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Magnetic solid phase extraction coupled with inductively coupled plasma mass spectrometry for the speciation of mercury in environmental water and human hair samples
Author’s Accepted Manuscript Magnetic solid phase extraction coupled with inductively coupled plasma mass spectrometry for the speciation of Mercury in environmental water and Human hair samples Shishuai Ma, Man He, Beibei Chen, Wenchao Deng, Qi Zheng, Bin Hu www.elsevier.com/locate/talanta
PII: DOI: Reference:
S0039-9140(15)30258-7 http://dx.doi.org/10.1016/j.talanta.2015.08.036 TAL15896
To appear in: Talanta Received date: 25 June 2015 Revised date: 12 August 2015 Accepted date: 16 August 2015 Cite this article as: Shishuai Ma, Man He, Beibei Chen, Wenchao Deng, Qi Zheng and Bin Hu, Magnetic solid phase extraction coupled with inductively coupled plasma mass spectrometry for the speciation of Mercury in environmental water and Human hair samples, Talanta, http://dx.doi.org/10.1016/j.talanta.2015.08.036 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Magnetic solid phase extraction coupled with inductively coupled plasma mass spectrometry for the speciation of mercury in environmental water and human hair samples Shishuai Ma1,2, Man He2, Beibei Chen2, Wenchao Deng1, Qi Zheng1*, Bin Hu2* 1
School of Chemical and Environmental Engineering, Jianghan University, Wuhan 430056, China 2
Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), Department of Chemistry, Wuhan University, Wuhan 430072, China
*
Corresponding
author.
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Abstract In
this
work,
γ-mercaptopropyltrimethoxysilane
(γ-MPTS)
modified
Fe3O4@SiO2 magnetic nanoparticles (MNPs) was successfully prepared, and characterized by Fourier transform infrared spectrometer (FT-IR), Transmission electron microscope (TEM) and Vibrating sample magnetometer (VSM). The sorption performance of the prepared Fe3O4@SiO2@γ-MPTS MNPs towards methylmercury (CH3Hg+) and inorganic mercury (Hg2+) was investigated. It was found that CH3Hg+ and Hg2+ could be simultaneously retained on the prepared Fe3O4@SiO2@γ-MPTS MNPs, and the quantitative elution of CH3Hg+ and total mercury (THg) was achieved by using 1.5 mol L-1 HCl containing 0.01% and 3% thiourea (m/v), respectively. And the levels of Hg2+ were obtained by subtracting CH3Hg+ from THg. Based on the above facts, a method of magnetic solid phase extraction (MSPE) combined with inductively coupled plasma mass spectrometry (ICP-MS) was developed for the speciation of CH3Hg+ and Hg2+. Various experimental parameters affecting MSPE of CH3Hg+ and Hg2+ such as pH, eluent, sample volume, and co-existing ions have been 1
studied. Under the optimized conditions, the limits of detection (LODs) for CH3Hg+ and THg were 1.6 and 1.9 ng L-1, respectively. The accuracy of the proposed method was validated by analysis of a Certified Reference Material NRCC DORM-2 dogfish muscle, and the determined values are in good agreement with the certified values. The proposed method has also been successfully applied for the speciation of CH3Hg+ and Hg2+ in environmental water and human hair samples. Keywords: Speciation; Magnetic solid phase extraction; Methylmercury; Inorganic mercury; Inductively coupled plasma mass spectrometry
1. Introduction Mercury is one of the highly toxic elements. Annually, more than 2000 tons of mercury is discharged into the environment by anthropogenic behavior, about two-thirds of which comes from coal, oil and other consumption [1]. In addition, about 2500 tons of mercury emission per year is caused by natural reason, such as volcanic eruption [2]. The majority of the released mercury is in the form of element, which can be oxidized to Hg2+ during the biochemistry transfer, and Hg2+ would be translated into CH3Hg+ after a biological methylation process [3]. Studies have shown that Hg2+ and CH3Hg+ are the main forms in environmental waters. However, the toxicity varies greatly in different mercury species and CH3Hg+ exhibits much greater toxicity than Hg2+ [4]. Moreover, because CH3Hg+ has a good affinity to protein and can easily pass through the cell wall, it is highly bioaccumulative and will reach very high concentrations in seafood such as fish during the food chain cycle. When entering into human bodies, CH3Hg+ can easily be absorbed by the gastrointestinal tract, finally gathered in the central nervous system [5, 6], jeopardizing human health. So it is of significance to monitor the pollution extent of CH3Hg+ in environmental water samples and the extent of human exposure to mercury [7]. Currently, the speciation of mercury is mostly realized by hyphenated techniques involving separation techniques and elemental specific detection techniques. Highly 2
sensitive and element-specific detection techniques such as atomic absorption spectrometry (AAS) [7], atomic fluorescence spectroscopy (AFS) [8], and inductively coupled plasma mass spectrometry (ICP-MS) [9, 10] are generally employed for mercury detection. Among these techniques, ICP-MS is the most widely employed because of its highest analytical sensitivity with a wide linear dynamic range and the capability of isotopic determination. Chromatographic techniques such as gas chromatography (GC) [7, 9-12], high performance liquid chromatography (HPLC) [8, 13-16], ion chromatography (IC) [17, 18] and capillary electrophoresis (CE) [19-22] are the most effective separation techniques for the speciation of mercury due to the merits of high separation resolution. Besides, a number of simple, sensitive and low cost non-chromatographic speciation schemes have been proposed for mercury speciation. These include solid phase extraction (SPE) [4, 23-25], solid phase microextraction (SPME) [26], liquid phase microextraction (LPME) [22, 27-29], cloud point extraction (CPE) [30, 31], and stir bar sorptive extraction (SBSE) [32]. Among them, SPE is one of the most commonly used alternatives, featuring with simple operation, fast separation, and various adsorbents available. In SPE, the choice of adsorbents is the key factor determining the selectivity and accuracy of the method. Among different SPE adsorbents, nanometer-sized materials have been widely used due to the large surface area and resulting high extraction efficiency as well as rapid extraction dynamics [33]. Nevertheless, high column pressure would generally occur in the conventional SPE column operation when nanometer-sized materials are packed into the column, along with the aggregation of the adsorbents. Magnetic solid phase extraction (MSPE) based on magnetic nanoparticles (MNPs) is another special operation mode for SPE. Compared with traditional SPE, the extraction time in MSPE is greatly decreased when dealing with large volume samples because MNPs can be well dispersed in samples by ultrasonication or vortex, and their large surface areas also accelerate the extraction process. Simultaneously, MNPs can be quickly separated from the matrix under an external magnetic field due to the high paramagnetic property [34]. Up to now, a few MSPE methods have been developed for the speciation of 3
mercury in environmental samples. Mehdinia et al. [34] employed polyaniline (PANI) modified Fe3O4 MNPs for rapid MSPE of methylmercury from sea water samples. This method is simple and low cost, while a derivatization process was needed. Najafi et
al.
[35]
prepared
a
magnetic
ion
imprinted
adsorbent
with
N-(pyridin-2-ylmethyl)prop-2-en-1-amine as the functional monomer, which can selectively extract Hg2+ from fish samples, but the elution dynamics was very slow, and 10 min was needed for the quantitative elution of target analyte. Zhang et al. [36] developed a novel and sensitive MSPE method for the preconcentration and determination of Hg2+ by using gold nanoparticles-coated Fe3O4 as adsorbents. The proposed method allowed Hg2+ determination at trace levels in water samples with high accuracy and reproducibility, but the sorption time is up to 15 min. Additionally, most of the established methods can only provide one specific mercury species information. Hence, fast and simple MSPE methods which could simultaneously separate/preconcentrate two or more mercury species are expected urgently. In our previous works, γ-mercaptopropyltrimethoxysilane (γ-MPTS) modified MNPs was prepared and proved to be a very efficient adsorbent for mercury ions [37, 38]. As well known, γ-MPTS is a mercapto group-containing coupling reagent, which has a good affinity towards mercury. It is speculated that γ-MPTS modified MNPs should be also an effective adsorbent for methymercury. Therefore, the aim of this work is to develop a MSPE-ICP-MS method with the use of γ-MPTS modified MNPs as adsorbent for the simultaneous analysis of methylmercury (MeHg+) and inorganic mercury (Hg2+) in environmental water and human hair samples. The experimental parameters affecting the extraction of MeHg+ and Hg2+ such as pH, sample volume, eluent concentration and volume, and interfering ions have been investigated in detail. The developed method was applied for the speciation of mercury in environmental water and human hair samples.
4
2. Experimental 2.1. Apparatus Determination of mercury concentration was carried out by a quadrupole (Q) ICP-MS (Model Agilent 7500a, Hewlett-Packard, Yokogawa Analytical Systems, Tokyo, Japan) with a Babington nebulizer. The optimal operation conditions for ICP-MS are similar to that specified in Ref. [38], and
200
Hg was employed for the
quantification. The pH values were controlled with a Mettler Toledo 320-S pH meter (Mettler Toledo Instruments Co. Ltd., Shanghai, China) supplied with a combined electrode. FT-IR spectra (4000-400 cm-1) in KBr were recorded using a NEXUS 870 FT-IR (Thermo, Madison, USA). The prepared Fe3O4@SiO2@γ-MPTS MNPs were characterized by a JEM-2010 transmission electron microscope (TEM, JEOL, Tokyo, Japan). Magnetic properties of the prepared materials were characterized by a PPMS-9 vibrating sample magnetometer (QUANTOM, USA). A KQ 5200DE model Ultrasonicator (Shumei Instrument Factory, Kunshan, China) was used to disperse the materials in solution. An Nd-Fe-B magnet (15.0 × 6.0 × 1.6 cm) was used for magnetic separation. 2.2. Standard solutions and reagents The stock standard solution of Hg2+ (1000 mg L-1) was prepared from mercury chloride (AR, Shanghai Chemicals Reagent Co. Ltd., Shanghai, China) in diluted nitric acid. The stock solution of MeHg+ (1000 mg L-1 as Hg) was prepared from methylmercury chloride (Alfa Aesar, Ward Hill, MA, USA) in methanol. Working standard solutions were prepared by diluting the stock solution with high purity deionized water. Tetraethoxysilane (TEOS) and γ-MPTS (Wuhan University Chemical Factory, Wuhan, China) were used for the preparation of adsorbent. High purity water (18.2 MΩ·cm) obtained from Milli-Q Element system (Millipore, Molsheim, France) was used throughout this work. All reagents were of analytical grade unless otherwise noted. Plastic and glass containers and all other laboratory 5
materials that could come into contact with samples and standards were stored in 20% (v/v) nitric acid over 24 h and rinsed with high purity water prior to use. 2.3. Synthesis of Fe3O4@SiO2@γ-MPTS The Fe3O4 nanoparticles were prepared by the conventional coprecipitation method [39]. Briefly, FeCl3·6H2O (11.68 g) and FeCl2·4H2O (4.30 g) were dissolved in 200 mL high purity water under nitrogen gas with vigorous stirring at 85 °C. When the solution turned into bright yellow, 30 mL of 30% NH3·H2O was added with further increased nitrogen passing rate and stirring speeds. The color of bulk solution changed from yellow to black immediately. The reaction was stopped after half an hour, and the suspension was cooled down to room temperature naturally. The magnetite precipitates were sequentially washed with high purity water, 0.02 mol L−1 NaCl and ethanol consecutively for several times, then stored in high purity water. Half of the obtained Fe3O4 MNPs were transferred into a beaker, spiked with 160 mL ethanol and 40 mL deionized water, and ultrasonicated for 30 min. The dispersion was then transferred to a three-necked flask, spiked with 5 mL of 30% NH3·H2O under stirring. After reacting for 30 min, 6 mL TEOS was added, then the mixture was stirred for 12 h at room temperature. After that, the dispersion was sequentially washed with high purity water and ethanol consecutively for several times, and then dried in an oven at 60 °C. Fe3O4@SiO2@γ-MPTS nanoparticles were prepared according to the method of Ref. [37] with minor modifications. 1 g of silica-coated magnetite prepared as mentioned above was weighed into a beaker, spiked with 200 mL ethanol. After ultrasonication for 30 min, the dispersion was transferred to a three-necked flask and 5 mL of 30% NH3·H2O was added under stirring. 2 mL of γ-MPTS was added after 30 min reaction, and the mixture was stirred for 12 h at room temperature. When the reaction was stopped, the obtained Fe3O4@SiO2@γ-MPTS was sequentially washed with ethanol and high purity water consecutively for several times, and then dried in an oven at 60 °C for further used. 6
2.4. MSPE procedure Two portions of 50 mL sample solutions containing Hg2+ and CH3Hg+ were placed in two 50 mL beakers, respectively, adjusted to pH 3.0 by 0.1 mol L-1 HNO3 and 0.1 mol L-1 NaOH, spiked with 10 mg Fe3O4@SiO2@γ-MPTS MNPs, and ultrasonicated for 5 min. Then the adsorbent was separated by using a strong flat permanent magnet and the supernatants were decanted directly. 20 mL of high purity water was added for washing and then decanted directly. For the one portion, 1.5 mol L-1 HCl containing 0.01 % (m/v) thiourea was employed for the desorption of CH3Hg+; for the other portion, 1.5 mol L-1 HCl containing 3 % (m/v) thiourea was used for the desorption of total mercury (THg). Both desorptions were processed by ultrasonicating for 5 min. Finally, the magnet was used again to settle the adsorbent, and the two eluents were pipetted into two test tubes, respectively, for subsequent ICP-MS analysis. High purity water was chosen as the blank solution and subjected to the same operation as described above. The determined values of target mercury species were obtained after subtracting the blank value. 2.5. Sample preparation A Certified Reference Material of NRCC DORM-2 dogfish muscle was analyzed to validate the accuracy of the developed method. The sample preparation procedure was referred to that reported by Ortiz et al. [40]. In brief, 0.05 g portion of fish tissue and 2 mL 5 mol L-1 HCl were placed in a 10 mL centrifuge tube and ultrasonicated for 10 min. After extraction, the suspension was centrifuged at 4000 rpm for 10 min and the supernatant was transferred into a 1000 mL flask. The residue was extracted again as above procedure. The two supernatant portions were combined and diluted to the calibrate with high purity water. Hair samples are a good indicator of one’s exposure to mercury besides blood and urine samples. Human hair samples, collected from the student volunteer of Wuhan, China, were finely cut from the nape of the neck near the scalp section. The 7
hairs were placed in a 250 mL beaker, and washed with nonionic surfactant, high purity water, and acetone sequentially, finally the hairs were dried (continuous weighing error is less than 5%) naturally. Before extraction, the hairs were cut to appr. 5-mm lengths with clean scissors, then 0.1 g portion of each sample and 4 mL of 5 mol L-1 HCl were placed in a 10 mL centrifuge tube and ultrasonicated for 30 min as described in Ref. [29]. After extraction, the suspension was centrifuged at 4000 rpm for 10 min and the supernatant was transferred into a 1000 mL flask. The residue was extracted again as above procedure. The two supernatant portions were combined and high purity water was added to the specified volume. Water samples include East Lake water collected from the middle part (Wuhan, China) and Yangtze River water collected from Wuhan, Hubei section. The two water samples were filtered through a 0.45 μm membrane filter (Tianjing Jinteng Instrument Factory, Tianjin, China) and were immediately subjected to the analytical procedure described above. The blank experiments were also prepared by the same procedure as described above without the sample involving. 3. Results and discussion 3.1. Characterization of Fe3O4@SiO2@γ-MPTS FT-IR spectra of Fe3O4@SiO2 and Fe3O4@SiO2@γ-MPTS MNPs are shown in Fig.S1, and the specific absorption peak of S-H was not clearly observed in the spectrum of Fe3O4@SiO2@γ-MPTS due to the low sensitivity of FT-IR to the thiol group. However, compared with the spectrum of Fe3O4@SiO2, an absorption peak around 2929 cm-1 which was attributed to the C-H stretch vibration in methylenes was observed in the spectrum of Fe3O4@SiO2@γ-MPTS, indicating a successful modification of γ-MPTS on Fe3O4@SiO2. The structure and morphology of the prepared Fe3O4@SiO2@γ-MPTS MNPs was characterized by TEM (Fig. S2), and it shows that the particles have a clear core-shell structure with an average diameter of about 30-40 nm. Besides, the 8
magnetic hysteresis loops of Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2@γ-MPTS were measured with a VSM (Fig. S3). The maximal saturation magnetic intensity of Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2@γ-MPTS were 68.6, 40.8 and 36.5 emu/g, respectively. Although the saturation magnetic intensity of Fe3O4@SiO2@γ-MPTS is lower than that of Fe3O4 and Fe3O4@SiO2, it is high enough to achieve rapid magnetic separation for MSPE. 3.2. Optimization of MSPE conditions 3.2.1. Effect of pH Sample pH value is an important factor in MSPE procedure. Therefore, the effect of
sample
pH
on
the
sorption
percentage
of
Hg2+
and
CH3Hg+
on
Fe3O4@SiO2@γ-MPTS MNPs was investigated over the pH range of 2-9, and the results are shown in Fig. 1. As can be seen, both Hg2+ and CH3Hg+ could be quantitatively adsorbed over the entire tested pH range. This is due to the strong affinity interaction between the -SH group and mercury. Considering that the hydrolysis of mercury ions may appear at high pH and the -SH groups could be oxidized in acidic media after a long time, pH 3 was selected for the following experiments. 3.2.2. Effect of eluent In our previous research work [37, 38], Hg adsorbed on γ-MPTMS modified MNPs can be quantitatively eluted by diluted acid containing thiourea. Herein, to separate Hg2+ and CH3Hg+ on the γ-MPTMS modified MNPs, diluted HCl was investigated as the eluent firstly, without the addition of thiourea. The effect of HCl concentration on the recovery of Hg2+ and CH3Hg+ was studied with its concentration varying in the range of 0.1-2.0 mol L-1, respectively. The experimental results showed that CH3Hg+ and Hg2+ cannot be separated only by use of HCl as elution solution. Specifically, CH3Hg+ could not be desorbed quantitatively in the whole investigated concentration range, its recovery was improved from 18 to 78% with increasing HCl 9
concentration from 0.1 to 2.0 mol L-1, while very low desorption efficiency of Hg2+ was found in the whole tested range. The discrepant elution behavior for Hg2+ and CH3Hg+ is mainly caused by the different affinity of -SH groups towards Hg2+ and CH3Hg+. Since only diluted HCl cannot elute target mercury species, a mixed solution of HCl and thiourea was investigated as the eluent for desorption of CH3Hg+ and Hg2+ by keeping the HCl concentration in the mixed solution of HCl and thiourea at 1.5 mol L-1. Figure 2 is the effect of thiourea concentration in the range of 0.01%-3.0% (m/v) on the recovery of Hg2+ and CH3Hg+. As can be seen, CH3Hg+ was completely eluted in the whole investigated thiourea concentration range; while Hg2+ was hardly eluted with 0.01% (m/V) thiourea in 1.5 mol L-1 HCl, the recovery of Hg2+ was increased rapidly with the increase of concentration of thiourea, and a quantitative recovery was obtained with the thiourea concentration higher than 1.0% (m/v) in 1.5 mol L-1 HCl. This means that CH3Hg+ and Hg2+ can be separated by using the different elution solutions. Specifically, CH3Hg+ can be quantitatively eluted with using the mixed solution of 1.5 mol L-1 HCl and 0.01% (m/v) thiourea as the eluent, while Hg2+ is still retained on the Fe3O4@SiO2@γ-MPTS MNPs. Hg2+ can be quantitatively desorbed with the mixed solution of 1.5 mol L-1 HCl and 1% (m/v) thiourea as the eluent. In real-world sample analysis, 1.5 mol L-1 HCl and 0.01% (m/v) thiourea was employed as the eluent to elute CH3Hg+, while 1.5 mol L-1 HCl and 3% (m/v) thiourea was chosen as the eluent for the desorption of total mercury (THg, CH3Hg+ and Hg2+). 3.2.3. Effect of elution volume and elution time The elution volume and the elution time can affect the elution efficiency significantly. To study the effect of elution volume on the desorption, three portions of 0.5 mL 1.5 mol L-1 HCl containing 0.01% (m/v) thiourea and three portions of 0.5 mL 1.5 mol L-1 HCl containing 3% (m/v) thiourea were used as the eluent to sequentially elute CH3Hg+ and Hg2+, respectively. The results in Fig. 3 show that 1.0 mL was 10
sufficient to quantitatively elute both CH3Hg+ and Hg2+, respectively. Moreover, the effect of elution time on the recovery of CH3Hg+ and Hg2+ was studied in the time range of 1-20 min, and the results are shown in Fig. S4. Elution was assisted by ultrasonication herein, and the elution time is defined as the ultrasonication time for desorption. It can be seen that the desorption kinetics was very quick and 2 min was enough for complete desorption of both CH3Hg+ and Hg2+. Finally, the elution time for CH3Hg+ and THg was set as 5 min, respectively. 3.2.4. Effect of sample volume and sorption time To obtain a higher enrichment factor, a lager sample volume is preferred. To investigate the effect of the sample volume on the recoveries of CH3Hg+ and Hg2+, the sample solutions of 5, 10, 25, 50, 100 and 150 mL containing 100 ng of CH3Hg+ or Hg2+ were prepared and subjected to the specified MSPE procedure, respectively. As can be seen from Fig. S5, a quantitative recovery of CH3Hg+ and Hg2+ was obtained in the tested sample volume range. Considering the convenience of operation, the sample volume was chosen as 50 mL. Furthermore, the effect of sorption time was studied in the range of 1-20 min. The sorption was also assisted by ultrasonication herein, and the sorption time is defined as the ultrasonication time for dispersing the MNPs in sample solution and trapping target analytes on MNPs. As shown in Fig. S6, 2 min was enough for quantitative sorption of the target analytes, which indicates the prepared adsorbent has fast sorption kinetics for CH3Hg+ and Hg2+. In the following experiments, the sorption time was chosen as 5 min. 3.3. Effect of the CH3Hg+/Hg2+ ratios In order to explore the application potential of the proposed method, the effect of CH3Hg+/Hg2+ ratios on the recovery of target mercury species was investigated in a range of 1:10-10:1 by fixing the total concentration of two species as 100 ng mL-1, and the results are shown in Fig.4. It can be seen that the recoveries of CH3Hg+ and THg were in the range of 87%-114% with the concentration ratio of CH3Hg+/Hg2+ 11
changing from 10:1 to 1:10, which indicates that the CH3Hg+/Hg2+ ratios hardly affects the recovery in the studied range and the proposed method has a wide application potential. 3.4. Effect of co-existing ions To study the effect of co-existing ions such as K+, Na+, Ca2+, Mg2+, Al3+, Zn2+, Fe3+, Cl-, NO3-, SO42-, H2PO4- and HPO42- on the extraction and determination of CH3Hg+ and Hg2+, 50 mL sample solutions containing 100 ng CH3Hg+ or Hg2+ and a certain amount of interfering ions were subjected to the proposed procedure. The tolerance limit was defined as the largest amount of co-existing ions with which the recovery of the target analytes could be maintained in the range of 85-115%. The results in Table 1 show that 30000 mg L-1 K+, Na+, Cl-, NO3-, 2000 mg L-1 Ca2+, Mg2+, SO42-, H2PO4-, HPO42-, 500 mg L-1 Al3+, Zn2+, or 100 mg L-1 Fe3+ had no significant effect on the extraction and determination of CH3Hg+ and Hg2+. Therefore, the method is potentially suitable for the speciation of mercury in the environmental samples. 3.5. Analytical performance Under the optimal experimental conditions, the analytical performance of the proposed method was evaluated. According to the IUPAC definition, the limits of detection (LODs) are defined as LOD = 3σ / k, where σ represents the standard deviation of blank signal intensity in 11 replicate extractions, and k represents the slope of the standard curve. As can be seen in Table 2, the LODs of this method were 1.6 and 1.9 ng L-1 for CH3Hg+ and THg, respectively. The relative standard deviations (RSDs) for seven replicate determinations of 20 ng L-1 CH3Hg+ and 50 ng L-1 THg were 3.4 and 2.6%, respectively. The linear range of the developed method is 5-10000 ng L-1 (R2 = 0.9999) and 10-10000 ng L-1 (R2 = 0.9996) for CH3Hg+ and THg, respectively. Table 3 is the comparison of the analytical performance data of the present 12
method with some other methods reported in the literatures. Compared with other MSPE based techniques [34-36], the present work has the lowest LODs for the target species, what is more, it can achieve the simultaneous analysis of CH3Hg+ and THg. Compared with the conventional SPE methods [4, 25], MSPE has a rapid analysis speed. Compared with other preconcentraction methods in Refs. [19, 29, 31], this method is less organic reagents consumption, simple operation, and low cost. It should be pointed out that simultaneous quantification of CH3Hg+ and Hg2+ was achieved by GC-ICP-MS with the assistant of isotope dilution and derivatization [41], and the analytical performance can be further improved if an appropriate preconcentration technique is involved. 3.6. Sample analysis The accuracy of the proposed method was verified by analysis of a Certified Reference Material of NRCC DORM-2 dogfish muscle. As can be seen in Table 4, the determined values for CH3Hg+ and THg are in good agreement with the certified values. The proposed method was applied to the determination of CH3Hg+ and THg in environmental water samples and human hair samples. The analytical results and the recoveries in the spiked samples are given in Table 5. As can be seen, the recoveries for the spiked samples were in the range of 75.6-99.6% for CH3Hg+ and 81.3-97.1% for THg, respectively. And the determined results for human hair sample are in accordance to that reported in Ref. [41]. 4. Conclusions In this work, a novel method of MSPE-ICP-MS with Fe3O4@SiO2@γ-MPTS MNPs as adsorbent was proposed for the speciation of mercury in environmental samples. This method is featured with fast sorption/desorption kinetics, good selectivity for the target species, strong resistance to co-existing ions, less reagents consumption, simple operation and low cost. The developed method has good 13
potential for the speciation of mercury in environmental and biological samples. Acknowledgements This work is financially supported by the National Nature Science Foundation of China (Nos. 21175102, 21075095, 21205090) and the Science Fund for Creative Research Groups of NSFC (Nos. 20621502, 20921062).
14
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1314 (2013) 86-93. [18] X. Chen, C. Han, H. Cheng, J. Liu, Z. Xu, X. Yin, Anal. Chim. Acta. 796 (2013) 7-13. [19] Y. Zhao, J. Zheng, L. Fang, Q. Lin, Y. Wu, Z. Xue, F. Fu, Talanta 89 (2012) 280-285. [20] P. Li, X. Zhang, B. Hu, J. Chromatogr. A 1218 (2011) 9414-9421. [21] Z. Fan, X. Liu, J. Chromatogr. A 1180 (2008) 187-192. [22] F. Yang, J. Li, W. Lu, Y. Wen, X. Cai, J. You, J. Ma, Y. Ding, L. Chen, Electrophoresis 35 (2014) 474-481. [23] T. Yordanova, I. Dakova, K. Balashev, I. Karadjova, Microchem. J. 113 (2014) 42-47. [24] F. Xu, L. Kou, J. Jia, X. Hou, Z. Long, S. Wang, Anal. Chim. Acta. 804 (2013) 240-245. [25] S. Buyuktiryaki, R. Say, A. Denizli, A. Ersoz, Talanta 71 (2007) 699-705. [26] S. Mishra, R.M. Tripathi, S. Bhalke, V.K. Shukla, V.D. Puranik, Anal. Chim. Acta. 551 (2005) 192-198. [27] F. Pena-Pereira, I. Lavilla, C. Bendicho, L. Vidal, A. Canals, Talanta 78 (2009) 537-541. [28] Z. Gao, X. Ma, Anal. Chim. Acta. 702 (2011) 50-55. [29] H. Jiang, B. Hu, B. Chen, W. Zu, Spectrochim. Acta. B 63 (2008) 770-776. [30] X.B. Yin, J. Chromatogr. A 1154 (2007) 437-443. [31] H. Chen, J. Chen, X. Jin, D. Wei, J. Hazard. Mater. 172 (2009) 1282-1287. [32] R. Ito, M. Kawaguchi, N. Sakui, H. Honda, N. Okanouchi, K. Saito, H. Nakazawa, J. Chromatogr. A 1209 (2008) 267-270. [33] J. Tian, J. Xu, F. Zhu, T. Lu, C. Su, G. Ouyang, J. Chromatogr. A 1300 (2013) 2-16. [34] A. Mehdinia, F. Roohi, A. Jabbari, J. Chromatogr. A 1218 (2011) 4269-4274. [35] E. Najafi, F. Aboufazeli, H.R. Lotfi Zadeh Zhad, O. Sadeghi, V. Amani, Food. Chem. 141 (2013) 4040-4045. [36] W.B. Zhang, C.X. Sun, X.A. Yang, Anal. Methods. 6 (2014) 2876-2880. 16
[37] C.Z. Huang, B. Hu, Spectrochim. Acta B 63 (2008) 437-444. [38] B.B. Chen, S.J. Heng, H.Y. Peng, B. Hu, X. Yu, Z.L. Zhang, D.W. Pang, X. Yue, Y. Zhu, J. Anal. At. Spectrom. 25 (2010) 1931-1938. [39] X. Liu, Z. Ma, J. Xing, H. Liu, J. Magn. Magn. Mater. 270 (2004) 1-6. [40] A.I.C. Ortiz, Y.M. Albarrán, C.C. Rica, J. Anal. At. Spectrom. 17 (2002) 1595-1601. [41] L. Laffont, L. Maurice, D. Amouroux, P. Navarro, M. Monperrus, J.E. Sonke, P. Behra, Anal. Bioanal. Chem. 405 (2013) 3001-3010. [42] E.K. Mladenova, I.G. Dakova, D.L. Tsalev, I.B. Karadjova, Cent. Eur. J. Chem. 10 (2012) 1175-1182. [43] M.V. Balarama Krishna, K. Chandrasekaran, D. Karunasagar, Talanta 81 (2010) 462-472.
17
Table 1 Tolerance limits of co-existing ions for the extraction and determination of CH3Hg+ and Hg2+ (2 μg L-1) Co-existing ions
Tolerance limits of ions (mg L-1)
K+
30000
Na+
30000
Ca2+
2000
Mg2+
2000
Al3+
500
Zn2+
500
Fe3+
100
Cl-
30000
NO3-
30000
SO42-
2000
H2PO4-
2000
HPO42-
2000
18
Table 2 Analytical performance of the proposed method for analysis of CH3Hg+ and THg
a
R2
Target
Linear equation
analytes
(ng L-1)
CH3Hg+
y=(41.4±0.6)x+(141±25)
THg
y=(78.1±0.5)x+(366±78)
Linear range
LOD
RSD (%,
(ng L-1)
(ng L-1)
n=7)
0.9999
5-10000
1.6
3.4a
0.9996
10-10000
1.9
2.6b
c (CH3Hg+) = 20 ng L-1, bc (THg ) = 50 ng L-1
Table 3 Comparison of the analytical performance for CH3Hg+ and THg LOD (ng L-1)
Methods
Enrichment factor
Ref.
CH3Hg+
THg
CH3Hg+
THg
MSPE-ICP-MS
1.6
1.9
50
50
This work
MSPE-GC-MS
100
-
91
-
[34]
MSPE-ICP-OES
-
30
-
20
[35]
MSPE-CVG-AFS
-
1.5
-
40
[36]
SPE-CVG-AAS
1.7
2.5
20
20
[4]
SPE-ICP-OES
20
50
1737
451
[25]
CE-ICP-MS
21
27
-
-
[19]
HF-LLLME-GFAAS
103
39
204
-
[29]
CPE-HPLC-CVAFS
10
4
21
42
[31]
Isotope dilution -GC-ICP-MS SPE-ICP-MS SPE-CVG-ICP-MS
12 ng g-1
27 ng g-1 for Hg2+ 1 for Hg2+ 25.2 for Hg2+
-
-
[41]
60-120
120-240 for Hg2+
[42] [43]
2.5 32.4
Note: CVG, chemical vapour generation
Table 4 Analytical results (mean ± S.D., n=3) for CH3Hg+ and THg in dogfish muscle (DORM-2, NRCC) Sample
CH3Hg+ (μg g-1)
THg (μg g-1) 19
Dogfish muscle (DORM-2, NRCC) a
Certified
Found
t-testa
Certified
Found
t-test
4.47±0.32
4.37±0.11
1.28
4.64±0.26
4.50±0.09
2.20
t0.05,2=4.43
Table 5 Analytical results (mean ± S.D., n=3) for CH3Hg+ and THg in water and human hair samples Add/ng L-1 Sample
East Lake water
Yangtze River water
Recovery/%
Hg2+/ng L-1
CH3Hg+
THg
CH3Hg+
THg
CH3Hg+
THg
Calculatedb
0
0
N.D.a
20.6±2.2
-
-
20.6±2.2
50
100
48.6±4.5
105±2
97.2
83.9
48.4±4.5
250
500
226±16
451±6
90.3
86.0
225±16
1000
2000
975±70
1883±45
97.5
93.1
908±70
0
0
N.D.
N.D.
-
-
-
50
100
46.8±5.0
85.7±3.6
93.7
85.7
38.9±5.0
250
500
219±17
461±6
87.6
92.1
242±17
1000
2000
756±59
1838±17
75.6
91.9
1082±59
Add/ng L-1 Sample
Found/ng L-1
Found/ng L-1
Recovery/%
Original samples/ng g -1
CH3Hg+
THg
CH3Hg+
THg
CH3Hg+
THg
CH3Hg+
Hg2+
0
0
18.7 ±2.6
37.2±4.2
-
-
187±26
185±42
50
100
68.5±4.9
125±3
99.6
87.9
685±49
565±49
250
500
251±21
444±13
88.5
81.3
2510±210
1930±210
1000
2000
904±23
1978±36
92.7
97.1
9040±230
10740±360
Human hair
a
N.D., not detected.
b
Calculated, the difference between THg and CH3Hg+.
20
Figure captions
Fig.1. Effect of pH on the sorption percentage of Hg2+ and CH3Hg+ (20 μg L-1) on the Fe3O4@SiO2@γ-MPTS. Sample volume: 5 mL, sorption time: 20 min, n=3. Fig.2. Effect of thiourea concentration in 1.5 mol L-1 HCl on the recovery of Hg2+ and CH3Hg+ (20 μg L-1). Conditions: sample volume: 5 mL, sorption time: 20 min, elution time: 20 min, elution volume: 1 mL, n=3. Fig. 3. Effect of the elution volume on the recovery of CH3Hg+ and Hg2+ (20 μg L-1). Conditions: sample volume: 5 mL, sorption time: 20 min, eluent for CH3Hg+: 1.5 mol L-1 HCl + 0.01% thiourea (m/v), eluent for Hg2+: 1.5 mol L-1 HCl + 3% thiourea (m/v), n=3. Fig. 4. Effect of the CH3Hg+/Hg2+ ratio on the recovery of CH3Hg+ and THg. Conditions: sample volume: 50 mL, sorption time: 5 min, elution time: 5 min, eluent for CH3Hg+: 1 mL 1.5 mol L-1 HCl + 0.01% thiourea (m/v), eluent for THg: 1 mL 1.5 mol L-1 HCl + 3% thiourea (m/v), n=3.
21
Hg2+ 110
CH3Hg+
100 90
Adsorption rate (%)
80 70 60 50 40 30 20 10 0 1
2
3
4
5
6
7
8
9
10
pH
Fig.1
22
100 90 80
Recovery (%)
70 60 50
2+
Hg
40
+
CH3Hg
30 20 10 0 0.00
0.05
0.10 0.5
1.0
1.5
2.0
2.5
3.0
Concentration of thiourea(%,m/v)
Fig. 2
23
100 90
CH3Hg
80
Hg
+
2+
Recovery (%)
70 60 50 40 30 20 10 0 1
2
3
Times of elution (0.5 mL for each time)
Fig.3
24
CH3Hg+ THg
120 110 100 90
Recovery (%)
80 70 60 50 40 30 20 10 0 1:10
1:5
1:1
5:1
10:1
+ 2+ CH3Hg /Hg ratios
Fig.4
Highlights CH3Hg+ and Hg2+ exhibited similar adsorption and different desorption behavior on Fe3O4@SiO2@γ-MPTS. 25
A method coupling MSPE with ICP-MS detection was proposed for the speciation of CH3Hg+ and Hg2+ It’s selective for target species with fast dynamics, good anti-interference ability and low reagents consumption.
26
PII: DOI: Reference:
S0039-9140(15)30258-7 http://dx.doi.org/10.1016/j.talanta.2015.08.036 TAL15896
To appear in: Talanta Received date: 25 June 2015 Revised date: 12 August 2015 Accepted date: 16 August 2015 Cite this article as: Shishuai Ma, Man He, Beibei Chen, Wenchao Deng, Qi Zheng and Bin Hu, Magnetic solid phase extraction coupled with inductively coupled plasma mass spectrometry for the speciation of Mercury in environmental water and Human hair samples, Talanta, http://dx.doi.org/10.1016/j.talanta.2015.08.036 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Magnetic solid phase extraction coupled with inductively coupled plasma mass spectrometry for the speciation of mercury in environmental water and human hair samples Shishuai Ma1,2, Man He2, Beibei Chen2, Wenchao Deng1, Qi Zheng1*, Bin Hu2* 1
School of Chemical and Environmental Engineering, Jianghan University, Wuhan 430056, China 2
Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), Department of Chemistry, Wuhan University, Wuhan 430072, China
*
Corresponding
author.
Tel:
0086-27-68752162;
Fax:
0086-27-68754067;
Email:
[email protected]
Abstract In
this
work,
γ-mercaptopropyltrimethoxysilane
(γ-MPTS)
modified
Fe3O4@SiO2 magnetic nanoparticles (MNPs) was successfully prepared, and characterized by Fourier transform infrared spectrometer (FT-IR), Transmission electron microscope (TEM) and Vibrating sample magnetometer (VSM). The sorption performance of the prepared Fe3O4@SiO2@γ-MPTS MNPs towards methylmercury (CH3Hg+) and inorganic mercury (Hg2+) was investigated. It was found that CH3Hg+ and Hg2+ could be simultaneously retained on the prepared Fe3O4@SiO2@γ-MPTS MNPs, and the quantitative elution of CH3Hg+ and total mercury (THg) was achieved by using 1.5 mol L-1 HCl containing 0.01% and 3% thiourea (m/v), respectively. And the levels of Hg2+ were obtained by subtracting CH3Hg+ from THg. Based on the above facts, a method of magnetic solid phase extraction (MSPE) combined with inductively coupled plasma mass spectrometry (ICP-MS) was developed for the speciation of CH3Hg+ and Hg2+. Various experimental parameters affecting MSPE of CH3Hg+ and Hg2+ such as pH, eluent, sample volume, and co-existing ions have been 1
studied. Under the optimized conditions, the limits of detection (LODs) for CH3Hg+ and THg were 1.6 and 1.9 ng L-1, respectively. The accuracy of the proposed method was validated by analysis of a Certified Reference Material NRCC DORM-2 dogfish muscle, and the determined values are in good agreement with the certified values. The proposed method has also been successfully applied for the speciation of CH3Hg+ and Hg2+ in environmental water and human hair samples. Keywords: Speciation; Magnetic solid phase extraction; Methylmercury; Inorganic mercury; Inductively coupled plasma mass spectrometry
1. Introduction Mercury is one of the highly toxic elements. Annually, more than 2000 tons of mercury is discharged into the environment by anthropogenic behavior, about two-thirds of which comes from coal, oil and other consumption [1]. In addition, about 2500 tons of mercury emission per year is caused by natural reason, such as volcanic eruption [2]. The majority of the released mercury is in the form of element, which can be oxidized to Hg2+ during the biochemistry transfer, and Hg2+ would be translated into CH3Hg+ after a biological methylation process [3]. Studies have shown that Hg2+ and CH3Hg+ are the main forms in environmental waters. However, the toxicity varies greatly in different mercury species and CH3Hg+ exhibits much greater toxicity than Hg2+ [4]. Moreover, because CH3Hg+ has a good affinity to protein and can easily pass through the cell wall, it is highly bioaccumulative and will reach very high concentrations in seafood such as fish during the food chain cycle. When entering into human bodies, CH3Hg+ can easily be absorbed by the gastrointestinal tract, finally gathered in the central nervous system [5, 6], jeopardizing human health. So it is of significance to monitor the pollution extent of CH3Hg+ in environmental water samples and the extent of human exposure to mercury [7]. Currently, the speciation of mercury is mostly realized by hyphenated techniques involving separation techniques and elemental specific detection techniques. Highly 2
sensitive and element-specific detection techniques such as atomic absorption spectrometry (AAS) [7], atomic fluorescence spectroscopy (AFS) [8], and inductively coupled plasma mass spectrometry (ICP-MS) [9, 10] are generally employed for mercury detection. Among these techniques, ICP-MS is the most widely employed because of its highest analytical sensitivity with a wide linear dynamic range and the capability of isotopic determination. Chromatographic techniques such as gas chromatography (GC) [7, 9-12], high performance liquid chromatography (HPLC) [8, 13-16], ion chromatography (IC) [17, 18] and capillary electrophoresis (CE) [19-22] are the most effective separation techniques for the speciation of mercury due to the merits of high separation resolution. Besides, a number of simple, sensitive and low cost non-chromatographic speciation schemes have been proposed for mercury speciation. These include solid phase extraction (SPE) [4, 23-25], solid phase microextraction (SPME) [26], liquid phase microextraction (LPME) [22, 27-29], cloud point extraction (CPE) [30, 31], and stir bar sorptive extraction (SBSE) [32]. Among them, SPE is one of the most commonly used alternatives, featuring with simple operation, fast separation, and various adsorbents available. In SPE, the choice of adsorbents is the key factor determining the selectivity and accuracy of the method. Among different SPE adsorbents, nanometer-sized materials have been widely used due to the large surface area and resulting high extraction efficiency as well as rapid extraction dynamics [33]. Nevertheless, high column pressure would generally occur in the conventional SPE column operation when nanometer-sized materials are packed into the column, along with the aggregation of the adsorbents. Magnetic solid phase extraction (MSPE) based on magnetic nanoparticles (MNPs) is another special operation mode for SPE. Compared with traditional SPE, the extraction time in MSPE is greatly decreased when dealing with large volume samples because MNPs can be well dispersed in samples by ultrasonication or vortex, and their large surface areas also accelerate the extraction process. Simultaneously, MNPs can be quickly separated from the matrix under an external magnetic field due to the high paramagnetic property [34]. Up to now, a few MSPE methods have been developed for the speciation of 3
mercury in environmental samples. Mehdinia et al. [34] employed polyaniline (PANI) modified Fe3O4 MNPs for rapid MSPE of methylmercury from sea water samples. This method is simple and low cost, while a derivatization process was needed. Najafi et
al.
[35]
prepared
a
magnetic
ion
imprinted
adsorbent
with
N-(pyridin-2-ylmethyl)prop-2-en-1-amine as the functional monomer, which can selectively extract Hg2+ from fish samples, but the elution dynamics was very slow, and 10 min was needed for the quantitative elution of target analyte. Zhang et al. [36] developed a novel and sensitive MSPE method for the preconcentration and determination of Hg2+ by using gold nanoparticles-coated Fe3O4 as adsorbents. The proposed method allowed Hg2+ determination at trace levels in water samples with high accuracy and reproducibility, but the sorption time is up to 15 min. Additionally, most of the established methods can only provide one specific mercury species information. Hence, fast and simple MSPE methods which could simultaneously separate/preconcentrate two or more mercury species are expected urgently. In our previous works, γ-mercaptopropyltrimethoxysilane (γ-MPTS) modified MNPs was prepared and proved to be a very efficient adsorbent for mercury ions [37, 38]. As well known, γ-MPTS is a mercapto group-containing coupling reagent, which has a good affinity towards mercury. It is speculated that γ-MPTS modified MNPs should be also an effective adsorbent for methymercury. Therefore, the aim of this work is to develop a MSPE-ICP-MS method with the use of γ-MPTS modified MNPs as adsorbent for the simultaneous analysis of methylmercury (MeHg+) and inorganic mercury (Hg2+) in environmental water and human hair samples. The experimental parameters affecting the extraction of MeHg+ and Hg2+ such as pH, sample volume, eluent concentration and volume, and interfering ions have been investigated in detail. The developed method was applied for the speciation of mercury in environmental water and human hair samples.
4
2. Experimental 2.1. Apparatus Determination of mercury concentration was carried out by a quadrupole (Q) ICP-MS (Model Agilent 7500a, Hewlett-Packard, Yokogawa Analytical Systems, Tokyo, Japan) with a Babington nebulizer. The optimal operation conditions for ICP-MS are similar to that specified in Ref. [38], and
200
Hg was employed for the
quantification. The pH values were controlled with a Mettler Toledo 320-S pH meter (Mettler Toledo Instruments Co. Ltd., Shanghai, China) supplied with a combined electrode. FT-IR spectra (4000-400 cm-1) in KBr were recorded using a NEXUS 870 FT-IR (Thermo, Madison, USA). The prepared Fe3O4@SiO2@γ-MPTS MNPs were characterized by a JEM-2010 transmission electron microscope (TEM, JEOL, Tokyo, Japan). Magnetic properties of the prepared materials were characterized by a PPMS-9 vibrating sample magnetometer (QUANTOM, USA). A KQ 5200DE model Ultrasonicator (Shumei Instrument Factory, Kunshan, China) was used to disperse the materials in solution. An Nd-Fe-B magnet (15.0 × 6.0 × 1.6 cm) was used for magnetic separation. 2.2. Standard solutions and reagents The stock standard solution of Hg2+ (1000 mg L-1) was prepared from mercury chloride (AR, Shanghai Chemicals Reagent Co. Ltd., Shanghai, China) in diluted nitric acid. The stock solution of MeHg+ (1000 mg L-1 as Hg) was prepared from methylmercury chloride (Alfa Aesar, Ward Hill, MA, USA) in methanol. Working standard solutions were prepared by diluting the stock solution with high purity deionized water. Tetraethoxysilane (TEOS) and γ-MPTS (Wuhan University Chemical Factory, Wuhan, China) were used for the preparation of adsorbent. High purity water (18.2 MΩ·cm) obtained from Milli-Q Element system (Millipore, Molsheim, France) was used throughout this work. All reagents were of analytical grade unless otherwise noted. Plastic and glass containers and all other laboratory 5
materials that could come into contact with samples and standards were stored in 20% (v/v) nitric acid over 24 h and rinsed with high purity water prior to use. 2.3. Synthesis of Fe3O4@SiO2@γ-MPTS The Fe3O4 nanoparticles were prepared by the conventional coprecipitation method [39]. Briefly, FeCl3·6H2O (11.68 g) and FeCl2·4H2O (4.30 g) were dissolved in 200 mL high purity water under nitrogen gas with vigorous stirring at 85 °C. When the solution turned into bright yellow, 30 mL of 30% NH3·H2O was added with further increased nitrogen passing rate and stirring speeds. The color of bulk solution changed from yellow to black immediately. The reaction was stopped after half an hour, and the suspension was cooled down to room temperature naturally. The magnetite precipitates were sequentially washed with high purity water, 0.02 mol L−1 NaCl and ethanol consecutively for several times, then stored in high purity water. Half of the obtained Fe3O4 MNPs were transferred into a beaker, spiked with 160 mL ethanol and 40 mL deionized water, and ultrasonicated for 30 min. The dispersion was then transferred to a three-necked flask, spiked with 5 mL of 30% NH3·H2O under stirring. After reacting for 30 min, 6 mL TEOS was added, then the mixture was stirred for 12 h at room temperature. After that, the dispersion was sequentially washed with high purity water and ethanol consecutively for several times, and then dried in an oven at 60 °C. Fe3O4@SiO2@γ-MPTS nanoparticles were prepared according to the method of Ref. [37] with minor modifications. 1 g of silica-coated magnetite prepared as mentioned above was weighed into a beaker, spiked with 200 mL ethanol. After ultrasonication for 30 min, the dispersion was transferred to a three-necked flask and 5 mL of 30% NH3·H2O was added under stirring. 2 mL of γ-MPTS was added after 30 min reaction, and the mixture was stirred for 12 h at room temperature. When the reaction was stopped, the obtained Fe3O4@SiO2@γ-MPTS was sequentially washed with ethanol and high purity water consecutively for several times, and then dried in an oven at 60 °C for further used. 6
2.4. MSPE procedure Two portions of 50 mL sample solutions containing Hg2+ and CH3Hg+ were placed in two 50 mL beakers, respectively, adjusted to pH 3.0 by 0.1 mol L-1 HNO3 and 0.1 mol L-1 NaOH, spiked with 10 mg Fe3O4@SiO2@γ-MPTS MNPs, and ultrasonicated for 5 min. Then the adsorbent was separated by using a strong flat permanent magnet and the supernatants were decanted directly. 20 mL of high purity water was added for washing and then decanted directly. For the one portion, 1.5 mol L-1 HCl containing 0.01 % (m/v) thiourea was employed for the desorption of CH3Hg+; for the other portion, 1.5 mol L-1 HCl containing 3 % (m/v) thiourea was used for the desorption of total mercury (THg). Both desorptions were processed by ultrasonicating for 5 min. Finally, the magnet was used again to settle the adsorbent, and the two eluents were pipetted into two test tubes, respectively, for subsequent ICP-MS analysis. High purity water was chosen as the blank solution and subjected to the same operation as described above. The determined values of target mercury species were obtained after subtracting the blank value. 2.5. Sample preparation A Certified Reference Material of NRCC DORM-2 dogfish muscle was analyzed to validate the accuracy of the developed method. The sample preparation procedure was referred to that reported by Ortiz et al. [40]. In brief, 0.05 g portion of fish tissue and 2 mL 5 mol L-1 HCl were placed in a 10 mL centrifuge tube and ultrasonicated for 10 min. After extraction, the suspension was centrifuged at 4000 rpm for 10 min and the supernatant was transferred into a 1000 mL flask. The residue was extracted again as above procedure. The two supernatant portions were combined and diluted to the calibrate with high purity water. Hair samples are a good indicator of one’s exposure to mercury besides blood and urine samples. Human hair samples, collected from the student volunteer of Wuhan, China, were finely cut from the nape of the neck near the scalp section. The 7
hairs were placed in a 250 mL beaker, and washed with nonionic surfactant, high purity water, and acetone sequentially, finally the hairs were dried (continuous weighing error is less than 5%) naturally. Before extraction, the hairs were cut to appr. 5-mm lengths with clean scissors, then 0.1 g portion of each sample and 4 mL of 5 mol L-1 HCl were placed in a 10 mL centrifuge tube and ultrasonicated for 30 min as described in Ref. [29]. After extraction, the suspension was centrifuged at 4000 rpm for 10 min and the supernatant was transferred into a 1000 mL flask. The residue was extracted again as above procedure. The two supernatant portions were combined and high purity water was added to the specified volume. Water samples include East Lake water collected from the middle part (Wuhan, China) and Yangtze River water collected from Wuhan, Hubei section. The two water samples were filtered through a 0.45 μm membrane filter (Tianjing Jinteng Instrument Factory, Tianjin, China) and were immediately subjected to the analytical procedure described above. The blank experiments were also prepared by the same procedure as described above without the sample involving. 3. Results and discussion 3.1. Characterization of Fe3O4@SiO2@γ-MPTS FT-IR spectra of Fe3O4@SiO2 and Fe3O4@SiO2@γ-MPTS MNPs are shown in Fig.S1, and the specific absorption peak of S-H was not clearly observed in the spectrum of Fe3O4@SiO2@γ-MPTS due to the low sensitivity of FT-IR to the thiol group. However, compared with the spectrum of Fe3O4@SiO2, an absorption peak around 2929 cm-1 which was attributed to the C-H stretch vibration in methylenes was observed in the spectrum of Fe3O4@SiO2@γ-MPTS, indicating a successful modification of γ-MPTS on Fe3O4@SiO2. The structure and morphology of the prepared Fe3O4@SiO2@γ-MPTS MNPs was characterized by TEM (Fig. S2), and it shows that the particles have a clear core-shell structure with an average diameter of about 30-40 nm. Besides, the 8
magnetic hysteresis loops of Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2@γ-MPTS were measured with a VSM (Fig. S3). The maximal saturation magnetic intensity of Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2@γ-MPTS were 68.6, 40.8 and 36.5 emu/g, respectively. Although the saturation magnetic intensity of Fe3O4@SiO2@γ-MPTS is lower than that of Fe3O4 and Fe3O4@SiO2, it is high enough to achieve rapid magnetic separation for MSPE. 3.2. Optimization of MSPE conditions 3.2.1. Effect of pH Sample pH value is an important factor in MSPE procedure. Therefore, the effect of
sample
pH
on
the
sorption
percentage
of
Hg2+
and
CH3Hg+
on
Fe3O4@SiO2@γ-MPTS MNPs was investigated over the pH range of 2-9, and the results are shown in Fig. 1. As can be seen, both Hg2+ and CH3Hg+ could be quantitatively adsorbed over the entire tested pH range. This is due to the strong affinity interaction between the -SH group and mercury. Considering that the hydrolysis of mercury ions may appear at high pH and the -SH groups could be oxidized in acidic media after a long time, pH 3 was selected for the following experiments. 3.2.2. Effect of eluent In our previous research work [37, 38], Hg adsorbed on γ-MPTMS modified MNPs can be quantitatively eluted by diluted acid containing thiourea. Herein, to separate Hg2+ and CH3Hg+ on the γ-MPTMS modified MNPs, diluted HCl was investigated as the eluent firstly, without the addition of thiourea. The effect of HCl concentration on the recovery of Hg2+ and CH3Hg+ was studied with its concentration varying in the range of 0.1-2.0 mol L-1, respectively. The experimental results showed that CH3Hg+ and Hg2+ cannot be separated only by use of HCl as elution solution. Specifically, CH3Hg+ could not be desorbed quantitatively in the whole investigated concentration range, its recovery was improved from 18 to 78% with increasing HCl 9
concentration from 0.1 to 2.0 mol L-1, while very low desorption efficiency of Hg2+ was found in the whole tested range. The discrepant elution behavior for Hg2+ and CH3Hg+ is mainly caused by the different affinity of -SH groups towards Hg2+ and CH3Hg+. Since only diluted HCl cannot elute target mercury species, a mixed solution of HCl and thiourea was investigated as the eluent for desorption of CH3Hg+ and Hg2+ by keeping the HCl concentration in the mixed solution of HCl and thiourea at 1.5 mol L-1. Figure 2 is the effect of thiourea concentration in the range of 0.01%-3.0% (m/v) on the recovery of Hg2+ and CH3Hg+. As can be seen, CH3Hg+ was completely eluted in the whole investigated thiourea concentration range; while Hg2+ was hardly eluted with 0.01% (m/V) thiourea in 1.5 mol L-1 HCl, the recovery of Hg2+ was increased rapidly with the increase of concentration of thiourea, and a quantitative recovery was obtained with the thiourea concentration higher than 1.0% (m/v) in 1.5 mol L-1 HCl. This means that CH3Hg+ and Hg2+ can be separated by using the different elution solutions. Specifically, CH3Hg+ can be quantitatively eluted with using the mixed solution of 1.5 mol L-1 HCl and 0.01% (m/v) thiourea as the eluent, while Hg2+ is still retained on the Fe3O4@SiO2@γ-MPTS MNPs. Hg2+ can be quantitatively desorbed with the mixed solution of 1.5 mol L-1 HCl and 1% (m/v) thiourea as the eluent. In real-world sample analysis, 1.5 mol L-1 HCl and 0.01% (m/v) thiourea was employed as the eluent to elute CH3Hg+, while 1.5 mol L-1 HCl and 3% (m/v) thiourea was chosen as the eluent for the desorption of total mercury (THg, CH3Hg+ and Hg2+). 3.2.3. Effect of elution volume and elution time The elution volume and the elution time can affect the elution efficiency significantly. To study the effect of elution volume on the desorption, three portions of 0.5 mL 1.5 mol L-1 HCl containing 0.01% (m/v) thiourea and three portions of 0.5 mL 1.5 mol L-1 HCl containing 3% (m/v) thiourea were used as the eluent to sequentially elute CH3Hg+ and Hg2+, respectively. The results in Fig. 3 show that 1.0 mL was 10
sufficient to quantitatively elute both CH3Hg+ and Hg2+, respectively. Moreover, the effect of elution time on the recovery of CH3Hg+ and Hg2+ was studied in the time range of 1-20 min, and the results are shown in Fig. S4. Elution was assisted by ultrasonication herein, and the elution time is defined as the ultrasonication time for desorption. It can be seen that the desorption kinetics was very quick and 2 min was enough for complete desorption of both CH3Hg+ and Hg2+. Finally, the elution time for CH3Hg+ and THg was set as 5 min, respectively. 3.2.4. Effect of sample volume and sorption time To obtain a higher enrichment factor, a lager sample volume is preferred. To investigate the effect of the sample volume on the recoveries of CH3Hg+ and Hg2+, the sample solutions of 5, 10, 25, 50, 100 and 150 mL containing 100 ng of CH3Hg+ or Hg2+ were prepared and subjected to the specified MSPE procedure, respectively. As can be seen from Fig. S5, a quantitative recovery of CH3Hg+ and Hg2+ was obtained in the tested sample volume range. Considering the convenience of operation, the sample volume was chosen as 50 mL. Furthermore, the effect of sorption time was studied in the range of 1-20 min. The sorption was also assisted by ultrasonication herein, and the sorption time is defined as the ultrasonication time for dispersing the MNPs in sample solution and trapping target analytes on MNPs. As shown in Fig. S6, 2 min was enough for quantitative sorption of the target analytes, which indicates the prepared adsorbent has fast sorption kinetics for CH3Hg+ and Hg2+. In the following experiments, the sorption time was chosen as 5 min. 3.3. Effect of the CH3Hg+/Hg2+ ratios In order to explore the application potential of the proposed method, the effect of CH3Hg+/Hg2+ ratios on the recovery of target mercury species was investigated in a range of 1:10-10:1 by fixing the total concentration of two species as 100 ng mL-1, and the results are shown in Fig.4. It can be seen that the recoveries of CH3Hg+ and THg were in the range of 87%-114% with the concentration ratio of CH3Hg+/Hg2+ 11
changing from 10:1 to 1:10, which indicates that the CH3Hg+/Hg2+ ratios hardly affects the recovery in the studied range and the proposed method has a wide application potential. 3.4. Effect of co-existing ions To study the effect of co-existing ions such as K+, Na+, Ca2+, Mg2+, Al3+, Zn2+, Fe3+, Cl-, NO3-, SO42-, H2PO4- and HPO42- on the extraction and determination of CH3Hg+ and Hg2+, 50 mL sample solutions containing 100 ng CH3Hg+ or Hg2+ and a certain amount of interfering ions were subjected to the proposed procedure. The tolerance limit was defined as the largest amount of co-existing ions with which the recovery of the target analytes could be maintained in the range of 85-115%. The results in Table 1 show that 30000 mg L-1 K+, Na+, Cl-, NO3-, 2000 mg L-1 Ca2+, Mg2+, SO42-, H2PO4-, HPO42-, 500 mg L-1 Al3+, Zn2+, or 100 mg L-1 Fe3+ had no significant effect on the extraction and determination of CH3Hg+ and Hg2+. Therefore, the method is potentially suitable for the speciation of mercury in the environmental samples. 3.5. Analytical performance Under the optimal experimental conditions, the analytical performance of the proposed method was evaluated. According to the IUPAC definition, the limits of detection (LODs) are defined as LOD = 3σ / k, where σ represents the standard deviation of blank signal intensity in 11 replicate extractions, and k represents the slope of the standard curve. As can be seen in Table 2, the LODs of this method were 1.6 and 1.9 ng L-1 for CH3Hg+ and THg, respectively. The relative standard deviations (RSDs) for seven replicate determinations of 20 ng L-1 CH3Hg+ and 50 ng L-1 THg were 3.4 and 2.6%, respectively. The linear range of the developed method is 5-10000 ng L-1 (R2 = 0.9999) and 10-10000 ng L-1 (R2 = 0.9996) for CH3Hg+ and THg, respectively. Table 3 is the comparison of the analytical performance data of the present 12
method with some other methods reported in the literatures. Compared with other MSPE based techniques [34-36], the present work has the lowest LODs for the target species, what is more, it can achieve the simultaneous analysis of CH3Hg+ and THg. Compared with the conventional SPE methods [4, 25], MSPE has a rapid analysis speed. Compared with other preconcentraction methods in Refs. [19, 29, 31], this method is less organic reagents consumption, simple operation, and low cost. It should be pointed out that simultaneous quantification of CH3Hg+ and Hg2+ was achieved by GC-ICP-MS with the assistant of isotope dilution and derivatization [41], and the analytical performance can be further improved if an appropriate preconcentration technique is involved. 3.6. Sample analysis The accuracy of the proposed method was verified by analysis of a Certified Reference Material of NRCC DORM-2 dogfish muscle. As can be seen in Table 4, the determined values for CH3Hg+ and THg are in good agreement with the certified values. The proposed method was applied to the determination of CH3Hg+ and THg in environmental water samples and human hair samples. The analytical results and the recoveries in the spiked samples are given in Table 5. As can be seen, the recoveries for the spiked samples were in the range of 75.6-99.6% for CH3Hg+ and 81.3-97.1% for THg, respectively. And the determined results for human hair sample are in accordance to that reported in Ref. [41]. 4. Conclusions In this work, a novel method of MSPE-ICP-MS with Fe3O4@SiO2@γ-MPTS MNPs as adsorbent was proposed for the speciation of mercury in environmental samples. This method is featured with fast sorption/desorption kinetics, good selectivity for the target species, strong resistance to co-existing ions, less reagents consumption, simple operation and low cost. The developed method has good 13
potential for the speciation of mercury in environmental and biological samples. Acknowledgements This work is financially supported by the National Nature Science Foundation of China (Nos. 21175102, 21075095, 21205090) and the Science Fund for Creative Research Groups of NSFC (Nos. 20621502, 20921062).
14
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1314 (2013) 86-93. [18] X. Chen, C. Han, H. Cheng, J. Liu, Z. Xu, X. Yin, Anal. Chim. Acta. 796 (2013) 7-13. [19] Y. Zhao, J. Zheng, L. Fang, Q. Lin, Y. Wu, Z. Xue, F. Fu, Talanta 89 (2012) 280-285. [20] P. Li, X. Zhang, B. Hu, J. Chromatogr. A 1218 (2011) 9414-9421. [21] Z. Fan, X. Liu, J. Chromatogr. A 1180 (2008) 187-192. [22] F. Yang, J. Li, W. Lu, Y. Wen, X. Cai, J. You, J. Ma, Y. Ding, L. Chen, Electrophoresis 35 (2014) 474-481. [23] T. Yordanova, I. Dakova, K. Balashev, I. Karadjova, Microchem. J. 113 (2014) 42-47. [24] F. Xu, L. Kou, J. Jia, X. Hou, Z. Long, S. Wang, Anal. Chim. Acta. 804 (2013) 240-245. [25] S. Buyuktiryaki, R. Say, A. Denizli, A. Ersoz, Talanta 71 (2007) 699-705. [26] S. Mishra, R.M. Tripathi, S. Bhalke, V.K. Shukla, V.D. Puranik, Anal. Chim. Acta. 551 (2005) 192-198. [27] F. Pena-Pereira, I. Lavilla, C. Bendicho, L. Vidal, A. Canals, Talanta 78 (2009) 537-541. [28] Z. Gao, X. Ma, Anal. Chim. Acta. 702 (2011) 50-55. [29] H. Jiang, B. Hu, B. Chen, W. Zu, Spectrochim. Acta. B 63 (2008) 770-776. [30] X.B. Yin, J. Chromatogr. A 1154 (2007) 437-443. [31] H. Chen, J. Chen, X. Jin, D. Wei, J. Hazard. Mater. 172 (2009) 1282-1287. [32] R. Ito, M. Kawaguchi, N. Sakui, H. Honda, N. Okanouchi, K. Saito, H. Nakazawa, J. Chromatogr. A 1209 (2008) 267-270. [33] J. Tian, J. Xu, F. Zhu, T. Lu, C. Su, G. Ouyang, J. Chromatogr. A 1300 (2013) 2-16. [34] A. Mehdinia, F. Roohi, A. Jabbari, J. Chromatogr. A 1218 (2011) 4269-4274. [35] E. Najafi, F. Aboufazeli, H.R. Lotfi Zadeh Zhad, O. Sadeghi, V. Amani, Food. Chem. 141 (2013) 4040-4045. [36] W.B. Zhang, C.X. Sun, X.A. Yang, Anal. Methods. 6 (2014) 2876-2880. 16
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17
Table 1 Tolerance limits of co-existing ions for the extraction and determination of CH3Hg+ and Hg2+ (2 μg L-1) Co-existing ions
Tolerance limits of ions (mg L-1)
K+
30000
Na+
30000
Ca2+
2000
Mg2+
2000
Al3+
500
Zn2+
500
Fe3+
100
Cl-
30000
NO3-
30000
SO42-
2000
H2PO4-
2000
HPO42-
2000
18
Table 2 Analytical performance of the proposed method for analysis of CH3Hg+ and THg
a
R2
Target
Linear equation
analytes
(ng L-1)
CH3Hg+
y=(41.4±0.6)x+(141±25)
THg
y=(78.1±0.5)x+(366±78)
Linear range
LOD
RSD (%,
(ng L-1)
(ng L-1)
n=7)
0.9999
5-10000
1.6
3.4a
0.9996
10-10000
1.9
2.6b
c (CH3Hg+) = 20 ng L-1, bc (THg ) = 50 ng L-1
Table 3 Comparison of the analytical performance for CH3Hg+ and THg LOD (ng L-1)
Methods
Enrichment factor
Ref.
CH3Hg+
THg
CH3Hg+
THg
MSPE-ICP-MS
1.6
1.9
50
50
This work
MSPE-GC-MS
100
-
91
-
[34]
MSPE-ICP-OES
-
30
-
20
[35]
MSPE-CVG-AFS
-
1.5
-
40
[36]
SPE-CVG-AAS
1.7
2.5
20
20
[4]
SPE-ICP-OES
20
50
1737
451
[25]
CE-ICP-MS
21
27
-
-
[19]
HF-LLLME-GFAAS
103
39
204
-
[29]
CPE-HPLC-CVAFS
10
4
21
42
[31]
Isotope dilution -GC-ICP-MS SPE-ICP-MS SPE-CVG-ICP-MS
12 ng g-1
27 ng g-1 for Hg2+ 1 for Hg2+ 25.2 for Hg2+
-
-
[41]
60-120
120-240 for Hg2+
[42] [43]
2.5 32.4
Note: CVG, chemical vapour generation
Table 4 Analytical results (mean ± S.D., n=3) for CH3Hg+ and THg in dogfish muscle (DORM-2, NRCC) Sample
CH3Hg+ (μg g-1)
THg (μg g-1) 19
Dogfish muscle (DORM-2, NRCC) a
Certified
Found
t-testa
Certified
Found
t-test
4.47±0.32
4.37±0.11
1.28
4.64±0.26
4.50±0.09
2.20
t0.05,2=4.43
Table 5 Analytical results (mean ± S.D., n=3) for CH3Hg+ and THg in water and human hair samples Add/ng L-1 Sample
East Lake water
Yangtze River water
Recovery/%
Hg2+/ng L-1
CH3Hg+
THg
CH3Hg+
THg
CH3Hg+
THg
Calculatedb
0
0
N.D.a
20.6±2.2
-
-
20.6±2.2
50
100
48.6±4.5
105±2
97.2
83.9
48.4±4.5
250
500
226±16
451±6
90.3
86.0
225±16
1000
2000
975±70
1883±45
97.5
93.1
908±70
0
0
N.D.
N.D.
-
-
-
50
100
46.8±5.0
85.7±3.6
93.7
85.7
38.9±5.0
250
500
219±17
461±6
87.6
92.1
242±17
1000
2000
756±59
1838±17
75.6
91.9
1082±59
Add/ng L-1 Sample
Found/ng L-1
Found/ng L-1
Recovery/%
Original samples/ng g -1
CH3Hg+
THg
CH3Hg+
THg
CH3Hg+
THg
CH3Hg+
Hg2+
0
0
18.7 ±2.6
37.2±4.2
-
-
187±26
185±42
50
100
68.5±4.9
125±3
99.6
87.9
685±49
565±49
250
500
251±21
444±13
88.5
81.3
2510±210
1930±210
1000
2000
904±23
1978±36
92.7
97.1
9040±230
10740±360
Human hair
a
N.D., not detected.
b
Calculated, the difference between THg and CH3Hg+.
20
Figure captions
Fig.1. Effect of pH on the sorption percentage of Hg2+ and CH3Hg+ (20 μg L-1) on the Fe3O4@SiO2@γ-MPTS. Sample volume: 5 mL, sorption time: 20 min, n=3. Fig.2. Effect of thiourea concentration in 1.5 mol L-1 HCl on the recovery of Hg2+ and CH3Hg+ (20 μg L-1). Conditions: sample volume: 5 mL, sorption time: 20 min, elution time: 20 min, elution volume: 1 mL, n=3. Fig. 3. Effect of the elution volume on the recovery of CH3Hg+ and Hg2+ (20 μg L-1). Conditions: sample volume: 5 mL, sorption time: 20 min, eluent for CH3Hg+: 1.5 mol L-1 HCl + 0.01% thiourea (m/v), eluent for Hg2+: 1.5 mol L-1 HCl + 3% thiourea (m/v), n=3. Fig. 4. Effect of the CH3Hg+/Hg2+ ratio on the recovery of CH3Hg+ and THg. Conditions: sample volume: 50 mL, sorption time: 5 min, elution time: 5 min, eluent for CH3Hg+: 1 mL 1.5 mol L-1 HCl + 0.01% thiourea (m/v), eluent for THg: 1 mL 1.5 mol L-1 HCl + 3% thiourea (m/v), n=3.
21
Hg2+ 110
CH3Hg+
100 90
Adsorption rate (%)
80 70 60 50 40 30 20 10 0 1
2
3
4
5
6
7
8
9
10
pH
Fig.1
22
100 90 80
Recovery (%)
70 60 50
2+
Hg
40
+
CH3Hg
30 20 10 0 0.00
0.05
0.10 0.5
1.0
1.5
2.0
2.5
3.0
Concentration of thiourea(%,m/v)
Fig. 2
23
100 90
CH3Hg
80
Hg
+
2+
Recovery (%)
70 60 50 40 30 20 10 0 1
2
3
Times of elution (0.5 mL for each time)
Fig.3
24
CH3Hg+ THg
120 110 100 90
Recovery (%)
80 70 60 50 40 30 20 10 0 1:10
1:5
1:1
5:1
10:1
+ 2+ CH3Hg /Hg ratios
Fig.4
Highlights CH3Hg+ and Hg2+ exhibited similar adsorption and different desorption behavior on Fe3O4@SiO2@γ-MPTS. 25
A method coupling MSPE with ICP-MS detection was proposed for the speciation of CH3Hg+ and Hg2+ It’s selective for target species with fast dynamics, good anti-interference ability and low reagents consumption.
26