Atomic Spectroscopy

THIOL-MODIFIED MAGNETIC SILICA SORBENT FORTHE DETERMINATION OF TRACE MERCURY INENVIRONMENTAL WATER SAMPLES COUPLED WITHCOLD VAPOR ATOMIC ABSORPTION SPECTROMETRY

Guoqiang Xiang, Lulu Li, Xiuming Jiang, Lijun He, andLu FanSchool of Chemistry and Chemical Engineering, Henan University ofTechnology, Zhengzhou, P. R. China

Core-shell magnetic silica has been prepared and then chemically modified with thiol groups. It

was used for magnetic solid phase extraction (MSPE) of trace Hg2 from environmental sam-

ples, followed by its determination by cold vapor atomic absorption spectrometry (CV-AAS).

The parameters for MSPE procedure, such as pH, elution conditions, ultrasonic time, and

effects of co-existing ions were investigated. The results showed that Hg2 ions are adsorbed

on the thiol-modified magnetic silica at pH 6.0 after a 10-min ultrasonic treatment and were

quantitatively eluted with 5.0mL thiourea (2%, m/v) in nitric acid solution (0.1M) after a

2-min ultrasonic treatment. Under the optimized experimental conditions, the adsorption

capacity of the new sorbent was 101.0mgg1 for Hg2. The detection limit of the proposed

method was 0.06 ngmL1 for Hg2 and the enrichment factor was 16.6. The analytical data

obtained from a certified reference water sample (GBW00809) were in good agreement with

the certified value. The method has also been successfully applied to the determination of trace

Hg2 in rain water, well water and treated water from a sewage treatment plant. The recovery

of Hg2 from spiked samples was between 96.0% and 110.0%.

Keywords: CV-AAS; Environmental samples; Magnetic silica; Magnetic solid phase extraction; Mercury

INTRODUCTION

Mercury contamination in the environment and in living organisms continuesto be a critical issue of concern on a global scale (Taylora et al. 2005). The UnitedStates Comprehensive Environmental Response, Compensation, and Liability Act(CERCLA) lists mercury and its compounds in third place on the Priority Lists

Received 26 June 2012; accepted 27 August 2012.

Financial supports fromNationalNature ScienceFoundationofChina (No. 21205028),Natural Science

Fundamental Research Fund of Science and Technology Department of Henan Province (112300410080), the

program for young teachers of universities in Henan province (2012GGJS-300) and Plan for Scientific

Innovation Talent of Henan University of Technology (No. 11CXRC11) are gratefully acknowledged.

Address correspondence to Guoqiang Xiang, School of Chemistry and Chemical Engineering,

Henan University of Technology, Zhengzhou, 450001, P. R. China. E-mail: xianggq@126.com

Analytical Letters, 46: 706716, 2013

Copyright # Taylor & Francis Group, LLCISSN: 0003-2719 print=1532-236X online

DOI: 10.1080/00032719.2012.726679

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of Hazardous Substances, and the European Water Framework Directive (2000=60=EG) classifies mercury as 1 of 30 precarious dangerous pollutants. With thetoxic effects of mercury on ecosystems and health well-established, the need to detectmercury at extremely low concentrations persists.

Several analytical techniques such as spectrometry (Rajesh and Gurulakshma-nan 2008), inductively coupled plasma atomic emission spectrometry=mass spec-trometry (ICP-AES=MS) (Zhu and Alexandratos 2007; Yin et al. 2010), atomicfluorescence spectrometry (AFS) (Margetinova et al. 2008), and cold vapor atomicabsorption spectrometry (CV-AAS) (Shamsipur et al. 2005; Rofouei et al. 2011) havebeen developed for the determination of mercury. Among these techniques, CV-AASis widely accepted technique for the determination of mercury due to its high sensi-tivity, simple operation, and relative freedom from matrix interferences as well aslow cost (Segade and Tyson 2007; Zhai et al. 2006). However, direct determinationof mercury in different samples is often difficult for both low concentration in samplesand complicated sample matrix. To solve this problem, a separationpreconcentrationprocedure prior to analysis is a popular choice.

Recently, solid phase extraction (SPE) technique has become increasinglypopular as an effective separation-preconcentration procedure for metal ions priorto their determination. Compared with traditional liquidliquid extraction, SPEhas the following advantages: (1) high enrichment factor; (2) convenient operation;(3) rapid phase separation; (4) can be coupled with different detectors; and (5) is timesaving and cost saving (Camel 2003). It is well-known that the adsorption mediumplays an important role in improving the selectivity of SPE and much more attentionhas been paid to the exploration of new adsorption materials for SPE in recent years(Turker 2007). Many kinds of adsorption materials, such as chelating resins, silicagel, carbon sorbents, inorganic based sorbents, biological adsorption material, andnanomaterials have been used in SPE (Camel 2003; Turker 2007; Lemos et al. 2008).

Magnetic materials, interesting advanced composite materials, have receivedincreasing attention in the past few decades due to the following characteristics: uniquemagnetic response, low cytotoxicity, easy chemical surface modification, and greatpotential application in various fields (Gardimalla et al. 2005; Zhao et al. 2008; Deng,Deng et al. 2005; Li et al. 2008). Magnetic separation technique (MST) is based on themagnetic response of the magnetic material, which means that the suspended magneticmaterial adheres to the target of interest (the container wall) and can be quickly separatedfrom amatrix by an external magnet. The application of magnetic materials in SPE couldgreatly simplify the conventional column SPE procedure. The important adsorption pro-cess and desorption process could simply be completed in a container assisted by a mag-net. The whole operation time is greatly reduced, and the SPE procedure, therefore,provides low cost and easy operation without SPE cartridge and pump.

However, it should be pointed out that pure magnetic inorganic materials eas-ily form a large aggregation and alter magnetic properties (Yang et al. 2004). And,the pure magnetic inorganic materials were not selective and not suitable for com-plex samples (Pu et al. 2004). To solve the aforementioned problems, coating themagnetic materials is a popular choice. Silica was considered to be one of the mostideal coating medium for magnetic materials due to its reliable chemical stability andeasy surface functionalization (Kang et al. 2009; Liang and Wang 2010; Ashtari et al.2005). For functionalization of the silica coating of magnetic materials, silylation

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reaction was the most popular method to combine the functional group on the silicacoating (Zheng and Hu 2010; Chen et al. 2009). This method has a shortcoming oflow concentration of the functional group on the surface of coated magnetic materi-als, which leads to a lower adsorption capacity of the modified magnetic materials.To improve the analytical performance of magnetic materials for magnetic solidphase extraction (MSPE), a new modification procedure should be developed.

The aim of this study was to produce thiol-modifiedmagnetic silica as a new typeof sorbent forMSPE, and to develop a rapid, selective, and sensitive analytical methodfor the determination of trace Hg2 in environmental water samples by CV-AAS.

EXPERIMENTAL

Apparatus

Determination of mercury was carried out on an atomic absorption spec-trometry (TAS 986; Beijing Purkinje General Instrument Co., Ltd., Beijing, China)equipped with a hydride generation part (WHG-103A; Beijing Hanshi InstrumentCo., Ltd., Beijing, China). The optimum operation conditions were as follow: Wave-length 253.7 nm, Spectrum band width 0.4 nm, Lamp current 3mA, Flow rate of Argas 200mLmin1, concentration of HCl 1%(v=v), concentration of KBH4 1.5%(m=v) in NaOH solution (0.3%, m=v). FT-IR spectra in KBr was recorded by aWQF-510 FT-IR spectrometer (Beijing Rayleigh Analytical Instrument Co., Ltd.,Beijing, China). The pH values were measured with a PHS-3C pH-meter (ShanghaiPrecision & Scientific Instrument Co., Ltd., Shanghai, China). A model ultrasonica-tor (KQ116, 40 kHz; Kun Shan Ultrasonic Instruments Co., Ltd., Beijing, China)was used to disperse the magnetic materials in the solution. A magnet (Nd-Fe-B,60mm 35mm 10mm) was used for magnetic separation.

Standard Solution and Reagents

The stock solution of Hg2 (1.0mgmL1) was prepared by dissolving appropri-ate amounts of Hg(NO3)2 (Shanghai Reagent Factory, Shanghai, China) in dilutednitric acid (5%, v=v) and made up to 100mL with deionized water. Calibration solu-tions and test solutions were prepared by stepwise dilution of the stock solution. Tet-raethoxysilane (TEOS) and Polyethylene-alt-maleic anhydride (Mw 100000500000)were purchased from Sigma-Aldrich. L-cysteine (AR; Shanghai Huixing BiochemicalReagent Company, Shanghai, China) and 3-aminopropyltrimethoxysilane (AladdinReagent Company, Shanghai, China) were used for reaction directly. HNO3(pH1.02.0), acetate-acetic acid buffer (pH 3.05.0) and NH3-NH4Cl buffer (pH8.010.0) were used for pH adjustment. All other chemicals were of analytical reagentgrade. All stock standard solutions were stored in polyethylene bottles in a refriger-ator at 6C. All glassware were kept in 10% nitric acid for at least 24 h and washedwith deionized water before use.

Synthesis of Thiol-Modified Magnetic Silica Sorbent (MSS)

The preparation of the thiol-modified MSS was illustrated in Fig. 1. First, themagnetic Fe3O4 was synthesized by co-precipitation of a mixture of FeCl3 6H2O

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and FeCl2 4H2O with concentrated aqueous ammonia. The obtained magneticFe3O4 was embedded in silica microsphere to obtain the monodispersed Fe3O4=SiO2 microsphere through a modified Stober method (Deng, Wang et al. 2005). Then,Fe3O4=SiO2 microsphere (1.0 g) and 3-aminopropyltrimethoxysilane (2.0mL) wereplaced into a round bottom flask containing anhydrous toluene (80mL), thenrefluxed at 110C for 10 h with nitrogen protection. The product obtained by a mag-net was washed by toluene and methanol, and then was dried at 80C. In anotherbranch, polyethylene-alt-maleic anhydride (2.0 g) and L-cysteine (1.2 g) was dissolvedin N,N-dimethylformamide (200mL), then refluxed at 130C for 1 h. After refluxing,the reactant was cooled to room temperature, and the aforementioned obtainedamino modified Fe3O4=SiO2 microphere (1.6 g) was added, and refluxed at 130

Cfor 1.5 h. After cooling to room temperature, the resultant product was collectedby an external magnet. The obtained particles was washed with methanol (100mL)and deionized water (100mL), and dried in vacuum.

Sample Preparation

Environmental water samples were collected from a sewage treatment plant(treated water; Wulongkou, Zhengzhou city, China) and a well in Zhengzhou city.The urban rainwater sample was collected in the wet season in Zhengzhou city.The water samples were filtered through filter paper, and then filtered through amembrane (0.45 mm). The pH of the samples was adjusted to 6.0 before analysis.

General Procedure for MSPE

The operation procedure of MSPE is described as follows: 100mL standardsolution (or sample solution) containing Hg2(2.5 ngmL1) was transferred to a250mL beaker, pH of the solution was adjusted to 6.0, and 0.2 g sorbent was added.After ultrasonication for 10min, the magnetic adsorbent was separated easily andquickly by a flat magnet and the supernatants were decanted directly. For elutionof the target ion, the sorbent was mixed with 5.0mL thiourea (2%, m=v) in nitric acid

Figure 1. The preparation of thiol-modified MNPs.

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solution (0.1M). The eluent was collected after 2min ultrasonication with a magnet.Finally, trace mercury in the collected eluent was detected by CV-AAS.

RESULTS AND DISCUSSION

Characterization of Thiol-Modified MSS

The thiol-modified MSS was characterized by FT-IR (Fig. 2). The typicalabsorption peaks of the modified MSS confirmed its chemical structure. The peaksappeared at 596 cm1 and 1109 cm1 and are attributed to the Fe-O stretching bandof Fe3O4 and the Si-O stretching band of silica on the surface of MSS, respectively.Furthermore, the presence of the SH-CH2-CH- group was reflected by the peaks at2856 cm1 and 2927 cm1, which corresponds to C-H vibrations in different hybridi-zations. And, the characteristic N-H and C=O bands of the amide group corre-sponded to the peaks at 3456 cm1(N-H) and 1643 cm1(C=O) (Ma et al. 2003;Jang and Lim 2010). The aforementioned details clearly demonstrated the successfulmodification of MSS with the thiol group by reaction between L-cysteine andpolyethylene-alt-maleic anhydride.

Obviously, the most important function group of the prepared sorbent forMSPE was the thiol group, and its amount on the surface of the sorbent was thekey factor for its analytical performance. The amount of thiol group on the obtainedMSS was determined according to the Ellman method (Ellman 1959). It was deter-mined that the amount of thiol group on the thiol-modified MSS was 0.61mmol g1,which brought great potential for the application of trace metal preconcentrationwith large adsorption capacity to the sorbent.

Effect of pH

The pH is an important factor for the adsorption of Hg2 on the thiol-modifiedMSS. The effect of pH on the adsorption efficiency of Hg2 was investigated by

Figure 2. FT-IR spectra of thiol-modified MNPs. (Figure available in color online.)

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testing Hg2 standard solution (25 ngmL1) with pH varying from 1.07.0 accordingto general procedure for MSPE. As showed in Fig. 3, the extraction efficiency ofHg2 on thiol-modified MSS was 100%, and kept constant within a pH range of2.07.0. At lower pH, extraction efficiency of Hg2 decreased. This may be explainedby the competitive adsorption and electrostatic repulsion between Hg2 and H atlower pH. For further experiments, pH 6.0 was chosen for preconcentration of Hg2.

Optimization of Elution Condition

From the results in Fig. 3, it could be seen that extraction efficiency of Hg2

was still larger than 90% at pH 1.0. Thus, typical acid eluent is possibly difficultfor quantitative elution of adsorbed Hg2. To obtain an effective elution of adsorbedHg2, the mixture of nitric acid and thiourea solution was chosen as eluent. Theeffect of the mixture concentration was optimized, and the results indicated thatquantitative elution could be obtained using thiourea (2%, m=v) in nitric acidsolution (0.1M) as the eluent.

By using thiourea (2%, m=v) in nitric acid (0.1M) solution as the eluent, theeffect of the eluent volume on the elution of Hg2 was studied with eluent volumevarying from 420mL. The results indicated that 5mL was sufficient to elute Hg2

quantitatively. Therefore, 5mL eluent was selected for the following experiment.

Effect of Ultrasonic Time

To minimize the pretreatment time, ultrasonic time for adsorption and elutionwas also investigated. For adsorption process, the effect of ultrasonic time on theextraction efficiency of Hg2 was studied according to general procedure for MSPEwith the ultrasonic time varying from 530min. The results indicated that quantitat-ive extraction could be obtained with ultrasonic time larger than 10min. The extrac-tion efficiency was obviously reduced when ultrasonic time was larger than 20min.These could possibly explain a certain time of ultrasonic agitation that made a

Figure 3. Effect of ultrasonic time on extraction efficiency of Hg2 on thiol-modified MNPs.

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homodispersed solution of adsorbent, which greatly improve the adsorptionefficiency; and, longer ultrasonic time may lead to an increasing temperature ofsolution by the ultrasonic heat effect, which may decrease the adsorption efficiency.For the elution process, the experimental results showed that 2min was adequate forquantitative recovery of Hg2 adsorbed on thiol-modified MSS with thiourea (2%,m=v) in nitric acid solution (0.1M) as the eluent. Finally, 10min of ultrasonic timefor adsorption and 2min of ultrasonic time for elution were chosen.

Effect of Concomitant

The effect of potentially interfering ions on the preconcentration and determi-nation of the target analyte was investigated. In this experiment, the Hg2 standardsolutions (2.5 ngmL1) containing the added interfering ions were treated accordingto general procedure for MSPE. The concentration of Hg2 in the eluent was deter-mined in order to calculate the recovery. The tolerances of the potentially interferingions, defined as the maximum concentration that could achieve >90% recovery ofthe target analyte, are given in Table 1. Table 1 shows that most of the presentcations had a large tolerable concentration (the ratio of concentrations betweeninterfering ions and analyte was more than 4000) under the selected conditions.However, the tolerable concentrations of Ag and Cu2 were much smaller than thatof other ions. It is well-known that the binding ability between metal ions and SHgroup could refer to the solubility product constants of the corresponding metal sul-fides. The solubility product constant of sulfide is smaller; the binding abilitybetween the corresponding metal ion and SH group is greater. The solubility pro-duct constants of metal sulfide for the presence of metal ions sorted in descendingorder were as follow: Ksp(HgS)

The adsorption capacity of thiol-modified MSS was studied in order toevaluate the amount of sorbent required to quantitatively concentrate the analytesfrom a given solution. The method used was adapted from the procedure recom-mended by Maquieira, Elmahadi, and Puchades (1994). The adsorption capacitiesof thiol-modified MSS for Hg2 was 101.0mg g1.

Analytical Performance

Under the optimized experimental conditions, the analytical performance ofthis method was evaluated. Based on the definition of IUPAC, the detection limit(3r) of this method was 0.06 ngmL1, and the relative standard deviation (RSD)was 3.9% (c 1.0 n gmL1, n 7). The calibration graph for the preconcentrationprocedure was A 0.0435C 0.0328 (r2 0.99839) with concentration range of0.25.0 ngmL1. The enrichment factor (EF, calculated as the ratio of the slopesfor the calibration graphs with preconcentration and without preconcentration,respectively) was 16.6.

For comparative purpose, the performance characteristics of this method andother off-line SPE-CV-AAS systems reported in the literature are listed in Table 2.The application of magnetic sorbent in SPE procedure greatly changed the SPEprocedure. The use of pump and SPE cartridge were avoided, and the magnetic sep-aration greatly reduced the column experiments time and labor. Compared with tra-ditional SPE procedure, the MSPE procedure was simple, effective, and quick. FromTable 2, it could be seen that the sorbent shows much larger adsorption capacity andcomparable detection limit with much smaller sample volume over other off-lineSPE-CV-AAS methods. Moreover, the sample volume and sample flow rate listedin Table 2 confirmed that the MSPE procedure was quick and simple.

Table 2. Comparative data from some recent studies on off-line SPE=FAAS system

Analyte Absorbent

Adsorption

capacity

Detection

limit

Sample volume

and flow rate

Hg2 Sulfur powder loaded with

N-(2-chloro benzoyl)-N-

phenylthiourea

35mg g1 3 pgmL1 1000mL 16mL

min1 (Pourreza

et al. 2009)

Hg2 Agar powder modified with

2-mercaptobenzimidazole

49 ug g1 20 pgmL1 250mL 6mL min1

(Pourreza and

Ghanemi 2009)

Hg2

MeHgOctadecyl-bonded silica membrane

disk modified by 2-[(2-

mercaptophyenylimino)methyl]

phenol (MPMP)

3.8 pgmL1 1500mL 45mL

min1 (Ashkenani

et al. 2009)

Hg2

MeHgStaphylococcus aureus loaded

Dowex Optipore V-493

6.5mg g1

(Hg2)

5.4mg g1

(MeHg)

2.5 pgmL1

(Hg2) 1.7

pgmL1

(MeHg)

250mL 4.0mL

min1 (Tuzen et al.

2009)

Hg2 Thiol-modified MSS 101.0mgg1 60 pgmL1 100mL ultrasoni-

cation for 10min

(this work)

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Sample Analysis

For real sample analysis, the standard calibration curve method was employed.In order to evaluate the validity of the procedure, the method has been applied to thedetermination of Hg2 in the certified reference sample (GBW0809 water sample).The analytical results showed a good agreement between the determined values(1.03 0.04 mgmL1) and the certified values (1.00 0.05 m gmL1).

The method was also applied to the determination of trace Hg2 in differentenvironmental water samples. The analytical results and the recoveries for the spikedsamples were given in Table 3. It could be seen that the recoveries for the spiked sam-ples were between 96.0% and 110.0%.

CONCLUSIONS

A new thiol-modified MSS was synthesized and it was applied to MSPEprocedure as sorbent. The adsorption behavior of Hg2 on the thiol-modifiedMSS was systematically investigated and it showed high selectivity and adsorptioncapacity for Hg2. A simple, rapid, selective, and reliable method was developedfor the determination of trace Hg2 in environmental water samples by couplingMSPE and CV-AAS.

REFERENCES

Ashkenani, H., S. Dadfarnia, A. M. H. Shabani, A. A. Jaffari, and A. Behjat. 2009. Precon-centration, speciation and determination of ultra trace amounts of mercury by modifiedoctadecyl silica membrane disk=electron beam irradiation and cold vapor atomic absorp-tion spectrometry. J. Hazard. Mater. 161: 276280.

Ashtari, P., X. X. He, K. Wang, and P. Gong. 2005. An efficient method for recovery of targetssDNA based on amino-modified silicacoated magnetic nanoparticles. Talanta 67: 548554.

Camel, V. S. 2003. Solid phase extraction of trace elements. Spectrochim. Acta. Part B. 58:11771233.

Chen, D. H., C. Z. Huang, M. He, and B. Hu. 2009. Separation and preconcentration of inor-ganic arsenic species in natural water samples with 3-(2-aminoethylamino) propyltrimethox-ysilane modified ordered mesoporous silica micro-column and their determination by

Table 3. Recovery for determination of Hg2 in environmental water samples. (n 3, ngmL1)

Samples Added (ngmL1) Determined (n gmL1) Recovery (%)

Rain water 0 1.41 0.17 1.0 2.51 0.16 110.02.0 3.46 0.10 102.5

Well water 0 0.34 0.02 1.0 1.40 0.12 106.02.0 2.37 0.05 109.5

Treated water

(sewage treatment plant)

0 1.30 0.17 1.0 2.26 0.14 96.02.0 3.45 0.03 107.5

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qian

g X

iang

] at

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2 Fe

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013

inductively coupled plasma optical emission spectrometry. J. Hazard. Mater. 164:11461151.

Deng, Y. H., C. H. Deng, D. Yang, C. C. Wang, S. K. Fu, and X. M. Zhang. 2005. Prep-aration, characterization and application of magnetic silica nanoparticle functionalizedmulti-walled carbon nanotubes. Chem. Commun. 55485550.

Deng, Y. H., C. C. Wang, J. H. Hu, W. L. Yang, and S. K. Fu. 2005. Investigation offormation of silica-coated magnetite nanoparticles via solgel approach. Colloids. Surf. A.262: 8793.

Ellman, G. L. 1959. Tissue sulfhydryl groups. Arch. Biochem. Biophys. 82: 7077.Gardimalla, H. M. R., D. Mandal, P. D. Stevens, M. Yen, and Y. Gao. 2005. Superparamag-netic nanoparticle-supported enzymatic resolution of racemic carboxylates. Chem. Commun.44324434.

Jang, J. H., and H. B. Lim. 2010. Characterization and analytical application of surfacemodified magnetic nanoparticles. Microchem. J. 94: 148158.

Kang, K., J. Choi, J. H. Nam, and S. C. Lee. 2009. Preparation and characterization ofchemically functionalized silica-coated magnetic nanoparticles as a DNA separator. J. Phys.Chem. B. 113: 536543.

Lemos, V. A., L. S. G. Teixeira, M. D. Bezerra, A. C. S. Costa, J. T. Castro, L. A. M.Cardoso, D. S. Jesus, E. S. Santos, P. X. Baliza, and L. N. Santosi. 2008. New materialsfor solid-phase extraction of trace elements. Appl. Spectrosc. Rev. Reviews. 43: 303334.

Li, J. D., X. L. Zhao, Y. L. Shi, Y. Q. Cai, S. F. Mou, and G. B. Jiang. 2008. Mixedhemimicelles solid-phase extraction based on cetyltrimethylammonium bromide-coatednano-magnets Fe3O4 for the determination of chlorophenols in environmental water sam-ples coupled with liquid chromatography=spectrophotometry detection. J. Chromatogr.A. 1180: 2431.

Liang, H. F., and Z. C. Wang. 2010. Adsorption of bovine serum albumin on functionalizedsilica-coated magnetic MnFe2O4 nanoparticles. Mater. Chem. Phys. 124: 964969.

Ma, M., Y. Zhang, W. Yu, H. Y. Shen, H. Q. Zhang, and N. Gu. 2003. Preparation andcharacterization of magnetic nanoparticles coated by amino silane. Colloid. Surface. A:212: 219226.

Maquieira, A., H. A. M. Elmahadi, and R. Puchades. 1994. Immobilized cyanobacteria foron-line trace metal enrichment by flow injection atomic absorption spectromentry. Anal.Chem. 66: 36323638.

Margetinova, J., P. Houserova-Pelcova, and V. Kuban. 2008. Speciation analysis of mercuryin sediments, zoobenthos and river water samples by high-performance liquid chromato-graphy hyphenated to atomic fluorescence spectrometry following preconcentration by solidphase extraction. Anal. Chim. Acta. 615: 115123.

Pourreza, N., and K. Ghanemi. 2009. Determination of mercury in water and fish samples bycold vapor atomic absorption spectrometry after solid phase extraction on agar modifiedwith 2-mercaptobenzimidazole. J. Hazard. Mater. 161: 982987.

Pourreza, N., H. Parham, A. R. Kiasat, K. Ghanemi, and N. Abdollahi. 2009. Solid phaseextraction of mercury on sulfur loaded with N-(2-chlorobenzoyl)-N-phenylthiourea as anew adsorbent and determination by cold vapor atomic absorption spectrometry. Talanta78: 12931297.

Pu, X. L., Z. C. Jiang, B. Hu, and H. B. Wang. 2004. g-MPTMS modified nanometer-sizedalumina micro-column separation and preconcentration of trace amounts of Hg, Cu, Auand Pd in biological, environmental and geological samples and their determination byinductively coupled plasma mass spectrometry. J. Anal. At. Spectrom. 19: 984989.

Rajesh, N., and G. Gurulakshmanan. 2008. Solid phase extraction and spectrophotometricdetermination of mercury by adsorption of its diphenylthiocarbazone complex on analumina column. Spectrochim. Acta. Part A. 69: 391395.

MAGNETIC SILICA SORBENT FOR EXTRACTION OF Hg 715

Dow

nloa

ded

by [

Guo

qian

g X

iang

] at

08:

16 2

2 Fe

brua

ry 2

013

Rofouei, M. K., A. Sabouri, A. Ahmadalinezhad, and H. Ferdowsi. 2011. Solid phase extrac-tion of ultra traces mercury (II) using octadecyl silica membrane disks modified by 1,3-bis(2-ethoxyphenyl)triazene (EPT) ligand and determination by cold vapor atomic absorptionspectrometry. J. Hazard. Mater. 192: 13581363.

Segade, S. R., and J. F. Tyson. 2007. Determination of methylmercury and inorganic mercuryin water samples by slurry sampling cold vapor atomic absorption spectrometry in a flowinjection system after preconcentration on silica C18 modified. Talanta 71: 16961702.

Shamsipur, M., A. Shokrollahi, H. Sharghi, and M. M. Eskandari. 2005. Solid phaseextraction and determination of sub-ppb levels of hazardous Hg2 ions. J. Hazard. Mater.117: 129133.

Taylora, H., J. D. Appletona, R. Listera, B. Smitha, D. Chitamwebab, O. Mkumbob, J. F.Machiwac, A. L. Teshad, and C. Beinhoffe. 2005. Environmental assessment of mercurycontamination from the Rwamagasa artisanal gold mining centre, Geita District, Tanzania.Sci. Total. Environ. 343: 111133.

Turker, A. R. 2007. New sorbents for solid-phase extraction for metal enrichment. Clean: SoilAir Water 35: 548557.

Tuzen, M., I. Karaman, D. Citak, and M. Soylak. 2009. Mercury(II) and methyl mercurydeterminations in water and fish samples by using solid phase extraction and cold vapouratomic absorption spectrometry combination. Food Chem. Toxicol. 47: 16481652.

Yang, H. H., S. Q. Zhang, X. L. Chen, Z. X. Zhuang, J. G. Xu, and X. R. Wang. 2004.Magnetite-containing spherical silica nanoparticles for biocatalysis and bioseparations.Anal. Chem. 76: 13161321.

Yin, Y. G., M. Chen, J. F. Peng, J. F. Liu, and G. B. Jiang. 2010. Dithizone-functionalizedsolid phase extractiondisplacement elution-high performance liquid chromatography inductively coupled plasma mass spectrometry for mercury speciation in water samples.Talanta 81: 17881792.

Zhai, Y., X. Chang, Y. Cui, N. Lian, S. Lai, H. Zhen, and Q. He. 2006. Selective determi-nation of trace mercury (II) after preconcentration with 4-(2-pyridylazo)-resorcinol modi-fied nanometer-sized SiO2 particles from sample solutions. Microchim. Acta. 154: 253259.

Zhao, X. L., Y. L. Shi, Y. Q. Cai, and S. F. Mou. 2008. Cetyltrimethylammonium bromide-coated magnetic nanoparticles for the preconcentration of phenolic compound fromenvironmental water samples. Environ. Sci. Technol. 42: 12011206.

Zheng, F., and B. Hu. 2010. Dual-column capillary microextraction (CME) combined withelectrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICP-MS) for the speciation of arsenic in human hair extracts. J. Mass. Spectrom. 45: 205214.

Zhu, X., and S. D. Alexandratos. 2007. Determination of trace levels of mercury in aqueoussolutions by inductively coupled plasma atomic emission spectrometry: elimination of thememory effect. Microchem. J. 86: 3741.

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