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Determination of lead at trace levels in mussel and sea water samples using vortex assisted dispersive liquid-liquid microextraction-slotted quartz tube-flame atomic absorption spectrometry
Chemosphere 189 (2017) 180e185
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Determination of lead at trace levels in mussel and sea water samples using vortex assisted dispersive liquid-liquid microextraction-slotted quartz tube-flame atomic absorption spectrometry € € zde Ozzeybek, Sezin Erarpat, Go Dotse Selali Chormey, Sezgin Bakırdere* _ Yıldız Technical University, Department of Chemistry, 34349 Istanbul, Turkey
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A novel DLLME-SQT-FAAS method was developed for the extraction and determination of lead at trace level. The applicability and accuracy of the method was verified with spiked recovery tests. Low %RSD values indicated high precision for the extraction process and instrumental measurements. The method was applied to mussel and sea water samples.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 August 2017 Received in revised form 9 September 2017 Accepted 15 September 2017 Available online 17 September 2017
In this study, dispersive liquid-liquid microextraction (DLLME) and slotted quartz tube (SQT) were coupled to flame atomic absorption spectrometry (FAAS) to increase the sensitivity of lead. Conditions such as the formation of the lead-dithizone complex, efficiency of the DLLME method and the output of the SQT were systematically optimized to improve the detection limit for the analyte. The conventional FAAS system was improved upon by about 3.0 times with SQT-FAAS, 32 times with DLLME-FAAS and 142 times with DLLME-SQT-FAAS. The method was applicable over a wide linear range (10e500 mg L1). The limit of detection (LOD) determined by DLLME-SQT-FAAS for seawater and mussel were 2.7 mg L1 and 270 mg kg1, respectively. The percent recoveries obtained for mussel and seawater samples (spiked at 20 and 50 mg L1) were 95e96% and 98e110%, respectively. © 2017 Elsevier Ltd. All rights reserved.
Handling Editor: Petra Petra Krystek Keywords: Lead Dispersive liquid-liquid microextraction Slotted quartz tube Flame atomic absorption spectrometry Seawater Biotissues
1. Introduction Lead can be found in the environment from natural sources and man-made activities, and it poses some risks to living organisms
* Corresponding author. E-mail address: [email protected] (S. Bakırdere). http://dx.doi.org/10.1016/j.chemosphere.2017.09.072 0045-6535/© 2017 Elsevier Ltd. All rights reserved.
and human being such as hematological, cerebral, and kidney malfunctions (Manahan, 1994; Huang and Keller, 2015). At the prenatal development of the brain, lead exposure might cause permanent adverse effects on neurocognitive function resulting in lower mental development and risk of schizophrenia later in life (Opler et al., 2004, 2008; Bellinger, 2013). Plumbism as a disease of lead poisoning has different symptoms such as hyperactivity, developmental delays, hearing loss, and even death (Hauptman
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et al., 2017). Lead can be entered into the human body in the form of inorganic lead and accumulated in soft tissues, bones, and teeth or as organic lead which accumulates in the brain because of its fatsoluble nature (Tchounwou et al., 2012). For this reason, there are several regulations on the release of lead into the environment. However, the automotive, ceramic, ink, pesticide and fertilizer producing industries make use of lead materials resulting in lead contamination (Comitre and Reis, 2005). In addition, lead can be easily transferred into drinking water by means of corrosion and old lead pipes found in several living places (Cartier et al., 2012; Frisbie et al., 2015). According to the World Health Organization (WHO), the tolerable weekly amount of lead for humans is 25 mg kg1 body weight (Mertz, 1983). In order to control the amount of lead in drinking water, the United States Environmental Protection Agency (USEPA) and WHO have published regulations including maximum levels of lead which are 0.015 and 0.010 mg L1, respectively (Etchie et al., 2013; Bakırdere et al., 2016). The maximum level of lead recommended for meat and fish by the European Union range between 0.10 and 0.30 mg g1 (EC, 2006). Hence, due to the several health effects, it is crucial to use sensitive analytical methods to determine this element at trace levels in water and food samples. Various techniques such as flame atomic absorption spectrometry (FAAS) (Bakirdere and Yaman, 2008), electrothermal atomic absorption spectrometry (ETAAS) (Cabon, 2002), hydride generation atomic absorption spectrometry (HGAAS) (Bakırdere et al., 2016) and inductively coupled plasma with optical emission spectrometry (ICP-OES) (Koksal, 2002) and mass spectrometry (Søndergaard et al., 2015) detection have been used for the determination of lead in various matrixes. FAAS has been commonly used due to its simplicity, low cost and operational ease (Santos et al., 2016). However, FAAS is not sensitive enough to obtain very low detection limits for several metals including lead due to low nebulization efficiency (Bakirdere et al., 2011). Hence, preconcentration methods or additional apparatus like a slotted quartz tube can be applied to overcome the sensitivity € limitation of this instrument (Ozzeybek et al., 2017). Slotted quartz tube developed by Watling in 1977 (Watling, 1977) could be easily fitted onto the flame burner head of FAAS to achieve low detection limits for heavy metals (Ataman, 2008; Demirtas et al., 2015). Residence time of analyte atoms in the flame and interaction between hollow cathode lamp light and metal atoms can be increased with the help of SQT (Kaya and Yaman, 2008; Fırat et al., 2017). In addition, SQTs can be used to trap atoms by coating the inner surface of the tube with metals that have higher melting points than the analyte of interest (Ataman, 2008). Flame furnace atomic absorption spectrometry is another technique which greatly improves the sensitivity over the conventional FAAS due to improved sample introduction systems such as thermospray and chemical vapor generation (Wu et al., 2008, 2009a, 2009b). Sample preparation and preconcentration methods such as extraction, co-precipitation, solvent evaporation and ion exchange for the determination of metals can be performed to alleviate matrix effects and improve the detection limit (Biparva and Matin, 2012). Microextraction methods are very popular due to their € several advantages (Ozzeybek et al., 2017). In literature, dispersive liquid-liquid microextraction (DLLME) as one of the microextraction methods has been successfully employed for the determination of polycyclic aromatic hydrocarbons (PAHs) (Rezaee et al., 2006), pesticides and endocrine disruptor compounds (Chormey et al., 2017), and metals (Biparva and Matin, 2012). DLLME has many advantages such as simple application, rapidity, high recovery and enrichment factor (Turan et al., 2017). This extraction method only requires a ternary component solvent system which consists of extraction solvent, dispersive solvent and aqueous
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€ sample (Ozzeybek et al., 2017). In DLLME method, a mixture of extraction and dispersive solvents is rapidly injected into aqueous solution and phase separation is accelerated by centrifugation. The analyte in the sediment phase can then be determined proper instrumentation (Farajzadeh et al., 2007; Nagaraju and Huang, 2007). In this study, a novel analytical method was developed for the determination of lead at trace levels. All the system parameters for DLLME-SQT-FAAS method were elaborately optimized to improve the sensitivity of the system. The developed method was applied to sea water and mussel samples to show the suitability of the method for selected matrices. 2. Materials and methods 2.1. Reagents All the chemicals and reagents used in this study were of analytical grade unless stated otherwise. Stock standard solution of lead with a concentration of 1000 mg L1 was supplied from High Purity Standards. Dithizone (ligand), acetonitrile, 1,2dichloroethane, sodium chloride, nitric acid (65%), hydrogen peroxide (30%), dichloromethane, isopropyl alcohol, carbon tetrachloride, ethanol, methanol, toluene, hydrochloric acid (37%), disodium hydrogen phosphate, sodium hydroxide, potassium chloride and potassium nitrate were purchased from Merck (Darmstadt, Germany). Ultrapure water from a MilliQ Reference System at the Central Laboratory of Yıldız Technical University was used throughout the experiments with the quality of 18.2 M-ohmcm. 2.2. Instrumentation An Analytik Jena AG NovAA 300 atomic absorption spectrometer with a flame burner and deuterium hollow cathode lamp (D2) was used throughout this study. The burner head of the FAAS was equipped with a lab-made quartz tube having the following dimensions; 16 mm internal diameter, 18 mm external diameter, 14 cm length, 5.5 cm entrance slot and 3.0 cm exit slot. The entrance slot was cut to fit the length of the burner flame which was 5.5 cm. The wavelength of a lead hollow cathode lamp operated at 6.0 mA was 283.3 nm and the monochromator spectral bandpass was 0.50 nm. Digestion of mussel samples was performed with a Milestone microwave digestion system. (see Fig. 1) 2.3. Procedure of DLLME A volume of 8.0 mL aqueous lead standard/sample solution was added to 0.50 g NaCl in a 15 mL centrifuge tube, after which 1.0 mL phosphate buffer solution (pH 12 and 1.0 mL of ligand solution (dithizone dissolved in ethanol) were added. The resulting solution was vortexed for 4.0 min to disperse the ligand homogeneously throughout the solution for a rapid and complete complexation. A mixture of 1,2-dichloroethane (200 mL) and acetonitrile (2.0 mL) were mixed in a separate tube and injected into the solution including lead-dithizone complex with a syringe. A cloudy solution was formed in the tube and it was mixed by vortex for 15 s. The solution was then centrifuged at 3460 g for 2.0 min. After the centrifugation, the settled organic phase at the bottom of the tube was transferred into a clean tube and placed in a water bath (100 C) for total evaporation. Concentrated nitric acid (150 mL) was added to dissolve the lead complex residue while heating in a water bath. Vapor stemming from the heating process was condensed in a refrigerator and centrifuged in order to settle all droplets at the bottom of the tube.
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Fig. 1. Schematic diagram of FAAS burner head fitted with a slotted quartz tube.
2.4. Samples Seawater sample was collected from the Marmara Sea (Bes¸iktas¸ coast) by washing the plastic bottle several times with the sample before filling to full. Mussel samples were purchased from local _ fishermen in Istanbul and then stored at 4.0 C. The digested mussel sample solution was adjusted to pH 12 by dropwise additions of 3.0 M NaOH. 2.5. Microwave assisted digestion A mixture of nitric acid and hydrogen peroxide was prepared in the optimum ratio of 2:1 for an effective digestion of mussel samples. In the digestion, about 1.0 g of mussel sample was taken and 12 mL of acid mixture was added. The heating program of microwave oven used for sample digestion is given in Table 1. After cooling to room temperature, the mussel samples were transferred into a different tube and the contents were diluted to 100 mL with de-ionized water for total lead determination. 3. Results and discussion The optimization process was performed with 0.50 mg L1 lead standard solutions. The optimum parameters of SQT-FAAS were selected based on the highest average absorbance of duplicate measurements. All extraction and system parameters were varied one after the other by keeping others constant. 3.1. SQT-FAAS optimizations The sample and acetylene flow rates were optimized to increase the amount of analyte reaching the flame and atomization efficiency, respectively. Optimization of these flow rates is essential because it improves instrumental measurements to a certain
Table 1 Temperature program of microwave system for sample digestion. Step
Temperature ( C) Ramp time (min) Hold time (min)
1
2
3
100 5 5
150 5 5
180 5 10
degree. The optimizations were performed with 5.0 mg L1 lead standard solutions. In order to obtain the highest nebulization efficiency, various positions on the nebulizer fixing knob were tested. The flow rate of the fuel gas was then adjusted to obtain optimum atomization temperature. The flow rate of acetylene was adjusted from 40 L h1 to 55 L h1 at 5.0 L h1 intervals. From the absorbance values recorded, the optimum sample and acetylene flow rates were 6.99 mL min1 and 65 L h1, respectively. Placing an SQT on top of the flame burner head holds the flame within a confined space and this extends the duration of free atoms in the flame compared to the open flame. The flow rate of sample remained unchanged for the SQT-FAAS system but the optimum fuel flow rate dropped to 40 L h1. This fuel flow rate was ideal because the exit flames at the hollow ends of the tube did not pose harm to electronic components of the instrument. The burner head was carefully adjusted to overlap with emissions from both deuterium lamp and lead hollow cathode lamp. The position of SQT on the burner flame has an impact on the entry rate of aspirated sample into the inner tube and the flame region where atomization takes place. The absorbance signals of the SQT at 0.0, 1.0 and 2.0 mm heights from the burner head were tested and the highest absorbance value was obtained for 1.0 mm height.
3.2. Optimization of complex formation In order to maximize lead-dithizone complex formation output, the pH and volume of buffer solution, dithizone concentration and complexing period were optimized. Buffer solutions in the pH range of 7e13 were prepared by adding dilute HCl/NaOH adjuster solutions to primary salt solutions, and kept at room temperature (25 C). pH is an essential factor for the formation of complex and extraction of complex formed. The highest absorbance values were recorded at pH 12 after which a gradual decrease occurred. The optimum amount of pH 12 buffer solution was then investigated by adding 0.40, 0.50, 1.0, 2.0 mLe8.0 mL aqueous standard solutions. Additionally, these buffer solution volumes were compared to the same standard concentration which was prepared with only pH 12 buffer solution. A linear increase in absorbance was obtained up to 1.0 mL, but the standard prepared solely with buffer solution recorded the lowest absorbance signal. Further optimizations were thus carried with 1.0 mL pH 12 buffer solution. Ligand concentration was also studied using 0.02, 0.05, 0.10 and 0.20% (w/v)
S. Erarpat et al. / Chemosphere 189 (2017) 180e185
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dithizone solutions. Containers including ligand solutions were kept in the dark because of its fast degradability when exposed to sunlight. A slight increase in absorbance values with increasing concentration occurred up to 0.10% after which it plateaued. For this reason, 0.10% was selected as optimum ligand concentration. Complexation period has an important effect on the formation of lead-dithizone complex. After adding the complexing agent to the lead standard solutions, 30, 60, 120, 180, 240 and 300 s mixing periods with the aid of vortex were studied. Peak height was increased with longer complexation period. Since there was no significant difference between the absorbance values of 240 and 300 s, the former was selected for further optimizations.
KI and BaCl2 were tested by adding 1.0 g of each salt to the leaddithizone complex, and their results compared to a saltless extraction. NaCl gave a higher absorbance signal than the other salt types and saltless extraction. Addition of NaCl and BaCl2 altered the density of the aqueous solution, resulting in 1,2 dichloroethane settling on the top of the water phase after extraction. KNO3, KI and saltless extractions did not show this effect. The optimum amount of NaCl was found as 0.50 g after testing 0.25, 0.50, 1.0 and 2.0 g. Despite the highest absorbance value of 1.0 g NaCl, 0.50 g was selected because the addition of 1.0 g to the aqueous solution led to the upper organic phase which was prone to evaporative losses.
3.3. Type and volume of extractor solvent
System analytical performance of the DLLME-SQT-FAAS method were obtained using the optimum conditions given in Table 2. Analytical figures of merit for all systems in this study are given in Table 3, together with other studies in literature. The limit of detection obtained in this study shows significant improvement for trace determination of lead. Each of the systems showed good linearity as indicated by their correlation coefficients (0.9972e0.9994). The relative standard deviations (4.45e11.7%) calculated from the lowest concentrations in each calibration plot also demonstrated high precision for both extraction process and instrumental measurements. When compared to the conventional FAAS, sensitivity improved by 2.9 times with SQT-FAAS, 31.7 times with DLLME-FAAS, and 141.3 times with DLLME-SQT-FAAS, respectively. Theoretically, the preconcentration factor of the DLLME method was calculated as 53 based on the initial sample volume and final volume of extracted analyte, and the actual enhancement recorded by this method was approximately 32. The detection limit for mussel by this method was determined as 0.27 mg g1 and this is comparable to that reported by Lavilla et al. as 0.37 mg g1 (Lavilla et al., 1999).
An extractor solvent should be immiscible with aqueous solution, have a higher or lower density than water, and should have an extraction capability for the analyte of interest (Dobrowska et al., 2016). Dichloromethane, 1,2-dichloroethane and chloroform were mixed with 2.0 mL ethanol and tested for their extraction efficiencies on the lead-dithizone complex. The absorbance signal recorded for 1,2-dichloroethane was about 1.5 and 1.3 times higher than dichloromethane and chloroform, respectively. This solvent was therefore selected and its optimum amount was tested. The extraction solvent volume is an important factor because it affects the extraction capability of the lead-dithizone complex. For this reason, 100, 200, 400 and 600 mL volumes of 1,2-dichloroethane were used to extract the lead-dithizone complex. The absorbance values obtained indicated that 100 mL extractor solvent was not enough to extract the lead-dithizone complex, but the other volumes had almost the same extraction output. In order to reduce the usage of the solvent, 200 mL was selected as the optimum one. 3.4. Type and volume of dispersive solvent The main criterion in the selection of the dispersive solvent is its miscibility with both organic and aqueous phases. Extraction solvent should be dispersed as fine droplets throughout the aqueous solution. Surface area between extraction solvent and aqueous solution is increased, and this subsequently improves the extraction output of the analyte (Fırat et al., 2017). Ethanol, acetone, acetonitrile, and isopropyl alcohol were examined for their dispersive efficiencies on 1,2-dichloroethane. Acetonitrile recorded the highest absorbance value and its optimum amount was also determined by testing 0.50, 1.0, 2.0 and 3.0 mL. The volume of dispersive solvent affects the degree of the dispersion of extraction solvent into the aqueous solution and the final volume of sedimented organic phase. It was observed that 1.0 mL acetonitrile volume was significantly lower than the higher volumes, suggesting an ineffective dispersion. A gradual decrease in absorbance occurred after 2.0 mL due to small losses of extraction solvent in aqueous solution. The volume of settled organic phase for 2.0 mL acetonitrile was conveniently transferred into clean tubes for further processing and was therefore selected as optimum volume. 3.5. Effect of ionic strength Ionic strength of an aqueous solution is another important factor affecting the extraction output of analyte(s). When the ionic strength is increased with the addition of salt, the solubility of analyte in aqueous solution is reduced, thereby enhancing the analyte's movement into the organic phase to obtain high extraction outcomes (Mohammadi et al., 2009). The effects of NaCl, KNO3,
3.6. Analytical performances
3.7. Real sample analysis The proposed extraction method was applied to sea water and Table 2 Optimized parameters of the DLLME-SQT-FAAS. Parameter
Value
Sample volume pH of buffer solution (amount) Ligand concentration (amount) Complexing period Extraction solvent (amount) Dispersive solvent (amount) Mixing type (period) Sample flow rate Acetylene flow rate Air flow rate SQT height
8.0 mL pH 12 (1.0 mL) 0.10% (w/v) (1.0 mL) 4.0 min (Vortex Assisted) 1,2-Dichloroethane (200 mL) Acetonitrile (2.0 mL) Vortex Assisted (15 s) 6.99 mL min1 40 L h1 685 L h1 1.0 mm
Table 3 Analytical performance of the FAAS systems and comparison to other methods. Method
LOD, mg L1
LOQ, mg L1
REF.
FAAS SQT-FAAS DLLME-FAAS DLLME-SQT-FAAS Co-precipitation-FAAS Off-line-SPE-FAAS SPE-ICP-OES
383 132 12 2.7 16 6.1 6.4
1275 440 40 9.0 -
This study This study This study This study (Doner and Ege, 2005) (Matoso, 2003) (Wei et al., 2015)
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Table 4 Percent recoveries of lead in sea water and mussel samples. Sample
20 mg L1
50 mg L1
Sea water, % Mussel, %
95.85 ± 1.89 110.28 ± 6.93
95.67 ± 1.89 98.45 ± 6.19
Uncertainties (±) are represented by the standard deviation for n ¼ 3.
mussel samples in order to demonstrate its applicability and reliability. Blank determination showed the absence of detectable lead in the samples and they were therefore spiked at 20 and 50 mg L1, and extracted under the optimum extraction conditions. Calibration standards of 10, 20, 50 and 100 mg L1 were measured together with the samples and used to determine the percent recoveries as presented in Table 4. The recoveries of the samples in the range of 95e110% were substantially good for trace determination of lead. 4. Conclusion In this study, determination of lead by conventional FAAS was enhanced by the combination with a slotted quartz tube and DLLME. This method is rapid, simple, environmentally friendly and applicable to complex matrixes like sea water and mussel samples. The sensitivity of this method was increased by about 141 times over the conventional FAAS. The limits of detection were found to be 2.7 mg L1 and 270 mg kg1 for sea water and mussel, respectively. Under the optimum conditions, the accuracy and precision of the proposed method are satisfactory as it provides high lead recovery and low relative standard deviation for sea water and mussel samples. Developed method could be applied for the determination of lead at trace levels in variety of matrix with high accuracy and precision. Competing interest statement We hereby confirm that no conflict of interest is associated with this study and that no substantial financial support was received to merit mentioning in this publication. References Ataman, O.Y., 2008. Vapor generation and atom traps: atomic absorption spectrometry at the ng/L level. Spectrochim. Acta Part B At. Spectrosc. 63, 825e834. _ Aydin, I., Yildirim, E., Bakirdere, S., Aydin, F., Bakirdere, E.G., Titretir, S., Akdeniz, I., Arslan, Y., 2011. From mg/kg to pg/kg levels: a story of trace element determination: a review. Appl. Spectrosc. Rev. 46, 38e66. Bakırdere, S., Chormey, D.S., Büyükpınar, Ç., San, N., Keyf, S., 2016. Determination of lead in drinking and wastewater by hydride generation atomic absorption spectrometry. Anal. Lett. 49, 1917e1925. Bakirdere, S., Yaman, M., 2008. Determination of lead, cadmium and copper in roadside soil and plants in Elazig, Turkey. Environ. Monit. Assess. 136, 401e410. Bellinger, D.C., 2013. Prenatal exposures to environmental chemicals and Children's neurodevelopment: an update. Saf. Health Work 4, 1e11. Biparva, P., Matin, A.A., 2012. Microextraction techniques as a sample preparation step for metal analysis. In: Farrukh, M.A. (Ed.), At. Absorpt. Spectrosc. InTech 61e88. Cabon, J.Y., 2002. Determination of Cd and Pb in seawater by graphite furnace atomic absorption spectrometry with the use of hydrofluoric acid as a chemical modifier. Spectrochim. Acta Part B At. Spectrosc. 57, 513e524. Cartier, C., Nour, S., Richer, B., Deshommes, E., Prevost, M., 2012. Impact of water treatment on the contribution of faucets to dissolved and particulate lead release at the tap. Water Res. 46, 5205e5216. Chormey, D.S., Büyükpınar, Ç., Turak, F., Komesli, O.T., Bakırdere, S., 2017. Simultaneous determination of selected hormones, endocrine disruptor compounds, and pesticides in water medium at trace levels by GC-MS after dispersive liquid-liquid microextraction. Environ. Monit. Assess. 189, 277. Comitre, A.L., Reis, B.F., 2005. Automatic flow procedure based on multicommutation exploiting liquid-liquid extraction for spectrophotometric lead determination in plant material. Talanta 65, 846e852. Demirtas, I., Bakirdere, S., Ataman, O.Y., 2015. Lead determination at ng/mL level by
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Determination of lead at trace levels in mussel and sea water samples using vortex assisted dispersive liquid-liquid microextraction-slotted quartz tube-flame atomic absorption spectrometry € € zde Ozzeybek, Sezin Erarpat, Go Dotse Selali Chormey, Sezgin Bakırdere* _ Yıldız Technical University, Department of Chemistry, 34349 Istanbul, Turkey
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A novel DLLME-SQT-FAAS method was developed for the extraction and determination of lead at trace level. The applicability and accuracy of the method was verified with spiked recovery tests. Low %RSD values indicated high precision for the extraction process and instrumental measurements. The method was applied to mussel and sea water samples.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 August 2017 Received in revised form 9 September 2017 Accepted 15 September 2017 Available online 17 September 2017
In this study, dispersive liquid-liquid microextraction (DLLME) and slotted quartz tube (SQT) were coupled to flame atomic absorption spectrometry (FAAS) to increase the sensitivity of lead. Conditions such as the formation of the lead-dithizone complex, efficiency of the DLLME method and the output of the SQT were systematically optimized to improve the detection limit for the analyte. The conventional FAAS system was improved upon by about 3.0 times with SQT-FAAS, 32 times with DLLME-FAAS and 142 times with DLLME-SQT-FAAS. The method was applicable over a wide linear range (10e500 mg L1). The limit of detection (LOD) determined by DLLME-SQT-FAAS for seawater and mussel were 2.7 mg L1 and 270 mg kg1, respectively. The percent recoveries obtained for mussel and seawater samples (spiked at 20 and 50 mg L1) were 95e96% and 98e110%, respectively. © 2017 Elsevier Ltd. All rights reserved.
Handling Editor: Petra Petra Krystek Keywords: Lead Dispersive liquid-liquid microextraction Slotted quartz tube Flame atomic absorption spectrometry Seawater Biotissues
1. Introduction Lead can be found in the environment from natural sources and man-made activities, and it poses some risks to living organisms
* Corresponding author. E-mail address: [email protected] (S. Bakırdere). http://dx.doi.org/10.1016/j.chemosphere.2017.09.072 0045-6535/© 2017 Elsevier Ltd. All rights reserved.
and human being such as hematological, cerebral, and kidney malfunctions (Manahan, 1994; Huang and Keller, 2015). At the prenatal development of the brain, lead exposure might cause permanent adverse effects on neurocognitive function resulting in lower mental development and risk of schizophrenia later in life (Opler et al., 2004, 2008; Bellinger, 2013). Plumbism as a disease of lead poisoning has different symptoms such as hyperactivity, developmental delays, hearing loss, and even death (Hauptman
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et al., 2017). Lead can be entered into the human body in the form of inorganic lead and accumulated in soft tissues, bones, and teeth or as organic lead which accumulates in the brain because of its fatsoluble nature (Tchounwou et al., 2012). For this reason, there are several regulations on the release of lead into the environment. However, the automotive, ceramic, ink, pesticide and fertilizer producing industries make use of lead materials resulting in lead contamination (Comitre and Reis, 2005). In addition, lead can be easily transferred into drinking water by means of corrosion and old lead pipes found in several living places (Cartier et al., 2012; Frisbie et al., 2015). According to the World Health Organization (WHO), the tolerable weekly amount of lead for humans is 25 mg kg1 body weight (Mertz, 1983). In order to control the amount of lead in drinking water, the United States Environmental Protection Agency (USEPA) and WHO have published regulations including maximum levels of lead which are 0.015 and 0.010 mg L1, respectively (Etchie et al., 2013; Bakırdere et al., 2016). The maximum level of lead recommended for meat and fish by the European Union range between 0.10 and 0.30 mg g1 (EC, 2006). Hence, due to the several health effects, it is crucial to use sensitive analytical methods to determine this element at trace levels in water and food samples. Various techniques such as flame atomic absorption spectrometry (FAAS) (Bakirdere and Yaman, 2008), electrothermal atomic absorption spectrometry (ETAAS) (Cabon, 2002), hydride generation atomic absorption spectrometry (HGAAS) (Bakırdere et al., 2016) and inductively coupled plasma with optical emission spectrometry (ICP-OES) (Koksal, 2002) and mass spectrometry (Søndergaard et al., 2015) detection have been used for the determination of lead in various matrixes. FAAS has been commonly used due to its simplicity, low cost and operational ease (Santos et al., 2016). However, FAAS is not sensitive enough to obtain very low detection limits for several metals including lead due to low nebulization efficiency (Bakirdere et al., 2011). Hence, preconcentration methods or additional apparatus like a slotted quartz tube can be applied to overcome the sensitivity € limitation of this instrument (Ozzeybek et al., 2017). Slotted quartz tube developed by Watling in 1977 (Watling, 1977) could be easily fitted onto the flame burner head of FAAS to achieve low detection limits for heavy metals (Ataman, 2008; Demirtas et al., 2015). Residence time of analyte atoms in the flame and interaction between hollow cathode lamp light and metal atoms can be increased with the help of SQT (Kaya and Yaman, 2008; Fırat et al., 2017). In addition, SQTs can be used to trap atoms by coating the inner surface of the tube with metals that have higher melting points than the analyte of interest (Ataman, 2008). Flame furnace atomic absorption spectrometry is another technique which greatly improves the sensitivity over the conventional FAAS due to improved sample introduction systems such as thermospray and chemical vapor generation (Wu et al., 2008, 2009a, 2009b). Sample preparation and preconcentration methods such as extraction, co-precipitation, solvent evaporation and ion exchange for the determination of metals can be performed to alleviate matrix effects and improve the detection limit (Biparva and Matin, 2012). Microextraction methods are very popular due to their € several advantages (Ozzeybek et al., 2017). In literature, dispersive liquid-liquid microextraction (DLLME) as one of the microextraction methods has been successfully employed for the determination of polycyclic aromatic hydrocarbons (PAHs) (Rezaee et al., 2006), pesticides and endocrine disruptor compounds (Chormey et al., 2017), and metals (Biparva and Matin, 2012). DLLME has many advantages such as simple application, rapidity, high recovery and enrichment factor (Turan et al., 2017). This extraction method only requires a ternary component solvent system which consists of extraction solvent, dispersive solvent and aqueous
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€ sample (Ozzeybek et al., 2017). In DLLME method, a mixture of extraction and dispersive solvents is rapidly injected into aqueous solution and phase separation is accelerated by centrifugation. The analyte in the sediment phase can then be determined proper instrumentation (Farajzadeh et al., 2007; Nagaraju and Huang, 2007). In this study, a novel analytical method was developed for the determination of lead at trace levels. All the system parameters for DLLME-SQT-FAAS method were elaborately optimized to improve the sensitivity of the system. The developed method was applied to sea water and mussel samples to show the suitability of the method for selected matrices. 2. Materials and methods 2.1. Reagents All the chemicals and reagents used in this study were of analytical grade unless stated otherwise. Stock standard solution of lead with a concentration of 1000 mg L1 was supplied from High Purity Standards. Dithizone (ligand), acetonitrile, 1,2dichloroethane, sodium chloride, nitric acid (65%), hydrogen peroxide (30%), dichloromethane, isopropyl alcohol, carbon tetrachloride, ethanol, methanol, toluene, hydrochloric acid (37%), disodium hydrogen phosphate, sodium hydroxide, potassium chloride and potassium nitrate were purchased from Merck (Darmstadt, Germany). Ultrapure water from a MilliQ Reference System at the Central Laboratory of Yıldız Technical University was used throughout the experiments with the quality of 18.2 M-ohmcm. 2.2. Instrumentation An Analytik Jena AG NovAA 300 atomic absorption spectrometer with a flame burner and deuterium hollow cathode lamp (D2) was used throughout this study. The burner head of the FAAS was equipped with a lab-made quartz tube having the following dimensions; 16 mm internal diameter, 18 mm external diameter, 14 cm length, 5.5 cm entrance slot and 3.0 cm exit slot. The entrance slot was cut to fit the length of the burner flame which was 5.5 cm. The wavelength of a lead hollow cathode lamp operated at 6.0 mA was 283.3 nm and the monochromator spectral bandpass was 0.50 nm. Digestion of mussel samples was performed with a Milestone microwave digestion system. (see Fig. 1) 2.3. Procedure of DLLME A volume of 8.0 mL aqueous lead standard/sample solution was added to 0.50 g NaCl in a 15 mL centrifuge tube, after which 1.0 mL phosphate buffer solution (pH 12 and 1.0 mL of ligand solution (dithizone dissolved in ethanol) were added. The resulting solution was vortexed for 4.0 min to disperse the ligand homogeneously throughout the solution for a rapid and complete complexation. A mixture of 1,2-dichloroethane (200 mL) and acetonitrile (2.0 mL) were mixed in a separate tube and injected into the solution including lead-dithizone complex with a syringe. A cloudy solution was formed in the tube and it was mixed by vortex for 15 s. The solution was then centrifuged at 3460 g for 2.0 min. After the centrifugation, the settled organic phase at the bottom of the tube was transferred into a clean tube and placed in a water bath (100 C) for total evaporation. Concentrated nitric acid (150 mL) was added to dissolve the lead complex residue while heating in a water bath. Vapor stemming from the heating process was condensed in a refrigerator and centrifuged in order to settle all droplets at the bottom of the tube.
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Fig. 1. Schematic diagram of FAAS burner head fitted with a slotted quartz tube.
2.4. Samples Seawater sample was collected from the Marmara Sea (Bes¸iktas¸ coast) by washing the plastic bottle several times with the sample before filling to full. Mussel samples were purchased from local _ fishermen in Istanbul and then stored at 4.0 C. The digested mussel sample solution was adjusted to pH 12 by dropwise additions of 3.0 M NaOH. 2.5. Microwave assisted digestion A mixture of nitric acid and hydrogen peroxide was prepared in the optimum ratio of 2:1 for an effective digestion of mussel samples. In the digestion, about 1.0 g of mussel sample was taken and 12 mL of acid mixture was added. The heating program of microwave oven used for sample digestion is given in Table 1. After cooling to room temperature, the mussel samples were transferred into a different tube and the contents were diluted to 100 mL with de-ionized water for total lead determination. 3. Results and discussion The optimization process was performed with 0.50 mg L1 lead standard solutions. The optimum parameters of SQT-FAAS were selected based on the highest average absorbance of duplicate measurements. All extraction and system parameters were varied one after the other by keeping others constant. 3.1. SQT-FAAS optimizations The sample and acetylene flow rates were optimized to increase the amount of analyte reaching the flame and atomization efficiency, respectively. Optimization of these flow rates is essential because it improves instrumental measurements to a certain
Table 1 Temperature program of microwave system for sample digestion. Step
Temperature ( C) Ramp time (min) Hold time (min)
1
2
3
100 5 5
150 5 5
180 5 10
degree. The optimizations were performed with 5.0 mg L1 lead standard solutions. In order to obtain the highest nebulization efficiency, various positions on the nebulizer fixing knob were tested. The flow rate of the fuel gas was then adjusted to obtain optimum atomization temperature. The flow rate of acetylene was adjusted from 40 L h1 to 55 L h1 at 5.0 L h1 intervals. From the absorbance values recorded, the optimum sample and acetylene flow rates were 6.99 mL min1 and 65 L h1, respectively. Placing an SQT on top of the flame burner head holds the flame within a confined space and this extends the duration of free atoms in the flame compared to the open flame. The flow rate of sample remained unchanged for the SQT-FAAS system but the optimum fuel flow rate dropped to 40 L h1. This fuel flow rate was ideal because the exit flames at the hollow ends of the tube did not pose harm to electronic components of the instrument. The burner head was carefully adjusted to overlap with emissions from both deuterium lamp and lead hollow cathode lamp. The position of SQT on the burner flame has an impact on the entry rate of aspirated sample into the inner tube and the flame region where atomization takes place. The absorbance signals of the SQT at 0.0, 1.0 and 2.0 mm heights from the burner head were tested and the highest absorbance value was obtained for 1.0 mm height.
3.2. Optimization of complex formation In order to maximize lead-dithizone complex formation output, the pH and volume of buffer solution, dithizone concentration and complexing period were optimized. Buffer solutions in the pH range of 7e13 were prepared by adding dilute HCl/NaOH adjuster solutions to primary salt solutions, and kept at room temperature (25 C). pH is an essential factor for the formation of complex and extraction of complex formed. The highest absorbance values were recorded at pH 12 after which a gradual decrease occurred. The optimum amount of pH 12 buffer solution was then investigated by adding 0.40, 0.50, 1.0, 2.0 mLe8.0 mL aqueous standard solutions. Additionally, these buffer solution volumes were compared to the same standard concentration which was prepared with only pH 12 buffer solution. A linear increase in absorbance was obtained up to 1.0 mL, but the standard prepared solely with buffer solution recorded the lowest absorbance signal. Further optimizations were thus carried with 1.0 mL pH 12 buffer solution. Ligand concentration was also studied using 0.02, 0.05, 0.10 and 0.20% (w/v)
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dithizone solutions. Containers including ligand solutions were kept in the dark because of its fast degradability when exposed to sunlight. A slight increase in absorbance values with increasing concentration occurred up to 0.10% after which it plateaued. For this reason, 0.10% was selected as optimum ligand concentration. Complexation period has an important effect on the formation of lead-dithizone complex. After adding the complexing agent to the lead standard solutions, 30, 60, 120, 180, 240 and 300 s mixing periods with the aid of vortex were studied. Peak height was increased with longer complexation period. Since there was no significant difference between the absorbance values of 240 and 300 s, the former was selected for further optimizations.
KI and BaCl2 were tested by adding 1.0 g of each salt to the leaddithizone complex, and their results compared to a saltless extraction. NaCl gave a higher absorbance signal than the other salt types and saltless extraction. Addition of NaCl and BaCl2 altered the density of the aqueous solution, resulting in 1,2 dichloroethane settling on the top of the water phase after extraction. KNO3, KI and saltless extractions did not show this effect. The optimum amount of NaCl was found as 0.50 g after testing 0.25, 0.50, 1.0 and 2.0 g. Despite the highest absorbance value of 1.0 g NaCl, 0.50 g was selected because the addition of 1.0 g to the aqueous solution led to the upper organic phase which was prone to evaporative losses.
3.3. Type and volume of extractor solvent
System analytical performance of the DLLME-SQT-FAAS method were obtained using the optimum conditions given in Table 2. Analytical figures of merit for all systems in this study are given in Table 3, together with other studies in literature. The limit of detection obtained in this study shows significant improvement for trace determination of lead. Each of the systems showed good linearity as indicated by their correlation coefficients (0.9972e0.9994). The relative standard deviations (4.45e11.7%) calculated from the lowest concentrations in each calibration plot also demonstrated high precision for both extraction process and instrumental measurements. When compared to the conventional FAAS, sensitivity improved by 2.9 times with SQT-FAAS, 31.7 times with DLLME-FAAS, and 141.3 times with DLLME-SQT-FAAS, respectively. Theoretically, the preconcentration factor of the DLLME method was calculated as 53 based on the initial sample volume and final volume of extracted analyte, and the actual enhancement recorded by this method was approximately 32. The detection limit for mussel by this method was determined as 0.27 mg g1 and this is comparable to that reported by Lavilla et al. as 0.37 mg g1 (Lavilla et al., 1999).
An extractor solvent should be immiscible with aqueous solution, have a higher or lower density than water, and should have an extraction capability for the analyte of interest (Dobrowska et al., 2016). Dichloromethane, 1,2-dichloroethane and chloroform were mixed with 2.0 mL ethanol and tested for their extraction efficiencies on the lead-dithizone complex. The absorbance signal recorded for 1,2-dichloroethane was about 1.5 and 1.3 times higher than dichloromethane and chloroform, respectively. This solvent was therefore selected and its optimum amount was tested. The extraction solvent volume is an important factor because it affects the extraction capability of the lead-dithizone complex. For this reason, 100, 200, 400 and 600 mL volumes of 1,2-dichloroethane were used to extract the lead-dithizone complex. The absorbance values obtained indicated that 100 mL extractor solvent was not enough to extract the lead-dithizone complex, but the other volumes had almost the same extraction output. In order to reduce the usage of the solvent, 200 mL was selected as the optimum one. 3.4. Type and volume of dispersive solvent The main criterion in the selection of the dispersive solvent is its miscibility with both organic and aqueous phases. Extraction solvent should be dispersed as fine droplets throughout the aqueous solution. Surface area between extraction solvent and aqueous solution is increased, and this subsequently improves the extraction output of the analyte (Fırat et al., 2017). Ethanol, acetone, acetonitrile, and isopropyl alcohol were examined for their dispersive efficiencies on 1,2-dichloroethane. Acetonitrile recorded the highest absorbance value and its optimum amount was also determined by testing 0.50, 1.0, 2.0 and 3.0 mL. The volume of dispersive solvent affects the degree of the dispersion of extraction solvent into the aqueous solution and the final volume of sedimented organic phase. It was observed that 1.0 mL acetonitrile volume was significantly lower than the higher volumes, suggesting an ineffective dispersion. A gradual decrease in absorbance occurred after 2.0 mL due to small losses of extraction solvent in aqueous solution. The volume of settled organic phase for 2.0 mL acetonitrile was conveniently transferred into clean tubes for further processing and was therefore selected as optimum volume. 3.5. Effect of ionic strength Ionic strength of an aqueous solution is another important factor affecting the extraction output of analyte(s). When the ionic strength is increased with the addition of salt, the solubility of analyte in aqueous solution is reduced, thereby enhancing the analyte's movement into the organic phase to obtain high extraction outcomes (Mohammadi et al., 2009). The effects of NaCl, KNO3,
3.6. Analytical performances
3.7. Real sample analysis The proposed extraction method was applied to sea water and Table 2 Optimized parameters of the DLLME-SQT-FAAS. Parameter
Value
Sample volume pH of buffer solution (amount) Ligand concentration (amount) Complexing period Extraction solvent (amount) Dispersive solvent (amount) Mixing type (period) Sample flow rate Acetylene flow rate Air flow rate SQT height
8.0 mL pH 12 (1.0 mL) 0.10% (w/v) (1.0 mL) 4.0 min (Vortex Assisted) 1,2-Dichloroethane (200 mL) Acetonitrile (2.0 mL) Vortex Assisted (15 s) 6.99 mL min1 40 L h1 685 L h1 1.0 mm
Table 3 Analytical performance of the FAAS systems and comparison to other methods. Method
LOD, mg L1
LOQ, mg L1
REF.
FAAS SQT-FAAS DLLME-FAAS DLLME-SQT-FAAS Co-precipitation-FAAS Off-line-SPE-FAAS SPE-ICP-OES
383 132 12 2.7 16 6.1 6.4
1275 440 40 9.0 -
This study This study This study This study (Doner and Ege, 2005) (Matoso, 2003) (Wei et al., 2015)
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Table 4 Percent recoveries of lead in sea water and mussel samples. Sample
20 mg L1
50 mg L1
Sea water, % Mussel, %
95.85 ± 1.89 110.28 ± 6.93
95.67 ± 1.89 98.45 ± 6.19
Uncertainties (±) are represented by the standard deviation for n ¼ 3.
mussel samples in order to demonstrate its applicability and reliability. Blank determination showed the absence of detectable lead in the samples and they were therefore spiked at 20 and 50 mg L1, and extracted under the optimum extraction conditions. Calibration standards of 10, 20, 50 and 100 mg L1 were measured together with the samples and used to determine the percent recoveries as presented in Table 4. The recoveries of the samples in the range of 95e110% were substantially good for trace determination of lead. 4. Conclusion In this study, determination of lead by conventional FAAS was enhanced by the combination with a slotted quartz tube and DLLME. This method is rapid, simple, environmentally friendly and applicable to complex matrixes like sea water and mussel samples. The sensitivity of this method was increased by about 141 times over the conventional FAAS. The limits of detection were found to be 2.7 mg L1 and 270 mg kg1 for sea water and mussel, respectively. Under the optimum conditions, the accuracy and precision of the proposed method are satisfactory as it provides high lead recovery and low relative standard deviation for sea water and mussel samples. Developed method could be applied for the determination of lead at trace levels in variety of matrix with high accuracy and precision. Competing interest statement We hereby confirm that no conflict of interest is associated with this study and that no substantial financial support was received to merit mentioning in this publication. References Ataman, O.Y., 2008. Vapor generation and atom traps: atomic absorption spectrometry at the ng/L level. Spectrochim. Acta Part B At. Spectrosc. 63, 825e834. _ Aydin, I., Yildirim, E., Bakirdere, S., Aydin, F., Bakirdere, E.G., Titretir, S., Akdeniz, I., Arslan, Y., 2011. From mg/kg to pg/kg levels: a story of trace element determination: a review. Appl. Spectrosc. Rev. 46, 38e66. Bakırdere, S., Chormey, D.S., Büyükpınar, Ç., San, N., Keyf, S., 2016. Determination of lead in drinking and wastewater by hydride generation atomic absorption spectrometry. Anal. Lett. 49, 1917e1925. Bakirdere, S., Yaman, M., 2008. Determination of lead, cadmium and copper in roadside soil and plants in Elazig, Turkey. Environ. Monit. Assess. 136, 401e410. Bellinger, D.C., 2013. Prenatal exposures to environmental chemicals and Children's neurodevelopment: an update. Saf. Health Work 4, 1e11. Biparva, P., Matin, A.A., 2012. Microextraction techniques as a sample preparation step for metal analysis. In: Farrukh, M.A. (Ed.), At. Absorpt. Spectrosc. InTech 61e88. Cabon, J.Y., 2002. Determination of Cd and Pb in seawater by graphite furnace atomic absorption spectrometry with the use of hydrofluoric acid as a chemical modifier. Spectrochim. Acta Part B At. Spectrosc. 57, 513e524. Cartier, C., Nour, S., Richer, B., Deshommes, E., Prevost, M., 2012. Impact of water treatment on the contribution of faucets to dissolved and particulate lead release at the tap. Water Res. 46, 5205e5216. Chormey, D.S., Büyükpınar, Ç., Turak, F., Komesli, O.T., Bakırdere, S., 2017. Simultaneous determination of selected hormones, endocrine disruptor compounds, and pesticides in water medium at trace levels by GC-MS after dispersive liquid-liquid microextraction. Environ. Monit. Assess. 189, 277. Comitre, A.L., Reis, B.F., 2005. Automatic flow procedure based on multicommutation exploiting liquid-liquid extraction for spectrophotometric lead determination in plant material. Talanta 65, 846e852. Demirtas, I., Bakirdere, S., Ataman, O.Y., 2015. Lead determination at ng/mL level by
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