- Email: [email protected]
Ultrasound assisted deep eutectic solvent based microextraction-slotted quartz tube-flame atomic absorption spectrometry for the determination of cadmium
Accepted Manuscript Ultrasound assisted deep eutectic solvent based microextractionslotted quartz tube-flame atomic absorption spectrometry for the determination of cadmium
Tuğçe Unutkan, Büşra Tışlı, Zeynep Tekin, Gülten Çetin, Sezgin Bakırdere PII: DOI: Reference:
S0584-8547(19)30051-5 https://doi.org/10.1016/j.sab.2019.03.001 SAB 5586
To appear in:
Spectrochimica Acta Part B: Atomic Spectroscopy
Received date: Revised date: Accepted date:
30 January 2019 5 March 2019 6 March 2019
Please cite this article as: T. Unutkan, B. Tışlı, Z. Tekin, et al., Ultrasound assisted deep eutectic solvent based microextraction-slotted quartz tube-flame atomic absorption spectrometry for the determination of cadmium, Spectrochimica Acta Part B: Atomic Spectroscopy, https://doi.org/10.1016/j.sab.2019.03.001
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Ultrasound assisted deep eutectic solvent based microextraction-slotted quartz tube-flame atomic absorption spectrometry for the determination of cadmium Tuğçe Unutkana, Büşra Tışlıb , Zeynep Tekinb , Gülten Çetinb , Sezgin Bakırdere b *
a
Yıldız Technical University, Department of Chemical Engineering, 34349 İstanbul, Turkey Yıldız Technical University, Department of Chemistry, 34349 İstanbul, Turkey
PT
b
RI
This study made use of the unique extraction properties of deep eutectic solvents (DESs) to determine trace levels of cadmium by slotted quartz tube flame atomic absorption
SC
spectrometry (SQT-FAAS). Ultrasound assisted (UA)-DES was applied in this study to
NU
improve the detection limit. The optimum extraction conditions were 1:2 ratio of choline chloride and phenol as DES, 0.50 mL DES volume, 1.0 mL emulsifier agent volume and 10 s
MA
of manual mixing. The linear calibration range of the optimum method was between 1.0 and 50 µg L-1 . Combining the DES extraction method and SQT-FAAS gave 0.35 and 1.2
ED
μg L− 1 as the limits of detection and quantitation (LOD and LOQ), respectively. The sensitivity was improved by about 80 folds when compared with the conventional FAAS
EP T
system. Accuracy and applicability of the method were also checked using celery and apple samples, and the recovery results calculated were close to 100%, respectively.
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Keywords: Cadmium; FAAS; SQT; Deep eutectic solvent Corresponding author: *Corresponding author: Sezgin Bakırdere, email: [email protected], Phone: +902123834245
ACCEPTED MANUSCRIPT 1.
Introduction
Cadmium occurs naturally in the crust of the earth and generally found in ores, rocks and soils. Due to its wide application in several industries (pigments, coating and plating, pesticides,
fertilizers,
thermoplastics,
nuclear reactors, and photoconductors/photoelectric
solar cells), it is easily released into the environment where humans and other organisms get
PT
exposed to [1-3]. Cadmium is known to be non-biodegradable, and a highly toxic element
RI
because even at trace levels, it has the potential to gather in the liver, lungs, and kidneys
SC
over time, and its adverse effects manifest when it reaches a threshold limit in the body [47]. For this reason, the United States Environmental Protection Agency (EPA) set the
NU
maximum concentration of this element in drinking water as 5.0 μg L-1 [8]. These low limits require sensitive, accurate and precise analytical methods for the analysis of variety
MA
environmental samples for their cadmium content.
Flame atomic absorption spectrometry (FAAS) is a widely used technique for metal
by much higher LOD's and matrix interferences. To overcome these
EP T
characterized
ED
determination due to its practical easy use and low cost [9, 10]. On the other hand, it is
drawbacks, cadmium preconcentration and matrix elimination
processes are applied in
agreement with the principles of green chemistry [11]. Deep eutectic solvents (DESs) are
AC C
preferably being employed for preconcentration studies due to their green nature, low cost and high extraction efficiency for analytes of interest. DESs consist of two chemical components, a hydrogen bond acceptor (HBA) and a hydrogen bond donor (HBD). The combination of these two chemicals results in a eutectic mixture with a lower melting point than the ingredients [12, 13]. Choline chloride (ChCl) has been mostly used as an HBA because it is easily synthesized, cheap, a non-toxic quaternary ammonium salt and biodegradable. Choline chloride efficiently forms DESs with HBDs such as phenol, oxalic acid, glycerol, sugars and urea [14-16].
ACCEPTED MANUSCRIPT In the present study, a sensitive and accurate analytical method (UA-DES-LPME-SQTFAAS) was developed for the quantification of cadmium. Tetrahydrofuran (THF) was used as emulsifier solvent to separate the DES used as water-soluble extraction solvent for the preconcentration of cadmium from the aqueous sample phase. Slotted quartz tube was fitted
Experimental
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2.
PT
to the FAAS to lower the detection limit of cadmium.
2.1.
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celery and apple samples are given in Table S2.
SC
Instrumental and SQT parameters are specified in Table S1, and the digestion parameters for
DES synthesis
MA
The DES used in this study was synthesized in a 50 mL centrifuge tube by mixing the halide salt (choline chloride) and hydrogen bond donor (phenol) in a 1:2 molar ratio. The mixture
Extraction procedure
EP T
2.2.
ED
was vortexed at room temperature until it became a colorless, homogeneous and clear liquid.
8.0 mL of cadmium standard/sample solution, 1.0 mL 0.05% (w/v) of diphenylcarbazone
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ligand solution and 0.50 mL pH 10 buffer solution were added successively into a 15 mL centrifuge tube. The mixture was shaken with a mechanical shaker for 15 s. Then, 0.50 mL of the DES was injected into the complex solution and placed in an ultrasonicator for 15 s. 1.0 mL THF as an emulsifier was added to the solution and the solution was hand shaken for 10 s. Centrifugation was performed for 120 s at 3461 g. The extraction phase containing cadmium complex was taken into a clean tube for instrumental measurements.
3.
Result and Discussion
ACCEPTED MANUSCRIPT All the optimization steps including complex formation, deep eutectic solvent extraction, emulsifier agent volume, FAAS and SQT-FAAS parameters were carried out with the 50 µg L-1 aqueous cadmium standard solutions. Optimum conditions were determined by testing different values of a parameter at a time and other parameters were maintained at one constant value. Three replicate extractions were performed for all optimization steps and the highest
Optimization of FAAS and SQT-FAAS
SC
3.1.
RI
PT
average absorbance signal with the lowest RSD value was selected as optimum value.
System parameters for FAAS and SQT-FAAS were optimized to enhance the sensitivity for
NU
cadmium determination. Optimum sample flow rate (7.24 mL min-1 ) for cadmium was taken from a previous study reported by our research group [17]. The influence of acetylene flow
MA
rate was determined by testing 40, 45, 50 and 55 L h-1 . 40 L h-1 was selected as the optimum flow rate. In addition to these, the location of the SQT on the flame was studied because it
ED
affects the interaction between the sample, the flame and analyte atomization. The height of the SQT from the burner head was also optimized by trying heights between 1.0 – 3.0 mm.
3.2.
AC C
EP T
The 1.0 mm position gave the highest absorbance value.
Optimization of deep eutectic solvent extraction
Optimum parameters for complex formation between Cd and DPC were obtained from a previous study [17] as given in Table S3. The same conditions were tested and high analytical signals were recorded for initial deep eutectic solvent extractions. High extraction affinity and electrostatic interactions for target analyte, hydrophobic nature and high selectivity are some significant properties of DESs that makes them ideal extraction solvents [18]. These properties however vary according to the molar ratio of the DES constituents used. There are many studies that have reported on the use choline chloride/phenol mixture in the literature
ACCEPTED MANUSCRIPT [19-21]. For this reason, choline chloride:phenol ratios of 1:2, 1:3, 1:4 and 1:5 were prepared. It was observed that an increase in the amount of phenol decreased the viscosity of the extraction solvent and this also led to decreased absorbance values. The highest absorbance values were obtained using the 1:2 ratio and it was therefore selected as optimum value for the following studies.
PT
The amount of an extraction solvent directly affects the extraction efficiency and
RI
preconcentration factor, thus, the volume of DES was examined in the range of 0.50 – 1.0 mL.
SC
An increase in the DES volume led to relatively high volumes of organic phase after extraction, and the low absorbance values recorded can be attributed to dilution by the excess
NU
volume. Among the volumes tested, 0.50 mL recorded the highest average absorbance value, but when DES volumes lower than 0.50 mL were tested, clear phase separation was not
MA
observed. For this reason, 0.50 mL of DES was selected as the most favorable amount for subsequent experiments. Different mixing types and period were also tested to ascertain their
ED
influence on extraction output after DES addition. After testing mechanical shaking, manual shaking, vortex, ultrasonication and no mixing, ultrasonication gave the highest absorbance
EP T
results. Its optimum period was determined as 15 s after testing different periods between 5.0 and 60 s, with 5.0 s increment intervals. Finally, the most efficient mode of DES addition to
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the aqueous solution was determined by comparing the extraction outputs recorded by using micropipette and syringe. The absorbance values obtained using the syringe for DES injection was significantly higher (about two times) than the absorbance values obtained for micropipette addition. The syringe injection resulted in a cloudy solution which signified homogenous distribution of DES throughout the solution. In addition, the relative standard deviation (triplicate extractions) obtained for DES addition by syringe injection was lower than addition by pipette. Subsequent extractions were therefore carried out using air-assisted syringe injections.
ACCEPTED MANUSCRIPT
3.3.
Optimization of emulsifier agent volume
In this study, THF was utilized as emulsifier agent to separate the DES phase (analyte rich phase) from the water phase. With the addition of THF, emulsification of DES occurs and as it separates from the water phase, analyte atoms are rapidly transferred into the relatively low
PT
volume settled phase. The amount of THF plays a significant role in DES extractions and for
RI
this reason, 0.50, 1.0, 1.5 and 2.0 mL volumes were tried to determine the optimum value. No
SC
separate phase was observed for the 0.50 mL volume tested. As the amount of THF volume increased, the amount of settled phase also increased, thus, absorbance values decreased due
NU
to dilution. The results obtained are depicted in Figure 1 and the 1.0 mL volume was selected for further experiments. After THF addition, mixing must be performed to ensure
MA
homogenous emulsification and effective mass transfer of the analyte. Ultrasonication, vortex, mechanical shaking and manual shaking were performed for 15 s and compared to an
ED
extraction performed without mixing. The lowest results were recorded for mechanical shaking while vortex and ultrasonication were quite close to each other. Manual shaking and
EP T
no mixing gave higher absorbance signals than the other mixing types. Manual shaking was chosen as optimum parameter for recording slightly higher absorbance values and lower
AC C
standard deviation values when compared with no mixing. Manual shaking periods between 5.0 and 60 s were performed. The highest absorbance values were obtained for 10 s, which was selected as the final optimum parameter.
3.4.
Analytical figures of merit
The final parameters of the method and calibration values of FAAS studies are presented in Table S3 and Table 1, respectively. The detection power of the FAAS resulted a 4.95 fold enhancement via using SQT. A better alternative to the SQT was the use of DES to
ACCEPTED MANUSCRIPT preconcentrate cadmium in its complex form, and this improved the detection power by 16 folds. When the two enhancement methods (SQT and DES microextraction) were combined, a very high increase in detection power (85 folds) was achieved and this corresponded to LOD of 0.35 μg L-1 . The enhancement factor in detection power was calculated as a ratio of FAAS LOD to DES-LPME-SQT-FAAS LOD. Using the slopes of the calibration plots, the
PT
sensitivity of the FAAS was improved by 80 folds using the optimum method. It is clear in
RI
Table 1 that the LOD values of this study are comparable with the literature values. In each calibration plot, the precisions of the systems performed were calculated with six replicates of
SC
the lowest concentrations and the relative standard deviations obtained ranged from 2.9 –
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9.6%, indicating high precisions.
MA
Table 1. Analytical figures of merit of the systems and comparison to other methods in literature. LOQ (μg L− 1 )
Range (μg L− 1 )
%RSD
R2
FAAS
30
99
100–1000
4.6
0.9995
SQT-FAAS
6.0
20.2
25–250
2.9
0.9993
DES-FAAS
1.85
6.17
5.0–100
6.6
1.0000
1.17
1.0–50
9.6
0.9994
0.74
-
5.0–25
-
0.99942
0.95
4.3
10-200
2.4
0.998
1.2
-
5-150
2.1
0.9997
EP T
DES-SQT-
0.35
FI-CPEFAAS [22]
AC C
FAAS
ED
LOD (μg L− 1 )
CPE-DPTHTriton X114- FAAS [23] DLLME-
ACCEPTED MANUSCRIPT FAAS [24]
3.5.
Recovery studies
For the purpose of testing not only the accuracy but also the applicability of the developed method, celery and apple samples were prepared according to the procedure given in section
PT
2.4. The prepared samples were then analyzed under the optimized conditions to determine its
RI
cadmium content. No analytical signal was observed for all triplicate blank extractions and this suggested that the samples did not contain cadmium within the detection limits of the
SC
method. The samples were also spiked to final concentrations as 5.0 and 10 μg L-1 for celery
NU
and apple, extracted under optimum DES conditions and analyzed by SQT-FAAS. The percent recoveries for celery samples calculated as 96.4 ± 4.2% and 101.5 ± 7.6%, for apple
MA
samples calculated as 92.1±4.0 and 116.8±8.6 respectively. The closeness of the recovery results to 100% validates its applicability and accuracy. The %RSDs were also lower than
Conclusion
EP T
4.
ED
10%, indicating high precision for replicate extractions/measurements.
In this study, a reliable, efficient and accurate analytical technique for the determination of Cd
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has been developed by using DES as a green extraction solvent in liquid phase microextraction method. Under optimum experimental conditions, about 85-fold enhancement in the detection power was achieved when compared to the conventional FAAS. Spike recovery tests were applied to celery and apple samples at different concentrations and the results obtained were close to 100%, validating the method’s accuracy and applicability to real samples. The developed method is simple, rapid, economical, green and produces high outputs.
ACCEPTED MANUSCRIPT Conflict of interest: None.
References
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[1] A. Afkhami, T. Madrakian, S.J. Sabounchei, M. Rezaei, S. Samiee, M. Pourshahbaz, Construction of a modified carbon paste electrode for the highly selective simultaneous electrochemical determination of trace amounts of mercury(II) and cadmium(II), Sensors and Actuators B: Chemical, 161 (2012) 542-548. [2] S. Nazari, Determination of trace amounts of cadmium by modified graphite furnace atomic absorption spectrometry after liquid phase microextraction, Microchemical Journal, 90 (2008) 107-112. [3] M.M. Al‐ Subu, E. Sahrhage, Uptake of cadmium from water by beech leaves AU Salim, Radi, Journal of Environmental Science and Health . Part A: Environmental Science and Engineering and Toxicology, 27 (1992) 603-627. [4] S. A. Gunn, T. Clark Gould, Selective Accumulation of Cd115 by Cortex of Rat Kidney, 1958. [5] K. Nishiyama, Whole-Body and Hair Retention of Cadmium in Mice AU - Norberg, Gunnar F, Archives of Environmental Health: An International Journal, 24 (1972) 209-214. [6] T. Nawrot, M. Plusquin, J. Hogervorst, H. Roels, H. Celis, L. Thijs, J. Vangronsveld, E. Hecke, J. A Staessen, Environmental exposure to cadmium and risk of cancer: A prospective population-based study, 2006. [7] S. Cerutti, M.F. Silva, J.A. Gásquez, R.A. Olsina, L.D. Martinez, On-line preconcentration/determination of cadmium in drinking water on activated carbon using 8hydroxyquinoline in a flow injection system coupled to an inductively coupled plasma optical emission spectrometer, Spectrochimica Acta Part B: Atomic Spectroscopy, 58 (2003) 43-50. [8] S. Moinfar, G. Khayatian, Continuous sample drop flow-based microextraction combined with graphite furnace atomic absorption spectrometry for determination of cadmium, Microchemical Journal, 132 (2017) 293-298. [9] M. Karimi, F. Aboufazeli, H. Reza Lotfi Zadeh Zhad, O. Sadeghi, E. Najafi, Cadmium Ions Determination in Sea Food Samples Using Dipyridyl-Functionalized Graphene Nanosheet, 2014. [10] M.A. Chamjangali, S.T. Farooji, B. Bahramian, Application of chloromethylated polystyrene functionalized with N,N-bis(naphthylideneimino)diethylenetriamine in an on-line preconcentration system for the determination of cadmium by FAAS, Journal of Hazardous Materials, 174 (2010) 843-850. [11] S. Dadfarnia, A.M. Haji Shabani, E. Kamranzadeh, Separation/preconcentration and determination of cadmium ions by solidification of floating organic drop microextraction and FI-AAS, Talanta, 79 (2009) 1061-1065. [12] S.C. Cunha, J.O. Fernandes, Extraction techniques with deep eutectic solvents, TrAC Trends in Analytical Chemistry, 105 (2018) 225-239. [13] E. Yilmaz, M. Soylak, Ultrasound assisted-deep eutectic solvent based on emulsification liquid phase microextraction combined with microsample injection flame atomic absorption spectrometry for valence speciation of chromium(III/VI) in environmental samples, Talanta, 160 (2016) 680-685.
ACCEPTED MANUSCRIPT
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[14] Q. Zhang, K. Vigier, S. Royer, F. jérôme, Deep eutectic solvents: Syntheses, properties and applications, 2012. [15] A.H. Panhwar, M. Tuzen, T.G. Kazi, Deep eutectic solvent based advance microextraction method for determination of aluminum in water and food samples: Multivariate study, Talanta, 178 (2018) 588-593. [16] E. Smith, A. Abbott, K. Ryder, Deep Eutectic Solvents (DESs) and Their Applications, 2014. [17] M. Fırat, S. Bakırdere, M.S. Fındıkoğlu, E.B. Kafa, E. Yazıcı, M. Yolcu, Ç. Büyükpınar, D.S. Chormey, S. Sel, F. Turak, Determination of trace amount of cadmium using dispersive liquid-liquid microextraction-slotted quartz tube-flame atomic absorption spectrometry, Spectrochimica Acta Part B: Atomic Spectroscopy, 129 (2017) 37-41. [18] R.A. Zounr, M. Tuzen, N. Deligonul, M.Y. Khuhawar, A highly selective and sensitive ultrasonic assisted dispersive liquid phase microextraction based on deep eutectic solvent for determination of cadmium in food and water samples prior to electrothermal atomic absorption spectrometry, Food Chemistry, 253 (2018) 277-283. [19] X. Li, M. Wang, J. Zhao, H. Guo, X. Gao, Z. Xiong, L. Zhao, Ultrasound-assisted emulsification liquid phase microextraction method based on deep eutectic solvent as extraction solvent for determination of five pesticides in traditional Chinese medicine, Journal of Pharmaceutical and Biomedical Analysis, 166 (2019) 213-221. [20] E. Yilmaz, M. Soylak, A novel and simple deep eutectic solvent based liquid phase microextraction method for rhodamine B in cosmetic products and water samples prior to its spectrophotometric determination, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 202 (2018) 81-86. [21] A. Shishov, N. Volodina, D. Nechaeva, S. Gagarinova, A. Bulatov, An automated homogeneous liquid-liquid microextraction based on deep eutectic solvent for the HPLC-UV determination of caffeine in beverages, Microchemical Journal, 144 (2019) 469-473. [22] E. Silva, P. Dos Santos Roldan, Simultaneous flow injection preconcentration of lead and cadmium using cloud point extraction and determination by atomic absorption spectrometry, 2008. [23] C. Bosch-Ojeda, F. Sánchez-Rojas, J.M. Cano-Pavón, Preconcentration of cadmium in environmental samples by cloud point extraction and determination by FAAS, 2010. [24] G. Karim-Nezhad, M. Ahmadi, B. Dizajdizi, Background Corrected Dispersive LiquidLiquid Microextraction of Cadmium Combined with Flame Atomic Absorption Spectrometry, 2011.
ACCEPTED MANUSCRIPT Highlights A sensitive and accurate method was developed using a green solvent.
SQT was used in order to enhance the sensitivity of the system.
The method provided 85-fold enhancement when compared to FAAS.
AC C
EP T
ED
MA
NU
SC
RI
PT
Figure 1
Tuğçe Unutkan, Büşra Tışlı, Zeynep Tekin, Gülten Çetin, Sezgin Bakırdere PII: DOI: Reference:
S0584-8547(19)30051-5 https://doi.org/10.1016/j.sab.2019.03.001 SAB 5586
To appear in:
Spectrochimica Acta Part B: Atomic Spectroscopy
Received date: Revised date: Accepted date:
30 January 2019 5 March 2019 6 March 2019
Please cite this article as: T. Unutkan, B. Tışlı, Z. Tekin, et al., Ultrasound assisted deep eutectic solvent based microextraction-slotted quartz tube-flame atomic absorption spectrometry for the determination of cadmium, Spectrochimica Acta Part B: Atomic Spectroscopy, https://doi.org/10.1016/j.sab.2019.03.001
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Ultrasound assisted deep eutectic solvent based microextraction-slotted quartz tube-flame atomic absorption spectrometry for the determination of cadmium Tuğçe Unutkana, Büşra Tışlıb , Zeynep Tekinb , Gülten Çetinb , Sezgin Bakırdere b *
a
Yıldız Technical University, Department of Chemical Engineering, 34349 İstanbul, Turkey Yıldız Technical University, Department of Chemistry, 34349 İstanbul, Turkey
PT
b
RI
This study made use of the unique extraction properties of deep eutectic solvents (DESs) to determine trace levels of cadmium by slotted quartz tube flame atomic absorption
SC
spectrometry (SQT-FAAS). Ultrasound assisted (UA)-DES was applied in this study to
NU
improve the detection limit. The optimum extraction conditions were 1:2 ratio of choline chloride and phenol as DES, 0.50 mL DES volume, 1.0 mL emulsifier agent volume and 10 s
MA
of manual mixing. The linear calibration range of the optimum method was between 1.0 and 50 µg L-1 . Combining the DES extraction method and SQT-FAAS gave 0.35 and 1.2
ED
μg L− 1 as the limits of detection and quantitation (LOD and LOQ), respectively. The sensitivity was improved by about 80 folds when compared with the conventional FAAS
EP T
system. Accuracy and applicability of the method were also checked using celery and apple samples, and the recovery results calculated were close to 100%, respectively.
AC C
Keywords: Cadmium; FAAS; SQT; Deep eutectic solvent Corresponding author: *Corresponding author: Sezgin Bakırdere, email: [email protected], Phone: +902123834245
ACCEPTED MANUSCRIPT 1.
Introduction
Cadmium occurs naturally in the crust of the earth and generally found in ores, rocks and soils. Due to its wide application in several industries (pigments, coating and plating, pesticides,
fertilizers,
thermoplastics,
nuclear reactors, and photoconductors/photoelectric
solar cells), it is easily released into the environment where humans and other organisms get
PT
exposed to [1-3]. Cadmium is known to be non-biodegradable, and a highly toxic element
RI
because even at trace levels, it has the potential to gather in the liver, lungs, and kidneys
SC
over time, and its adverse effects manifest when it reaches a threshold limit in the body [47]. For this reason, the United States Environmental Protection Agency (EPA) set the
NU
maximum concentration of this element in drinking water as 5.0 μg L-1 [8]. These low limits require sensitive, accurate and precise analytical methods for the analysis of variety
MA
environmental samples for their cadmium content.
Flame atomic absorption spectrometry (FAAS) is a widely used technique for metal
by much higher LOD's and matrix interferences. To overcome these
EP T
characterized
ED
determination due to its practical easy use and low cost [9, 10]. On the other hand, it is
drawbacks, cadmium preconcentration and matrix elimination
processes are applied in
agreement with the principles of green chemistry [11]. Deep eutectic solvents (DESs) are
AC C
preferably being employed for preconcentration studies due to their green nature, low cost and high extraction efficiency for analytes of interest. DESs consist of two chemical components, a hydrogen bond acceptor (HBA) and a hydrogen bond donor (HBD). The combination of these two chemicals results in a eutectic mixture with a lower melting point than the ingredients [12, 13]. Choline chloride (ChCl) has been mostly used as an HBA because it is easily synthesized, cheap, a non-toxic quaternary ammonium salt and biodegradable. Choline chloride efficiently forms DESs with HBDs such as phenol, oxalic acid, glycerol, sugars and urea [14-16].
ACCEPTED MANUSCRIPT In the present study, a sensitive and accurate analytical method (UA-DES-LPME-SQTFAAS) was developed for the quantification of cadmium. Tetrahydrofuran (THF) was used as emulsifier solvent to separate the DES used as water-soluble extraction solvent for the preconcentration of cadmium from the aqueous sample phase. Slotted quartz tube was fitted
Experimental
RI
2.
PT
to the FAAS to lower the detection limit of cadmium.
2.1.
NU
celery and apple samples are given in Table S2.
SC
Instrumental and SQT parameters are specified in Table S1, and the digestion parameters for
DES synthesis
MA
The DES used in this study was synthesized in a 50 mL centrifuge tube by mixing the halide salt (choline chloride) and hydrogen bond donor (phenol) in a 1:2 molar ratio. The mixture
Extraction procedure
EP T
2.2.
ED
was vortexed at room temperature until it became a colorless, homogeneous and clear liquid.
8.0 mL of cadmium standard/sample solution, 1.0 mL 0.05% (w/v) of diphenylcarbazone
AC C
ligand solution and 0.50 mL pH 10 buffer solution were added successively into a 15 mL centrifuge tube. The mixture was shaken with a mechanical shaker for 15 s. Then, 0.50 mL of the DES was injected into the complex solution and placed in an ultrasonicator for 15 s. 1.0 mL THF as an emulsifier was added to the solution and the solution was hand shaken for 10 s. Centrifugation was performed for 120 s at 3461 g. The extraction phase containing cadmium complex was taken into a clean tube for instrumental measurements.
3.
Result and Discussion
ACCEPTED MANUSCRIPT All the optimization steps including complex formation, deep eutectic solvent extraction, emulsifier agent volume, FAAS and SQT-FAAS parameters were carried out with the 50 µg L-1 aqueous cadmium standard solutions. Optimum conditions were determined by testing different values of a parameter at a time and other parameters were maintained at one constant value. Three replicate extractions were performed for all optimization steps and the highest
Optimization of FAAS and SQT-FAAS
SC
3.1.
RI
PT
average absorbance signal with the lowest RSD value was selected as optimum value.
System parameters for FAAS and SQT-FAAS were optimized to enhance the sensitivity for
NU
cadmium determination. Optimum sample flow rate (7.24 mL min-1 ) for cadmium was taken from a previous study reported by our research group [17]. The influence of acetylene flow
MA
rate was determined by testing 40, 45, 50 and 55 L h-1 . 40 L h-1 was selected as the optimum flow rate. In addition to these, the location of the SQT on the flame was studied because it
ED
affects the interaction between the sample, the flame and analyte atomization. The height of the SQT from the burner head was also optimized by trying heights between 1.0 – 3.0 mm.
3.2.
AC C
EP T
The 1.0 mm position gave the highest absorbance value.
Optimization of deep eutectic solvent extraction
Optimum parameters for complex formation between Cd and DPC were obtained from a previous study [17] as given in Table S3. The same conditions were tested and high analytical signals were recorded for initial deep eutectic solvent extractions. High extraction affinity and electrostatic interactions for target analyte, hydrophobic nature and high selectivity are some significant properties of DESs that makes them ideal extraction solvents [18]. These properties however vary according to the molar ratio of the DES constituents used. There are many studies that have reported on the use choline chloride/phenol mixture in the literature
ACCEPTED MANUSCRIPT [19-21]. For this reason, choline chloride:phenol ratios of 1:2, 1:3, 1:4 and 1:5 were prepared. It was observed that an increase in the amount of phenol decreased the viscosity of the extraction solvent and this also led to decreased absorbance values. The highest absorbance values were obtained using the 1:2 ratio and it was therefore selected as optimum value for the following studies.
PT
The amount of an extraction solvent directly affects the extraction efficiency and
RI
preconcentration factor, thus, the volume of DES was examined in the range of 0.50 – 1.0 mL.
SC
An increase in the DES volume led to relatively high volumes of organic phase after extraction, and the low absorbance values recorded can be attributed to dilution by the excess
NU
volume. Among the volumes tested, 0.50 mL recorded the highest average absorbance value, but when DES volumes lower than 0.50 mL were tested, clear phase separation was not
MA
observed. For this reason, 0.50 mL of DES was selected as the most favorable amount for subsequent experiments. Different mixing types and period were also tested to ascertain their
ED
influence on extraction output after DES addition. After testing mechanical shaking, manual shaking, vortex, ultrasonication and no mixing, ultrasonication gave the highest absorbance
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results. Its optimum period was determined as 15 s after testing different periods between 5.0 and 60 s, with 5.0 s increment intervals. Finally, the most efficient mode of DES addition to
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the aqueous solution was determined by comparing the extraction outputs recorded by using micropipette and syringe. The absorbance values obtained using the syringe for DES injection was significantly higher (about two times) than the absorbance values obtained for micropipette addition. The syringe injection resulted in a cloudy solution which signified homogenous distribution of DES throughout the solution. In addition, the relative standard deviation (triplicate extractions) obtained for DES addition by syringe injection was lower than addition by pipette. Subsequent extractions were therefore carried out using air-assisted syringe injections.
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3.3.
Optimization of emulsifier agent volume
In this study, THF was utilized as emulsifier agent to separate the DES phase (analyte rich phase) from the water phase. With the addition of THF, emulsification of DES occurs and as it separates from the water phase, analyte atoms are rapidly transferred into the relatively low
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volume settled phase. The amount of THF plays a significant role in DES extractions and for
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this reason, 0.50, 1.0, 1.5 and 2.0 mL volumes were tried to determine the optimum value. No
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separate phase was observed for the 0.50 mL volume tested. As the amount of THF volume increased, the amount of settled phase also increased, thus, absorbance values decreased due
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to dilution. The results obtained are depicted in Figure 1 and the 1.0 mL volume was selected for further experiments. After THF addition, mixing must be performed to ensure
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homogenous emulsification and effective mass transfer of the analyte. Ultrasonication, vortex, mechanical shaking and manual shaking were performed for 15 s and compared to an
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extraction performed without mixing. The lowest results were recorded for mechanical shaking while vortex and ultrasonication were quite close to each other. Manual shaking and
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no mixing gave higher absorbance signals than the other mixing types. Manual shaking was chosen as optimum parameter for recording slightly higher absorbance values and lower
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standard deviation values when compared with no mixing. Manual shaking periods between 5.0 and 60 s were performed. The highest absorbance values were obtained for 10 s, which was selected as the final optimum parameter.
3.4.
Analytical figures of merit
The final parameters of the method and calibration values of FAAS studies are presented in Table S3 and Table 1, respectively. The detection power of the FAAS resulted a 4.95 fold enhancement via using SQT. A better alternative to the SQT was the use of DES to
ACCEPTED MANUSCRIPT preconcentrate cadmium in its complex form, and this improved the detection power by 16 folds. When the two enhancement methods (SQT and DES microextraction) were combined, a very high increase in detection power (85 folds) was achieved and this corresponded to LOD of 0.35 μg L-1 . The enhancement factor in detection power was calculated as a ratio of FAAS LOD to DES-LPME-SQT-FAAS LOD. Using the slopes of the calibration plots, the
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sensitivity of the FAAS was improved by 80 folds using the optimum method. It is clear in
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Table 1 that the LOD values of this study are comparable with the literature values. In each calibration plot, the precisions of the systems performed were calculated with six replicates of
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the lowest concentrations and the relative standard deviations obtained ranged from 2.9 –
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9.6%, indicating high precisions.
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Table 1. Analytical figures of merit of the systems and comparison to other methods in literature. LOQ (μg L− 1 )
Range (μg L− 1 )
%RSD
R2
FAAS
30
99
100–1000
4.6
0.9995
SQT-FAAS
6.0
20.2
25–250
2.9
0.9993
DES-FAAS
1.85
6.17
5.0–100
6.6
1.0000
1.17
1.0–50
9.6
0.9994
0.74
-
5.0–25
-
0.99942
0.95
4.3
10-200
2.4
0.998
1.2
-
5-150
2.1
0.9997
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DES-SQT-
0.35
FI-CPEFAAS [22]
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FAAS
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LOD (μg L− 1 )
CPE-DPTHTriton X114- FAAS [23] DLLME-
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3.5.
Recovery studies
For the purpose of testing not only the accuracy but also the applicability of the developed method, celery and apple samples were prepared according to the procedure given in section
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2.4. The prepared samples were then analyzed under the optimized conditions to determine its
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cadmium content. No analytical signal was observed for all triplicate blank extractions and this suggested that the samples did not contain cadmium within the detection limits of the
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method. The samples were also spiked to final concentrations as 5.0 and 10 μg L-1 for celery
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and apple, extracted under optimum DES conditions and analyzed by SQT-FAAS. The percent recoveries for celery samples calculated as 96.4 ± 4.2% and 101.5 ± 7.6%, for apple
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samples calculated as 92.1±4.0 and 116.8±8.6 respectively. The closeness of the recovery results to 100% validates its applicability and accuracy. The %RSDs were also lower than
Conclusion
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4.
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10%, indicating high precision for replicate extractions/measurements.
In this study, a reliable, efficient and accurate analytical technique for the determination of Cd
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has been developed by using DES as a green extraction solvent in liquid phase microextraction method. Under optimum experimental conditions, about 85-fold enhancement in the detection power was achieved when compared to the conventional FAAS. Spike recovery tests were applied to celery and apple samples at different concentrations and the results obtained were close to 100%, validating the method’s accuracy and applicability to real samples. The developed method is simple, rapid, economical, green and produces high outputs.
ACCEPTED MANUSCRIPT Conflict of interest: None.
References
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[14] Q. Zhang, K. Vigier, S. Royer, F. jérôme, Deep eutectic solvents: Syntheses, properties and applications, 2012. [15] A.H. Panhwar, M. Tuzen, T.G. Kazi, Deep eutectic solvent based advance microextraction method for determination of aluminum in water and food samples: Multivariate study, Talanta, 178 (2018) 588-593. [16] E. Smith, A. Abbott, K. Ryder, Deep Eutectic Solvents (DESs) and Their Applications, 2014. [17] M. Fırat, S. Bakırdere, M.S. Fındıkoğlu, E.B. Kafa, E. Yazıcı, M. Yolcu, Ç. Büyükpınar, D.S. Chormey, S. Sel, F. Turak, Determination of trace amount of cadmium using dispersive liquid-liquid microextraction-slotted quartz tube-flame atomic absorption spectrometry, Spectrochimica Acta Part B: Atomic Spectroscopy, 129 (2017) 37-41. [18] R.A. Zounr, M. Tuzen, N. Deligonul, M.Y. Khuhawar, A highly selective and sensitive ultrasonic assisted dispersive liquid phase microextraction based on deep eutectic solvent for determination of cadmium in food and water samples prior to electrothermal atomic absorption spectrometry, Food Chemistry, 253 (2018) 277-283. [19] X. Li, M. Wang, J. Zhao, H. Guo, X. Gao, Z. Xiong, L. Zhao, Ultrasound-assisted emulsification liquid phase microextraction method based on deep eutectic solvent as extraction solvent for determination of five pesticides in traditional Chinese medicine, Journal of Pharmaceutical and Biomedical Analysis, 166 (2019) 213-221. [20] E. Yilmaz, M. Soylak, A novel and simple deep eutectic solvent based liquid phase microextraction method for rhodamine B in cosmetic products and water samples prior to its spectrophotometric determination, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 202 (2018) 81-86. [21] A. Shishov, N. Volodina, D. Nechaeva, S. Gagarinova, A. Bulatov, An automated homogeneous liquid-liquid microextraction based on deep eutectic solvent for the HPLC-UV determination of caffeine in beverages, Microchemical Journal, 144 (2019) 469-473. [22] E. Silva, P. Dos Santos Roldan, Simultaneous flow injection preconcentration of lead and cadmium using cloud point extraction and determination by atomic absorption spectrometry, 2008. [23] C. Bosch-Ojeda, F. Sánchez-Rojas, J.M. Cano-Pavón, Preconcentration of cadmium in environmental samples by cloud point extraction and determination by FAAS, 2010. [24] G. Karim-Nezhad, M. Ahmadi, B. Dizajdizi, Background Corrected Dispersive LiquidLiquid Microextraction of Cadmium Combined with Flame Atomic Absorption Spectrometry, 2011.
ACCEPTED MANUSCRIPT Highlights A sensitive and accurate method was developed using a green solvent.
SQT was used in order to enhance the sensitivity of the system.
The method provided 85-fold enhancement when compared to FAAS.
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Figure 1