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A green, accurate and sensitive analytical method based on vortex assisted deep eutectic solvent-liquid phase microextraction for the determination of cobalt by slotted quartz tube flame atomic absorption spectrometry
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A green, accurate and sensitive analytical method based on vortex assisted deep eutectic solvent-liquid phase microextraction for the determination of cobalt by slotted quartz tube flame atomic absorption spectrometry Zeynep Tekina, Tuğçe Unutkanb, Fatih Erulaşc, Emine Gülhan Bakırdered, Sezgin Bakırderea,
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Yıldız Technical University, Department of Chemistry, 34349 İstanbul, Turkey Yıldız Technical University, Department of Chemical Engineering, 34349 İstanbul, Turkey c Siirt University, Faculty of Education, Department of Science Education, Siirt, Turkey d Department of Science Education, Yıldız Technical University, Faculty of Education, 34210 İstanbul, Turkey a
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A R T I C LE I N FO
A B S T R A C T
Keywords: Cobalt Deep eutectic solvent Liquid phase microextraction Flame atomic absorption spectrometry
Preconcentration of cobalt was carried out with deep eutectic solvent based liquid phase microextraction (DESLPME) for trace determination by a slotted quartz tube (SQT) attached flame atomic absorption spectrometry (FAAS) system. Choline chloride and phenol in a 1:2 M ratio was used as a green solvent to extract cobalt from the aqueous sample solution. Key parameters influencing the extraction efficiency of cobalt were examined and optimized. Under the conditions optimized, the linear dynamic range was found between 5.0 and 50 µg L−1, and the limits of detection and quantification (LOD and LOQ) were calculated as 2.0 and 6.6 µg L−1, respectively. The detection power of the conventional FAAS was improved upon by 67 folds using the optimized DES-LPMESQT-FAAS method. The developed analytical method was successfully applied for the determination of cobalt in linden tea samples and the recovery results obtained for different spiked concentrations (20, 30 and 40 µg L−1) were remarkable (≈100%).
1. Introduction Cobalt is known as an essential element for human beings and numerous living organisms at trace levels. The primary component of cyanocobalamin or vitamin B12 is cobalt (Ghoochani Moghadam, Rajabi, Hemmati, & Asghari, 2017). In addition to being a significant part of vitamin B12, cobalt also plays a significant role in the production of blood cells (Arain, Yilmaz, & Soylak, 2016). The required cobalt is supplied to humans through foods such as fish, oysters, eggs, milk and green vegetables because humans cannot synthesize the minerals that include cobalt (Chaparro, Ferrer, Leal, & Cerdà, 2015; Khoddami & Shemirani, 2016). Cobalt deficiency results in some diseases including metabolic disorders, retarded growth, anemia and degeneration of nerve cells (Casarett, Klaassen, Amdur, & Doull, 1996; Lison, 2007; Öztürk Er, Bakırdere, Unutkan, & Bakırdere, 2018). However, excessive amounts of this analyte can cause toxic effects and may lead to asthma, decreased cardiac output, cardiac enlargement, heart and lung diseases, dermatitis and vasodilation (Park et al., 2016; Wang et al., 2018). Both low and high amounts of cobalt have negative impacts on human health. Tea is the second most consumed beverage in the world only
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after water and concentrations of cobalt in the range of 0.10–0.60 µg/g has been reported in some tea samples (Brzezicha-Cirocka, Grembecka, & Szefer, 2016; Kim & Gibb, 2006). For this reasons, it is necessary to detect cobalt at low concentration levels with sensitive analytical techniques in food and drink samples. For decades, several analytical methods including flame atomic absorption spectrometry (FAAS) (Jamali, Soleimani, Rahnama, & Rahimi, 2017), electrothermal atomic absorption spectrometry (ETAAS) (Chen, Lei, Yang, & Wen, 2017), inductively coupled plasma optical emission spectrometry (ICP-OES) (Bartosiak, Jankowski, & Giersz, 2018), inductively coupled plasma mass spectrometry (ICP-MS) (Zhao, Wei, Shu, Kong, & Yang, 2016), voltammetry (Mettakoonpitak, MillerLionberg, Reilly, Volckens, & Henry, 2017) and ultraviolet–visible spectrophotometry (UV–VIS) (Guo et al., 2017) have been used for the determination of several metals in literature. FAAS is a widely used instrumental method which has important advantages such as robustness, accuracy and precision. In addition, it is a relatively cheap instrument and offers low operational cost (Thompson & Davidow, 2009). However, FAAS has some limitations such as low sensitivity which hinders determination of metals at trace concentrations (Tokalıoğlu,
Corresponding author. E-mail address: [email protected] (S. Bakırdere).
https://doi.org/10.1016/j.foodchem.2019.125825 Received 20 June 2019; Received in revised form 18 October 2019; Accepted 28 October 2019 0308-8146/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Zeynep Tekin, et al., Food Chemistry, https://doi.org/10.1016/j.foodchem.2019.125825
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entry slot (5.7 cm) to cover the flame and an exit slot (1.2 cm) situated at 180° angle with respect to each other. The SQT was carefully placed on the head of the burner so that the optical path of the lamp light passed through without blockage. Centrifugation was performed with a Hettich EBA 20 model centrifuge. A Hapa M-100 ultrasonic water bath and a WiseMix VM-10 vortex were used for mixing throughout extraction studies.
Papak, & Kartal, 2017). In this regard, slotted quartz tube (introduced by Watling in 1977) is a commonly used device to enhance the absorbance signal of FAAS (Watling, 1977). The SQT is placed on the flame burner and it helps to improve the residence period of analyte atoms in the light path coming from hallow cathode lamp (HCL). Therefore, more interactions between light from HCL and analyte atoms results in high absorbance values (Kaya & Yaman, 2008). In addition to SQT modification of FAAS for enhanced absorbance signal, a variety of pretreatment techniques can be carried out to extract analytes of interest from sample matrixes (Shokoufi, Shemirani, & Assadi, 2007). There are several preconcentration/microextraction procedures which are used for heavy metals such as solid phase extraction (SPE) (Amin, 2014), liquid–liquid extraction (LLE) (Reza Jamali, Assadi, & Shemirani, 2007), flow injection extraction (FIE) (Bakircioglu, Topraksever, & Kurtulus, 2014), cloud point extraction (CPE) (Ghaedi, Shokrollahi, Ahmadi, Rajabi, & Soylak, 2008), dispersive liquid–liquid microextraction (DLLME) (El-Shahawi & Al-Saidi, 2013) and liquid phase microextraction (LPME) (Memon, Yilmaz, & Soylak, 2017). The use of economical and non-toxic solvent is a main goal in microextraction methods (Milanova, Chambers, Bahga, & Santiago, 2011; Yilmaz & Soylak, 2018). Hence, deep eutectic solvents (DESs) are attracting great attention as replacement for traditional organic solvents. DESs consist of two or more compounds with the capability to interact with each other via hydrogen bonds which are called eutectic mixtures. The main characteristic property of DESs is having lower melting points than each of the discrete constituents because of the self-association of hydrogen bond donors and acceptors (Wagle, Zhao, & Baker, 2014; Wang et al., 2017; Yilmaz & Soylak, 2016). They are generally obtained from a mixture of choline chloride salt such as ChCl and Vitamin B4, and a metal salt or hydrogen bond donor (HBDs) such as urea, sugars and glycerol (Arain et al., 2016; Smith, Abbott, & Ryder, 2014). Because of its cheapness, biodegradability and green properties, choline chloride (ChCl) which is a quaternary ammonium salt is the most widely utilized component in the synthesis of DESs (Abbott, Capper, Davies, McKenzie, & Obi, 2006; Zounr, Tuzen, Deligonul, & Khuhawar, 2018). DESs and ionic liquids (ILs) have similar physicochemical properties such as non-flammability and high solubility not only for organic sompounds, but also for inorganic species (Francisco, van den Bruinhorst, & Kroon, 2013; Zounr, Tuzen, & Khuhawar, 2018). On the other hand, DESs surpasses organic solvents and ILs in various aspects such as economical cost, easy preparation by mixing components, no requirement of additional pretreatment, biodegradability, biologically compatibility and non-toxicity (Arain et al., 2016; Smith et al., 2014). Finally, DESs are useful in a variety of application areas such as synthesis of nanomaterials, extraction of inorganic and organic species, drugs and metal oxide dissolution and purification of precious organic compounds like oil and biodiesel (Karimi, Dadfarnia, Shabani, Tamaddon, & Azadi, 2015; Yilmaz & Soylak, 2016, 2018). In this study, DES-LPME method was developed for both extraction and preconcentration of cobalt prior to determination by the SQT-FAAS system. The proposed method was successfully applied to linden samples for the determination of cobalt at trace levels with high accuracy and precision.
2.2. Chemicals and reagents Reagents used in the study were of analytical grade and high purity. All working/calibration standards of cobalt were prepared from a 1000 mg L−1 cobalt stock solution purchased from High-Purity Standards, USA. DES solvent was prepared by mixing choline chloride (ChCl, Acros, 99%) and phenol (PhOl, Merck). Tetrahydrofuran (THF) with a purity of ≥99.9% was obtained from Merck. Ultrapure water from a Milli-Q® Reference Ultrapure Water Purification System with 18.2 M-ohm-cm resistivity was used in the preparation of standard/ sample solutions. 2.3. Synthesis of complexing agent Synthesis of the ligand used in the study was achieved in accordance with the following procedure as reported by our research group (Öztürk Er et al., 2018): 10 mmol of 5-bromosalicylaldehyde was dissolved in a proper amount of ethanol and then p-toluenesulfonic acid (0.01 mg) was added and the mixture was heated to 60 °C. Afterwards, 20 mmol of p-toluidine in 25 mL of ethanol was added to the final solution. A dark orange reaction color was observed, indicating precipitation of the ligand ((Z)-3-bromo-5-((p-tolylimino)methyl)phenol)). The final product was filtered and then dried at 50 °C. 2.4. Preparation of DES Choline chloride as an organic salt (HBA-ChCl) and phenol (HBDPhOl) as a hydrogen bond donor were used in preparing DES for the extraction of cobalt. In order to find the optimal ratio, different DESs were prepared by simply mixing the ChCl and PhOl in 50 mL centrifuge tubes at 1:2, 1:3, 1:4 and 1:5 M ratios by vortexing (3000 rpm) at room conditions until a colorless and homogeneous liquid was observed. The prepared DES was left in the ultrasonic bath for 15–20 s to get rid of air bubbles. The DESs were prepared fresh each day for optimization studies and method validation. 2.5. Procedure The extraction procedure was carried out in 15 mL of centrifuge tubes containing 10 mL of aqueous Co standard/sample solution. The addition of 1.0 mL 0.20% (w/v) (Z)-3-bromo-5-((p-tolylimino)methyl) phenol ligand was preceded by adding 1.0 mL of pH 10 buffer, and the resulting solution was sonicated in the ultrasonic bath for 10 s for the complexation of ligand with cobalt. Then, 0.60 mL of DES with a molar ratio of 1:2 was injected into the cobalt complex solution and the cloudy solution formed was then vortexed for 5.0 s. Afterwards, 1.0 mL of THF was added to the solution as an emulsifier agent and hand shaken for 15 s for homogeneous distribution. The final solution was centrifuged at 6000 rpm for 120 s, and 125 µL was taken from the upper organic phase into a clean tube. The separated phase was held in a hot water bath to prevent clogging of the nebulizer while performing measurements with the FAAS instrument.
2. Materials and methods 2.1. Instrumentation An ATI UNICAM 929 AA model FAAS having a deuterium background corrector was used for cobalt absorbance measurement at the 240.7 nm analytical line of a Varian SpectrAA hollow cathode lamp. The respective slit width and operational current of the hollow cathode lamp were 0.5 nm and 15 mA. An air-acetylene flame was used for the atomization of cobalt. A lab-made SQT was 15.3 cm long, with 1.75 cm internal diameter and 2.05 cm outer diameter specifically cut with an
3. Result and discussion In this study, the ligand was synthesized to be a complexing agent in the extraction process. In order to increase the sensitivity of the method, all parameters having effects cobalt complex formation, 2
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vortexing and manual shaking, after which the results were compared to complexing without mixing. Ultrasonication recorded the highest absorbance signals and its optimum period was determined as 10 s after testing 5.0, 10, 15, 30 and 60 s mixing periods.
extraction and determination signal were optimized. Three replicate measurements were done for each optimization. Different optimization approaches are reported in literature such as experimental design and univariate optimizations (Leardi, 2009). In this study, one parameter was varied at different values while all other parameters were kept constant. Optimum phase volume was examined by transferring 50, 75, 100 and 125 µL from the upper phase obtained after extraction into clean tubes for absorbance measurements. The highest signal was recorded for the 125 µL volume. The separated volume after addition of THF was very low and difficult to collect for instrumental measurement. Trace amounts of water phase were collected with the organic phase when higher volumes were tested, and this led to high standard deviation values. In addition, no significant increase was observed for volumes tested up to 125 µL. Subsequent experiments were therefore performed with 125 µL volume of extract from the upper phase.
3.3. Optimization of deep eutectic solvent extraction An ideal extraction solvent should be selected considering the properties of being immiscible with aqueous solution, having a higher/ lower density than water, and high selectivity for the analyte of interest. Additionally, the solvent should be green, that is, have low toxicity and generate waste that can be safely disposed into the environment (Ghoochani Moghadam et al., 2017). The use of DES as an extraction solvent in this study was considered to meet these requirements. Choline chloride and phenol at the ratio of 1:2, 1:3, 1:4 and 1:5 were tested in extracting the cobalt complex from aqueous solution. As the phenol ratio decreased, there was a decrease in the viscosity of the solution and an increase in absorbance values as shown in Fig. 2. This can be explained by the efficiency of the nebulizer unit of the FAAS which reduces with increasing viscosity. Therefore, 1:2 was chosen as optimum value for the DES molar ratio. DES volume has a significant influence on the extraction power and the enrichment amount of cobalt with regard to the amount of analytes collected. The amount of settled phases varied for each volume of DES as a result of different solubility in water. The volumes of DES tested were 0.30, 0.50, 0.60, 0.75 and 1.0 mL, and the amount of settled phase increased with the amount of extractor solvent. As a result of this, concentration of the analyte in high volume settled phases were low by reason of dilution. Although the highest signal was observed for 0.50 mL, the optimum value was chosen as 0.60 mL because it gave more reproducible absorbances (low standard deviation) and the final volume was convenient for withdrawal and processing for absorbance measurements. There was no settled phase observed for 0.30 mL of DES due to total miscibility with water. In order to determine the best method for adding DES to the aqueous solution, micropipette and syringe were compared. Absorbance values obtained after injection with syringe was about two times higher than that obtained with micropipette. The injection process brought about efficient distribution of DES throughout the sample solution. Similarly to complex formation, mixing effect was studied on extraction output by testing mechanical shaking, ultrasonication, vortex, manual shaking and no mixing. Vortex emerged as the most efficient mixing type and 5.0 s was obtained as optimum period after testing up to 60 s.
3.1. Optimization of FAAS and SQT-FAAS Instrumental parameters of the FAAS and SQT-FAAS systems including acetylene flow rate, sample flow rate and SQT height from the burner head were optimized the boost cobalt absorbance signals. All instrumental optimization studies were carried on with 5.0 mg L−1 of cobalt standard solution without preconcentration. The sample and acetylene flow rates affect the efficiency of nebulization and increase the amount of atomized analyte. Nebulizer set button and fuel gas scale were adjusted to various positions to find the optimum flow rates that gives the highest absorbance. Thus, the optimum flow rates for sample and acetylene were found to be 4.39 mL min−1 and 12.5 L h−1, respectively. Acetylene flow rates beyond 12.5 L h−1 were not tested to avoid the risk of harming the some parts of the instrument due to horn shaped flames exits from the slotted quartz tube which were close to the quartz windows on both sides. The SQT heights of 0.0, 1.0 and 2.0 mm from the burner head were tried and the highest signal was obtained at 1.0 mm height. 3.2. Optimization of complex formation Variables including concentration/amount of ligand, pH/amount of buffer solution and complexing period were optimized to increase the efficiency of complex formation. pH of the solution affects the state of an analyte in solution. Buffer solutions in the pH range of 3.0–13 were studied and the highest absorbance value was obtained for pH 10. Lower absorbance values were observed at the other pH values. In addition, the optimum amount of the pH 10 buffer solution was determined by adding 0.50, 1.0, 1.5 and 2.0 mL volumes. In addition, an extraction was repeated without the addition of buffer solution. As a result of the analysis, a significant increase was observed for 1.0 mL over 0.50 mL, while there was no big difference between 1.0, 1.5 and 2.0 mL. The lowest absorbance value was recorded for the extraction performed without buffer addition. 1.0 mL was selected as optimum amount due to lower standard deviation. The optimum concentration of the ligand was determined as 0.20% after testing it together with 0.50, 1.0 and 2.0% (w/v). Higher concentrations were not workable due to sedimentation depending on the solubility of the ligand in ethanol. The resulting phases from the high ligand concentrations were allowed to stand in hot water bath to prevent any tubing blockage or clogging of nebulizer unit by the precipitates/sediments formed. The amount of ligand was also optimized to attain a high complex formation output. The amounts of 0.20% (w/v) ligand solution tested were 0.50, 1.0, 1.5 and 2.0 mL, and these were compared to a ligandless extraction (Fig. 1). The signal increased proportionately with increasing ligand volume up to 2.0 mL. There was no phase separation observed for the 2.0 mL volume, and 1.5 mL was selected as the optimum amount. For homogenous dispersion of ligand solution, the complex solution was subjected to 15 s each of mechanical shaking, ultrasonication,
3.4. Optimization of emulsifier agent Emulsifier agent forms a film around the droplets of the immiscible phase reducing the interfacial tension between the phases and prevents reunion of droplets (Thompson & Davidow, 2009). In this study, THF as emulsifier agent was added with aid of a micropipette into aqueous sample containing analytes. The extraction solution after addition of THF became very cloudy. As a result of this, cobalt was easily transferred into the extracted phase in a very short time. The volumes of THF tested were 0.50, 1.0, 1.5 and 2.0 mL but no phase separation was observed for the 0.50 mL. The final volume obtained using 1.0 mL was quite low but workable enough to obtain reproducible results. Furthermore, the effect of type and period of mixing was determined. Manual shaking gave the highest dispersion over mechanical shaker, ultrasonication, vortex and no mixing. The optimum period of 15 s was assigned after testing 5.0, 10, 15, 30 and 60 s. 3.5. Analytical performance of DES-LPME method Under the optimized conditions given in Table 1, analytical performance of the systems was determined using calibration plots of aqueous Co standards. Blank extractions and measurements were 3
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Fig. 1. Effect of ligand volume on the absorbance of cobalt (Conditions: 8.0 mL of sample volume, 1.0 mL of pH 10 buffer solution, 0.20% (w/v) ligand solution, 0.60 mL of DES (1:2; ChCl:PhOl), 1.0 mL of THF).
Fig. 2. Effect of DES molar ratio on the absorbance of cobalt (Conditions: 8.0 mL of sample volume, 1.0 mL of pH 10 buffer solution, 0.20% (w/v) 1.5 mL ligand solution, 0.60 mL of DES, 1.0 mL of THF).
LOD = 3 × SD/Slope LOQ = 10 × SD/Slope
Table 1 Optimized parameters of the DES-LPME–SQT–FAAS. Parameter
Value
Sample flow rate Acetylene flow rate SQT height pH of buffer solution/volume Ligand concentration/volume DES molar ratio/volume Emulsifier agent volume
4.39 mL min−1 12.5 L h−1 1.0 mm 10/1.0 mL 0.20% (w/v)/1.5 mL ChCl:PhOl; 1:2/0.60 mL THF/1.0 mL
The calibration plots of the systems were linear over broad concentration ranges with R2 values greater than 0.9994, indicating good linearity. Analytical figures of merit of the four systems in this study and comparison with other literature findings are presented in Table 2. The improvements in detection powers for the developed systems were calculated as a ratio of the detection limit of the FAAS system to the optimized systems. The detection power of FAAS system was enhanced by about 2.0 folds with the attachment of SQT. In addition to this, applying the DES-LPME before FAAS measurement resulted in about 33 folds increase in detection power. Combining the two enhancement methods with FAAS resulted in about 67 folds increase in detection power with an LOD value of 2.0 µg L−1. The %RSD values were calculated from six replicate measurements of the lowest concentrations in each calibration plot and the low values obtained indicate good precision.
performed in the development of calibration plots for all systems studied. The absence of analytical signals for blank measurements suggested Co impurities were below detection limits. The parameters used to validate the systems were limit of detection and quantification (LOD and LOQ), linear dynamic range (LDR), coefficient of determination (R2) and precision (%RSD). The LOD and LOQ values were calculated with the expressions below, using the standard deviation (SD) from 6 replicate measurements of the lowest calibration standard and calibration slope. 4
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Table 2 Analytical performance of the systems under the optimum conditions and detection limit comparison with other methods reported in literature. Method
LOD (µg L−1)
LOQ (µg L−1)
%RSD
Range (µg L−1)
FAAS SQT–FAAS DES-LPME-FAAS DES-LPME-SQT-FAAS SPE-FAAS (Habila, ALOthman, Yilmaz, & Soylak, 2018) On-Line Sorbent Preconcentration-FAAS (Ye, Ali, & Yin, 2002) ETAAS(Felipe-Sotelo et al., 2004)
134 71 4.1 2.0 2.9 3.2 1.7
445 235 14 6.6
5.9 5.6 2.9 7.1
350–10,000 200–10,000 10–1000 5.0–50
5.6
interference can be mitigated using matrix matching calibration strategy, to obtain accurate and precise quantification results. An absorbance signal obtained for linden tea extract spiked at 30 µg L−1 and analyzed under the optimum method is given in Fig. 3. In addition, ICP-MS instrument was used to check the accuracy of the developed method. 30 µg L−1 spiked linden tea extract was prepared and the concentration of Co was measured with both ICP-MS and the developed DES-LPME-SQT-FAAS systems. For the ICP-MS instrument, Co concentration was found to be 30.0 ± 0.7 µg L−1 while it was 30.9 ± 1.6 µg L−1 using DES-LPME-SQT-FAAS. It is clear that there is no difference in the results indication the high accuracy of the developed method.
Table 3 Recovery values for spiked linden tea. Matrix
Co concentration, µg L−1
Recovery, %
Linden Tea
20 30 40
98.6 ± 4.4 97.1 ± 5.1 100.0 ± 3.2
3.6. Accuracy and applicability check Consumption of the linden plant extract as traditional tea is very high in Turkey especially during winter. Hence, accurate determination of cobalt in this matrix is very crucial. For the purpose of demonstrating applicability of the developed method, linden samples were purchased from five different herbalists and used for spike recovery experiments. The linden samples (20 g) were boiled in water for 45 min at 100 °C. Afterwards, the brewed sample was allowed to cool and filtered while warm to remove solid particulates before making up the volume to 100 mL. Blank samples were firstly analyzed to determine whether or not cobalt was present. The results showed that cobalt was not present in all of the samples, or below the method’s detection limit if present. They were therefore spiked at 20, 30 and 40 μg L−1 and analyzed under the optimized extraction/instrumental conditions. When calculated against aqueous calibration standards, the percent recoveries were lower than 80% due to matrix interference. To overcome these effects, a different linden tea extract was used to prepare matrix matched calibration standards and run alongside the spiked samples. The percent recovery results as presented in Table 3 using matrix matching technique were in the range of 97–100%. This certifies that sample matrix
4. Conclusion Deep eutectic solvent is a green extraction solution used for the determination of different analytes in recent years. In this study, a DESLPME method was optimized for the preconcentration of cobalt complex for determination by slotted quartz tube flame atomic absorption spectrometry. Due to its low cost and non-toxicity, choline chloride was used as an organic salt to produce DES with phenol. All the effective parameters were optimized and the analytical performance of each system was determined. The detection power of the FAAS was enhanced by about 67 folds using the combined DES-LPME and SQT-FAAS system. The developed method was successfully applied to the linden tea matrix. Five different linden tea samples were analyzed but cobalt was not detected. Even though matrix interference hampered recovery of cobalt, matrix matching was used to overcome the effects and enhance accuracy of quantification. Recovery results obtained were satisfactory.
Fig. 3. Analytical signal of Co in spiked sample at 30 µg L−1 under optimum DES-LPME-SQT-FAAS conditions. 5
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In addition, the result obtained using the developed method was compared with the ICP-MS measurement and it was in good agreement with the ICP-MS result. It is clear that combination of high preconcentration factor of the DES-LPME and enhanced detection power of SQT-FAAS created a sensitive analytical method having high accuracy and precision.
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Deep eutectic solvent-based ultrasound-assisted dispersive liquid-liquid microextraction coupled with high-performance liquid chromatography for the determination of ultraviolet filters in water samples. Journal of Chromatography A, 1516, 1–8. https://doi.org/10. 1016/J.CHROMA.2017.07.073. Wang, Z., Xing, X., Yang, Y., Zhao, R., Zou, T., Wang, Z., & Wang, Y. (2018). One-step hydrothermal synthesis of thioglycolic acid capped CdS quantum dots as fluorescence determination of cobalt ion. Scientific Reports, 8(1), 8953. https://doi.org/10.1038/ s41598-018-27244-0. Watling, R. J. (1977). The use of a slotted quartz tube for the determination 0f arsenic, antimony, selenium and mercury. Analytica Chimica Acta, 94(1), 181–186. https:// doi.org/10.1016/S0003-2670(01)83645-X. Ye, Y., Ali, A., & Yin, X. (2002). Cobalt determination with FI-FAAS after on-line sorbent preconcentration using 1-nitroso-2-naphthol. Talanta, 57(5), 945–951. https://doi. org/10.1016/S0039-9140(02)00141-8. 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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.foodchem.2019.125825. References Amin, A. S. (2014). Study on the solid phase extraction and spectrophotometric determination of cobalt with 5-(2-benzothiazolylazo)-8-hydroxyquinolene. Arabian Journal of Chemistry, 7(5), 715–721. https://doi.org/10.1016/J.ARABJC.2010.12. 008. Abbott, A. P., Capper, G., Davies, D. L., McKenzie, K. J., & Obi, S. U. (2006). Solubility of metal oxides in deep eutectic solvents based on choline chloride. Journal of Chemical and Engineering Data https://doi.org/10.1021/JE060038C. Arain, M. B., Yilmaz, E., & Soylak, M. (2016). Deep eutectic solvent based ultrasonic assisted liquid phase microextraction for the FAAS determination of cobalt. Journal of Molecular Liquids, 224, 538–543. https://doi.org/10.1016/J.MOLLIQ.2016.10.005. Bakircioglu, D., Topraksever, N., & Kurtulus, Y. B. (2014). Determination of zinc in edible oils by flow injection FAAS after extraction induced by emulsion breaking procedure. Food Chemistry, 151, 219–224. https://doi.org/10.1016/J.FOODCHEM.2013.11.069. Bartosiak, M., Jankowski, K., & Giersz, J. (2018). Determination of cobalt species in nutritional supplements using ICP-OES after microwave-assisted extraction and solidphase extraction. Journal of Pharmaceutical and Biomedical Analysis, 155, 135–140. https://doi.org/10.1016/J.JPBA.2018.03.058. Brzezicha-Cirocka, J., Grembecka, M., & Szefer, P. (2016). Monitoring of essential and heavy metals in green tea from different geographical origins. Environmental Monitoring and Assessment, 188(3), https://doi.org/10.1007/s10661-016-5157-y. Casarett, L. J., Klaassen, C. D., Amdur, M. O., & Doull, J. (1996). Casarett and Doull’s toxicology: The basic science of poisons. McGraw-Hill, Health Professions Division. Chaparro, L., Ferrer, L., Leal, L., & Cerdà, V. (2015). A multisyringe flow-based system for kinetic–catalytic determination of cobalt(II). Talanta, 133, 94–99. https://doi.org/10. 1016/J.TALANTA.2014.06.071. Chen, L., Lei, Z., Yang, S., & Wen, X. (2017). Application of portable tungsten coil electrothermal atomic absorption spectrometer for the determination of trace cobalt after ultrasound-assisted rapidly synergistic cloud point extraction. Microchemical Journal, 130, 452–457. https://doi.org/10.1016/J.MICROC.2016.11.003. El-Shahawi, M. S., & Al-Saidi, H. M. (2013). Dispersive liquid-liquid microextraction for chemical speciation and determination of ultra-trace concentrations of metal ions. TrAC Trends in Analytical Chemistry, 44, 12–24. https://doi.org/10.1016/J.TRAC. 2012.10.011. Felipe-Sotelo, M., Carlosena, A., Andrade, J. M., Fernández, E., López-Mahı́a, P., Muniategui, S., & Prada, D. (2004). Development of a slurry-extraction procedure for direct determination of cobalt by electrothermal atomic absorption spectrometry in complex environmental samples. Analytica Chimica Acta, 522(2), 259–266. https:// doi.org/10.1016/j.aca.2004.06.005. Francisco, M., van den Bruinhorst, A., & Kroon, M. C. (2013). Low-transition-temperature mixtures (LTTMs): A new generation of designer solvents. Angewandte Chemie International Edition, 52(11), 3074–3085. https://doi.org/10.1002/anie.201207548. Ghaedi, M., Shokrollahi, A., Ahmadi, F., Rajabi, H. R., & Soylak, M. (2008). Cloud point extraction for the determination of copper, nickel and cobalt ions in environmental samples by flame atomic absorption spectrometry. Journal of Hazardous Materials, 150(3), 533–540. https://doi.org/10.1016/J.JHAZMAT.2007.05.029. Ghoochani Moghadam, A., Rajabi, M., Hemmati, M., & Asghari, A. (2017). Development of effervescence-assisted liquid phase microextraction based on fatty acid for determination of silver and cobalt ions using micro-sampling flame atomic absorption spectrometry. Journal of Molecular Liquids, 242, 1176–1183. https://doi.org/10. 1016/J.MOLLIQ.2017.07.038. Guo, Y., Zhao, H., Han, Y., Liu, X., Guan, S., Zhang, Q., & Bian, X. (2017). Simultaneous spectrophotometric determination of trace copper, nickel, and cobalt ions in water samples using solid phase extraction coupled with partial least squares approaches. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 173, 532–536. https://doi.org/10.1016/J.SAA.2016.10.003. Habila, M. A., ALOthman, Z. A., Yilmaz, E., & Soylak, M. (2018). Activated carbon cloth filled pipette tip for solid phase extraction of nickel(II), lead(II), cadmium(II), copper (II) and cobalt(II) as 1,3,4-thiadiazole-2,5-dithiol chelates for ultra-trace detection by FAAS. International Journal of Environmental Analytical Chemistry, 98(2), 171–181. https://doi.org/10.1080/03067319.2018.1430794.
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Food Chemistry journal homepage: www.elsevier.com/locate/foodchem
A green, accurate and sensitive analytical method based on vortex assisted deep eutectic solvent-liquid phase microextraction for the determination of cobalt by slotted quartz tube flame atomic absorption spectrometry Zeynep Tekina, Tuğçe Unutkanb, Fatih Erulaşc, Emine Gülhan Bakırdered, Sezgin Bakırderea,
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Yıldız Technical University, Department of Chemistry, 34349 İstanbul, Turkey Yıldız Technical University, Department of Chemical Engineering, 34349 İstanbul, Turkey c Siirt University, Faculty of Education, Department of Science Education, Siirt, Turkey d Department of Science Education, Yıldız Technical University, Faculty of Education, 34210 İstanbul, Turkey a
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Cobalt Deep eutectic solvent Liquid phase microextraction Flame atomic absorption spectrometry
Preconcentration of cobalt was carried out with deep eutectic solvent based liquid phase microextraction (DESLPME) for trace determination by a slotted quartz tube (SQT) attached flame atomic absorption spectrometry (FAAS) system. Choline chloride and phenol in a 1:2 M ratio was used as a green solvent to extract cobalt from the aqueous sample solution. Key parameters influencing the extraction efficiency of cobalt were examined and optimized. Under the conditions optimized, the linear dynamic range was found between 5.0 and 50 µg L−1, and the limits of detection and quantification (LOD and LOQ) were calculated as 2.0 and 6.6 µg L−1, respectively. The detection power of the conventional FAAS was improved upon by 67 folds using the optimized DES-LPMESQT-FAAS method. The developed analytical method was successfully applied for the determination of cobalt in linden tea samples and the recovery results obtained for different spiked concentrations (20, 30 and 40 µg L−1) were remarkable (≈100%).
1. Introduction Cobalt is known as an essential element for human beings and numerous living organisms at trace levels. The primary component of cyanocobalamin or vitamin B12 is cobalt (Ghoochani Moghadam, Rajabi, Hemmati, & Asghari, 2017). In addition to being a significant part of vitamin B12, cobalt also plays a significant role in the production of blood cells (Arain, Yilmaz, & Soylak, 2016). The required cobalt is supplied to humans through foods such as fish, oysters, eggs, milk and green vegetables because humans cannot synthesize the minerals that include cobalt (Chaparro, Ferrer, Leal, & Cerdà, 2015; Khoddami & Shemirani, 2016). Cobalt deficiency results in some diseases including metabolic disorders, retarded growth, anemia and degeneration of nerve cells (Casarett, Klaassen, Amdur, & Doull, 1996; Lison, 2007; Öztürk Er, Bakırdere, Unutkan, & Bakırdere, 2018). However, excessive amounts of this analyte can cause toxic effects and may lead to asthma, decreased cardiac output, cardiac enlargement, heart and lung diseases, dermatitis and vasodilation (Park et al., 2016; Wang et al., 2018). Both low and high amounts of cobalt have negative impacts on human health. Tea is the second most consumed beverage in the world only
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after water and concentrations of cobalt in the range of 0.10–0.60 µg/g has been reported in some tea samples (Brzezicha-Cirocka, Grembecka, & Szefer, 2016; Kim & Gibb, 2006). For this reasons, it is necessary to detect cobalt at low concentration levels with sensitive analytical techniques in food and drink samples. For decades, several analytical methods including flame atomic absorption spectrometry (FAAS) (Jamali, Soleimani, Rahnama, & Rahimi, 2017), electrothermal atomic absorption spectrometry (ETAAS) (Chen, Lei, Yang, & Wen, 2017), inductively coupled plasma optical emission spectrometry (ICP-OES) (Bartosiak, Jankowski, & Giersz, 2018), inductively coupled plasma mass spectrometry (ICP-MS) (Zhao, Wei, Shu, Kong, & Yang, 2016), voltammetry (Mettakoonpitak, MillerLionberg, Reilly, Volckens, & Henry, 2017) and ultraviolet–visible spectrophotometry (UV–VIS) (Guo et al., 2017) have been used for the determination of several metals in literature. FAAS is a widely used instrumental method which has important advantages such as robustness, accuracy and precision. In addition, it is a relatively cheap instrument and offers low operational cost (Thompson & Davidow, 2009). However, FAAS has some limitations such as low sensitivity which hinders determination of metals at trace concentrations (Tokalıoğlu,
Corresponding author. E-mail address: [email protected] (S. Bakırdere).
https://doi.org/10.1016/j.foodchem.2019.125825 Received 20 June 2019; Received in revised form 18 October 2019; Accepted 28 October 2019 0308-8146/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Zeynep Tekin, et al., Food Chemistry, https://doi.org/10.1016/j.foodchem.2019.125825
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entry slot (5.7 cm) to cover the flame and an exit slot (1.2 cm) situated at 180° angle with respect to each other. The SQT was carefully placed on the head of the burner so that the optical path of the lamp light passed through without blockage. Centrifugation was performed with a Hettich EBA 20 model centrifuge. A Hapa M-100 ultrasonic water bath and a WiseMix VM-10 vortex were used for mixing throughout extraction studies.
Papak, & Kartal, 2017). In this regard, slotted quartz tube (introduced by Watling in 1977) is a commonly used device to enhance the absorbance signal of FAAS (Watling, 1977). The SQT is placed on the flame burner and it helps to improve the residence period of analyte atoms in the light path coming from hallow cathode lamp (HCL). Therefore, more interactions between light from HCL and analyte atoms results in high absorbance values (Kaya & Yaman, 2008). In addition to SQT modification of FAAS for enhanced absorbance signal, a variety of pretreatment techniques can be carried out to extract analytes of interest from sample matrixes (Shokoufi, Shemirani, & Assadi, 2007). There are several preconcentration/microextraction procedures which are used for heavy metals such as solid phase extraction (SPE) (Amin, 2014), liquid–liquid extraction (LLE) (Reza Jamali, Assadi, & Shemirani, 2007), flow injection extraction (FIE) (Bakircioglu, Topraksever, & Kurtulus, 2014), cloud point extraction (CPE) (Ghaedi, Shokrollahi, Ahmadi, Rajabi, & Soylak, 2008), dispersive liquid–liquid microextraction (DLLME) (El-Shahawi & Al-Saidi, 2013) and liquid phase microextraction (LPME) (Memon, Yilmaz, & Soylak, 2017). The use of economical and non-toxic solvent is a main goal in microextraction methods (Milanova, Chambers, Bahga, & Santiago, 2011; Yilmaz & Soylak, 2018). Hence, deep eutectic solvents (DESs) are attracting great attention as replacement for traditional organic solvents. DESs consist of two or more compounds with the capability to interact with each other via hydrogen bonds which are called eutectic mixtures. The main characteristic property of DESs is having lower melting points than each of the discrete constituents because of the self-association of hydrogen bond donors and acceptors (Wagle, Zhao, & Baker, 2014; Wang et al., 2017; Yilmaz & Soylak, 2016). They are generally obtained from a mixture of choline chloride salt such as ChCl and Vitamin B4, and a metal salt or hydrogen bond donor (HBDs) such as urea, sugars and glycerol (Arain et al., 2016; Smith, Abbott, & Ryder, 2014). Because of its cheapness, biodegradability and green properties, choline chloride (ChCl) which is a quaternary ammonium salt is the most widely utilized component in the synthesis of DESs (Abbott, Capper, Davies, McKenzie, & Obi, 2006; Zounr, Tuzen, Deligonul, & Khuhawar, 2018). DESs and ionic liquids (ILs) have similar physicochemical properties such as non-flammability and high solubility not only for organic sompounds, but also for inorganic species (Francisco, van den Bruinhorst, & Kroon, 2013; Zounr, Tuzen, & Khuhawar, 2018). On the other hand, DESs surpasses organic solvents and ILs in various aspects such as economical cost, easy preparation by mixing components, no requirement of additional pretreatment, biodegradability, biologically compatibility and non-toxicity (Arain et al., 2016; Smith et al., 2014). Finally, DESs are useful in a variety of application areas such as synthesis of nanomaterials, extraction of inorganic and organic species, drugs and metal oxide dissolution and purification of precious organic compounds like oil and biodiesel (Karimi, Dadfarnia, Shabani, Tamaddon, & Azadi, 2015; Yilmaz & Soylak, 2016, 2018). In this study, DES-LPME method was developed for both extraction and preconcentration of cobalt prior to determination by the SQT-FAAS system. The proposed method was successfully applied to linden samples for the determination of cobalt at trace levels with high accuracy and precision.
2.2. Chemicals and reagents Reagents used in the study were of analytical grade and high purity. All working/calibration standards of cobalt were prepared from a 1000 mg L−1 cobalt stock solution purchased from High-Purity Standards, USA. DES solvent was prepared by mixing choline chloride (ChCl, Acros, 99%) and phenol (PhOl, Merck). Tetrahydrofuran (THF) with a purity of ≥99.9% was obtained from Merck. Ultrapure water from a Milli-Q® Reference Ultrapure Water Purification System with 18.2 M-ohm-cm resistivity was used in the preparation of standard/ sample solutions. 2.3. Synthesis of complexing agent Synthesis of the ligand used in the study was achieved in accordance with the following procedure as reported by our research group (Öztürk Er et al., 2018): 10 mmol of 5-bromosalicylaldehyde was dissolved in a proper amount of ethanol and then p-toluenesulfonic acid (0.01 mg) was added and the mixture was heated to 60 °C. Afterwards, 20 mmol of p-toluidine in 25 mL of ethanol was added to the final solution. A dark orange reaction color was observed, indicating precipitation of the ligand ((Z)-3-bromo-5-((p-tolylimino)methyl)phenol)). The final product was filtered and then dried at 50 °C. 2.4. Preparation of DES Choline chloride as an organic salt (HBA-ChCl) and phenol (HBDPhOl) as a hydrogen bond donor were used in preparing DES for the extraction of cobalt. In order to find the optimal ratio, different DESs were prepared by simply mixing the ChCl and PhOl in 50 mL centrifuge tubes at 1:2, 1:3, 1:4 and 1:5 M ratios by vortexing (3000 rpm) at room conditions until a colorless and homogeneous liquid was observed. The prepared DES was left in the ultrasonic bath for 15–20 s to get rid of air bubbles. The DESs were prepared fresh each day for optimization studies and method validation. 2.5. Procedure The extraction procedure was carried out in 15 mL of centrifuge tubes containing 10 mL of aqueous Co standard/sample solution. The addition of 1.0 mL 0.20% (w/v) (Z)-3-bromo-5-((p-tolylimino)methyl) phenol ligand was preceded by adding 1.0 mL of pH 10 buffer, and the resulting solution was sonicated in the ultrasonic bath for 10 s for the complexation of ligand with cobalt. Then, 0.60 mL of DES with a molar ratio of 1:2 was injected into the cobalt complex solution and the cloudy solution formed was then vortexed for 5.0 s. Afterwards, 1.0 mL of THF was added to the solution as an emulsifier agent and hand shaken for 15 s for homogeneous distribution. The final solution was centrifuged at 6000 rpm for 120 s, and 125 µL was taken from the upper organic phase into a clean tube. The separated phase was held in a hot water bath to prevent clogging of the nebulizer while performing measurements with the FAAS instrument.
2. Materials and methods 2.1. Instrumentation An ATI UNICAM 929 AA model FAAS having a deuterium background corrector was used for cobalt absorbance measurement at the 240.7 nm analytical line of a Varian SpectrAA hollow cathode lamp. The respective slit width and operational current of the hollow cathode lamp were 0.5 nm and 15 mA. An air-acetylene flame was used for the atomization of cobalt. A lab-made SQT was 15.3 cm long, with 1.75 cm internal diameter and 2.05 cm outer diameter specifically cut with an
3. Result and discussion In this study, the ligand was synthesized to be a complexing agent in the extraction process. In order to increase the sensitivity of the method, all parameters having effects cobalt complex formation, 2
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vortexing and manual shaking, after which the results were compared to complexing without mixing. Ultrasonication recorded the highest absorbance signals and its optimum period was determined as 10 s after testing 5.0, 10, 15, 30 and 60 s mixing periods.
extraction and determination signal were optimized. Three replicate measurements were done for each optimization. Different optimization approaches are reported in literature such as experimental design and univariate optimizations (Leardi, 2009). In this study, one parameter was varied at different values while all other parameters were kept constant. Optimum phase volume was examined by transferring 50, 75, 100 and 125 µL from the upper phase obtained after extraction into clean tubes for absorbance measurements. The highest signal was recorded for the 125 µL volume. The separated volume after addition of THF was very low and difficult to collect for instrumental measurement. Trace amounts of water phase were collected with the organic phase when higher volumes were tested, and this led to high standard deviation values. In addition, no significant increase was observed for volumes tested up to 125 µL. Subsequent experiments were therefore performed with 125 µL volume of extract from the upper phase.
3.3. Optimization of deep eutectic solvent extraction An ideal extraction solvent should be selected considering the properties of being immiscible with aqueous solution, having a higher/ lower density than water, and high selectivity for the analyte of interest. Additionally, the solvent should be green, that is, have low toxicity and generate waste that can be safely disposed into the environment (Ghoochani Moghadam et al., 2017). The use of DES as an extraction solvent in this study was considered to meet these requirements. Choline chloride and phenol at the ratio of 1:2, 1:3, 1:4 and 1:5 were tested in extracting the cobalt complex from aqueous solution. As the phenol ratio decreased, there was a decrease in the viscosity of the solution and an increase in absorbance values as shown in Fig. 2. This can be explained by the efficiency of the nebulizer unit of the FAAS which reduces with increasing viscosity. Therefore, 1:2 was chosen as optimum value for the DES molar ratio. DES volume has a significant influence on the extraction power and the enrichment amount of cobalt with regard to the amount of analytes collected. The amount of settled phases varied for each volume of DES as a result of different solubility in water. The volumes of DES tested were 0.30, 0.50, 0.60, 0.75 and 1.0 mL, and the amount of settled phase increased with the amount of extractor solvent. As a result of this, concentration of the analyte in high volume settled phases were low by reason of dilution. Although the highest signal was observed for 0.50 mL, the optimum value was chosen as 0.60 mL because it gave more reproducible absorbances (low standard deviation) and the final volume was convenient for withdrawal and processing for absorbance measurements. There was no settled phase observed for 0.30 mL of DES due to total miscibility with water. In order to determine the best method for adding DES to the aqueous solution, micropipette and syringe were compared. Absorbance values obtained after injection with syringe was about two times higher than that obtained with micropipette. The injection process brought about efficient distribution of DES throughout the sample solution. Similarly to complex formation, mixing effect was studied on extraction output by testing mechanical shaking, ultrasonication, vortex, manual shaking and no mixing. Vortex emerged as the most efficient mixing type and 5.0 s was obtained as optimum period after testing up to 60 s.
3.1. Optimization of FAAS and SQT-FAAS Instrumental parameters of the FAAS and SQT-FAAS systems including acetylene flow rate, sample flow rate and SQT height from the burner head were optimized the boost cobalt absorbance signals. All instrumental optimization studies were carried on with 5.0 mg L−1 of cobalt standard solution without preconcentration. The sample and acetylene flow rates affect the efficiency of nebulization and increase the amount of atomized analyte. Nebulizer set button and fuel gas scale were adjusted to various positions to find the optimum flow rates that gives the highest absorbance. Thus, the optimum flow rates for sample and acetylene were found to be 4.39 mL min−1 and 12.5 L h−1, respectively. Acetylene flow rates beyond 12.5 L h−1 were not tested to avoid the risk of harming the some parts of the instrument due to horn shaped flames exits from the slotted quartz tube which were close to the quartz windows on both sides. The SQT heights of 0.0, 1.0 and 2.0 mm from the burner head were tried and the highest signal was obtained at 1.0 mm height. 3.2. Optimization of complex formation Variables including concentration/amount of ligand, pH/amount of buffer solution and complexing period were optimized to increase the efficiency of complex formation. pH of the solution affects the state of an analyte in solution. Buffer solutions in the pH range of 3.0–13 were studied and the highest absorbance value was obtained for pH 10. Lower absorbance values were observed at the other pH values. In addition, the optimum amount of the pH 10 buffer solution was determined by adding 0.50, 1.0, 1.5 and 2.0 mL volumes. In addition, an extraction was repeated without the addition of buffer solution. As a result of the analysis, a significant increase was observed for 1.0 mL over 0.50 mL, while there was no big difference between 1.0, 1.5 and 2.0 mL. The lowest absorbance value was recorded for the extraction performed without buffer addition. 1.0 mL was selected as optimum amount due to lower standard deviation. The optimum concentration of the ligand was determined as 0.20% after testing it together with 0.50, 1.0 and 2.0% (w/v). Higher concentrations were not workable due to sedimentation depending on the solubility of the ligand in ethanol. The resulting phases from the high ligand concentrations were allowed to stand in hot water bath to prevent any tubing blockage or clogging of nebulizer unit by the precipitates/sediments formed. The amount of ligand was also optimized to attain a high complex formation output. The amounts of 0.20% (w/v) ligand solution tested were 0.50, 1.0, 1.5 and 2.0 mL, and these were compared to a ligandless extraction (Fig. 1). The signal increased proportionately with increasing ligand volume up to 2.0 mL. There was no phase separation observed for the 2.0 mL volume, and 1.5 mL was selected as the optimum amount. For homogenous dispersion of ligand solution, the complex solution was subjected to 15 s each of mechanical shaking, ultrasonication,
3.4. Optimization of emulsifier agent Emulsifier agent forms a film around the droplets of the immiscible phase reducing the interfacial tension between the phases and prevents reunion of droplets (Thompson & Davidow, 2009). In this study, THF as emulsifier agent was added with aid of a micropipette into aqueous sample containing analytes. The extraction solution after addition of THF became very cloudy. As a result of this, cobalt was easily transferred into the extracted phase in a very short time. The volumes of THF tested were 0.50, 1.0, 1.5 and 2.0 mL but no phase separation was observed for the 0.50 mL. The final volume obtained using 1.0 mL was quite low but workable enough to obtain reproducible results. Furthermore, the effect of type and period of mixing was determined. Manual shaking gave the highest dispersion over mechanical shaker, ultrasonication, vortex and no mixing. The optimum period of 15 s was assigned after testing 5.0, 10, 15, 30 and 60 s. 3.5. Analytical performance of DES-LPME method Under the optimized conditions given in Table 1, analytical performance of the systems was determined using calibration plots of aqueous Co standards. Blank extractions and measurements were 3
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Fig. 1. Effect of ligand volume on the absorbance of cobalt (Conditions: 8.0 mL of sample volume, 1.0 mL of pH 10 buffer solution, 0.20% (w/v) ligand solution, 0.60 mL of DES (1:2; ChCl:PhOl), 1.0 mL of THF).
Fig. 2. Effect of DES molar ratio on the absorbance of cobalt (Conditions: 8.0 mL of sample volume, 1.0 mL of pH 10 buffer solution, 0.20% (w/v) 1.5 mL ligand solution, 0.60 mL of DES, 1.0 mL of THF).
LOD = 3 × SD/Slope LOQ = 10 × SD/Slope
Table 1 Optimized parameters of the DES-LPME–SQT–FAAS. Parameter
Value
Sample flow rate Acetylene flow rate SQT height pH of buffer solution/volume Ligand concentration/volume DES molar ratio/volume Emulsifier agent volume
4.39 mL min−1 12.5 L h−1 1.0 mm 10/1.0 mL 0.20% (w/v)/1.5 mL ChCl:PhOl; 1:2/0.60 mL THF/1.0 mL
The calibration plots of the systems were linear over broad concentration ranges with R2 values greater than 0.9994, indicating good linearity. Analytical figures of merit of the four systems in this study and comparison with other literature findings are presented in Table 2. The improvements in detection powers for the developed systems were calculated as a ratio of the detection limit of the FAAS system to the optimized systems. The detection power of FAAS system was enhanced by about 2.0 folds with the attachment of SQT. In addition to this, applying the DES-LPME before FAAS measurement resulted in about 33 folds increase in detection power. Combining the two enhancement methods with FAAS resulted in about 67 folds increase in detection power with an LOD value of 2.0 µg L−1. The %RSD values were calculated from six replicate measurements of the lowest concentrations in each calibration plot and the low values obtained indicate good precision.
performed in the development of calibration plots for all systems studied. The absence of analytical signals for blank measurements suggested Co impurities were below detection limits. The parameters used to validate the systems were limit of detection and quantification (LOD and LOQ), linear dynamic range (LDR), coefficient of determination (R2) and precision (%RSD). The LOD and LOQ values were calculated with the expressions below, using the standard deviation (SD) from 6 replicate measurements of the lowest calibration standard and calibration slope. 4
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Table 2 Analytical performance of the systems under the optimum conditions and detection limit comparison with other methods reported in literature. Method
LOD (µg L−1)
LOQ (µg L−1)
%RSD
Range (µg L−1)
FAAS SQT–FAAS DES-LPME-FAAS DES-LPME-SQT-FAAS SPE-FAAS (Habila, ALOthman, Yilmaz, & Soylak, 2018) On-Line Sorbent Preconcentration-FAAS (Ye, Ali, & Yin, 2002) ETAAS(Felipe-Sotelo et al., 2004)
134 71 4.1 2.0 2.9 3.2 1.7
445 235 14 6.6
5.9 5.6 2.9 7.1
350–10,000 200–10,000 10–1000 5.0–50
5.6
interference can be mitigated using matrix matching calibration strategy, to obtain accurate and precise quantification results. An absorbance signal obtained for linden tea extract spiked at 30 µg L−1 and analyzed under the optimum method is given in Fig. 3. In addition, ICP-MS instrument was used to check the accuracy of the developed method. 30 µg L−1 spiked linden tea extract was prepared and the concentration of Co was measured with both ICP-MS and the developed DES-LPME-SQT-FAAS systems. For the ICP-MS instrument, Co concentration was found to be 30.0 ± 0.7 µg L−1 while it was 30.9 ± 1.6 µg L−1 using DES-LPME-SQT-FAAS. It is clear that there is no difference in the results indication the high accuracy of the developed method.
Table 3 Recovery values for spiked linden tea. Matrix
Co concentration, µg L−1
Recovery, %
Linden Tea
20 30 40
98.6 ± 4.4 97.1 ± 5.1 100.0 ± 3.2
3.6. Accuracy and applicability check Consumption of the linden plant extract as traditional tea is very high in Turkey especially during winter. Hence, accurate determination of cobalt in this matrix is very crucial. For the purpose of demonstrating applicability of the developed method, linden samples were purchased from five different herbalists and used for spike recovery experiments. The linden samples (20 g) were boiled in water for 45 min at 100 °C. Afterwards, the brewed sample was allowed to cool and filtered while warm to remove solid particulates before making up the volume to 100 mL. Blank samples were firstly analyzed to determine whether or not cobalt was present. The results showed that cobalt was not present in all of the samples, or below the method’s detection limit if present. They were therefore spiked at 20, 30 and 40 μg L−1 and analyzed under the optimized extraction/instrumental conditions. When calculated against aqueous calibration standards, the percent recoveries were lower than 80% due to matrix interference. To overcome these effects, a different linden tea extract was used to prepare matrix matched calibration standards and run alongside the spiked samples. The percent recovery results as presented in Table 3 using matrix matching technique were in the range of 97–100%. This certifies that sample matrix
4. Conclusion Deep eutectic solvent is a green extraction solution used for the determination of different analytes in recent years. In this study, a DESLPME method was optimized for the preconcentration of cobalt complex for determination by slotted quartz tube flame atomic absorption spectrometry. Due to its low cost and non-toxicity, choline chloride was used as an organic salt to produce DES with phenol. All the effective parameters were optimized and the analytical performance of each system was determined. The detection power of the FAAS was enhanced by about 67 folds using the combined DES-LPME and SQT-FAAS system. The developed method was successfully applied to the linden tea matrix. Five different linden tea samples were analyzed but cobalt was not detected. Even though matrix interference hampered recovery of cobalt, matrix matching was used to overcome the effects and enhance accuracy of quantification. Recovery results obtained were satisfactory.
Fig. 3. Analytical signal of Co in spiked sample at 30 µg L−1 under optimum DES-LPME-SQT-FAAS conditions. 5
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In addition, the result obtained using the developed method was compared with the ICP-MS measurement and it was in good agreement with the ICP-MS result. It is clear that combination of high preconcentration factor of the DES-LPME and enhanced detection power of SQT-FAAS created a sensitive analytical method having high accuracy and precision.
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