Nova Biotechnol Chim (2017) 16(2): 124-131 DOI: 10.1515/nbec-2017-0017  Corresponding author: wipharatc@nu.ac.th  Nova Biotechnologica et Chimica Microextraction based on solidified floating organic drop coupled with ETAAS for the determination of lead in herbs Arnon Thongsaw, Ratana Sananmuang, Gareth M. Ross and Wipharat Chuachuad Chaiyasith Department of Chemistry, Research Center for Academic Excellence in Petroleum, Petrochemical and Advanced Materials, Faculty of Science, Naresuan University, Phitsanulok, Thailand 65000 Article info Article history: Received: 19th July 2017 Accepted: 27th November 2017 Keywords: ETAAS Herb samples Lead Solidified of floating organic droplet Microextraction Abstract A rapid, inexpensive and practical solidified of floating organic droplet microextraction (SFODME) prior to electrothermal atomic absorption spectrometry (ETAAS) was proposed for lead (Pb) determination in herb samples. For SFODME procedure, 1-(2-pysidylazo)-2-naphthol was used as a complexing agent. Analytical parameters influencing the extraction efficiency, i.e. types and volume of extracting solvent, concentration of 1-(2-pyridylazo)-2-naphthol, pH, extraction temperature and time were optimized. Under the optimized conditions, LOD and LOQ were 0.064 and 0.214 µg L-1, respectively, and an enrichment factor was achieved at 18.71 with the relative standard deviation ranging from 1.3 to 2.5% (n=6). The proposed method was effectively applied to the determination of lead in Spinach leaves (SRM-1570a) and Thai herb samples with acceptable results.  University of SS. Cyril and Methodius in Trnava           Introduction In recent years, consumption of herbs for health benefits are not only common in Asia but are also widely used around the world. They can also have used for phytotherapy, which is using plants for relieve and treat of diseases. Medicinal plants have been increasingly used due to their mild and low side effects (Rates 2001; Oviedo et al. 2013). Manufacturers are only required to carry out the analysis of contaminants such as hazardous metals in the raw plant materials. However, the extracts normally do not define the quality and amount of contaminate present. In order to help the quality estimation of these products, the analysis of contaminates can reduce health risk, especially by toxic metals. Therefore, it is necessary to monitor the amount of toxic heavy metals in these samples (Khan et al. 2008). Lead (Pb) results in harmful effects to the renal, endocrine, digestive, cardiovascular, reproductive systems as well as affecting the development of neurologic system (Solidum 2014). The World Health Organization (WHO) has suggested 10 mg kg-1 as the maximum acceptable concentration of Pb in therapeutic plants. Literature review found that Pb was mostly found in a variety of herbs and showed Pb contents between 0.2 to 2.7 mg kg-1 (Divrikli et al. 2014). Consequently, traceable and reliable measurement of Pb is essential for the safety and risk assessment for the wide use of traditional and medicinal herbs. Currently, there are some variety methods related to modern instrumentation that have been widely used for heavy metal determination; these include ICP (MS and AES) (Zhao et al. 2012; Sorbo et al. 2014; Deng et al. 2015; Tai et al. 2016) and AAS (Demirtaş et al. 2015; Batista et al. 2016; Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 28.02.20 08:11 UTC Nova Biotechnol Chim (2017) 16(2): 124-131 125 Zhong et al. 2016). However, trace determination of Pb in herb samples is relatively inconvenient because of the matrix effect and low concentration of Pb. To answer this problem, separation or pre-concentration techniques are required. Numerous techniques have been examined for pre-concentration/determination of trace Pb, namely liquid phase extraction (Ikeda et al. 1998; Dapaah et al. 1999; Zendelovska et al. 2001) and solid phase extraction (Dadfarnia et al. 2007; Ghaedi et al. 2007; Tharakeswar et al. 2012). Although, conventional extraction methods can be obtaining reliable results, they usually have some disadvantages such as required long time, labor intensive and/or required large volumes of extracting solvent. In order to find a solution to pre-concentration, microextraction techniques, namely single drop microextraction (SDME) (Maltes et al. 2008; Manzoori et al. 2009), solid phase microextraction (SPME) (Zhang et al. 1994; Górecki and Pawliszyn 1996; Djozan and Assadi 2004) dispersive liquid liquid microextraction (DLLME) (López-García et al. 2014; Jalbani and Soylak 2015; Rosa et al. 2015) and hollow fiber liquid phase microextraction (HFLPME) (Xia et al. 2007; Jiang et al. 2009; Saleh et al. 2009) have been reported. Solidified of floating organic droplet microextraction (SFODME) was developed in 2007 (Lam et al. 2010). This microextraction methodis different from other liquid phase micro- extraction that uses a microliter volume of the extractant )melting point around 10 – 30 °C( delivered to the sample solution, stirred for an appropriate time, and then relocated into an ice bath. After the extracting solvent becomes solid,it is transferred into another vial and melted immediately. Finally, the solution is directly determined the analyte concentration. From the literature, there are only few reports related to separation/pre-concentration of Pb in herb samples (Solidum 2014; Tai et al. 2016, Lam et al. 2010; Yavuz et al. 2016). Therefore, SFODME technique combination with ETAAS for the Pb determination in herb samples was conducted in this work. We selected 1-(2-pyridylazo)-2-naphthol as the complexing agent and the several parameters influencing the extraction efficiency were optimized. Experimental Standard solution of Pb 1,000 mg L-1 was obtained from AVS Titrinorm (VWR International, Belgium) and working standard solutions were freshly prepared with deionized water. Chelating reagent was prepared by dissolving an appropriate amount of 1-(2-pyridylazo)-2-naphthol (Acros organics, US) in ethanol. 1-undecanol was purchased from Merck (OHG, Germany). Buffer solutions of ammonium acetate (pH 4, 0.2 mol L-1), phosphate (pH 7, 0.2 mol L-1), and ammonium chloride (pH 9, 0.2 mol L-1) were used to adjust the pH in the range of 3.0 – 12.0. The laboratory glassware were soaked in nitric acid (10%) for 24 h and rinsed with deionized water before use. The certified reference material was used for method validation: Spinach leaves (SRM-1570a) was purchased from National Institution of Standard and Technology. The experiments were performed with an electrothermal atomic absorption spectrometer (Varian Model SpectrAA 220Z) and the instrument parameters were demonstrated in Table 1. The pH of solution were measured by pH meter (Metrohm, 827 pH lab) combined glass electrode. For SFODME extraction procedure, blank solution, 15.00 μg L-1 of standard (Pb) or sample solution (13.0 mL) was adjusted to pH 6 with phosphate buffer and 0.5 mL of 1-(2-pyridylazo)-2-naphthol (3.0 mmol L-1) were mixed in a screw-cap glass vial (15 mL). Then, 90 μL of 1-undecanol was added to the aqueous solution and the solution was stirred for 40 min at 750 rpm at room temperature. After this procedure, the glass vial was transferred into an ice bath for 5 min and the organic phase eventually turned to solid. This solvent was simply relocated into the ETAAS vial by a small spatula and the droplet was melted immediately. Finally, the organic drop was diluted to 500 μL with ethanol and determined by ETAAS. Thai herb samples were collected from Phitsanulok (Thailand) and dehydrated with oven (70°C) for one hour. After that, samples were sieved through a 100 µm sieve (Oviedo et al. 2013). 4 mL of concentrated HNO3 and H2O2 at a ratio 3:1 was added and thoroughly mixed with 0.01 g of the sieved sample. The solution was heated on the Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 28.02.20 08:11 UTC Nova Biotechnol Chim (2017) 16(2): 124-131 126 hotplate at 120 °C until the red brown smoke disappeared. The residue was filtrated and diluted to 25 mL with volumetric flask. This solution was then used for extraction by SFODME technique. The proposed method was validated with recovery study from spiked known amount of Pb standard solution at the concentration of 5.00 and 10.00 µg L-1 to herb samples before digestion. Table 1. Instrumental parameters and furnace heating program for determination of Pb. Instrumental parameter Value Lamp current (mA) 10 Wavelength (nm) 283.3 Spectral resolution (nm) 0.5 Background correction Zeeman background correction Injected sample volume (µL) 10 Chemical modifier Pd(NO3)2 1000 mg L-1 5 µL Furnace program Stage Temperature (°C) Time (s) Drying 95 15.0 Ashing 400 19.0 Atomization* 2 100 2.9 Cleaning 2 100 2.0 *Reading step Results and Discussion 1-(2-pyridylazo)-2-naphthol is a ligand that can interact with many metal ions to form complexes, such as, Cd(II) or Pb(II) (Tharakeswar et al. 2012), as shown in Fig. 1. 1-(2-pyridylazo)-2-naphthol is relatively soluble in aqueous condition but its complexes have low polarity. Also, in the preliminary experiment, the Pb-1-(2-pyridylazo)-2- naphthol complexes are well soluble in 1-undecanol and this ligand can be used for the separation and extraction of Pb by SFODME method followed by ETAAS detection. In order to get high extraction efficiency, the effect of different parameters was considered and optimized. Selection of extracting solvent In the SFODME technique, the suitable organic solvent has a remarkable effect on the formation of metal-ion complexes. Furthermore, it must have a melting point around 10 to 30°C (near room temperature), low toxicity, and high enrichment factor (Yavuz et al. 2016). In this experiment, two extracting solvents, 1-dodecanol and 1-undecanol, were compared and 1-undecanol was chosen as it gave the highest relative absorbance. Since the volume of organic solvent could affect the pre-concentration factor (Li et al. 2006), therefore, different volumes (20 – 150 μL) of 1-undecanol were investigated in the extraction procedure. When increasing the volume of 1-undecanol, the relative absorbance increased until 90 μL and then relative absorbance decreased. This could be owing to the transportation of analyte, which is associated with increasing the surface contact between the layers of two liquids. From the results, 90 μL of 1-undecanol was considered as optimized extraction volume. pH value Generally, the extraction efficiency depends on pH value of the aqueous solution and have a significant effect to the complex formation of Pb-1-(2-pyridylazo)-2-naphthol and subsequently the SFODME extraction. In this experiment, pH of solution was controlled in the range of 3.0 – 12.0 Fig. 1. Chemical structure of metal-PAN complex. Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 28.02.20 08:11 UTC Nova Biotechnol Chim (2017) 16(2): 124-131 127           Fig. 2. Effect of the pH. Extraction conditions: Sample volume – 13 mL; extraction solvent – 1-undecanol; PAN concentration – 3.0 mmol L-1 – 0.5 mL; extraction time – 30 min; stirring rate – 625 rpm at room temperature. using different buffer solutions. The result is illustrated in Fig. 2 and it is clearly seen that the best relative absorbance was obtained at pH 9. At this pH, Pb-1-(2-pyridylazo)-2-naphthol complexes were effectively extracted into the organic solvent. Therefore, pH 9 was chosen for further experiments. 1-(2-pyridylazo)-2-naphthol concentration Due to the amount of 1-(2-pyridylazo)-2-naphthol can affect to the extraction efficiency, the concentration of 1-(2-pyridylazo)-2-naphthol was consequently investigated. The result showed that the relative absorbance was increased when increasing the concentration of 1-(2-pyridylazo)-2- naphthol up to 3.0 mmol L-1 and this continued constant even as the amount of 1-(2-pyridylazo)-2- naphthol is increased further. Therefore, 3.0 mmol L-1 of 1-(2-pyridylazo)-2-naphthol was selected for further studies. Extraction time In order to obtain high sensitivity, good precision and speed up the extraction process, it is required to investigate the effect of the extraction time to ensure the completion of extraction between organic phase and sample solution. The extraction time was studied between of 10 – 120 minutes. The result revealed that the relative absorbance was increased and steadied after 40 minutes (Fig. 3). Therefore, 40 minutes of extraction time was selected for subsequent experimentations. Fig. 3. Effect of the extraction time. Conditions are the same as in Fig. 2. except extraction time, at pH 9 and concentration of PAN, 3.0 mmol L-1. Stirring rate To reduce the time required to reach the equilibrium between the extracting solvent and aqueous solution (Li et al. 2006), the effect of stirring rate was studied between of 250 – 1250 rpm. For the optimized experimental conditions, 750 rpm was selected. Extraction temperature At higher temperatures, the extraction kinetics influence the mass transfer of the analyte between the aqueous solution and the extracting solvent (Ghanbarian et al. 2013). Extraction temperature was studied over a temperature range of 20 – 80°C. From the results, the relative absorbance increased until 50°C, after that relative absorbance decreased when increasing extraction temperature. This phenomenon may be due to the solubility of organic solvent increased and the breakdown of organic solvent at high temperature (Bidabadi et al. 2009). Therefore, the extraction temperature was controlled at 35°C. Table 2. Effect of interfering ions on the quantitative recovery of 10 μg L-1 Pb. Ions Mole ratio (Ion/analyte) Na+, K+, Cr3+, NO3-, SO42-, PO43- 10 000 Cd2+, Mg2+, Fe3+, As3+ 100 Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 28.02.20 08:11 UTC Nova Biotechnol Chim (2017) 16(2): 124-131 128 Table 3. Application of presented method in certified reference material and Thai herb. Samples Certified (µg g-1) Added (µg L-1) Concentration in digested solution (µg L-1 ± SD) Concentration in sample (µg g-1 ± SD) Recovery (% ± SD) SRM-1570a (Spinach leaves) 0.2 – – 0.20 ± 0.01 100.0 ± 1.1 Zingiber officinale – 3.91 ± 0.55 0.48 ± 0.07 – 5.00 9.43 ± 0.67 – 110.5 ± 1.7 10.00 14.19 ± 0.41 – 102.9 ± 3.3 Coscinium fenestratum – 0.76 ± 0.02 0.09 ± 0.01 – 5.00 5.74 ± 0.19 – 99.5 ± 4.2 10.00 10.80 ± 0.39 – 100.3 ± 3.9 Curcuma longa – 1.72 ± 0.34 0.20 ± 0.04 – 5.00 6.62 ± 0.28 – 98.1 ± 1.3 10.00 11.96 ± 0.47 – 102.4 ± 3.7 Piper retrofractum – 2.64 ± 0.68 0.31 ± 0.09 – 5.00 7.79 ± 0.97 – 103.1 ± 4.6 10.00 13.01 ± 0.89 – 103.7 ± 9.1 Tinospora crispa – 1.54 ± 0.26 0.18 ± 0.03 – 5.00 6.77 ± 0.18 – 104.7 ± 1.0 10.00 12.00 ± 0.34 – 104.6 ± 3.1 Cryptolepis buchanani – 2.32 ± 0.26 0.27 ± 0.03 – 5.00 7.43 ± 0.25 – 102.3 ± 0.8 10.00 12.78 ± 0.30 – 104.6 ± 1.4 Butea superba – 1.42 ± 0.20 0.16 ± 0.02 – 5.00 6.44 ± 0.23 – 100.4 ± 0.7 10.00 11.48 ± 0.19 – 100.6 ± 1.0 Moringa oleifera – 2.14 ± 0.38 0.25 ± 0.05 – 5.00 7.26 ± 0.34 – 102.4 ± 1.9 10.00 12.33 ± 0.78 – 101.9 ± 5.0 Kaempferia parviflora – 1.65 ± 0.02 0.19 ± 0.01 – 5.00 6.80 ± 0.23 – 103.1 ± 1.9 10.00 11.79 ± 0.21 – 101.4 ± 4.3 Addition of salt The addition of salt in aqueous solution can change the physical properties of the Nernst diffusion film and reduce the rate of diffusion of the interest analytes into the drop (Sobhi et al. 2009). Also, the effect of salt addition on SFODME technique was studied by adding different quantities of NaCl and KCl at 0–5% (w/v). The results exhibited that when increasing amount of salt, the relative absorbance decreased. Consequently, the extraction procedures were accomplished without salt addition. Interfering ions In the SFODME procedure (Wang et al. 2011; Chen et al. 2013; Dadfarnia et al. 2013), many other heavy metal ions can form complexes with the chelating reagent and can co-extraction with interested metal. To investigate the selectivity of the proposed procedure, the effect of interfering ions on Pb determination in herb samples by SFODME was investigated considering from the percentage recovery. These coexisting ions were considered to interfere in Pb determination if the percentage recovery was over ±5%. The results were demonstrated in Table 2. Analytical performance Under the optimized condition for SFODME, the analytical performance for Pb determination was evaluated. The linear range of Pb concentration was 0.5 – 30.0 µg L-1 (Y = 0.0131X + 0.0100, where Y is the concentration of analyte) with r2 = 0.9996. The LOD and LOQ were 0.064 and 0.214 µg L-1, respectively and %RSDs varied from 1.27% to 2.48% (n=6). The enrichment Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 28.02.20 08:11 UTC Nova Biotechnol Chim (2017) 16(2): 124-131 129 Table 4. Comparison of analytical performance of the proposed method with other methods for determination of Pb. Method Sample LOD (μg L-1) Linear range (μg L-1) RSD (%) Reference CPE-GFAAS Water 0.08 up to 30.0 2.8 Chen et al., 2005 SDME-ETAAS Water 0.2 1.0 – 15.0 4 Maltes et al., 2008 DLLME-ETAAS Water 0.02 0.05 – 1.00 2.5 Naseri et al., 2008 SPE-ETAAS Seawater 0.12 0.1 – 10.0 3.2 Alonso et al., 2006 SFODME-ETAAS Water and infant formula 0.058 0.2 – 10.0 8.8 Chamsaz et al., 2013 SFODME-ETAAS Herbs 0.064 0.5 – 30.0 2.48 This work factor, EF (Dadfarnia et al. 2007; Durukan et al. 2011) defined by the division of the slope of calibration equation after pre-concentration and that of previous pre-concentration by ETAAS was accomplished at 18.71 for 13 mL of sample solution. To guarantee the accuracy of the method, the determination of certified reference material SRM-1570a (Spinach leaves) was used for method validation through recovery experiments. The results are presented in Table 3 and it was indicated that a found concentration agreed well with certified value. Satisfactory recoveries revealed that SFODME-ETAAS method was effective for the pre-concentration/determination of lead. A comparison of analytical performance of the proposed method with others is shown in Table 4 and indicates its higher effectiveness to Pb determination at lower detection limit and better precision than previously reported methods. Determination of Pb in Thai herb samples The procedure was applied to the determination of Pb in Thai herb samples. The samples were prepared as described and 13 mL of it was treated according to the same procedure. The proposed method was validated with recovery study from spiked known amounts of Pb standard solutions. Recoveries from spiked standard solution to the samples are shown in Table 3. It was found that recoveries were in the range of 98 – 110.5% which confirms the validity and accuracy with acceptable results. Conclusions SFODME-ETAAS method was assessed for the pre-concentration and determination at trace μg L-1 level of Pb in Thai herb samples. This method provided several advantages such as simple to operate, minimum consumption of extraction solvent and the solidified organic phase is simply collected and separated from the aqueous sample. Additionally, on-going research related to the application of SFODME method for pre-concentration/determination of other heavy metals in different types of sample matrices using new chelating reagents are subjects on ongoing research. Acknowledgement Financial support from the Science Achievement Scholarship of Thailand and Research Center for Academic Excellence in Petroleum, Petrochemical and Advanced Materials, Faculty of science, Naresuan University in Thailand is gratefully acknowledged. References Alonso EV, Cordero MS, De Torres AG, Pavón JC (2006) Lead ultra-trace on-line pre-concentration and determination using selective solid phase extraction and electrothermal atomic absorption spectrometry: applications in seawaters and biological samples. Anal. Bioanal. Chem. 385: 1178-1185. Batista ÉF, Dos Santos Augusto A, Pereira-Filho ER (2016) Chemometric evaluation of Cd, Co, Cr, Cu, Ni (inductively coupled plasma optical emission spectrometry) and Pb (graphite furnace atomic absorption spectrometry) concentrations in lipstick samples intended to be used by adults and children. Talanta 150: 206-212. Bidabadi MS, Dadfarnia S, Shabani AMH (2009) Solidified floating organic drop microextraction (SFODME) for simultaneous separation/precon-centration and determination of cobalt and nickel by graphite furnace atomic absorption spectrometry (GFAAS). J. Hazard. Mater. 166: 291-296. Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 28.02.20 08:11 UTC Nova Biotechnol Chim (2017) 16(2): 124-131 130 Chamsaz M, Akhoundzadeh J, Arbab-Zavar MH (2013) Pre- concentration of lead using solidification of floating organic drop and its determination by electrothermal atomic absorption spectrometry. J. Adv. Res. 4: 361- 366. Chen J, Xiao S, Wu X, Fang K, Liu W (2005) Determination of lead in water samples by graphite furnace atomic absorption spectrometry after cloud point extraction. Talanta 67: 992-996. Chen S, Cheng X, He Y, Zhu S, Lu D (2013) Determination of the rare earth elements La, Eu, and Yb using solidified floating organic drop microextraction and electrothermal vaporization ICP-MS. Microchim. Acta 180: 1479-1486. Dadfarnia S, Shabani AMH (2013) Solidified floating organic drop micro-extraction–electrothermal atomic absorption spectrometry for ultra trace determination of antimony species in tea, basil and water samples. J. Iran. Chem. Soc. 10: 289-296. Dadfarnia S, Talebi M, Shabani AMH, Beni Z (2007) Determination of lead and cadmium in different samples by flow injection atomic absorption spectrometry incorporating a microcolumn of immobilized ammonium pyrrolidine dithiocarbamate on microcrys- talline naphthalene. Croat. Chem. Acta 80: 17-23. Dapaah AR, Takano N, Ayame A (1999) Solvent extraction of Pb (II) from acid medium with zinc hexamamethylenedithiocarbamate followed by back- extraction and subsequent determination by FAAS. Anal. Chim. Acta 386: 281-286. Demirtaş İ, Bakirdere S, Ataman OY (2015) Lead determination at ng/mL level by flame atomic absorption spectrometry using a tantalum coated slotted quartz tube atom trap. Talanta 138: 218-224. Deng B, Xu X, Xiao Y, Zhu P, Wang Y (2015) Understanding the effects of potassium ferricyanide on lead hydride formation in tetrahydroborate system and its application for determination of lead in milk using hydride generation inductively coupled plasma optical emission spectrometry. Anal. Chim. Acta 853: 179-186. Divrikli U, Horzum N, Soylak M, Elci L (2006) Trace heavy metal contents of some spices and herbal plants from western Anatolia, Turkey. Int. J. Food Sci. Technol. 41: 712-716. Djozan D, Assadi Y (2004) Modified pencil lead as a new fiber for solid-phase microextraction. Chromatographia 60: 313-317. Durukan İ, Şahin ÇA, Bektaş S (2011) Determination of copper traces in water samples by flow injection-flame atomic absorption spectrometry using a novel solidified floating organic drop microextraction method. Microchem. J. 98: 215-219. Ghaedi M, Montazerozohori M, Soylak M (2007) Solid phase extraction method for selective determination of Pb (II) in water samples using 4-(4-methoxybenzylidenimine) thiophenole. J. Hazard. Mater. 142: 368-373. Ghambarian M, Yamini Y, Esrafili A (2013) Liquid-phase microextraction based on solidified floating drops of organic solvents. Microchim. Acta 180: 519-535. Górecki T, Pawliszyn J (1996) Determination of tetraethyllead and inorganic lead in water by solid phase microextraction/gas chromatography. Anal. Chem. 68: 3008-3014. Ikeda K, Abe S (1998) Liquid–liquid extraction of cationic metal complexes with p-nonylphenol solvent: Application to crown and thiacrown ether complexes of lead (II) and copper (II). Anal. Chim. Acta 363: 165- 170. Jalbani N, Soylak M (2015) Separation–pre-concentration of nickel and lead in food samples by a combination of solid–liquid–solid dispersive extraction using SiO2 nanoparticles, ionic liquid-based dispersive liquid–liquid micro-extraction. Talanta 131: 361-365. Jiang H, Hu B, Chen B, Xia L (2009) Hollow fiber liquid phase microextraction combined with electrothermal atomic absorption spectrometry for the speciation of arsenic (III) and arsenic (V) in fresh waters and human hair extracts. Anal. Chim. Acta 634: 15-21. Khan S, Cao Q, Zheng YM, Huang YZ, Zhu YG (2008) Health risks of heavy metals in contaminated soils and food crops irrigated with wastewater in Beijing, China. Environ. Pollut. 152: 686-692. Lam JCW, Chan KK, Yip YC, Tong WF, Sin DWM (2010) Accurate determination of lead in Chinese herbs using isotope dilution inductively coupled plasma mass spectrometry (ID-ICP-MS). Food Chem. 121: 552-560. Li L, Hu B, Xia L, Jiang Z (2006) Determination of trace Cd and Pb in environmental and biological samples by ETV-ICP-MS after single-drop microextraction. Talanta 70: 468-473. López-García I, Vicente-Martínez Y, Hernández-Córdoba M (2014) Determination of cadmium and lead in edible oils by electrothermal atomic absorption spectrometry after reverse dispersive liquid–liquid microextraction. Talanta 124: 106-110. Maltez HF, Borges DL, Carasek E, Welz B, Curtius AJ (2008) Single drop micro-extraction with O, O-diethyl dithiophosphate for the determination of lead by electro- thermal atomic absorption spectrometry. Talanta 74: 800-805. Manzoori JL, Amjadi M, Abulhassani J (2009) Ultra-trace determination of lead in water and food samples by using ionic liquid-based single drop microextraction- electrothermal atomic absorption spectrometry. Anal. Chim. Acta 644: 48-52. Naseri MT, Hosseini MRM, Assadi Y, Kiani A (2008) Rapid determination of lead in water samples by dispersive liquid–liquid microextraction coupled with electrothermal atomic absorption spectrometry. Talanta 75: 56-62. Oviedo JA, Fialho LL, Nóbrega JA (2013) Determination of molybdenum in plants by vortex-assisted emulsification solidified floating organic drop microextraction and flame atomic absorption spectrometry. Spectrochim. Acta B 86: 142-145. Rates SMK (2001) Plants as source of drugs. Toxicon 39: 603-613. Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 28.02.20 08:11 UTC Nova Biotechnol Chim (2017) 16(2): 124-131 131 Rosa FC, Duarte FA, Paniz JN, Heidrich GM, Nunes MA, Flores EM, Dressler VL (2015) Dispersive liquid–liquid microextraction: An efficient approach for the extraction of Cd and Pb from honey and determination by flame atomic absorption spectrometry. Microchem. J. 123: 211-217. Saleh A, Yamini Y, Faraji M, Shariati S, Rezaee M (2009) Hollow fiber liquid phase microextraction followed by high performance liquid chromatography for determination of ultra-trace levels of Se (IV) after derivatization in urine, plasma and natural water samples. J. Chromatogr. B 877: 1758-1764. Sobhi HR, Yamini Y, Esrafili A, Adib M (2008) Extraction and determination of 2-pyrazoline derivatives using liquid phase microextraction based on solidification of floating organic drop. J. Pharm. Biomed. Anal. 48: 1059-1063. Solidum JN (2014) Lead levels in fresh medicinal herbs and commercial tea products from Manila, Philippines. APCBEE Procedia 10: 281-285. Sorbo A, Turco AC, Di Gregorio M, Ciaralli L (2014) Development and validation of an analytical method for the determination of arsenic, cadmium and lead content in powdered infant formula by means of quadrupole inductively coupled plasma mass spectrometry. Food Control. 44: 159-165. Tai CY, Jiang SJ, Sahayam AC (2016) Determination of As, Hg and Pb in herbs using slurry sampling flow injection chemical vapor generation inductively coupled plasma mass spectrometry. Food Chem. 192: 274-279. Tharakeswar Y, Kalyan Y, Gangadhar B, Kumar KS, Naidu GR (2012) Optical chemical sensor for screening cadmium (II) in natural waters. J Sens Technol 2: 68-74. Wang Y, Zhang J, Zhao B, Du X, Ma J, Li J (2011) Development of dispersive liquid–liquid microextraction based on solidification of floating organic drop for the determination of trace nickel. Biol. Trace Elem. Res. 144: 1381-1393. Xia L, Wu Y, Hu B (2007) Hollow‐fiber liquid‐phase microextraction prior to low‐temperature electrothermal vaporization ICP‐MS for trace element analysis in environmental and biological samples. J. Mass Spectrom. 42: 803-810. Yavuz E, Tokalioğlu Ş, Şahan H, Patat Ş (2016) Nanosized spongelike Mn3O4 as an adsorbent for pre-concentration by vortex assisted solid phase extraction of copper and lead in various food and herb samples. Food Chem. 194: 463-469. Zendelovska D, Pavlovska G, Cundeva K, Stafilov T (2001) Electrothermal atomic absorption spectrometric determination of cobalt, copper, lead and nickel traces in aragonite following flotation and extraction separation. Talanta 54: 139-146. Zhang Z, Yang MJ, Pawliszyn J (1994) Solid-phase microextraction. A solvent-free alternative for sample preparation. Anal. Chem. 66: 844A-853A. Zhao L, Zhong S, Fang K, Qian Z, Chen J (2012) Determination of cadmium (II), cobalt (II), nickel (II), lead (II), zinc (II), and copper (II) in water samples using dual-cloud point extraction and inductively coupled plasma emission spectrometry. J. Hazard. Mater. 239: 206-212. Zhong WS, Ren T, Zhao LJ (2016) Determination of Pb (Lead), Cd (Cadmium), Cr (Chromium), Cu (Copper), and Ni (Nickel) in Chinese tea with high-resolution continuum source graphite furnace atomic absorption spectrometry. J. Food Drug Anal. 24: 46-55. Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 28.02.20 08:11 UTC