M.D. Lilly, Biotechnol. Bioeng. 15 (1973) 603.[2] X.D. Tong, Y. Sun, Biotechnol. Prog. 17 (2001) 738.[3] C. Grüttner, S. Rudershausen, J. Teller, J. Magn. Magn. Mater. 225 (2001) 1.[4] Y. Li, Y.C. Liu, J. Tang, H.Q. Lin, N. Yao, X.Z. Shen, C.H. Deng, P.Y. Yang, X.M. Zhang,

J. Chromatogr. A 1172 (2007) 57.[5] N. Yao, H.M. Chen, H.Q. Lin, C.H. Deng, X.M. Zhang, J. Chromatogr. A 1185 (2008)

93.[6] J.C. Liu, P.J. Tsai, Y.C. Lee, Y.C. Chen, Anal. Chem. 80 (2008) 5425.[7] F. Merino, S. Rubio, D. Pérez-Bendito, Anal. Chem. 75 (2003) 6799.[8] J.D. Li, Y.Q. Cai, Y.L. Shi, S.F. Mou, G.B. Jiang, Talanta 74 (2008) 498.[9] M. Cantero, S. Rubio, D. Pérez-Bendito, J. Chromatogr. A 1120 (2006) 260.10] M. Cantero, S. Rubio, D. Pérez-Bendito, J. Chromatogr. A 1067 (2005) 161.

[11] X.L. Zhao, J.D. Li, Y.L. Shi, Y.Q. Cai, S.F. Mou, G.B. Jiang, J. Chromatogr. A 1154(2007) 52.

12] F.J. López-Jimónez, S. Rubio, D. Pérez-Bendito, Anal. Chim. Acta 551 (2005) 142.13] N. Luque, F. Merino, S. Rubio, D. Pérez-Bendito, J. Chromatogr. A 1094 (2005) 17.14] A. Moral, M.D. Sicilia, S. Rubio, D. Pérez-Bendito, J. Chromatogr. A 1100 (2005)

8.15] T. Saitoh, Y. Nakayama, M. Hiraide, J. Chromatogr. A 972 (2002) 205.16] J.D. Li, Y.Q. Cai, Y.L. Shi, S.F. Mou, G.B. Jiang, J. Chromatogr. A 1139 (2007) 178.

[17] A. Moral, M.D. Sicilia, S. Rubio, D. Pérez-Bendito, Anal. Chim. Acta 569 (2006)132.

18] X.L. Zhao, Y.L. Shi, Y.Q. Cai, S.F. Mou, Environ. Sci. Technol. 42 (2008) 1201.19] J.D. Li, X.L. Zhao, Y.L. Shi, Y.Q. Cai, S.F. Mou, G.B. Jiang, J. Chromatogr. A 1180

(2008) 24.

20] X.L. Zhao, Y.L. Shi, T. Wang, Y.Q. Cai, G.B. Jiang, J. Chromatogr. A 1188 (2008) 140.21] S. Babic, D. Asperger, D. Mutavdzic, A.J.M. Horvat, M. Kastelan-Macan, Talanta

70 (2006) 732.22] J.E. Renew, C.H. Huang, J. Chromatogr. A 1042 (2004) 113.23] D.C.G. Bedor, T.M. Goncalves, M.L.L. Ferreira, C.E.M. de Sousa, A.L. Menezes, E.J.

Oliveira, D.P. de Santana, J. Chromatogr. B 863 (2008) 46.

1 imica

[[[[[

68 L. Sun et al. / Analytica Ch

24] S. Croubels, S.D. Baere, P.D. Backer, J. Chromatogr. B 788 (2003) 167.25] F.C.C.R. de Paula, A.C. de Pietro, Q.B. Cass, J. Chromatogr. A 1189 (2008) 221.26] S.G. Hu, L. Li, X.W. He, Anal. Chim. Acta 537 (2005) 215.27] R.N. Rao, N. Venkateswarlu, R. Narsimha, J. Chromatogr. A 1187 (2008) 151.28] E.L. McClure, C.S. Wong, J. Chromatogr. A 1169 (2007) 53.

[

[[[

Acta 638 (2009) 162–168

29] A. Göbel, A. Thomsen, C.S. McArdell, A.C. Alder, W. Gige, N. Theiß, D. Löffler, T.A.Ternes, J. Chromatogr. A 1085 (2005) 179.

30] A. Göbel, C.S. McArdell, M.J.F. Suter, W. Giger, Anal. Chem. 76 (2004) 4756.31] H. Chang, J.Y. Hua, M. Asami, S. Kunikane, J. Chromatogr. A 1190 (2008) 390.32] Z.F. Wang, H.S. Guo, Y.L. Yu, N.Y. He, J. Magn. Magn. Mater. 302 (2006) 397.