Extraction of phthalate esters from water and beverages using a graphene
ORIGINAL PAPER
Extraction of phthalate esters from water and beverages using a graphene-based magnetic nanocomposite prior to their determination by HPLC
Qiuhua Wu &Min Liu &Xiaoxing Ma &Weina Wang &Chun Wang &Xiaohuan Zang &Zhi Wang
Received:16September 2011/Accepted:12December 2011/Published online:24December 2011#Springer-Verlag 2011
Abstract We have developed a highly sensitive microex-traction method for the preconcentration of some phthalate esters such as diethyl phthalate,di-n -propylphthalate,di-n -butyl-phthalate,dicyclohexyl-phthalate,and diethyl-hexyl phthalate prior to their determination by HPLC.It is based on a magnetic graphene nanocomposite as an effective adsorbent.The effects of the amount of the extractant com-posite employed,extraction time,pH values,salt concentra-tion and desorption conditions were investigated.Under the optimum conditions,the enrichment factors range from 1574to 2880.Response is linear in the concentration range from 0.1to 50ng mL ?1.The limits of detection (at S /N 03)were between 0.01and 0.04ng mL ?1.The method was successfully applied to the determination of five phthalate esters in water and beverage samples.
Keywords Magnetic graphene nanocomposite .
Microextraction .Phthalate esters .High performance liquid chromatography
Introduction
Each year,large quantities of phthalate esters (PAEs)are produced all over the world for the manufacture of a wide variety of common consumer goods.PAEs are used princi-pally as plasticizers to improve flexibility,workability and durability of polymeric materials,but they can also be found
in products such as paints,adhesives,inks,and cosmetics [1,2].PAEs are bound to polymers through weak secondary molecular interactions with polymer chains.Since they are not chemically but only physically bound to the polymer,they can be released easily from products,migrate into environment,and,consequently,pollute water,soil,air and food products [3].PAEs have become ubiquitous pollutants in the environment,and several PAEs are suspected to be human cancer causing agents and endocrine disruptors.The United States Environmental Protection Agency (US EPA)has listed PAEs as the priority contaminants [4,5].Very recently,a variety of food contamination cases have been reported by different media in Taiwan due to the addition of cloudy agents and emulsifiers in food processing,and many foods including some beverages were heavily contaminated with PAEs,such as DEP,DEHP and DBP.The intensive use of PAEs and their pollutions have become a more and more serious problem worldwide and also a major public health concern.Therefore,for the sake of human health and envi-ronment protection,the development of simple,sensitive and reliable analytical methods for the determination of these compounds in different samples is necessary.
To measure a trace level of contaminants in a sample,sample preconcentration is usually necessary before instru-mental analysis.Up to now,various pretreatment techniques have been attempted to extract PAEs from different samples,such as liquid-liquid extraction (LLE)[6,7],solid-phase extraction (SPE)[8–13],solid-phase microextraction (SPME)[14–18],liquid-phase microextraction (LPME)[19],LPME method based on the solidification of a floating organic microdrop (LPME-SFO)[20],accelerated solvent extraction (ASE)[21],stir-bar sorptive extraction (SBSE)[22],single-drop microextraction (SDME)[23],dispersive liquid-liquid microextraction (DLLME)[24–26],hollow
Q.Wu :M.Liu :X.Ma :W.Wang :C.Wang :X.Zang :Z.Wang (*)
Key Laboratory of Bioinorganic Chemistry,College of Science,Agricultural University of Hebei,Baoding 071001,Hebei,China
e-mail:zhiwang1963@https://www.sodocs.net/doc/0419017085.html,
Microchim Acta (2012)177:23–30DOI 10.1007/s00604-011-0752-7
fiber-based liquid-phase microextraction(HF-LPME)[27], polymer monolith microextraction(PMME)[28],super-critical fluid extraction[29],and ultrasound-assisted emulsification–microextraction(USAEME)[30].Among them,SPE and SPME are the mostly used techniques for environmental analysis.However,SPE could be tedious and time-consuming;for SPME,the SPME fibers are relatively expensive and the polymer coatings are fragile and easily broken.Recently,a new mode for SPE,based on the use of magnetic or magnetically modified adsorbents called magnetic solid-phase extraction(MSPE)has been developed. Compared with traditional adsorbents,magnetic adsorbents can make separation process easier and faster without the need of additional centrifugation or filtration procedures.MSPE can avoid the time-consuming column passing operations encountered in SPE.Thus far,magnetic adsorbents have been widely applied in many fields including analytical chemistry, medicine,biotechnology,and so on.However,there are only a very few reports about their use for the extraction of PAEs.Niu et al.have prepared alginate-polymer-caged, C18-functionalized magnetic titanate nanotubes for the fast and efficient extraction of PAEs from water samples [31].Zhang et al.have prepared barium alginate caged Fe3O4@C18magnetic nanoparticles for the preconcentra-tion of PAEs from environmental water samples[32]. More recently,Meng et al.have reported applying polypyrrole-coated magnetic particles for the micro solid-phase extraction of PAEs in water[33].However,the magnetic adsorbents in these studies were mainly applied to the analysis of water samples,and their applications for more complex matrix sam-ples were not reported.
Graphene(G),a novel carbon material,is one-atom-thick two-dimensional(2D)layers of sp2-bonded carbon[34]. With a large delocalizedπ-electron system,G can form a strongπ-stacking interaction with the benzene ring[35],so it could be a good adsorbent for the adsorption of benzenoid form compounds.So far,G-based composites have been applied for the extraction of polycyclic aromatic hydrocar-bons[36]and pyrethroid pesticides[37].However,the applications of G-based magnetic nanocomposite as the adsorbent for the extraction or removal of organic pollutants are still very few in the literature although the introduction of magnetic properties into G could combine the high adsorption capacity of the G and the separation conve-nience of the magnetic materials.Chandra et al.have prepared magnetite-reduced graphene oxide composites for arsenic removal[38].Cao et al.have applied mag-netic CoFe2O4-functionalized graphene sheets to remove methyl orange[39].Luo et al.have used Fe3O4@SiO2/ graphene for the extraction and determination of sulfon-amide antibiotics in water samples[40].
The aim of the present work was to explore the potential application of G-based magnetic nanocomposite(G-Fe3O4)for the extraction of PAEs in water and beverage samples. Five PAEs including diethyl-phthalate(DEP),di-n-propyl-phthalate(DPP),di-n-butyl phthalate(DBP),dicyclohexyl phthalate(DCP),and diethylhexyl-phthalate(DEHP)were selected as model compounds.Several important experi-mental parameters affecting the extraction efficiencies such as the amount of the G-Fe3O4,extraction time,desorption conditions,sample pH,and salt addition were studied.To the best of our knowledge,this is the first report about the use of G-Fe3O4for the determination of PAEs in water and beverage samples.
Experimental
Reagents and materials
Graphite powder(50meshes)was purchased from the Boaixin Chemical Reagents Company(Baoding,China). Standards of PAEs(DEP,DPP,DBP,DCP and DEHP)were purchased from Aladdin-reagent(https://www.sodocs.net/doc/0419017085.html,/).Ammonium ferrous sulfate and ammonium ferric sulfate were obtained from Chengxin Chemical Reagents Company(Baoding,China).Acetonitrile,acetone, hydrochloric acid(HCl),sodium hydroxide(NaOH),and all other reagents were purchased from Beijing Chemical Reagents Company(https://www.sodocs.net/doc/0419017085.html,/).The water used throughout the work was double-distilled on a SZ-93 automatic double-distiller purchased from Shanghai Yarong Biochemistry Instrumental Factory(http://yarong.instrument. https://www.sodocs.net/doc/0419017085.html,/).
A mixture stock solution containing each of DEP,DPP, DBP,DCP,and DEHP at100.0μg mL?1was prepared in methanol.A series of standard solutions were prepared by mixing an appropriate amount of the stock solution with double-distilled water in a10mL volumetric flask.All the standard solutions were stored at4°C and protected from light.
G and G-Fe3O4was synthesized according to the method reported in our previous work[41],and characterized by scanning electron microscopy.
River water was collected from Tang River(Baoding, China);bottled water and beverages were purchased from local supermarket.
Apparatus and HPLC procedures
HPLC was carried out on a LC-20AT liquid chromatogra-phy(Shimadzu,https://www.sodocs.net/doc/0419017085.html,/)with two LC-20AT VP pumps and a SPD-20A UV/vis detector. Chromatographic separations were performed on a Promosil C18column(150mm×4.6mm I.D.,5.0μm)from Agela Technologies(https://www.sodocs.net/doc/0419017085.html,/).The
24Q.Wu et al.
mobile phase was methanol-water with the following gradient elution:0–4min,from 68%methanol to 70%methanol;4–45min,from 70%methanol to 95%meth-anol;45–50min,95%methanol to 68%methanol.The flow rate of the mobile phase was 1mL min ?1.The UV monitoring wavelength was chosen at 225nm.
The size and morphology of the magnetic nanoparticles were observed by scanning electron microscopy (SEM)using a FEI Quanta 200F field emission electron microscope (https://www.sodocs.net/doc/0419017085.html,/default.aspx )operated at 30kV .The magnetic property was analyzed using a JDM-13vibrating sample magnetometer (https://www.sodocs.net/doc/0419017085.html,/newjlu/)at room temperature.MSPE procedure
The MSPE procedure for the extraction of the five PAEs from the samples:Firstly,25mg G-Fe 3O 4was added into 300mL of sample solution,and then the mixture was placed on a slow-moving platform shaker and equilibrated for 15min.Secondly,a strong magnet was deposited at the bottom of the beaker and the G-Fe 3O 4was isolated from the solution.After about 5min,the solution became limpid and the supernatant was decanted.Then the residual solution and G-Fe 3O 4was totally transferred to a 10mL centrifuge tube.The G-Fe 3O 4was aggregated again by positioning a magnet to the outside of tube wall so that the residual solution could be completely removed by pipette.Finally,the preconcentrated analytes were eluted from the isolated particles with 0.5mL acetone by vortexing for 10s.After positioning a magnet to the outside of the centrifuge tube,the supernatant solution was collected using a micropipette.Three replicate desorptions were performed.The desorption solutions were combined together and transferred to a 2mL microcentrifuge tube,and then evaporated to dryness under a mild nitrogen stream.The residue was dissolved in 100.0μL methanol and 20.0μL was injected into the HPLC system for analysis.
For calibration curve,a series of working solutions con-taining each of the PAEs at seven concentration levels of 0.1,0.5,2.0,5.0,10.0,20.0and 50.0ng mL ?1were pre-pared.For each level,five replicate experiments were
performed according to the MSPE procedure.The recovery test was carried out by spiking the PAEs standard solution into the samples at two concentration levels (0.5and 5.0ng mL ?1).
Results and discussion
Characterization of the magnetic graphene nanoparticles The SEM images of both the G and G-Fe 3O 4are shown in Fig.1.As can be seen from Fig.1a ,G has a crumpled silk wave-like carbon sheet structure,which is a characteris-tic feature of the single-layer graphene sheets.Iron oxide nanoparticles were also successfully coated on the sur-face of the G to form a G-Fe 3O 4nanocomposite (Fig.1b ).The average size of the Fe 3O 4nanoparticle estimated from the SEM observation was about 20nm.The Fe 3O 4nanoparticles were well distributed on graphene sheets,which were nearly flat and had a big area up to several square micrometers.Some nanoparticles were slightly aggregated due to the close to saturation loading degree.
It is important that the adsorbents possess sufficient mag-netic properties to realize rapid separation under a magnetic field.Figure 2shows the VSM magnetization curves of the Fe 3O 4and G-Fe 3O 4nanoparticles at 298K.Both Fe 3O 4and G-Fe 3O 4exhibit typical superparamagnetic behavior.The saturation magnetization intensity of Fe 3O 4and G-Fe 3O 4were 71.9and 72.8emu g ?1,respectively,which are suffi-cient for their magnetic separation from a solution with a strong magnet.
Effect of the amount of G-Fe 3O 4
In order to select the optimum MSPE conditions for the extraction of the PAEs,300mL double-distilled water spiked with 5ng mL ?1each of the five PAEs was used to study the extraction performance of the MSPE under differ-ent experimental conditions.All the experiments were per-formed in triplicate and the means of the results were used for
optimization.
Fig.1Scanning electron micrographs of G (a )and G-Fe 3O 4composite (b )
Extraction of phthalate esters from water and beverages 25
The equilibrium adsorption quantity (q eq in mg g ?1)of the analyte by the magnetic graphene nanocomposite was calculated by q eq ?
C 0àC eq
m
V Where C 0(mg L ?1)represents the initial concentration of the analyte,C eq (mg L ?1)is the equilibrium concentration of the analyte remaining in the solution,V (L)is the volume of the aqueous solution,m (g)is the weight of the magnetic graphene nanocomposite.The value of q eq is related to C 0.To investigate the adsorption capacity of the G-Fe 3O 4toward the target compounds,25mg G-Fe 3O 4was added into 300mL of aqueous solution containing each of DEP,DPP,DBP,DCP and DEHP at 100.0μg mL ?1.The values of q eq for DEP,DPP,DBP,DCP,and DEHP were 122,440,526,443,and 581mg g ?1,respectively.
To choose the optimum amount of the adsorbent (G-Fe 3O 4),the amount of the G-Fe 3O 4required for the quantitative extraction of the PAEs was investigated with the extraction time of 20min.According to the results shown in Fig.3,among the amounts investigated,i.e.,6,12,18,25,30and 50mg,the maximum extraction efficiency was obtained
at 25mg of G-Fe 3O 4(corresponding to its concentration 0.083mg/mL in the sample).When the amount of the adsorb-ents was above 25mg,the curves turned out to be flat,and there was no distinct increase of the extraction efficiency.Therefore,25mg G-Fe 3O 4was selected.The results proved that the G-Fe 3O 4sorbent has a high adsorption capacity and a good extraction efficiency can be achieved by using only a small amount of the sorbent.Extraction time
Extraction time is also an important parameter affecting the extraction efficiency to a large extent.For the study of the effect of the extraction time on the extraction efficiency of the PAEs,different extraction times were investigated.The results indicated that the recoveries of all the PAEs were enhanced with increased extraction time from 1to 15min,and then remained almost unchanged.Hence,the extraction time of 15min was selected.This result showed that the extraction equilibrium could be attained in a very short time.Influence of sample solution pH and salinity
The pH of the sample solution could play an important role for the adsorption of the analytes to the sorbents by both affecting the existing form of the compounds and their charge species and densities on the sorbents surface.The effect of sample pH on the extraction efficiency was inves-tigated in the pH range of 2.0–10.The experimental results showed that the extraction efficiency almost had no signif-icant changes with the changes of the pH of the sample solution.This could be because the PAEs exist as neutral molecules under ordinary conditions and are relatively insusceptible to the changes of sample solution pH.Normally,the pH of the water samples was in the range from 6.0to 7.0;the pH of Coca-Cola and green tea was about 2.5and 6.0.Therefore,there is no need to adjust the pH of the sample solution.
In most cases,the addition of salt can decrease the solubil-ity of organic analytes (salting-out effect)and increase the
-80
-60-40-200204060
80M a g n e t i z a t i o n (e m u g -1
)
Applied magnetic field (Oc)
Fig.2VSM magnetization curves of Fe 3O 4and G-Fe 3O 4
20
40
60
80
100
R e c o v e r y (%)
The amount of G - Fe 3O 4 (mg)
Fig.3Effect of G-Fe 3O 4dosage on the extraction efficiency of the PAEs
Table 1Analytical performance data for the PAEs by the MSPE technique PAEs
LR (ng mL ?1)r
RSD (%)(n 06)EF LOD (ng mL ?1)
DEP 0.1-500.9986 5.622680.02DPP 0.1-500.9999 4.528200.02DBP 0.1-500.9999 5.628800.01DCP 0.1-500.9993 4.625560.03DEHP
0.1-50
0.9998
5.2
1574
0.04
LR linear range.
26
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distribution constant,but on the other hand,it can also in-crease the viscosity of the solution,which will reduce the extraction capability and the diffusion coefficient.To investi-gate the effect of sample salinity on the extraction recoveries of the analytes,different concentrations of NaCl,i.e.0,0.5, 1.0,2.0,5.0%(w/v)were added into the solution.The result showed that no significant effect on the extraction recoveries of the PAEs was observed with the addition of NaCl from0to 5%.So,there is no need to add salt to the sample solution for the experiment and the effect of salinity in real samples on the extraction of the analytes should be negligible. Desorption conditions
It is necessary to completely desorb the analytes from the G-Fe3O4particles for further HPLC–UV analysis.In this work, acetonitrile,methanol and acetone were tried as the desorp-tion solvent for the desorption of the analytes from the
Table2Comparison of presented method with other microextraction techniques
Methods Linearity(ng mL?1)LOD(ng mL?1)RSD(%)EF Samples Ref
SPE-HPLC-UV0.6–500.12–0.17 4.1–5.9600water[9]
SPE-HPLC-UV 2.0–1000.18–0.86––water[10]
SPE-HPLC-UV0.01–200.002–0.033 3.7–6.7–water[44] SPME-GC-MS0.1–200.003–0.085 1.38–21.7–bottled water[18] SPME-GC-FID a–0.003–3.429 1.69–13.51–bottled beer[16]
MIP b-SPME-GC-MS0.01–100.0022–0.021 1.5–8.04–water[42]
SFO-LPME-GC-MS0.05–1000.02–0.05 5.5–7.7307–412water[45]
SPE-HPLC-FD c0.1–200.019–0.039–500water[11] DLLME-HPLC-UV2–1000.68–1.36 2.2–3.7174–212water[43] PMME-HPLC-UV3–50000.7–3.7 1.4–7.7–Cosmetic product[28] MSPE-HPLC-UV0.1–200.019–0.059–500–1000water[32] MSPE-HPLC-UV0.1–500.01–0.04 4.5–5.61574–2880water and beverage This method
a FID:flame ionization detector.
b MIP:molecularly imprinted polymer.
c FD:fluorescence detector.
Table3Recoveries obtained in the determination of PAEs in spiked water and beverage samples
PAEs Spiked
(ng mL?1)
Bottled water(n05)River water(n05)Cola(n05)Green tea(n05)
Found (ng mL?1)R b(%)RSD(%)Found
(ng mL?1)
R b(%)RSD(%)Found
(ng mL?1)
R b(%)RSD(%)Found
(ng mL?1)
R b(%)RSD(%)
DEP0.0nd a nd a nd a nd a
0.50.4386.0 5.10.4080.0 5.70.4386.0 5.90.4182.0 5.5
5.0 4.4288.4 4.8 4.3587.0 5.9 4.2585.0
6.2 4.1583.0 5.8 DPP0.0nd a nd a nd a nd a
0.50.4488.0 5.20.4284.0 6.20.4590.0 4.20.4794.0 4.9
5.0 4.5791.4 4.1 4.5891.6 5.4 4.6392.6 4.4 4.6192.2 4.7 DBP0.0nd a0.12nd a nd a
0.50.4590.0 5.50.65106.0 5.20.4488.0 5.40.4182.0 4.6
5.0 4.6192.2 4.6 5.17101.0
6.2 4.7294.4 6.1 4.3086.0 5.1 DCP0.0nd a nd a nd a nd a
0.50.4794.0 4.80.52104.0 5.90.4590.0 5.60.4692.0 5.4
5.0 4.829
6.4 4.2 4.889
7.6 6.3 4.6893.6 5.5 4.7294.4 5.2 DEHP0.0nd a0.15nd a nd a
0.50.4386.0 5.70.6498.0 4.60.52104.0 4.70.4896.0 4.7
5.0 4.7895.6 5.0 4.7592.0 5.1 4.7595.0 5.8 4.5891.6 5.0
a nd:not detected;
b R:recovery of the method.
Extraction of phthalate esters from water and beverages27
magnetic adsorbents.The results showed that the eluting power of acetone was much stronger than methanol and acetonitrile.Thus acetone was selected as the desorption solvent.The influence of the acetone volume on the desorp-tion efficiency of the analytes was also investigated.It was found that the quantitative desorption of the analytes were achieved with 1.5mL (0.5mL each time and three times)of acetone.The combined desorption solution (1.5mL)in a 2mL microcentrifuge tube was evaporated to dryness under a mild nitrogen stream.The residues were dissolved in 100.0μL methanol and 20.0μL was injected into the HPLC system for analysis.Analytical performance
Under the above optimized conditions,some analytical per-formance parameters,including linear range (LR),correla-tion coefficients (r ),and limit of detection (LOD)were investigated.The characteristic calibration data obtained are listed in Table 1.The linear response was observed over the concentration range of 0.10–50ng mL ?1,with the r ranging from 0.9986to 0.9999.The LODs (S /N 03)ranged
between 0.01and 0.04ng mL ?1for the PAEs.The enrich-ment factors (EF ),defined as the ratio between the analyte concentration in 0.1mL methanol and the initial analyte concentration in the aqueous samples,were in the range between 1574and 2880,indicating that the graphene-based magnetic nanocomposite exhibited a high adsorption capacity for the analytes.
To assess the precision of the measurement,the repeat-ability study was carried out by performing six parallel experiments at the concentration of 2ng mL ?1for each of
0Fig.4The typical chromatograms of (a )blank sample and (b )sample spiked with PAEs at each concentration of 0.5ng mL ?1(225nm).(A )River water sample;(B )Coca Cola sample;Peak identification:(1)DEP,(2)DPP,(3)DBP,(4)DCP,(5)DEHP
Table 4Analytical results in the determination of the PAEs in certified reference materials (n 04)Sample
Certified (ng mL ?1)Found (ng mL ?1)Recovery (%)
RSD (%)
DEP 10.09.393.0 5.1DPP 10.09.595.0 4.5DBP 10.09.595.0 4.3DC P 10.09.797.0 4.2DEHP
10.0
9.6
96.0
3.8
28
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the PAEs.The resultant repeatabilities expressed as the relative standard deviations(RSDs)varied from4.5%to 5.6%.The above results suggest that the present method has a high sensitivity,wide linear range and good precision.
Comparison with other extraction methods
The performance of the current method for the extraction and determination of the PAEs in water samples were com-pared with the other reported extraction methods from the viewpoint of LR,LODs,RSDs and EF.The comparison results are shown in Table2.It can be seen from Table2 that the EF of the current method is much better than that obtained with SPE,DLLME and SFO-LPME methods.For the MSPE-HPLC-UV method with Fe3O4@C18@barium alginate(Ba2+-ALG)as the adsorbent[32],its sensitivity was similar to the present method,but the dosage used (20mg per100mL sample)was higher than that of the present method(8.3mg per100mL sample).This indicated that the G-Fe3O4has a high adsorption ability for the PAEs. Compared with traditional adsorbents,magnetic adsorbents can make separation process easier and faster without the need of additional centrifugation or filtration procedures and also can avoid the time-consuming column passing opera-tions encountered in SPE.Therefore,it was much easier to deal with large volume samples to obtain high enrichment factors and high https://www.sodocs.net/doc/0419017085.html,pared with the non-selective extraction methods,such as LPME and DLLME, SPE,MSPE and MIP-SPE have a better sample clean-up ability,which are more suitable for the extraction of the analytes in complicated matrix samples.
Analysis of real samples
In order to validate the applicability of the developed method, it was applied to determine the analytes in bottled water,river water,Cola and green tea samples.The results are shown in Table3.Among the four samples,no PAEs were detected in bottled water,Coca-Cola or green tea samples.Trace levels of DBP(0.12ng mL?1)and DEHP(0.15ng mL?1)were found in river water.The recoveries for the PAEs were in the range from80.0%to106.0%,which showed that the developed method was feasible in the application of real sample analysis. The typical chromatograms of the PAEs for the river and Coca Cola samples are shown in Fig.4.
The accuracy of the developed method was further eval-uated by determining the PAEs in Standard Reference Material for Environment Water after an appropriate dilu-tion.The results obtained with the current method are in good agreement with the certified values(Table4),indicat-ing that the method is reliable for the determination of the PAEs in real samples.Conclusions
In the present study,a G-based magnetic nanocomposite was synthesized and used as an effective adsorbent for the pre-concentration of some PAEs in water and beverage samples. The sorbent could extract and enrich the PAEs from the samples efficiently and the use of magnetic nanocomposite as the adsorbent endowed the method with an easy separation of the adsorbent from sample solution.The G-Fe3O4used in this work has the merits of excellent adsorption capacity and strong magnetism.The results indicated that the developed method was fast and efficient for the preconcentration of trace levels of PAEs in water and beverage samples and the adsor-bent could have a great potential for the enrichment of other environmental pollutants in other aqueous samples. Acknowledgements Financial support from the National Natural Science Foundation of China(No.31171698),the Natural Science Foundations of Hebei(B2010000657)and the the Scientific Research Foundation of Education Department of Hebei Province(2009132)is gratefully acknowledged.
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