Anionic Surfactant Micelle-Mediated Extraction Coupled with

Article pubs.acs.org/JAFC

Anionic Surfactant Micelle-Mediated Extraction Coupled with Dispersive Magnetic Microextraction for the Determination of Phthalate Esters Hao Wu, Hong Tian, Ming-Fei Chen, Jun-Chen You, Li-Ming Du,* and Yun-Long Fu Analytical and Testing Center, Shanxi Normal University, Linfen, Shanxi 041004, P. R. China S Supporting Information *

ABSTRACT: A novel and easy two-step microextraction technique combining anionic surfactant coacervation phase (CAP) extraction and dispersive microsolid-phase extraction (D-μ-SPE) was developed for the high-performance liquid chromatography−ultraviolet detection to determination of phthalate esters (PEs) in water samples. The method started with the phase separation of sodium dodecylbenzenesulfonic acid (SDBSA) obtained by adding NaCl, whereas the target analytes were extracted in the CAP. The CAP was then retrieved using diatomaceous earth-supported magnetite nanoparticles. The effects of solution acidity, SDBSA, and electrolyte concentration, extraction time, magnetic material quantity, and elution solvent volume were discussed. Under optimal extraction conditions, the extraction recoveries ranged from 48.6 to 84.8%, and relative standard deviations ranged from 3.9 to 5.7% (n = 10). The detection limits ranged from 0.5 to 5.0 ng mL−1 for the five PEs. The proposed method was used to determine the five PEs in the water samples and recoveries between 85.7 and 105.5%. KEYWORDS: sodium dodecylbenzenesulfonic acid, diatomaceous earth-supported magnetite nanoparticle, dispersive microsolid-phase extraction, high-performance liquid chromatography, phthalate ester

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INTRODUCTION Phthalate esters (PEs) are widely used as plasticizers and additives in many products, such as pesticides, plastics, paints, and cosmetics.1 The most important use of PEs by far is as plasticizers in polymeric materials. PEs can easily migrate from plastic materials to the environment, even to the food they come in contact with, because they are only physically, not chemically, bound to the polymer structure.1,2 For example, the US Environmental Protection Agency (EPA)3 has indicated that the major source of di-2-ethylhexyl phthalate (DEHP) in drinking water is discharge from rubber and chemical factories. The limit for DEHP in drinking water is 6 ng mL−1. Certain PEs and their metabolites and degradation products have adverse effects on human health, specifically on the liver, kidney, and testicles.4−6 The intensive use and the pollutive effects of PEs have become a serious problem and a major public health concern worldwide. Therefore, simple, sensitive, and reliable analytical methods for evaluating and monitoring trace amounts of PEs in different water samples must be developed for human health protection and environmental control. Sample preparation of PEs is necessary before instrumental analysis to obtain sensitive and accurate results because of the extremely low concentration level of PEs, as well as the complex matrix of the environment and food water samples. Various pretreatment techniques have been attempted to extract PEs from water samples, such as liquid−liquid extraction (LLE),7 solid-phase extraction (SPE),8−13 solidphase microextraction (SPME),14−19 liquid-phase microextraction (LPME),20−25 and cloud-point extraction (CPE).26 However, LLE is considered tedious and time-consuming and often results in high blank values.20,22 Other methods, although © 2014 American Chemical Society

they offer certain advantages such as the use of small volumes of nontoxic organic solvents, high reproducibility, fast sample treatment, and compatibility with most analytical instrumentation, also encounter specific limitations, a feature that gives rise to the incessant efforts to develop new procedures or reinstate conventional methods resolving their drawbacks.27 CPE is an effective separation/preconcentration technique first studied by Watanabe et al.28 in early 1978. Compared with the traditional LLE, CPE requires a very small amount of relatively nonflammable and nonvolatile surfactants that are environmentally friendly. Another important merit is that no solvent concentration procedures that may cause analyte loss are needed under appropriate conditions. The solution containing the surfactant becomes turbid and separates into two phases: a surfactant-rich phase (at a very small volume) and a larger volume aqueous solution phase (bulk amount) with a diluted surfactant concentration, which approximates to its critical micelle concentration. The hydrophobic analytes of the solution are extracted into the small volume of the surfactantrich phase with high enrichment factor.26 Meanwhile, CPE is easily used in conjunction with spectral, atomic absorption, chromatographic, and electrochemical determinations to develop highly sensitive and convenient analytical methods.29 Compared with dispersive liquid−liquid microextraction (DLLME), CPE does not use toxic and environmentally pollutive extraction and disperser solvents. Received: Revised: Accepted: Published: 7682

February 7, 2014 July 13, 2014 July 16, 2014 July 16, 2014 dx.doi.org/10.1021/jf502364x | J. Agric. Food Chem. 2014, 62, 7682−7689

Journal of Agricultural and Food Chemistry

Article

Figure 1. Simplified scheme of the two-step extraction process. convenience of the analysis and for minimizing reagent consumption, 10 mL aqueous standard solutions containing each analyte at 50 ng mL−1 were used for the optimization study Methanol and acetonitrile (HPLC grade) were purchased from J&K Scientific Ltd. (Beijing, China). Iron(III) chloride hexahydrate (FeCl3· 6H2O) (Acros Organics, New Jersey, USA), ethylene glycol (EG; Acros Organics, New Jersey, USA), 1,2-ethylenediamine (ETH; J&K Scientific Ltd., Beijing, China), sodium acetate (NaAc, J&K Scientific Ltd., Beijing, China), and diatomaceous earth (Qingdao, China) were used to prepare the DSMNPs. Milli-Q water (Millipore, Bedford, MA, USA) was used throughout the study. All solutions prepared for HPLC were filtered through 0.45 μm membranes (Tianjing Jinteng Instrument Factory, Tianjin, China) before use, and all labware are glass products that do not come into contact with any plastic products. All tubes in HPLC are made of Teflon and stainless steel. Synthesis and Characterization of DSMNPs. Raw diatomaceous earth from Qingdao, China was purified by a previously described method.33 In a typical synthesis, purified diatomaceous earth (0.3 g) was added to 24 mL of EG. Subsequently, 0.6 g of FeCl3·6H2O and 1.2 g of NaAc were dissolved in the EG solution at ambient temperature. After stirring for about 30 min, the solution was transferred to a 50 mL Teflon-lined stainless-steel autoclave kept at 200 °C for 8 h and then naturally cooled to ambient temperature. The black precipitate was isolated using an external magnetic field, and the supernatant was decanted. The obtained solid part was washed with ethanol several times and then dried at 60 °C in a vacuum oven. Using this method, 100 g of DSMNPs could be synthesized simultaneously using multiple Teflon-lined stainless-steel autoclaves. The nonmodified MNPs were prepared according to the literature.34 The photography of scanning electronic microscope for MNPs and DSMNPs has been shown in Figure S1 in the Supporting Information. Liquid Chromatography Analysis. The chromatography equipment was a 1525 binary HPLC pump and a 2489 dual λ UV detector from Waters (Waters Corporation, USA). Waters Breeze software was used to control the instruments and collect the data. Chromatographic separation of the analytes was carried out on a Gemini C18 column (5 μm; 4.6 mm × 250 mm; Phenomenex, Torrance, CA, USA) maintained at 25 °C. The mobile phases A and B were water and acetonitrile. The gradient conditions are as follows: 0−15 min, 70% B; 15−20 min, 70%−100% B; 20−30 min, 100% B. The flow rate of the mobile phase was maintained at 1.0 mL/min; the sample injection volume was 10 μL; and the detection wavelength was 280 nm. Extraction Procedure. In a typical procedure, 0.8 mL of SDBSA solution was put into a 10 mL glass tube containing 10 mL of sample and was thoroughly mixed. Afterward, 1.2 g of NaCl was added, and the tube was sealed and shaken for 4 min. Then, 25 mg of DSMNPs was added to the tube that was sonicated for another 2 min. The DSMNPs were rapidly collected from the tube wall using an magnetic field provided by a strong Nd−Fe−B magnet (Supporting Information Figure S2), and the sample solution was discarded by decanting. After removing the magnet, 0.2 mL of methanol was introduced to the tube

CPE can be used to determine PEs in environmental water samples by high-performance liquid chromatography (HPLC) and ultraviolet (UV) detection, using the nonionic surfactant Trinton X-114 as the extraction solvent.26 However, the nonionic surfactants most frequently used for extraction and preconcentration prior to HPLC analysis have high background absorbance in the UV region and long retention times.30 The results also overlap between the chromatographic peaks of the surfactant and the analytes, thereby requiring a cleanup step to obtain adequate selectivity.31 Gradient elution has also been used to separate the surfactant and PEs to obtain quality chromatograms.26 In 1999, Casero et al.30 proposed the acidinduced CPE using anionic surfactants to overcome the aforementioned problems because anionic surfactants have appropriate ionic groups that lead to low retention times. Anionic surfactant solutions exhibiting phase separation in the presence of electrolytes is a phenomenon called coacervation.32 In this work, anionic surfactants were used as extraction solvents, and then electrolytes were added to separate the two phases, coacervation phase (CAP) and aqueous phase. The CAP was found to be loose solid instead of liquid, which became the top part and cannot be accurately transferred after centrifugation. For the complete separation of the CAP, a twostep extraction technique that combines the CAP and further dispersive microsolid phase extraction (D-μ-SPE) was developed. The diatomaceous earth-supported magnetite nanoparticles (DSMNPs) were used as the D-μ-SPE material to retrieve the anionic surfactant CAP instead of centrifugation. The new method was used for preconcentration of PEs in water prior to HPLC-UV determination.

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MATERIALS AND METHODS

Reagents and Materials. All PE standards including dimethyl phthalate (DMP; 98%), diethyl phthalate (DEP; 99.9%), butylbenzyl phthalate (BBP; 99%), dibutyl phthalate (DBP; 96.8%), and DEHP (99.6%) were obtained from AccuStandard, Inc. (New Haven, USA). Sodium dodecylbenzenesulfonic acid (SDBSA) (Acros Organics, New Jersey, USA) accounted for 88% and sodium dodecyl sulfate (SDS) (Acros Organics, New Jersey, USA) accounted for 99% of the main component and were used as received. A 7% (w/v) surfactant solution was prepared by dissolving the reagents in water. Individual stock solutions of each PE compound were prepared in methanol, and a standard mixture solution of all target compounds was prepared in methanol at a final concentration of 0.1 mg mL−1. The working solution was freshly prepared daily by appropriately diluting the methanolic stock standard solution. All standard solutions were stored at 4 °C and brought to ambient temperature prior to use. For the 7683

dx.doi.org/10.1021/jf502364x | J. Agric. Food Chem. 2014, 62, 7682−7689


Article

to desorb the SDBSA by sonication for 2 min. Finally, the magnet was again placed next to the tube, and the supernatant was collected into an Eppendorf tube. All optimized experiments were performed in triplicate. A schematic of the two-step extraction process is shown in Figure 1. Real Samples. To evaluate the feasibility of the developed method, four real-world water samples were used, including tap, river, tanked, well, and bottled water. Tap water was obtained from a water tap after flowing for 10 min in our laboratory. River water was obtained from the Fen River (Linfen, China) in May 2013. Tanked and well water were simultaneously collected from a local area (Linfen, China). Bottled water was obtained from a local market (Linfen, China). All water samples were collected into precleaned, light-colored glass bottles, and each sample was filtered through 0.45 μm micropore membranes to remove suspended solids. All samples were stored in a refrigerator (at 4 °C) in the dark until analysis. Calibration of the EF and Extraction Recovery Percentage (ER%). To evaluate the performance of the proposed method, EF and ER% were determined using eqs 1 and 2 EF =

Cdes C0

Figure 3. Effect of volume of SDBSA. Experiment conditions: amount of DSMNPs, 25 mg; extraction time, 4 min; amount of NaCl, 1.2 g; sonication time, 2 min; volume of methanol, 0.2 mL.

0.8 mL, beyond which the ER% remained constant. Thus, 0.8 mL of SDBSA solution was used in the subsequent experiments. The coacervation of anionic surfactants is achieved only at a certain salt concentration. Addition of NaCl to the water sample may have various effects (salting out, salting in, or no effect) on extraction. The effect of NaCl on the coacervation of the system was investigated. The effect of NaCl was studied within the quality range of 0.6−1.8 g to 10 mL of sample. The results showed (Supporting Information Figure S3) that the ER % of the four PEs increased with increased NaCl until the amount was 1.2 g, beyond which ER% remained almost constant. Thus, 1.2 g of NaCl was deemed optimum. The pH of the working solutions or samples has an important role in the extraction because it determines the existing state of the analytes and the EE of the target analytes. In this experiment, the effect of pH was studied within the range of 2.0−11.0, and the results show (Supporting Information Figure S4) the EE almost had no significant change within pH 4.0−9.0 of the sample solution. Therefore, the sample solution pH was not adjusted because the pH of the normal water samples ranged within 6.0−8.0. Extraction time is also an important parameter affecting EE. Increasing the extraction repeatability necessitates an extraction period within which the equilibrium between the aqueous phase and CAPs is reached. The effect of the extraction time was examined within the range of 0−10 min, whereas other experimental conditions were kept constant. The results showed (Supporting Information Figure S5) that the EE of PEs increased with increased extraction time within 0−4 min and remained almost constant within 4−10 min. Therefore, 4 min was selected as the optimum extraction time. Magnetic Material Selection. The magnetic material used in this method has a strong interaction effect on the SDBSA CAP so that target analytes extracted by SDBSA can be completely transferred. Two types of magnetic material, i.e., nonmodified MNPs and DSMNPs, were chosen in terms of their adsorption behavior to the CAP. Compared with MNPs, DSMNPs can completely retrieve the SDBSA CAP and have better dispersion and less coaggregation.35 Thus, DSMNPs were chosen for further experiments. Sonication was performed for the complete adsorption of CAP to DSMNPs. The sonication time was studied within the time range of 0.5−5 min in 0.5 min intervals. The results showed (Supporting Information Figure S6) that the EE increased with increased sonication time from 0.5 to 2 min.

(1)

where Cdes and C0 are the concentrations of the analytes in the desorbed phase and the initial concentration of the analytes in the sample solution, respectively.

ER% =

Cdes × Vdes V × 100% = EF × des × 100% C0 × V0 V0

(2)

where Vdes and V0 are the volumes of the desorbed phase and sample solution, respectively.

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RESULTS AND DISCUSSION Optimization of CAP Extraction Parameters. Selecting an appropriate extraction solvent is important for optimizing

Figure 2. Effect of types of anionic surfactants. Experiment conditions: volume of surfactants, 0.8 mL; amount of DSMNs, 25 mg; extraction time, 4 min; amount of NaCl, 1.2 g; sonication time, 2 min; volume of methanol, 0.2 mL.

the extraction process. Thus, the inexpensive and highly watersoluble anionic surfactants, SDBSA and SDS, were used. The working solution concentration of the surfactants was 7% (w/ v). The experimental results showed (Figure 2) that the percentage of the extracted PEs was high for SDBSA with a phenyl group. This effect was consistent with the affinity of PEs to a more hydrophobic CAP. Thus, SDBSA was selected as the extraction solvent. To study the effect of the extraction solvent volume of 7% SDBSA on the extraction efficiency (EE) of PEs, other experimental conditions were fixed, and different SDBSA volumes were tested (0.3−1.2 mL). The results are shown in Figure 3. ER% increased with increased SDBSA volumes up to 7684



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Table 1. Analytical Performance of the Proposed Method for Determination of PEs compounds

retention time (min)

DMP DEP BBP DBP DEHP

4.1 5.4 12.5 14.1 26.7

calibration equation (ng/mL) y y y y y

= = = = =

599x 800x 538x 699x 348x

+ + + + +

1503 2300 3834 2514 1607

LRs (ng/mL)

LODs (ng/mL)

LOQs (ng/mL)

R2

ER%

EFs

RSD (%, n = 10)

5−1000 2−1000 5−1000 5−1000 10−1000

1.0 0.5 1.0 1.0 5.0

3.0 1.5 3.0 3.0 10.0

0.9952 0.9987 0.9992 0.9991 0.9965

48.6 72.7 82.5 82.7 84.8.

14.6 21.8 24.8 24.8 25.4

4.6 3.9 5.5 4.7 5.7

Table 2. Intra- and Interday Precision intraday precisiona added PAEs

concentration (ng/mL)

found (ng/mL)

RSD (%)

recovery (%)

found (ng/mL)

RSD (%)

recovery (%)

DMP

10 20 10 20 10 20 10 20 10 20

8.8 20.9 9.4 20.2 8.7 19.3 10.3 18.3 8.7 20.8

3.2 2.9 4.4 2.6 3.5 1.8 2.4 3.9 3.8 2.3

88.2 104.7 94.4 101.1 86.5 96.7 102.6 91.4 87.3 103.9

8.9 19.1 10.6 20.0 10.1 18.4 8.6 20.7 9.4 19.7

5.1 4.7 5.6 4.4 3.8 4.2 5.8 4.4 4.3 3.9

89.2 95.7 105.6 99.9 101.3 92.2 86.2 103.3 93.8 98.7

DEP BBP DBP DEHP a

interday precisiona

Based on five sample extractions.

Table 3. Comparison of the Presented Method with Other Microextraction Techniquesa extraction mehod

determination technique

SPE SPME SPME SDME LPME HFM-LPME IL-CIA-DLLME

HPLC/UV GC/FID GC/FID GC/FID GC/MS GC/MS HPLC/VWD

LDS-VSLLME CPE CAP-D-μ-SPE

GC/MS HPLC/UV HPLC/UV

extracting solvent/material

consumption of organic solvent (μL)

extraction time (min)b

LRs (ng mL−1)

LODs (ng mL−1)

R2

2000 2 7 3 750

50 60 >10 25 25 20 13

1−200 0.8−8000 0.4−100 0.1−50 0.05−100 0.02−10 2−100

0.35−0.43 0.06−3.429 0.4 0.02−0.21 0.02−0.05 0.005−0.01 0.68−1.36

0.9974−0.9999 0.9965−0.9998 0.998 0.9940−0.9971 0.9984−0.9999 0.9968−0.9994

9 16 19 20 21 22 24

30 -

6 65 8

0.05−25 5−200 2−1000

0.008−0.020 1.0−3.8 0.5−1.0

0.9823−0.9984 0.9964−0.9998 0.9952−0.9992

25 26 this work

bamboo charcoal calix(6)arene fiber Fe3O4@ polythiophene MNPs dichloromethane:hexane:toluene 1-dodecanol toluene 1-octyl-3-methylimidazolium hexafluorophosphate toluene triton X-114 SDBSA

ref

UV, ultraviolet detector; FID, flame ionization detector; HFM, Hollow-fiber membrane; VWD, variable wavelength detector; IL-CIA-DLLME, Ionic liquid cold-induced aggregation dispersive liquid−liquid microextraction; LDS−VSLLME, low-density solvent-based vortex-assisted surfactantenhanced-emulsification liquid−liquid microextraction. LPME, liquid-phase microextraction; SDME, single-drop microextraction. bThe time interval between added extraction agent to before chromatographic analysis. a

the desorption of the target analytes from the DSMNP surface. The methanol volume investigated ranged from 0.1 to 0.6 mL. The results showed (Supporting Information Figure S9) that the increase in methanol volume from 0.1 to 0.2 mL increased the EE, whereas the ER% remained almost constant, except when the volume was 0.2 mL. However, in this case, EF decreased because the volume of the desorption solvent increased. The application of 0.2 mL of methanol was finally chosen, and the total volume of the desorption solution was almost 0.33 mL. Influence of Interfering Substances for CAP-D-μ-SPE. Aliquots of aqueous solutions containing 50 ng mL−1 PEs and certain amounts of other chemical species were obtained, and the proposed procedure was followed to study the selective separation and determination of PEs from the water samples with various chemical species. The tolerance limit was defined as the concentration of the added interfering substance that

Prolonged sonication time had little effect on EE. Thus, the sonication time of 2 min was deemed suitable for subsequent experiments. The effect of the amount of DSMNPs was studied using 15, 20, 25, 30, 35, and 40 mg amounts of retrieval agent. The results showed (Supporting Information Figure S7) that, with increased amount of DSMNPs from 15 to 40 mg, the maximum EE was obtained at 25 mg of DSMNPs (corresponding to its concentration of 2.5 mg mL−1 in the sample). When the amount of the adsorbents was >25 mg, the plots were flat, and no distinct increase in EE was observed. Therefore, 25 mg of DSMNPs was used in the subsequent experiments. Desorption Conditions. Different organic solvents were studied including methanol, ethanol, and acetonitrile. The results (Supporting Information Figure S8) showed that methanol gave the highest overall EE for the five PEs, followed by ethanol and acetonitrile. Therefore, methanol was used for 7685



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Table 4. Determination of PEs in Water Samples water sample tap water founda (ng mL−1) recoverya (%) river water founda (ng mL−1) recoverya (%) well water founda (ng mL−1) recoverya (%) tanked water founda (ng mL−1) recoverya (%) bottled water 1 founda (ng mL−1) recoverya (%) bottled water 2 founda (ng mL−1) recoverya (%) bottled water 3 founda (ng mL−1) recoverya (%) bottled water 4 founda (ng mL−1) recoverya (%) bottled water 5 founda (ng mL−1) recoverya (%) bottled water 6 founda (ng mL−1) recoverya (%) bottled water 7 founda (ng mL−1) recoverya (%) bottled water 8 founda (ng mL−1) recoverya (%) bottled water 9 founda (ng mL−1) recoverya (%) bottled water 10 founda (ng mL−1) recoverya (%) a

DMP

DEP

BBP

DBP

DEHP

ndb 86.0 ± 2.1

ndb 103.1 ± 2.8

ndb 101.7 ± 2.7

ndb 93.0 ± 2.1

ndb 90.1 ± 4.5

ndb 87.7 ± 2.4

ndb 97.6 ± 2.3

ndb 89.8 ± 3.8

ndb 89.0 ± 2.1

ndb 97.1 ± 2.4

ndb 85.7 ± 0.9

ndb 94.8 ± 2.4

ndb 95.7 ± 1.3

ndb 98.9 ± 2.3

ndb 90.8 ± 3.2

ndb 93.0 ± 3.6

ndb 87.5 ± 2.2

ndb 92.2 ± 2.0

6.13 ± 0.32 93.2 ± 5.1

ndb 93.1 ± 3.4

ndb 91.7 ± 5.1

ndb 93.7 ± 1.4

ndb 105.8 ± 4.0

5.51 ± 0.18 102.5 ± 2.1

ndb 88.9 ± 2.4

ndb 87.1 ± 1.8

3.24 ± 0.14 95.8 ± 3.3

ndb 103.7 ± 1.6

ndb 91.9 ± 1.3

ndb 95.8 ± 3.2

6.38 ± 0.29 104.1 ± 3.8

ndb 103.2 ± 4.0

ndb 94.7 ± 2.7

5.24 ± 0.24 105.5 ± 2.0

ndb 89.8 ± 1.4

ndb 104.1 ± 2.2

2.77 ± 0.11 91.9 ± 3.5

ndb 93.3 ± 2.5

6.03 ± 0.23 100.4 ± 3.8

ndb 90.1 ± 3.0

5.36 ± 0.17 88.7 ± 4.3

ndb 99.5 ± 3.3

ndb 89.3 ± 2.4

ndb 91.8 ± 1.3

ndb 89.4 ± 2.9

ndb 94.4 ± 5.1

ndb 86.5 ± 3.8

ndb 93.3 ± 3.5

7.43 ± 0.32 91.5 ± 3.5

ndb 86.4 ± 2.0

5.94 ± 0.21 94.4 ± 3.1

ndb 104.6 ± 1.9

ndb 90.3 ± 2.3

ndb 97.9 ± 1.8

ndb 89.6 ± 2.7

ndb 92.9 ± 4.1

ndb 97.1 ± 2.1

ndb 95.8 ± 2.2

6.18 ± 0.25 94.2 ± 2.9

ndb 96.6 ± 3.1

ndb 87.2 ± 3.6

ndb 95.6 ± 2.2

ndb 98.4 ± 4.7

5.71 ± 0.16 103.1 ± 3.2

ndb 87.1 ± 4.0

ndb 89.7 ± 2.0

ndb 95.3 ± 2.1

ndb 95.3 ± 3.4

6.46 ± 0.27 90.3 ± 3.2

ndb 95.0 ± 1.8

Based on five sample extractions. bNot detected.

caused less than ±5% relative error in the determination of the PEs. The results show that 1500-fold Na+, K+, Ba2+, Ca2+, Mg2+, Al3+, Ni2+, Cu2+, Cd2+, Hg2+, Co2+, Cr3+, NH4+, Cl−, NO2−, CH3O−, CrO42−, Cr2O72−, F−, Br−, I−, NO3−, SO42−, S2O32−, S2O82−, BO33−, B4O72−, CH3COO−, CO32−, HCO3−, C2O42−, HPO42−, and H2PO4−, and 500-fold Zn2+ and Sr2+, and 100-fold Sn2+, and 50-fold Pb2+, Fe2+, and Fe3+, and 8-fold MnO4− did not interfere with the determination, indicating the high selectivity of the proposed method. Determination by HPLC-UV. The background chromatograms of the blank water, reagent blank, and spiked water samples were determined. The results (Supporting Information Figure S10) show the absence of interfering peaks in the background chromatograms and highlight the selectivity of the HPLC-UV method. The matrix effect during HPLC-UV determination was studied by comparing the slopes of two

calibration sets prepared in methanol and in the matrix extract. The results (Supporting Information Table S1) show no significant differences between the slopes. Therefore, HPLCUV determination is not affected by matrix effects. Method Validation. The external calibration curves were prepared by 10 concentration levels from 0.02 μg mL−1, 0.05 μg mL−1, or 0.1 μg mL−1 to 10 μg mL−1 of analytes to blank water samples with working solutions, calibration equations calculated using the peak areas of the substances, linear ranges (LRs), determination coefficient (R2), limits of detection (LODs), limits of quantification (LOQs), and ER%, and EFs for PE determination were obtained under optimum conditions (Table 1). The linearity of the method was explored at PE concentrations from 2.0 ng mL−1, 5.0 ng mL−1, or 10.0 ng mL−1 to 1000 ng mL−1 with R2 between 0.9952 and 0.9991. The LODs based on a signal-to-noise ratio (S/N) of 3 ranged 7686



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samples collected from various sources, including tap, river, tanked, well, and bottled water. All water samples were spiked with standards of the 10 ng mL−1 of PEs to assess the matrix effect. The results in Table 4 show that DMP, DEP, and DBP may be detected in tank and bottled water; here, the containers were made of polyethylene terephthalate (PET). PAEs with low molecular weights are fairly soluble in water and are not chemically bonded to the polymer.36 Thus, PAEs easily migrate from bottles, especially PET ones, to the water.37 Table 4 also shows that the average results of the five replicate analyses of each water sample obtained by the proposed method satisfactorily agreed with one another (relative recoveries between 85.7% and 105.5%). This result indicated that the method was feasible for the determination of PEs in water samples. The HPLC chromatograms of river water are shown in Figure 4. A simple, low-cost, and reliable method for enriching and determining four PEs in environmental water samples was established using SDBSA as an extraction solvent and DSMNPs as an adsorption solvent in a two-step extraction technique. Compared with the nonionic surfactant which due to the nonpolar character and large retention times causes the chromatographic overlapping between the surfactant and the analytes, the anionic surfactant SDBSA had an ionic group which resulted in low retention time and did not interfere with the determination of analytes. No toxic organic solvent or dispersant was used throughout the entire extraction procedure, in addition to diatomaceous earth. The DSMNPs used to retrieve the SDBSA CAP in the magnetic field simplified the phase-separation process, rendering unnecessary the use of time-consuming procedures of centrifugation. Overall, the proposed method can expand the applicability of anionic surfactants as extraction solvents because of their low toxicity, high preconcentration factors, and low retention time. The automation of such an extraction procedure is possible, and related work by our group is underway.

Figure 4. Typical chromatograms for determination of PEs in river water. (A) Standard solution; (B) river water after extraction; (C) spiked river water after extraction, PEs, 10 ng mL−1. Peak 1, DMP; 2, DEP; 3, BBP; 4, DBP; 5, DEHP.

between 0.5 and 5.0 ng mL−1. The repeatability study was carried out by extracting spiked water samples at a concentration of 5 ng mL−1 for the DMP, DEP, DBP, and BBP and of 10 ng mL−1 for the DEHP, and the relative standard deviations (RSDs) were between 3.9% and 5.7% (n = 10). The results confirmed that the proposed method had good reproducibility and stability and potential for use in the analysis of PEs at trace levels. Intra- and interday variations were chosen to measure the precision of the developed method by analyzing spiked water samples at a concentration of 10 and 20 ng mL−1. Intraday precision was assessed by five extractions and determinations in 1 day, whereas interday precision was evaluated by five extractions and determinations in different days. The RSDs obtained for both of them presented acceptable precisions (Table 2); i.e., they were obtained in the range of 1.8−4.4% and 3.8−5.8% for intra- and interday assay, respectively. Comparison of the Proposed Technique with Other Methods. The performance of the proposed method in extracting and determining PEs in water samples was compared with those of other methods, and the results are shown in Table 3. The analytical performance of the proposed method has the following advantages. First, the utilization of the anionic surfactant SDBSA as an extractant without adding any toxic organic solvent and dispersant enabled true “green” microextraction. Second, SDBSA is a front eluting compound than nonionic surfactants and did not interfere with the analytes. Third, using DSMNPs as an adsorbent of the CAP simplified the phase-separation procedures that commonly use centrifugation or filtration during extraction. Although the LOD and linear range of the proposed method are poorer than GC methods,16,19−22,25 especially the LOD of DEHP (10 ng mL−1) is higher than the maximum limit established by the EPA (6 ng mL−1)3 but similar to other HPLC methods.9,24,26 These results revealed that this technique was environmentally friendly, simple, rapid, and effective. Moreover, this technique can also be used for PE preconcentration in water samples and extended to other applications. Analysis of Real Water Samples. The practical applicability of the developed method was evaluated by the extraction of DMP, DEP, BBP, DBP, and DEHP from water

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ASSOCIATED CONTENT

S Supporting Information *

The SEM figure for the MNPs and DSMNPs (Figure S1), the figure of Nd−Fe−B magnet (Figure S2), the figures of optimization steps (Figure S3−S9), the background chromatogram of the blank water sample, reagent blank sample and spiked water sample (Figure S10), and the comparison of the slopes (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 357 2057969. Fax: +86 357 2057969. E-mail: lmd@ dns.sxnu.edu.cn. Funding

Financial support by the National Natural Science Foundation of China (No. 21171110), the Research Fund for the Doctoral Program of Higher Education of China (No.20091404110001), and the Shanxi Province Natural Science Foundation for Youths (No. 2014021018−3) is gratefully acknowledged. Notes

The authors declare no competing financial interest. 7687



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