Simultaneous speciation of inorganic selenium and antimony in water samples by electrothermal vaporization inductively coupled plasma mass spectrometry following selective cloud point extraction

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42 (2008) 1195 – 1203

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Simultaneous speciation of inorganic selenium and antimony in water samples by electrothermal vaporization inductively coupled plasma mass spectrometry following selective cloud point extraction Yingjie Li, Bin Hu, Man He, Guoqiang Xiang Department of Chemistry, Wuhan University, Wuhan 430072, China

art i cle info

ab st rac t

Article history:

A new method was developed for the simultaneous speciation of inorganic selenium and

Received 12 December 2006

antimony in water by electrothermal vaporization inductively coupled plasma mass

Received in revised form

spectrometry (ETV-ICP-MS) following selective cloud point extraction (CPE). The method is

31 August 2007

based on the fact that Se(IV) and Sb(III) could form complexes with diethyldithiocarbamate

Accepted 5 September 2007

(DDTC) at pH 6.00, and the complexes were quantitatively extracted into the non-ionic

Available online 11 September 2007

surfactant-rich phase of octylphenoxypolyethoxyethanol (Triton X-114), whereas the Se(VI)

Keywords: Simultaneous speciation Selenium and antimony ETV-ICP-MS Cloud point extraction Water

and Sb(V) remained as free species in aqueous solutions. Sb(III) and Se(IV) in concentrate were determined by ETV-ICP-MS after proper disposal. The total Se and total Sb were determined by the same protocol after Se(VI) and Sb(V) were reduced by L-cysteine, and Se(VI) and Sb(V) concentrations were obtained by respectively subtracting Se(IV) and Sb(III) from the total Se and the total Sb. Under the optimized conditions, the limits of detection (LODs) were 0.05 mg L1 for Se(IV) and 0.03 mg L1 for Sb(III), the relative standard deviations (RSDs) were 3.5% for Se(IV) and 4.2% for Sb(III) (C ¼ 1.00 mg L1, n ¼ 5). The proposed method was applied to the speciation of inorganic selenium and antimony in different water samples with satisfactory results. & 2007 Elsevier Ltd. All rights reserved.

1.

Introduction

The different chemical forms of trace elements can change their biologic effects. Information about the oxidation state of trace elements is as important to know as its chemical structure (Smichowski et al., 1998). Elemental toxicity depends more strongly on the chemical forms of elements rather than on the total element content. Furthermore, there is often only a modest difference between essential and toxic levels of trace elements. For example, selenium is well-known as an essential and a toxic element to most mammalian species, including humans, depending on its chemical form

and concentration, because of its dual role as both an essential nutrient at low concentrations and a toxic substance at higher levels. The concentration range between Se as an essential element and a toxic one is moreover rather narrow, only in the range of concentrations of selenium 50.0–200 mg d1 is beneficial to human health (Ferri et al., 2001; Li and Li, 1995). In the majority of environmental matrices, such as natural water and fly ash, selenium is usually present as selenite, Se(IV) and selenate, Se(VI), because these oxidation states are the most environmentally mobile and geochemically important forms of this element, and inorganic Se(VI) is much more toxic than Se(IV). Different from

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E-mail addresses: [email protected] (Y. Li), [email protected] (B. Hu), [email protected] (M. He), [email protected] (G. Xiang). 0043-1354/$ - see front matter & 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2007.09.002

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selenium, antimony is non-essential for life and is a cumulative toxic element, and it has chemical and toxicological properties similar to those of arsenic (Gebel, 1997). Antimony is especially originated from industrial treatment facilities and was accepted as a significant contaminant by the United Nations Protecting of the Environment Organization (Koch et al., 1997), and the European Union has established a maximum admissible concentration of a total of 5.00 mg L1 antimony in drinking water (Miravet et al., 2004). Inorganic compounds of antimony are more toxic than its organic forms. Toxicity of Sb(III) has been shown to be 10 times higher than that of Sb(V), and Sb(III) oxide has been shown to cause lung cancer (Poon et al., 1998). Thus, in order to obtain correct information on toxicity and biotransformation of selenium and antimony, it is necessary not only to determine the total selenium and antimony in the different environmental and biological samples but also to determine selenium and antimony in its different oxidation states. The speciation of inorganic selenium and antimony has been generally achieved by combing a very effective separation method with a sensitive detection technique. Many separation/preconcentration methods such as coprecipitation (Sun and Yang, 1999), liquid–liquid extraction (Garbos et al., 2000), solid-phase extraction (Yu et al., 2002; Bueno and PotinGautier, 2002), high-performance liquid chromatography (HPLC) (Sayago et al., 2002; Go´mez-Ariza et al., 2004; Vin˜as et al., 2006) and capillary electrophoresis (CE) (Lu and Yan, 2005; Michalke and Schramel, 1999) have been used for the speciation of selenium and antimony. However, these techniques are often complicated and time-consuming or have high operation costs. Thus, a new, simple separation technique for the speciation of inorganic selenium and antimony is essential. In the last decades, increasing attention has been paid to the development of surfactant-based methods in all fields of analytical chemistry (Stalikas, 2002; Zygoura et al., 2005), and the most widely explored surfactant-based preconcentration process for analytical purposes is the cloud point extraction (CPE) (Stalikas, 2002; Zygoura et al., 2005; Paleologos et al., 2003). This technique is based on the turbid phenomenon of non-ionic surfactant when heated to a temperature known as the cloud point. Above this temperature, the isotropic micellar solution separates into two transparent liquid phases: one of them, with the smallest volume (surfactant-rich phase), contains most of the surfactant whereas the other (referred to as the aqueous phase) contains a surfactant concentration close to critical micellar concentration. The small volume of the surfactant-rich phase obtained with this methodology permits the design of extraction schemes that are simple, cheap, highly efficient, speedy and of lower toxicity to the environment than those conventional extractions that use organic solvents. From an analytical point of view, any hydrophobic species originally present in water is able to react with and bind to micellars and become concentrated in a small volume of surfactantrich phase (Saitoh et al., 1995). The use of CPE for preconcentration of metals was pioneered by Watanabe and co-workers, who studied the extraction of Ni with 1-(2-thiazdylazo)-2napthol in Triton X-100 micellar solution (Miuar et al., 1976). Until now, CPE has been used for the extraction and preconcentration of many metal ions (Shemirani et al.,

2005a–c; Luconi et al., 2006), Cr(III)/Cr(VI) (Paleologos et al., 2000; Tang et al., 2004; Li et al., 2006), Fe(II)/Fe(III) (Giokas et al., 2002), As(III)/As(V) (Shemirani et al., 2005a–c), etc. Several techniques, e.g., graphite furnace atomic absorption spectrometry (Ojeda et al., 2005), inductively coupled plasma mass spectrometry (ICP-MS) (Yu et al., 2002; Xia et al., 2006), inductively coupled plasma optical emission spectrometry (ICP-OES) (Feng et al., 1999), hydride generation atomic absorption spectrometry (HG-AAS) (Niedzielski, 2005), etc., have been proposed to determine antimony and selenium species. Of all these detection methods, ICP-MS has been considered to be one of the most efficient and robust element-specific techniques due to its high sensitivity, large dynamic linear range, multi-element capability and the possibility to perform isotopic measurements. Given its excellent detection ability, ICP-MS can be used directly to determine selenium and antimony at mg L1 levels without preconcentration. However, ICP-MS has mainly been used for single elemental speciation (Yu et al., 2002; Xia et al., 2006) until recently. In recent years, the simultaneous speciation analysis of different elements has received extensive attention, and some studies on simultaneous multielemental speciation of As, Se, Sb, Te by on-line coupling of anion exchange HPLC (Lindemann et al., 1999; Guerin et al., 1997) or CE (Prange and Schaumlo¨ffel, 1999) with ICP-MS have been performed. Electrothermal vaporization (ETV), as one of the sample introduction techniques currently employed in ICP-OES/MS, has been demonstrated to have the distinct merits of low sample consuming, high transport efficiency and low absolute detection limit. It can be assumed that the CPE in connection with ETV-ICP-MS would be a powerful analytical technique for simultaneous speciation analysis of multielement. To the best of our knowledge, the use of CPE as a preconcentration step for ETV-ICP-MS simultaneous speciation of inorganic selenium and antimony has not been reported before. The aim of the present work was to develop a new method—simultaneous speciation of inorganic selenium and antimony in water by ETV-ICP-MS following selective CPE. The basis of the developed method is the use of Triton X-114 as the extractant for the selective extraction of diethyldithiocarbamate (DDTC) complexes Sb(III) and Se(IV), while Sb(V) and Se(VI) did not form DDTC complexes and were not extracted by Triton X-114 as free species in the solution. Thus, an in situ separation of Se(IV) and Se(VI), Sb(III) and Sb(V) could be realized. The main factors affecting CPE and the vaporization behavior of analytes in graphite tube were investigated in detail. The proposed method was applied to the speciation of inorganic antimony and selenium in two certified reference materials and in different water samples with satisfactory results.

2.

Experimental

2.1.

Standard solution and reagents

All chemicals and reagents used in this study were of analytical-reagent grade or higher purity. A total of

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1000 mg L1 stock standard solutions of Sb(III) was prepared by dissolving appropriate amounts of potassium antimonyl tartrate (Tianjin Reagent Factory, Tianjin, China) in highpurity deionized water and diluting to 100 mL. A total of 1000 mg L1 stock standard solution of Sb(V) was prepared by dissolving appropriate amounts of potassium pyroantimonate (Shanghai Reagent Factory, Shanghai, China ) in boiling water and diluting to 100 mL. Se(IV) and Se(VI) stock solutions (1000 mg L1) were prepared from analytical reagent-grade sodium selenite (Wako Pure Chemical Industries, Ltd, Osaka, Japan) and sodium selenate (Wako Pure Chemical Industries, Ltd, Osaka, Japan) by dissolving their appropriate amounts in high-purity deionized water, respectively. Solution (2.00%, w/v) of DDTC (Shanghai SSS Reagent Co., Ltd, Shanghai, China) was prepared in high-purity deionized water. Solution (2.00%, w/v) of L-cysteine (Shanghai Boao Biological Technological Limited Company, Shanghai, China) was prepared in high-purity deionized water. Solution (1.00%, w/v) of the nonionic surfactant Triton X-114 (Acros Organics, NJ, USA) was prepared in high-purity deionized water and was used without further purification. All stock standard solutions were stored in polyethylene bottles in a refrigerator with a temperature of 4 1C. All glassware was kept in 10% nitric acid for at least 24 h and washed three times with high-purity deionized water before use. The standard reference materials of GBW(E)080545 were supplied by National Research Center for CRMs (Beijing, China), and GSB07-1253-2000 was supplied by Institute for Reference Materials of SEPA (Beijing, China).

2.2.

Procedures

2.2.1.

Sample pretreatment

Lake water sample (pH 6.20, East Lake, Wuhan, China), river water (pH 7.99, Yangtse River, Wuhan, China) and pool water (pH 7.34, Wuhan University, Wuhan, China) were collected in a 50 mL polyethylene container. Local tap water (pH 6.15) was collected after 5 min of free outflow from the tap. All water samples were filtered through a 0.45-mm membrane filter (Tianjin Jinteng Instrument Factory, Tianjin, China), and then the samples were stored at 4 1C in polyethylene bottles when the analysis of water samples was not carried out. The storage period was kept as short as possible.

2.2.2.

CPE procedure

For CPE preconcentration, aliquots of 10.0 mL of the solution (buffered to pH 6.00 with 0.01 mol L–1 HCl) containing the analytes (Se(IV) and Sb(III)), 0.40% (w/v) DDTC and 0.10% (w/v) Triton X-114 were heated in a thermostatted water bath at 30 1C for 5 min. The mixture was centrifuged at 4000 rpm for 5 min for phase separation, and then cooled in an ice-bath for 10 min to increase the viscosity of the surfactant-rich phase. The supernatant aqueous phase was carefully removed with a pipette. To decrease the viscosity of the extract and to allow its pipetting, 100 mL of ethanol was added to the surfactantrich phase and 10 mL sample solution was injected into the graphite furnace for analysis.

2.2.3.

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ICP-MS procedure

An Agilent 7500a ICP-MS (Agilent, Tokyo, Japan), equipped with a modified commercially available WF-4C graphite

furnace (Beijing Second Optics, China) as the electrothermal vaporizer, was used. The transfer line consisted of a laboratory-built connecting interface and a polyethylene tube (6 mm id) with a total length of 70 cm. The pH values were controlled with a Mettler Toledo 320-S pH meter (Mettler Toledo Instruments Co. Ltd., Shanghai, China)and the phase separation was assisted with a centrifuge (Shanghai Scientific Instruments Co., Ltd, Shanghai, China). The optimized operating conditions for ICP-MS and ETV are given in Table 1. After the ETV unit was connected to the ICP-MS and the system was stabilized, 10 mL of the analytes was injected into the graphite furnace. During the drying step of the temperature program, the dosing hole of the graphite furnace was kept open to remove water and other vapors. Then it was sealed with a graphite probe during the high-temperature vaporization step, the vaporized analytes were swept into the plasma by a carrier gas of argon and the peak-hop transient mode for data acquisition was used to detect the ions selected.

2.3.

Data analysis

(1) Se(IV) and Sb(III): After the extracted analytes were treated with ethanol, it was introduced into ETV-ICP-MS for the determination of Se(IV) and Sb(III). (2) Se(VI) and Sb(V): Total Se and total Sb were determined after the reduction of Se(VI) to Se(IV) and Sb(V) to Sb(III), the concentrations of Se(VI) and Sb(V) were calculated by the respective differences between the total Se and Se(IV), and the total Sb and Sb(III) concentrations. Prereductions of Se(VI) to Se(IV) and Sb(V) to Sb(III) were carried out in 0.50% (w/v) L-cysteine media for 25 min in a boiling-water bath. The determined values for both inorganic selenium and antimony species were obtained after subtracting the blank values.

Table 1 – Equipment and operating parameters ICP-MS plasma Rf power Outer gas flow rate Intermediate gas flow rate Nebulizer gas flow rate Sampling depth Sampler/skimmer diameter orifice

1250 W 15 L min1 0.90 L min1 0.70 L min1 7.00 mm Nickel 1.00 mm/0.40 mm

Time-resolved data acquisition Scanning mode Dwell time Integration mode Points per spectral peak Elements and isotopes

Peak-hopping 20 ms Peak area 1 121 Sb, 82Se

Electrothermal vaporizer Sample volume Carrier gas flow rate Drying step Vaporization step Clean step

10 mL 0.40 L min1 100 1C ramp 10 s hold 10 s 2200 1C hold 5 s 2600 1C hold 4 s

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Calibration curve

A series of standard solutions with different target analytes concentrations (0.25, 0.50, 1.00, 5.00, 10.00, 50.00 mg L1), 0.40% (w/v) DDTC and 0.10% (w/v) Triton X-114 were prepared and subjected to the same analytical procedure as described previously. The calibration curve was obtained by plotting signal intensity (y-axis) versus analytes concentrations (x-axis), and the concentrations of analytes in the samples were calculated based on the calibration curve.

2.5.

LOD and LOQ experiments

For limit of detection (LOD) and limit of quantification (LOQ), a blank test solution was measured (n ¼ 8) under the optimized conditions. And the blank test was performed as follows: 10 mL mixed solution of 0.40% (w/v) DDTC, 0.10% (w/v) Triton X-114, chosen as the blank solution, was subjected to CPE. The surfactant-rich phase obtained was diluted to 100 mL with ethanol. The blank values of analytes were obtained by determining the diluted surfactant-rich phase. In accordance with IUPAC recommendations, the LODs were calculated as three times the standard deviation of the blank signal, and the LOQ was calculated according to ten times the standard deviation of the blank signal.

2.6.

Spike-addition experiment

2.00 mg L1 Se(IV), Se(VI), Sb(III) and Sb(V) were added to different water samples, and the spiked samples were adjusted to 6.00 before CPE separation. The surfactant-rich phase obtained was diluted to 100 mL with ethanol. The concentrations of the target species in the spiked samples were obtained by determining the diluted surfactant-rich phase.

600000

counts / s

Se

3.

Results and discussion

3.1.

Optimization of ETV parameters

3.1.1.

Optimization of vaporization temperature and time

A drying temperature of 100 1C and a drying time of 10 s were selected as the drying conditions in this work. Under the selected drying conditions, the effect of vaporization temperature on the signal intensities of Se(IV) and Sb(III) with/without the addition of DDTC was studied and the result is shown in Fig. 1. As can be seen, in the absence of DDTC, the signal intensity of Se(IV) and Sb(III) enhanced with the increase of temperature, and no plateau for Se(IV) (a) and Sb(III) (c) is found in the range of the tested vaporization temperature. On the contrary, the addition of DDTC greatly affected the vaporization behavior of Se(IV) (b) and Sb(III) (d): a weaker signal intensity could be detected for Se(IV) and Sb(III) at a vaporization temperature of 1200 1C, the signals of the elements studied increased gradually with the increase of vaporization temperature and the maximum analytical signals were obtained at 1800 1C for Se(IV) and 2200 1C for Sb(III), and this maximum signal was kept constant with the further increase of vaporization temperature to 2400 1C. In this work, a temperature of 2200 1C was selected as the appropriate vaporization temperature for Se(IV) and Sb(III). By applying the established heating program, the effect of vaporization time on the analytical signal of the analytes was studied. It was found that the signal increased with the increase of vaporization time, and the maximum signal intensity was achieved after the vaporization time approached to 4 s, and it remained constant up to 6 s. Thus, a vaporization time of 5 s was chosen in further experiments.

3.1.2.

Vaporization behavior of Se(IV) and Sb(III) in ETV

From Fig. 1, it could be seen that there was no plateau of signal intensity for Se(IV) (a) and Sb(III) (c) in the whole tested

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d

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0

1000 1200 1400 1600 1800 2000 2200 2400 2600 2800

1000 1200 1400 1600 1800 2000 2200 2400 2600 2800

Vaporization temperature (°C)

Vaporization temperature (°C)

Fig. 1 – The effect of vaporization temperature on the signal intensity of selenium (IV) and antimony (III). (a) Se(IV) aqueous solution, (b) Se(IV) with DDTC and Triton X-114 in ethanol medium with a CPE step (pH 6.00), (c) Sb(III) aqueous solution and (d) Sb(III) with DDTC and Triton X-114 in ethanol medium with a CPE step (pH 6.00). Condition: (a, c), Sb(III)/Se(IV): 50.00 lg L1; (b, d), Se(IV)/Sb(III): 1.00 lg L1; DDTC: 0.40% (w/v); Triton X-114: 0.10% (w/v); equilibration temperature, 30 1C; equilibration time, 5 min. All other conditions as in Table 1.

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vaporization temperature range when DDTC was not added. This means that a higher vaporization temperature is required for the quantitative vaporization of selenium and antimony. However, the situation was changed after DDTC was added. The plateau of signal intensity was obtained at a vaporization temperature of 2000 1C for Se(IV) and 2200 1C for Sb(III). Fig. 2 shows the typical signal profiles of Se(IV) and Sb(III) with/without the addition of DDTC as the chemical modifier. As can be seen, without the addition of DDTC as the chemical modifier, Se(IV) was only partly vaporized at 2200 1C (d) and a severe memory signal was observed (d0 ), while an intense and sharp signal profile for Sb(III) could be observed at 2200 1C (i) and a weak memory signal was observed (i0 ). However, with the addition of DDTC, at the same vaporization temperature (2200 1C) a much sharper analytical signal profile could be detected for Se(IV) (b) and for Sb(III) (f), and no memory signal for Se(IV) (b0 ) and Sb(III) (f0 ) could be observed at 2600 1C. Meanwhile, the blank signals of the corresponding solution are also given in Figs. 2a and e. As could be seen, a very low value of blank signal could be detected. All these results indicated that the use of DDTC as a chemical modifier can effectively improve the vaporization behaviors, eliminate the memory effect, improve transportation efficiency and therefore improve analytical performance of the method.

3.2.

Optimization of the CPE

3.2.1.

Effect of pH

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pH plays a unique role on metal-chelate formation and subsequent extraction. In this part of the study, the medium pH was evaluated for its effect on the signal intensity of Se(VI) and Se(IV), Sb (V) and Sb (III). The examined pH ranged from 2.00 to 7.00. As shown in Fig. 3, the maximum signal intensity for Se(IV) and Sb(III) was achieved in the range of 5.50–7.00 and 5.50–6.50, respectively, whereas Se(VI) and Sb (V) were not extracted in the whole examined pH range. The reason for the extraction of Se(IV) and Sb(III) was that they can form the

Triton X-114 extractable Se(IV)–DDTC and Sb(III)–DDTC complexes with DDTC, but Se (VI) and Sb(V) did not form DDTC complexes and are not extracted by Triton X-114. Thus, pH 6.00 was used for the selective separation of different species of inorganic selenium and antimony.

3.2.2.

Effect of DDTC concentration

The variation of the analytical signal as a function of the concentration of DDTC in the range of 0.01–1.00% (w/v) was studied, and the experimental results in Fig. 4 demonstrated that the signal intensity of the analytes were accentuated by DDTC at concentrations up to about 0.20% (w/v). The maximum signal intensity achieved with this concentration remained constant up to the highest amount studied. Considering the competitive complexation with other metal ions in practical cases, 0.40% (w/v) DDTC was selected for further research.

3.2.3.

Effect of Triton X-114 concentration

The effect of Triton X-114 concentration upon sensitivity and extraction was studied within the surfactant concentration range of 0.01–0.50% (w/v). Fig. 5 showed the effect of Triton X-114 concentration on signal intensity. It is obvious that a quantitative extraction was observed with the Triton X-114 concentration in the range of 0.08–0.15% (w/v). However, at higher concentrations the signal intensity deteriorates, probably due to a larger volume of surfactant-rich phase when higher amounts of surfactants are used, resulting in a dilution of concentration of the extracted analytes. Accordingly, a Triton X-114 concentration of 0.10% (m/v) was employed for further studies.

3.2.4.

Effects of equilibration temperature and time

To achieve easy phase separation and preconcentration as efficiently as possible, optimal equilibration temperature and incubation time are necessary to complete reactions. The effect of the equilibration temperature was investigated with

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f'

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times /s

Fig. 2 – Signal profiles obtained for selenium (IV) and antimony (III) vaporized at 2200 1C. (a, e) DDTC and Triton X-114 in ethanol medium with a CPE step (pH 6.00); (a0 , e0 ) clean step of ‘a, e’ at 2600 1C; (b, f) Se(IV)/Sb(III) with DDTC and Triton X-114 in ethanol medium with a CPE step (pH 6.00); (b0 , f0 ) clean step of ‘b, f’ at 2600 1C; (c, h) Only high-purity deionized water (the blank of Se(IV)/Sb(III) aqueous solution); (c0 , h0 ) clean step of ‘c, h’ at 2600 1C; (d, i) Se(IV)/Sb(III) aqueous solution; (d0 ,i0 ) clean step of ‘A’ at 2600 1C. Condition: (b, f), Se(IV)/Sb(III), 1.0 lg L1; (d, i), Se(IV)/Sb(III), 50.00 lg L1; DDTC, 0.40% (w/v); Triton X-114, 0.10% (w/v); equilibration temperature, 30 1C; equilibration time, 5 min. All other conditions as in Table 1.

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100

80 60 Sb(III)+DDTC Se(IV)+DDTC Sb(V)+DDTC Se(VI)+DDTC

40 20 0 2

3

4

5

6

Relative signal intensity (%)

Relative signal intensity (%)

100

7

Relative signal intensity (%)

100

40

20

Se(IV)+DDTC Sb(III)+DDTC

60

40

20

0 0.8

0.2

0.3

0.4

0.5

Fig. 5 – Effect of Triton X-114 concentration on analytical signal intensity. Conditions: Sb(III)/Se(IV), 1.00 lg L1; DDTC, 0.40%(w/v); pH ¼ 6.00; equilibration temperature, 30 1C; equilibration time, 5 min. All other conditions as in Table 1.

Effect of coexisting ions

The effects of common coexisting ions were investigated. In these experiments, solutions of 1.00 mg L1 of Se(IV) and Sb(III) and the added interfering ions were treated according to the recommended procedure. The tolerance limits (mg L1), defined as interferent concentration varying the analyte signal by 10%, and the results indicated that 1000 mg L1 of 1 1 of Mg2+ and SO2 of Na+, K+ and NO 3 ; 500 mg L 4 ; 100 mg L 2+ 1 2+ 2+ 3+ 1 3+ Ca ; 50 mg L of Zn , Cu and Al ; 20 mg L of Fe and Pb2+ had no obvious influence on the signal intensity of analytes. These results permit the application of the developed methods for determining Se and Sb species in water samples.

80

0.6

0.1

TX-114 concentration (%)

3.3.

0.4

60

0.0

Fig. 3 – Effect of pH on analytical signal intensity. Conditions: Sb(III)/Sb(V), Se(IV)/Se(VI), 1.00 lg L1; DDTC, 0.40%(w/v); Triton X-114, 0.10%(w/v); equilibration temperature, 30 1C; equilibration time, 5 min. All other conditions as in Table 1.

0.2

80

0

pH

0.0

Se(IV)+DDTC Sb(III)+DDTC

1.0

DDTC concentration (w/v,%) Fig. 4 – Effect of DDTC concentration on analytical signal intensity. Conditions: Sb(III)/Se(IV), 1.00 lg L1; Triton X-114, 0.10% (w/v); pH ¼ 6.00; equilibration temperature, 30 1C; equilibration time, 5 min. All other conditions as in Table 1.

the temperature varying from 22 to 60 1C, and the experimental results showed that the maximum signal intensity for Se(IV) and Sb(III) was achieved in the range of 30–40 and 22–40 1C, respectively. Over 40 1C, the CPE efficiency of the two elements decreased. So, an equilibration temperature of 30 1C was used. Studies on the effect of the incubation time showed that the maximum extraction efficiency was observed from 5 to 20 min for Se(IV) and from 5 to 15 min for Sb(III), and further increase in the incubation time resulted in a decrease of the efficiencies probably due to the thermal instability of the formed DDTC complexes. For the rest of the experiments, an incubation time of 5 min was used.

3.4.

Calibration curve, detection limit and precision

Using the proposed method, the calibration curves for Se(IV)and Sb(III) were established with calibration levels of 0.25, 0.50, 1.00, 5.00, 10.00 and 50.00 mg L1, and the correlation coefficient (r2) of 0.99 for Se(IV) and 0.99 for Sb(III) was obtained. The blank values of Sb(III) and Se(IV) were 0.0870.01 and 0.1070.02 mg L1, respectively. The detection limits (LODs), calculated according to 3s, for eight replicate measurements of a blank solution, were 0.03 mg L1 for Sb(III) and 0.05 mg L1 for Se(IV). The LOQ, calculated according to 10s, for eight replicate measurements of a blank solution, were 0.09 mg L1 for Sb(III) and 0.17 mg L1 for Se(IV). The relative standard deviations (RSDs) for five replicate determinations of 1.00 mg L1 of Se(IV) and Sb(III) were 3.5% and 4.2%, respectively, and the enhancement factor was 50. A comparison of the analytical performance (including LOD, RSD, sample preparation and cost) obtained by this method with several other approaches reported in literatures using HPLCatomic spectrometry techniques for the speciation of antimony and selenium is shown in Table 2. As could be

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seen: (1) the LODs of this method are much lower than that reported in the literatures; (2) the cost of the developed method is still comparable with the literatures methods, considering that the proposed method is a multielemental speciation method, while the above-mentioned methods (HPLC-HG-AAS, HPLC- HG–AFS and HPAEC–AFS) are single elemental speciation methods; (3) the precisions (RSDs) were better than that reported in the references (Lindemann et al., 1999; Niedzielski, 2005), and were comparable with the references (Sayago et al., 2002; Liang et al., 2006; Vin˜as et al., 2006). Moreover, the speciation methodology for Se and Sb using HPLC-atomic spectrometry techniques is always associated with high dilution factors, and chromatographic techniques also suffer from potential risks in long retention times and serious peak-tailing. In addition, the other advantages of CPE-ETV-ICP-MS include that the small volume of the surfactant-rich phase is very compatible with ETV-ICP-MS detection, DDTC is used not only as an extraction chelating reagent but also as an chemical modifier for ETV, and the capability of multielement analysis of ICP-MS is exploited.

3.5.

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Validation and applications

The proposed method was validated by analyzing the Environmental Water Reference Material GBW(E) 080545 and GSB071253-2000 after appropriate dilution with high-purity deionized water. The determined values for both inorganic selenium and antimony species were obtained after subtracting the blank

values. As could be seen from Table 3, the analytical results were in good agreement with the certified values. The developed method was also applied to determine Se(IV)/Se(VI), Sb(III)/Sb(V) in different water samples (pool water, lake water, tap water, Yangtse River water) and recovery experiments were also carried out by spiking 2.00 mg L1 of target species in the water samples. Table 4 shows the analytical results along with the recoveries for inorganic Sb and Se species in various water samples. From Table 4, it could be concluded that (1) the difference in concentration of two inorganic Se species is very small for all analyzed water samples, and the same situation is found for two inorganic Sb species; (2) the concentrations of high oxidation state species (Sb(V) and Se(VI)) in all water samples were a little bit higher than that of low oxidation state species (Sb(III) and Se(IV)); (3) the total inorganic Sb or Se in these water samples is at the level of mg L1. The precisions (RSD, n ¼ 3) for the real sample analysis range from 1.9 to 10.2% for Se species and 2.7–8.2% for Sb species. The recovery is defined as the amount of analyte found in spiked sample divided by the sum of original amount and spiked amount, times 100. The recoveries for the spiked water samples were 95–109% for Se species and 90–104% for Sb species.

4.

Conclusion

In this paper, a new method is proposed for the simultaneous speciation of inorganic selenium and antimony by CPE

Table 2 – Comparison of the performance found in the literature for Se and Sb speciation using HPLC-atomic detector Analytical approach

HPLC-ICP-MS HPLC-HG-AFS HPLC-HG-AFS HPLC-HG-AAS HPAEC–AFS CPE-ETV-ICP-MS

LOD (mg L1)

RSD (%)

Se(IV)

Sb(III)

Se(IV)

Sb(III)

3.70 – – 2.40 1.00 0.050

1.70 0.26 0.91 – – 0.03

7.00 – – 14.20 3.80 3.50

5.50 3.30 3.00 – – 4.20

Sample preparation

Cost

Ref.

– – – – – CPE

+++ + + + + ++

Lindemann et al. (1999) Sayago et al. (2002) Vin˜as et al. (2006) Niedzielski (2005) Liang et al. (2006) This work

HPAEC–AFS: high-performance anion-exchange chromatography coupled with atomic fluorescence spectrometry. ‘‘+++’’ means ‘‘more expensive’’. ‘‘++’’ means ‘‘expensive’’. ‘‘+’’ means ‘‘normal’’.

Table 3 – The results of certified reference material (CRM) of GBW(E)080545 and GSB07-1253-2000 water samples (mean7sd, n ¼ 3) Sample GBW(E)080545

GSB07-1253-2000

ND: not determined.

Element

Determined (mg L1)

Certified (mg L1)

Sb(III) Sb(V)

ND 95.6072.90

100.00

Se(IV) Se(VI)

ND 109.0074.70

100.00

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Table 4 – Analytical data of antimony and selenium species determined in water samples (mean7sd, n ¼ 3) Elements

Sample Added (mg L1)

Pool water

East lake water

Tap water

Yangtse River water

Found (mg L1)

Recovery (%)

Found (mg L1)

Recovery (%)

Found (mg L1)

Recovery (%)

Found (mg L1)

Recovery (%)

Se(IV)

0.0 2.0

0.8970.05 2.870.1

– 96

1.070.07 3.170.2

– 102

0.8070.02 2.670.2

– 95

0.7570.04 3.070.07

– 109

Se(VI)

0.0 2.0

1.270.1 3.470.2

– 104

1.270.1 3.170.06

– 97

1.670.1 3.870.2

– 105

1.170.1 3.270.1

– 104

Sb(III)

0.0 2.0

1.170.03 3.270.2

– 104

0.870.05 2.970.1

– 101

0.970.06 3.070.2

– 104

0.7770.05 2.670.09

– 96

Sb(V)

0.0 2.0

1.270.1 3.170.2

– 97

1.470.08 3.270.2

– 94

1.470.1 3.370.09

– 99

1.170.08 2.870.2

– 90

combined with ETV-ICP-MS with the use of DDTC as the chelating reagent and Triton X-114 as the extractant. The proposed method is characterized with mild separation conditions (pH 6.00 is close to the original pH of natural water), simplicity (does not require separation by chromatography), selectivity (good anti-disturbance ability), safety (without organic solvent), low cost (cheap reagent) and rapid (batch operation mode), and it could be adapted for rugged and routine use by the contract lab population and practitioners in water field who utilize such analyses.

Acknowledgment Financial supports from the Science Fund for Creative Research Groups of NSFC (No. 20621502) and MOE of China (NCET-04-0658) are gratefully acknowledged. R E F E R E N C E S

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