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Magnetic Solid-Phase Extraction Clean-Up Combined with Solidified Floating Organic Drop Microextraction for Determination of Trace Mercury (II) in Tea Samples.

Byline: Yi He 2Ning Li and Jing Jun Ma

Summary: Magnetic solid-phase extraction (MSPE) clean-up combined with solidified floating organic drop microextraction (SFODME) has been developed as a new approach for extraction and determination of trace amounts of mercury (II) in tea samples. After purification of the samples by MSPE with the use of Fe3O4@SiO2/graphene as sorbents SFODME technique was used to concentrate the target analytes. Various experimental parameters that could affect the extraction efficiencies were investigated. Under optimized experimental conditions a preconcentration factor of 23.0 was obtained using 15.0 mL sample solution. The calibration graph was linear from 0.05g/L to 5.0 g/L with a detection limit of 4.0 ng/L. The relative standard deviation for 10 replicate measurements of 0.1 and 1.0 g/L of mercury (II) were 4.27% and 3.95% respectively.

The proposed method was applied in the analysis of four tea samples and the accuracy of the method was assessed through the analysis of certified reference materials and recovery experiments.

Keywords: Magnetic solid-phase extraction; Solidified floating organic drop microextraction; Hydride generation atomic fluorescence spectrometry; Graphene; Mercury

Introduction

Magnetic solid-phase extraction (MSPE) has elicited much interest in complex matrix sample preparations [1-3]. MSPE adopts magnetic particles as sorbents which endow some unique features. For instance the magnetic particles can be readily isolated from the sample matrix with the use of a magnet. Compared with isolation of conventional sorbents by filtration or centrifugation magnetic isolation is significantly more convenient economic and efficient [4]. In addition the sorbents in MSPE are universally dispersed into the sample solution to achieve purification. In such dispersive mode the contact area between the sorbents and the sample solution is sufficiently large to remove the interfering substances. These merits render MSPE as a promising technique for sample clean-up.

In addition MSPE kernel is used as sorbent. In general pure magnetic particles (Fe3O4) have some inherent limitations because they form aggregates and their magnetic properties are altered in complex environmental and biological system [5]. As a result a suitable protective coating on a magnetic core is often used. Silica has been considered as ideal shell materials because of its reliable chemical stability biocompatibility and versatility in surface modification [6]. Thus silica-coated Fe3O4 nanoparticles (Fe3O4@SiO2) could be used for rapid separation. Graphene is an electron-rich hydrophobic nanomaterial with p-p electrostatic stacking property [7] it has been used as an effective adsorbent material since its discovery in 2004. However separating or retrieving graphene from aquatic phase is difficult because of its small size [8].

If the advantages of graphene and magnetic-silica nanoparticles are combined to fabricate nanosized MSPE adsorbents with high surface area high chemical stability and good magnetic separability a new kind of nanocomposite MSPE sorbent (Fe3O4@SiO2 /graphene) can be obtained to enhance the clean-up effect and remove the coloring interfering substances.

Solidified floating organic drop microextraction (SFODME) [9] is a new liquid-phase microextraction technique in which small volume of an organic solvent with a melting point near room temperature (in the range of 10 oC to 30oC) floated on the surface of aqueous solution. SFODME has been applied in the preconcentration of organic compounds and metal ions [10-14]. SFODME is simple and low cost with minimum organic solvent consumption and high enrichment factor [15]. However SFODME is not a selective extraction method. This is the main reason why most of the reported applications of SFODME have been focused on simple water samples [16-20]. Therefore exploration of the potential applications of SFODME technique in more complex matrix samples is desirable.

In the current work the applicability of the MSPE cleanup followed by SFODME for the extraction of trace mercury (II) in tea samples prior to their analysis by hydride generation atomic fluorescence spectrometry (HG-AFS) was explored. Fe3O4@SiO2/graphene was used as sorbent in MSPE to remove the color-interfering substances from tea samples. This study aims to establish a preconcentration method in tea samples which can isolate the analytes from environmental matrices and reduce the matrix effects effectively.

Results and Discussion

Optimal Extraction Conditions

After samples purification by MSPE the sample solutions were subjected to SFODME. The optimum values of the parameters affecting the SFODME procedure such as extraction solvent type and extraction volume time temperature pH amount of chelating agent and salt effect were determined.

Selection of the Extraction Solvent

The extracting solvent for SFODME should form a cloudy solution in aqueous phase. In addition it must have a lower density than water high extraction capability for the compounds of interest low volatility low water solubility and melting point near room temperature. Several extracting solvents including 1-bromohexadecane (melting point 17.3C) 1-undecanol (melting point 13C to 15C) hexadecane (melting point 18C) and 1-dodecanol melting point 24C) were investigated. 1-Undecanol was selected because of its stability lower price low water solubility and low vapor pressure. 1-Undecanol also has the highest extraction efficiency. By contrast with 1-bromohexadecane and hexadecane the dispersed drop could not be aggregated completely after centrifugation. With 1-dodecanol the extraction efficiency was approximately 64% that of 1-undecanol. Thus 1-undecanol was selected as extracting solvent.

Effect of Extraction Solvent Volume

To determine the effect of the extraction solvent volume different volumes of 1-undecanol ( 30 L to 100 L at 10 L intervals) were examined in the preconcentration procedure. The results are shown in Fig. 1. The fluorescence intensity increased with the increase of 1-undecanol volume from 30 L to 60 L and then remained constant with further increase in volume. Therefore 80 L 1-undedanol was selected to achieve higher enrichment factor and better repeatability.Extraction conditions: sample volume 15.0 mL; APDC volume 60 L; pH 5.0; Hg2+ concentration 1.0 g L ; extraction time 2.0 min; and extraction temperature 25 C.

Effect of pH

The pH has a unique function on metal-chelate formation and its subsequent extraction. Therefore the effect of pH on the extraction of hydrophobic chelate of Hg with APDC from 15.0 mL of aqueous phase into 80 L of 1-undecanol was studied from pH 2.0 to pH 10.0. The pH was adjusted using nitric acid ammonium acetate phosphate and ammonium chloride. The results show that the analytical signal is nearly constant from pH 5.0 to pH 7.0 (Fig. 2). The progressive decrease in extraction of interested metal ions at low pH is due to competition of hydrogen ion with analytes for reaction with APDC. Accordingly pH 5.0 was selected for subsequent work and tea samples analysis.

Effect of the Amount of APDC

The effect of the amount of APDC (10-5 mol/L) on the fluorescence intensity was studied and the results are shown in Fig. 3. The fluorescence intensity was stable when the APDC volume wasgreater than 50 L indicating complete extraction. When the volume of APDC was greater than 70 L the analytical signal was decreased. This effect is probably caused by competition between complexing agent molecules which are in excess in the solution and by Hg-complex molecules for extraction solvent interaction. Thus APDC volume of 60 L was chosen as the optimum amount for the mercury (II) determination.

Extraction conditions: sample volume 15.0 mL; 1-undecanol volume 80 L; pH 5.0; Hg2+ concentration 1.0 g L-1; extraction time 2.0 min; extraction temperature 25oC. Effect of the Extraction Time

In SFODME extraction time is defined as the time between injection of extraction solvent and the end of the sonication stage [21]. The effect of extraction time on the extraction efficiency was examined by varying the extraction time from 2.0 min to 60.0 min at constant experimental condition. The extraction of Hg2+ was found to be significantly rapid and independent of the time in these periods. Thus 2.0 min was selected as the extraction time in the following experiments.

Effect of Extraction Temperature

The effect of temperature on extraction was studied by varying the temperature between 10 and 60C; the results show that the extraction are quantitative(greater than 95%) from 20C to 30C; but a further increase in temperature cause a slight decrease in recovery which may be due to increase in solubility of organic phase at higher temperature and complex degradation. The decreases in extraction at temperature less than 20 C may be due to improper dispersion of the organic phase at low temperature. Thus further experiments were performed at room temperature.

Effect of Salt

Salting-out is a process of addition of electrolytes to an aqueous phase to increase the distribution ratio of a particular solute. The term also connotes reduction of mutual miscibility of two liquids by adding of electrolytes. Accordingly the effect of salt on extraction efficiency was studied by varying the NaCl concentration from 0% to 5% (w/v). The results showed that the addition of salt has no significant effect on extraction efficiency.

Optimal Back- Extraction Conditions

Back-Extraction Solutions Type and its Volume

Given the high viscosity and low volatility of 1-undecanol subsequent hydride generation analysis was unfavorable. Therefore ultrasound-assisted back extraction was employed as a supplement stage for coupling SFODME to HG-AFS.

Various back-extraction solutions were studied to evaluate their efficiencies by adding 600 L of the solutions to the coacervate phase with sonication of the resulting mixture for 3.0 min. Among the various back extraction solutions examined including HCl HNO3 H2SO4 and CH3COOH the highest sensitivity was obtained with the use of HNO3. Therefore HNO3 was used as the back-extraction solution. Moreover the effect of the concentration of nitric acid (2.0 mol/L to 8.0 mol/L) was also studied.

Effect of Coexisting Ions

The efficiency of the method in extraction and preconcentration of Hg in the presence of various cations and anions were examined (Table 1). The tolerance limit was considered as the concentration of the interfering ions that caused a variation in the fluorescence intensity of the Hg(II) in the sample within 5%. At the given level no significant interference was observed.

Figures of Merit

The analytical characteristics of the MSPE-SFODME-HG-AFS method for the determination of Hg(II) were evaluated. Under the optimized conditions the calibration graph was linear from 0.05 g/L to 5.0 g/L. The calibration equation is I=779.62C + 25.376 with a correlation coefficient of 0.9995 where I is the fluorescence intensity of Hg obtained by peak area; and C is the concentration (g/L) of Hg2+ in the sample solution. On the basis of three fold background noise the limit of detection (LOD) was found to be 4.0 ng L-1 for Hg(II). The relative standard deviation (RSD) for 10 replicate measurements of 0.1 and 1.0 g/L of Hg(II) were 4.27% and 3.95% respectively.

Table-1: Effect of interfering ions on recovery of 1.0 g L-1 Hg2+ in samples using MSPE-SFODMEAFS.

Interferent###Concentration###Interferent/Hg2+###Recovery

###(g L-1)###ratio###(%)

Na +###100000###100000###95.0

K+###100000###100000###96.3

Ag+###1000###1000###95.4

Mg2+###1000###1000###95.8

Ca2+###1000###1000###96.2

Ba2+###500###500###96.7

Pb2+###500###500###96.4

Fe2+###100###100###97.2

Cd2+###100###100###96.8

Co2+###80###80###96.0

Ni2+###80###80###96.5

Al3+###50###50###97.2

As3+###50###50###96.4

Fe3+###50###50###96.3

Cr3+###50###50###97.1

Bi3+###40###40###95.4

Cu2+###20###20###95.0

Zn2+###20###20###95.5

Cl-###100000###100000###96.5

NO3-###100000###100000###96.2

I-###100000###100000###96.7

CH3COO-###100000###100000###96.0

SO42-###1000###1000###95.5

Cr2O72-###1000###1000###95.6

PO43-###1000###1000###95.8

Equation 1 was used to calculate the preconcentration factor. In this equation CS is the Hg(II) concentration (g/L) in the floating organic drop after phase separation and C0 is the initial concentration of Hg(II) (g/L) [22].Equation

For the determination of the fluorescence intensity for Hg(II) in the floating organic drop the extraction solvent was diluted with nitric acid to a volume of up to 600 L; the preconcentration factor for the proposed method is 23.00.53.

Analysis of Samples

The proposed method was used for Hg determination in several tea samples and the results along with the recovery for the spiked samples are given in Table 2. The recoveries for the addition of different Hg concentrations to tea samples were from 90.0% to 97.3%. To verify the accuracy of the proposed procedure the method was then used for the determination of Hg content in the National Information Center for tea leaf (GBW07605) and tomato leaf (GSBZ 51001-94). The results are also given in Table 2. Student t-test was used to examine two certified reference materials. The t0.05 2 values for Hg are also listed in Table 2. Good agreement was found between the determined values and the certified values.

Table-2: Analytical results of mercury (II) determination in certified reference materials and spiked tea samples using MSPE-SFODMEAFS method (n=3).

Sample###Certified###Added###Founda###Recovery (%)###t-testb

GBW07605

###0.013###0.0120.0007###92.3###2.47

###(g/g)

GSBZ 51001-94###

###0.1400.022###0.1340.006###95.7###1.73

###(g/g)###

###0.00###less than LOD###

###c

###Jasmine tea

###0.10###0.0940.004###94.0

###(g/g)

###0.15###0.1460.005###97.3

Chrysanthemum###0.00###less than LOD###

###d

###tea###0.10###0.0930.004###93.0###

###(g/g)###0.15###0.1420.005###94.7

###0.00###less than LOD###

###Longjing teae

###(g/g)

###0.10###0.0900.005###90.0

###0.15###0.1450.006###96.7###

###0.00###less than LOD###

Tie Guanyin teaf

###(g/g)

###0.10###0.0930.004###93.0

###0.15###0.1460.006###97.3

solutions were prepared daily by serial dilutions of the stock solution with deionized water immediately prior to analysis. An ammonium pyrrolidine dithiocarbamate (APDC) solution (10-5 mol/L) was prepared by dissolving the appropriate amount of APDC (Shanghai Chemistry Reagent Company Shanghai China) in deionized water. The extraction solvents namely 1-Bromohexadecane 1-undecanol hexadecane and 1-dodecanol were obtained from Aladdin Chemistry Reagent Company (Beijing China). A 1.0% (m/v) NaBH4 solution was prepared by dissolving NaBH4 in 0.5% (m/v) NaOH solution. Ferric chloride hexahydrate (FeCl36H2O) iron dichloride tetrahydrate (FeCl24H2O) and graphite powder (50 mesh) were obtained from Tianjin Tianda Chemical Reagent Company (Tianjin China).

Nitric acid (0.2 mol/L) was used to adjust the pH levels from 2.0 to 3.0. Ammonium acetate buffers (0.2 mol/L) were prepared by adding an appropriate amount of acetic acid to ammonium acetate solutions to produce pH 4.0 to pH 5.0 buffers. For pH 6.0 to pH 8.0 phosphate (0.2 mol/L) buffer solution was prepared by adding an appropriate amount of disodium hydrogen phosphate to sodium dihydrogen phosphate. Ammonium chloride buffer solutions (0.2 mol/L) were prepared by adding an appropriate amount of ammonia to ammonium chloride solutions resulting in solutions of pH 9.0 to pH 10.0 solutions.

All of the reagents used were analytical reagent grade. Deionized water was used in the glassware were kept in 10% nitric acid for at least 24 h and subsequently washed four times with deionized water before application Apparatus A continuous flow AFS-3100 atomic fluorescence spectrometer (Beijing Haiguang Instrument Company Beijing China) was employed for the analytical determinations. The optimal the pH measurements were conducted out using a pH3-3C digital pH meter equipped with a combined glass-calomel electrode (Hangzhou Dongxing Instrument Factory Hangzhou China). A Model LD5-2A centrifuge (Beijing Jingli Instrument Factory Beijing China) was used to accelerate the phase separation. A 53 kHz 200 W ultrasonic bath with temperature control (Shanghai Kudos Ultrasonic instrument Co Ltd. Shanghai P.R.China) was used for assisting the extraction process.

Table-3: Instrumental parameters for HG-AFS.

###Parameters###Setting

###Negative high voltage of PMT (V)###300

###Lamp current (mA)###50

###Atomizer height (mm)###8

###Carrier argon flow (mL min-1)###500

###Shield argon flow (mL min-1)###1000

###Read time (s)###10

###Delay time (s)###2

###Injection volume (L)###600

Synthesis of Magnetic Microspheres and Graphene

Fe3O4 nanoparticles were prepared by chemical coprecipitation method [23]. Magnetic microspheres coated with silica layer (Fe3O4@SiO2) were synthesized using previously reported methods [24 25]. Graphene nanoparticles were synthesized in accordance with our previously reported study [26].

Fe3O4@SiO2 (4.0 mg) and graphene (2.0 mg) were placed in a 20.0 mL vial and washed sequentially with pure water and acetone. After adding 5.0 mL pure water and 5.0 mL DMF the Fe3O4@SiO2/graphene dispersive solution was obtained by vortexing vigorously for 1.0 min (designated MGDS).

Preparation of Real Samples

Portions (0.5 g) of GBW07605 tea leaf (National Information Center for Certified Reference Materials Beijing China) were weighed into the beaker and 10.0 mL concentrated HNO3 and 3.0 mL of H2O2 were added The solution was heated until it become transparent after which it was continuously heated to near dryness. The residue dissolved in 0.1 mol/L HNO3 was diluted with deionized water to 100.0 mL. Tea samples were purchased from a local supermarket (Baoding China). After being dried and sieved through a small mesh size each tea sample was accurately weighed and prepared using the same procedure as GBW07605 tea leaf. The blank sample without analytes but with the same amount of digestion solution was prepared using the same procedure and subjected to digestion procedure.

MSPE-SFODME Procedure

An aliquot of MGDS was added into the sample solution and the mixture was sonicated vigorously for a prescribed period. An external magnet was then attached to the outside bottom of the vial and the Fe3O4@SiO2/graphene was gathered to the bottom of the vial. The supernatant was decanted and used in the subsequent SFODME procedures.

After MSPE purification 15.0 mL of the the standard or sample solution containing mercury was adjusted to pH 5.0 using ammonium acetate buffers and then transferred into a 20.0 mL screw-cap glass test tube with conical bottom. Up to 60 L APDC (10-5 mol/L) and 80 L of 1-undecanol were added sequentially. The conical tube was sonicated for 2.0 min at 25C to ensure complete extraction. The mixture was then centrifuged for 2.0 min at 3000 rpm. After this process fine droplets of 1-undecanol coalesced and the organic solvent was collected at the upper surface of the sample solution. The conical test tube was transferred into an ice bath and the organic solvent was solidified after 1.0 min. The solidified solvent was then transferred into another conical vial where it melted immediately. Ultrasound-assisted back-extraction was performed by adding 600 L of nitric acid (5.0 mol/L) in the resulting coacervate phase and sonicating the system for 3.0 min.

Finally the solution was transferred into an ice bath to remove the solidified solvent and the remaining liquid phase (~600 L) was introduced into the AFS with the use of the flow injection system.

Conclusion

In this study a MSPE cleanup method combined with SFODME was developed as a new mode for the extraction of trace amounts of Hg from tea samples prior to HG-AFS detection. The combination of the MSPE procedure with SFODME enables selective determination of the trace analytes in complex matrix samples. The method offer several advantages such as simplicity ease of operation relatively short analysis time and low consumption of organic solvent. Accordingly the proposed method possesses significant potential in the analysis of trace metal elements in complex matrix samples.

Acknowledgements

This project was sponsored by grants from the National Natural Science Foundation of China (No. 31401759); the Sci-technical Support Project of Hebei Provience (No. 13227124); the Youth Foundation of the Department of Education of Hebei Province (No. QN20131014); and the Science Foundation of the Agriculture University of Hebei(No. LG201305).

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