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Cost-effective voltammetric determination of boron in dried fruits and nuts using modified electrodes
Journal Pre-proofs Cost-effective voltammetric determination of boron in dried fruits and nuts using modified electrodes Lokman Liv, Nuri Nakiboğlu PII: DOI: Reference:
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Please cite this article as: Liv, L., Nakiboğlu, N., Cost-effective voltammetric determination of boron in dried fruits and nuts using modified electrodes, Food Chemistry (2019), doi: https://doi.org/10.1016/j.foodchem.2019.126013
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Cost-effective voltammetric determination of boron in dried fruits and nuts using modified electrodes Lokman Liva* and Nuri Nakiboğlub aElectrochemistry
Laboratory, Chemistry Group, National Metrology Institute, (TUBITAK UME), Kocaeli, Turkey
Barış Dist. Dr. Zeki Acar St. No:1, P. Box: 41470, TUBITAK UME, Gebze, Kocaeli, Turkey. Phone: +90 262 679 50 00-6504. Fax: +90 262 679 50 01. Email: [email protected] bDepartment
of Chemistry, Faculty of Arts and Sciences, Balıkesir University, Balıkesir, Turkey
Balıkesir University Çağış Campus, Bigadiç Road 17th km, P. Box: 10145, Balıkesir, Turkey. Phone: +90 266 612 10 00-1113. Fax: +90 266 612 15 15. Email: [email protected] ABSTRACT Cost effective, simple and accurate two voltammetric methods for determination of boron in hazelnut, peanut, almond, raisin, prune and date samples were described. Metal nanoparticles-carbon nanotube modified glassy carbon electrode (MNP/CNT/GCE, M= Au or Cu) and poly xylenol orange modified pencil graphite electrode (p-XO/PGE) were used as working electrodes. The oxidation of alizarin red s (ARS) in the boron-ARS complex at MNP/CNT/GCE and the oxidation of tiron in the B-tiron complex at p-XO/PGE were monitored as response. The limit of determination values (based on visual evaluation) for CuNP/CNT/GCE, AuNP/CNT/GCE and p-XO/PGE were calculated as 100 µg/L, 125 µg/L and 80 µg/L, respectively. The results were compared with the results obtained by inductively coupled plasma mass spectrometric method and no significant difference between the results was observed. The accuracy experiments of the methods and uncertainty calculations 1
were also performed using a certified reference material (UME CRM 1202 Elements in Hazelnut). Keywords: Boron; Alizarin red s; Tiron; Dried fruits and nuts; Modified electrode; Validation; Voltammetry. 1. Introduction Boron is a significant element for human health and some of the properties of boron can be listed as follows; preventing arthritis, reducing the allergenic and inflammatory conditions, increasing testosterone levels in men, estrogen production in menopausal women, embryonic development, cancer treatment, providing appropriate cell membrane function, preventing blood clots, alleviating the effect on congestive heart failure, decreasing lipid levels and fungal infections, improving brain functions and mental performance and eliminating harmful enzymes (Organicfacts, 2017). Consuming boron over 500 mg/day can lead to vomiting, diarrhoea, nausea, skin rash and breathing problems (Von Burg, 1992). Exposure to large amounts of boron (about 30 g of boric acid) over short periods can affect the stomach intestines, liver, kidney, and brain and can eventually lead to death (Agency for Toxic Substances and Disease Registry, 2010). An acceptable safe oral boron intake for adults could be between 1 and 20 mg/day. Tolerable upper intake levels (UL) for boron vary from 3 to 20 mg/day according to age (Drugs.com, 2017). Dried fruits, nuts, avocados and legumes are the excellence boron sources (Kuoppala, 2017). Dried fruits and nuts contain crucial bioactive compounds which are minerals (calcium, magnesium, sodium, potassium, copper and/or phosphorus), vitamins (vitamin A, 2
vitamin E, choline, niacin, and/or folic acid), carotenoids, phenolic compounds, fibers, phytosterols and/or lutein-zeaxanthin. Although nuts contain mostly unsaturated fat, dried fruits contain mainly carbohydrates. It is thought that dried fruits and nuts have the synergic effect of treating cardiovascular diseases (CVD), hypertension, hypercholesterolemia and type 2 diabetes (T2D) (Hernández-Alonso, Camacho-Barcia, Bulló, & Salas-Salvadó, 2017). Moreover, it was reported that nuts help to control body weight and dried fruits, especially prunes, improve the gastrointestinal function (INC-International Nut & Dried Fruit, 2016). Because of these features of the boron, simple, cheap, sufficiently sensitive and accurate methods for determination of boron in real samples are important. Techniques used in the existing literature for determination of boron in dried fruits and nuts are volumetric titrimetry, UV–visible spectrophotometry, neutron capture prompt gamma-ray activation analysis and plasma-derived atomic spectrometry. The methods using these techniques are summarized in Table 1. Of these, volumetric titrimetry is not sensitive enough, and spectrophotometric methods require complex procedures based on the formation of coloured compounds in concentrated acidic medium. Neutron capture prompt gamma-ray activation analysis is a non-destructive nuclear method that requires special expertise, has a high investment cost and is not easy to provide a source of neutrons and is therefore not widely used. Plasma-derived atomic spectroscopic techniques, namely ICP-OES and ICP-MS, consist of cumbersome devices with high investment and operating costs. However, voltammetry is a technique at which a time-dependent potential is applied to between working electrode and reference electrode, and the resulting current is measured as an analytical signal. This technique has some advantages such as simplicity, cheapness, rapidity, sensitivity and accuracy over the aforementioned techniques. In addition, the measurement system is portable and can be moved if desired. For these reasons, 3
voltammetry has become a preferred technique in recent years. Since the boric acid is electrochemically inactive in the potential range of the electrode used, electroactive complexing agent is required such as alizarin red s (ARS) (Şahin & Nakiboglu, 2006a; Tunay, Şahin, & Nakiboglu, 2011), beryllon(III) (Jin, Jiao, & Metzner, 1993; Lu, Li, & Deng, 1994; Thunus, 1996; Tanaka, Nishu, Nabekawa, & Hayashi, 2006), mannitol (Lewis, 1956; Boyd, 1965; Şahin & Nakiboglu, 2006b), azomethine-H (Isbir, 2006) and tiron (Fujimori, Akimoto, Tsuji, Takehara, & Yoshimura, 2010; Liv & Nakiboglu, 2016; 2018) for its voltammetric determination. In our recent studies, we reported two different voltammetric methods for the determination of boron in various water samples and eye lotion samples using tiron and ARS as an electroactive ligand (Liv & Nakiboglu, 2018; Liv, Dursun, & Nakiboglu, 2018). In the first method, the oxidation current of the tiron in the boron-tiron complex in phosphate buffered medium (pH 8) was measured as a signal at the poly xylenol orange modified pencil graphite electrode (p-XO/PGE). In the second method, ARS was used as the ligand, and the oxidation current of the ARS in the boron-ARS complex at metal nanoparticles (Au or Cu)/carbon nanotube modified glassy carbon electrode (MNP/CNT/GCE) in the ammonium/ammonia buffered medium (pH 8.5) was measured. Depending on our best knowledge, there is no voltammetric method for determination of boron in dried fruits and nuts. Therefore, it is aimed to develop an accurate, precise, fast and cost effective voltammetric method for determination of boron. The results were compared with the ICP-MS technique, and no statistically significant difference between the methods was observed for 95% confidence level. Besides the accuracy of the methods were approved with UME CRM 1202 (Elements in Hazelnut).
4
2. Materials and methods 2.1. Chemicals and apparatus HAuCl4.3H₂O in 12.7% HCl (Titrisol Standard) standard solution was used for gold modification. NIST (National Institute of Standards and Technology) SRM 3107 boron standard reference solution was used in ICP-MS measurements. NIST SRM 2387 (peanut butter) was used to detect the limit of determination values. The other chemicals were analytical reagent grade. Ultrapure water (18.2 MΩ) provided from ELGA PureLab Flex 2 water purification system was used for all preparation of solutions. High-density polyethylene (HDPE) falcon tubes and bottles were used for storage of all solutions. Metrohm Autolab PGSTAT 128N instrument with FRA32M impedance module and BASi C3 stand was used for voltammetric measurements. The instrument is controlled by a computer. A three-electrode system consisting of CuNP/CNT/GCE, AuNP/CNT/GCE (supporting surface: BASi MF-2012, 3.0 mm dia.) and p-XO/PGE as working electrodes, Ag/AgCl/3 M NaCl (BASi MF-2052 RE-5B) as reference electrode and platinum wire electrode (BASi MW-1032 (7.5 cm)) as counter electrode was used. 0.5 mm H type pencil graphite lead was supplied from Tombow brand, and MP 775 model graphite lead holder was supplied from ONAS. pH measurements were performed using Mettler Toledo Seven Compact pH meter with temperature controlled circulating bath (Thermo Haake DC 10 K20) for stable and accurate readings. ISOLAB 3 L Ultrasonic Bath was used to clean the surfaces of platinum wire and glassy carbon electrodes. Milestone Ethos SEL (Solvent extraction labstation) microwave system was used for digestion of the dried fruits and nuts.
5
Thermo brand Finnigan Element 2 model High-Resolution ICP-MS was used as a comparison technique for determination of boron in dried fruits and nuts. 2.2. Preparation of modified electrodes 2.2.1. Preparation of MNP/CNT/GCE 100 mg of multi-walled carbon nanotube (MWCNT) was weighed in a small beaker, and 3 mL H2SO4 and 1 mL HNO3 (3:1, 98%, 65%, v/v) were added into this beaker. The suspension was boiled for 3 hours at 90 °C. The nanotubes were washed many times with ultrapure water until obtaining neutral supernatant. Lastly, MWCNTs were dispersed in dimethylformamide and ultrasonicated for 30 minutes. GCE surface was polished with 0.05 µm alumina on synthetic cloth and rinsed many times with ultrapure water. Finally, it was ultrasonicated in 1:1 ethanol: ultrapure water and ultrapure water, respectively. The activation of GCE was performed in 0.5 M pH 8.5 ammonium/ammonia buffer by waiting at + 1.2 V for 120 s in stirring media then cyclic voltammetry was used by sweeping the potential between - 1 V and + 1 V with 50 mV.s-1 until obtaining a stable background. GCE surface was dried with infrared light, and 10 µL of MWCNTs suspension was dropped on the GCE, and the solvent was evaporated with infrared light for 5 minutes. Notation of this electrode was expressed as CNT/GCE. The gold modification was made in a solution consisting of 5x10-5 M HAuCl4 and 0.25 M H2SO4 using cyclic voltammetry with a potential sweeping from + 0.25 V to + 1.30 V with 50 mV.s-1 scan rate and 4 cycles. Notation of this electrode was expressed as AuNP/CNT/GCE. Copper modification was made in a solution consisting of 5x10-5 M CuSO4 and 0.15 M H2SO4 using cyclic voltammetry with a potential sweeping from - 0.80 V to + 0.70 V with 50 mV.s-1 scan rate and 4 cycles. Notation of this electrode
was
expressed
as
CuNP/CNT/GCE. 6
Characterization
of
the
prepared
MNP/CNT/GCEs were performed by scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), electrochemical impedance spectroscopy (EIS) and x-ray photoelectron spectroscopy (XPS) techniques and the results obtained were given in our previous study (Liv, Dursun, & Nakiboglu, 2018). 2.2.2. Preparation of p-XO/PGE modified electrode The bare pencil graphite electrode was activated in 0.1 M pH 7 phosphate buffer solution at + 2.0 V for 180 s. The electrode is stated as oxidized pencil graphite electrode. Oxidized pencil graphite electrode was modified with poly xylenol orange in 0.1 M pH 10 phosphate and 10-4 M xylenol orange solution using cyclic voltammetry via 7 scans and 150 mV.s-1 scan rate between - 0.4 V, and + 2.0 V. The current increase was observed between + 1.2 and + 2.0 V, indicating the electro-polymerization. The modified electrode is expressed as poly xylenol orange modified and oxidized pencil graphite electrode (p-XO/PGE). The modified electrode was prepared daily. The results related to the characterization of the electrode by SEM, EDX and EIS techniques were presented in our previous study (Liv &
Nakiboglu, 2018). 2.3. Measurement procedure All voltammetric measurements were performed in a quartz voltammetric cell, which has 5/4 oz. volume. GCE and PGE were activated and modified before the measurements. For CuNP/CNT/GCE and AuNP/CNT/GCE, the solution consisting of a decomposed sample, 0.1 M KCl and 0.07 mM ARS in 0.5 M pH 8.5 ammonium/ammonia buffer solution was set to 10 mL with ultrapure water and purged with argon gas for 5 minutes. The potential was swept from - 0.9 V to - 0.4 V using differential pulse scanning mode. The optimal values of
7
interval time, step and pulse amplitude, pulse time, deposition potential and time were used as 0.275 s, 1 mV, 70 mV, 0.05 s, - 900 mV and 40 s, respectively. For p-XO/PGE, the solution consisting of decomposed sample, 0.3 M KCl and 4 mM tiron in 0.055 M pH 8 phosphate buffer was set to 10 mL with ultrapure water and purged with argon gas for 5 minutes. The optimal values of interval time, step amplitude, pulse amplitude and pulse time were used as 0.3 s, 4 mV, 130 mV, 0.03 s, respectively. The potential was swept from 0 V to + 1.3 V using differential pulse scanning mode. All measurements were carried out at 21 ± 3°C. ICP-MS measurements were performed according to the US EPA (United States Environmental Protection Agency) 6020B method (U.S. Environmental Protection Agency, 2014). The measurement conditions of ICP-MS can be seen in Table S1. The samples diluted approximately 30-fold by microwave digestion system were analyzed by ICP-MS by further diluting as 100-fold with 2% of nitric acid. Matrix-matched standard addition method was used as a calibration method and each sample was measured 3 times. Each measurement includes 10 repetitive results from the instrument and 50 repetitive mass scans for each repetitive result. The calibration curves for B10 and B11 and the signals of the samples and the standard additions were shown in Figure S1 and Figure S2, respectively. 2.4. Sample preparation procedure Different microwave digestion methods involving different acid mixtures such as HNO3 or HNO3 and HF (U.S. Environmental Protection Agency, 1996), HNO3 and H2O2 (Hosseinzadeh, Abbasi & Ahmadi, 2007; Dündar, Altundag & Tunca, 2018) and HNO3, HClO4 and HF (Gonzalez & Watters, 2015) can be found in the literature. Only nitric acid solution, which is the digestion method recommended for UME CRM 1202, has been used for 8
digestion of the samples. Almond, date hazelnut, peanut, prune, raisin and hazelnut CRM (UME CRM 1202) are separately chopped with a food processor, and approximately 1 g of each sample was weighed. All samples were transferred into the microwave digestion cells, and 10 mL HNO3 (65%, v/v) was added into each cell. It was waited for 2 hours to provide complete penetration of the acid into the samples then the microwave digestion process was performed. The program consists of 4 steps. The first step is to reach 170 °C in 6 minutes. The second step is to hold at 170 °C for 4 minutes. The third step is to reach 210 °C in 10 minutes, and lastly, the fourth step is to hold at 210 °C for 25 minutes. The power used for microwave digestion was 1000 watt for all steps. The decomposed samples were diluted with some ultrapure water, and pH values of these samples were set to 7.5 with the addition of solid sodium hydroxide, and the final volume of the solutions was set to 25 mL (20 mL for UME CRM 1202) with ultrapure water. These samples were diluted with ultrapure water as 1:1 and 1:100 ratio and analysed with voltammetry using all modified electrodes and ICP-MS (except UME CRM 1202, the certificated value traceable to SI was used for accuracy evaluation), respectively. 3. Results and discussion 3.1. Principles of the methods Figure 1A shows the differential pulse adsorptive stripping voltammograms of free ARS and ARS in the B-ARS complex for CuNP/CNT/GCE and AuNP/CNT/GCE electrodes. This figure shows that free ARS in the solution gives an oxidation peak at - 0.73 V. When boron is added to the solution, a new peak at - 0.63 V emerges. This new peak increases with boron concentration and, both of the electrodes exhibited similar behaviour in the same solutions.
9
The principle of the other voltammetric method based on the oxidation of tiron in boron-tiron complex at p-XO/PGE is shown in Figure 1B. In solution containing 0.3 M KCl, 4 mM tiron and 0.055 M phosphate buffer, the free tiron gives an oxidation peak of about + 0.82 V (Figure 1B-a). When boron is added to this solution, the peak potential remains unchanged while the peak height increases (Figure 1B-b, c). This increase in peak height is directly proportional to the boron concentration, allowing for the voltammetric determination of boron. The detailed information about the voltammetric methods used in this study can be seen in our previous studies (Liv & Nakiboglu, 2018; Liv, Dursun, & Nakiboglu, 2018) and in Figure 2. Herein, only the studies related to the application of these methods for the determination of boron in dried fruits and nuts were evaluated. 3.2. Application of the methods to dried fruits and nuts samples and validation The boron contents in hazelnut, peanut, almond, raisin, prune and date were determined with the voltammetric methods described above (N= 9). Voltammograms related to the oxidation of ARS in the B-ARS complex at AuNP/CNT/GCE were shown as an example in Figure 1A. The voltammograms belong to the dried fruits and nuts can be seen in Figure S3 for CuNP/CNT/GCE and Figure S4 for AuNP/CNT/GCE, respectively. In this method, the results were calculated by plotting the external calibration graphs which are 𝐼(𝜇𝐴) = 0.229𝐶𝐵𝑜𝑟𝑜𝑛(𝜇𝑔/𝐿) ― 21.421 (𝑅2 = 0.998) for AuNP/CNT/GCE and 𝐼(𝜇𝐴) = 0.124 𝐶𝐵𝑜𝑟𝑜𝑛(𝜇𝑔/𝐿) + 3.467 (𝑅2 = 0.991) for CuNP/CNT/GCE, respectively. However, in the Btiron system where p-XO/PGE was used, the standard addition method was used because the external calibration graph did not yield good results. The voltammograms of the standard addition method used for the determination of boron in UME CRM 1202 were 10
based on the oxidation of tiron in the B-tiron complex at p-XO/PGE (Figure 1B). The voltammograms and standard addition equations belong to the dried fruits and nuts can be seen in Figure S5 and Table S2 for p-XO/PGE, respectively. ICP-MS method was also used as a comparison method. The results obtained from each method were given in Table 2. These results were compared with the ICP-MS method using F and t-tests. No significant difference between the methods for the 95% confidence level was found. Moreover, one way ANOVA test was performed for four groups and the averages of the results were the same for the 95% confidence level, as seen in Table 3. Besides the accuracy experiments of the methods (Table 4) and uncertainty calculations were performed using CRM named as Elements in Hazelnut (UME CRM 1202). For accuracy evaluation, F and t-tests were applied, and no significant difference for the 95% confidence level was found. For the estimation of the LOD values of the methods, boron-free NIST SRM 2387 was used as a matrix. For this purpose, the voltammogram of the sample was recorded and it was ensured that there was no signal related to boron. Then, standard boron solutions were added to the sample and estimated as LOD, where a significant boron signal was observed. The LOD values thus obtained were 80 μg/L, 100 μg/L and 125 μg/L for p-XO/PGE, AuNP/CNT/GCE and CuNP/CNT/GCE electrodes, respectively. In our previous studies, LOD values were estimated as 28, 55 and 83 μg/L for p-XO/PGE, AuNP/CNT/GCE and CuNP/CNT/GCE, respectively. It is seen that LOD values of the methods using the same electrodes are slightly increased due to the matrix effect. Obtaining the best LOD value with p-XO/PGE was attributed that there were more heteroatoms in the p-XO molecule, such as O and N, and therefore more boron complexes could be adsorbed on the electrode surface.
11
Additionally, uncertainty calculations were applied according to equation (1), where U is expanded uncertainty, T is trueness and RSD is relative standard deviation. The first part of the equation (100-T%), indicates the total systematic error and the second part of the equation (2×RSD%), indicates the total random error (Menditto, Patriarca, & Magnusson, 2007). UME CRM 1202 was used to calculate T% and RSD% values of each electrode. For this purpose, the average of nine independent measurements was divided by the value specified in UME CRM 1202 and then multiplied by 100. RSD% values were found by dividing the standard deviation of these five measurements by the mean value and then multiplying by 100. The uncertainties for CuNP/CNT/GCE, AuNP/CNT/GCE and p-XO/PGE were obtained as 0.38 mg/kg, 0.32 mg/kg and 0.50 mg/kg, respectively.
U%
100 T % 2 xRSD%
(1)
4. Conclusions The voltammetric methods proposed here are the first studies using voltammetry for the determination of boron in dried fruits and nuts. The results of voltammetric methods were compared with those obtained from the ICP-MS method, and no statistically significant difference between the methods was found for 95% confidence level. The accuracy of the proposed methods was approved, and uncertainty calculations were performed using CRM named as Elements in Hazelnut (UME CRM 1202). The lowest uncertainty and bias values were obtained with AuNP/CNT/GCE. Moreover LOD values were investigated successfully in NIST 2387 SRM, “Peanut Butter”, which has a similar matrix with dried nuts. These reference material studies prove that the methods were not affected by the interferences in the dried fruit and nut matrices.
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On the other hand, it is crucial to indicate that the method using p-XO/PGE is more sensitive to boron than bare PGE and the electrode material is very cheap compared to other solid electrodes. The boron contents of the dried fruits and nuts from the literature (between 10–50 mg/L) are comparable with the results of proposed methods (between 12– 26 mg/L). Finally, the voltammetric methods given in this study can be suggested as a simple, inexpensive and accurate method as an alternative to other methods in the literature for the determination of boron in dried fruits and nuts. Conflicts of interest The authors declare no conflict of interest. Acknowledgements This article is a part of the PhD thesis of Lokman Liv. This study was performed in the Electrochemistry Laboratory of TUBITAK UME. ICP-MS measurements were performed in the Inorganic Chemistry Laboratory of TUBITAK UME. The proposed study was supported by the scientific research project division of Balıkesir University (Contract number: 2015/130) and TUBITAK UME. References Agency for Toxic Substances and Disease Registry. Toxicological profile for boron. (2010). https://www.atsdr.cdc.gov/ToxProfiles/tp26.pdf Accessed 02.07.2019. Al-Warthan, A. A., Al-Showiman, S. S., Al-Tamrah, S. A. & Baosman, A. A. (1993). Spectrophotometric determination of boron in dates of some cultivars grown in Saudi Arabia. Journal of AOAC International, 76, 601-603.
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Table 1. The methods in the literature for the determination of boron in dried fruits and nuts
Sample
Raisins Peanut
Boron content (mg/kg) 100-220 24.9
Limit of detection
Technique
Reference
Volumetry
Brown (1936)
UV-visible
Salazar and Young
spectrophotometry
(1984)
(LOD) -
Al-Warthan, AlDates
32.6-56.4
-
UV-visible
Showiman, Al-
spectrophotometry
Tamrah and Baosman (1993)
Raisin, Prune & Peanut
19.0, 21.5 & 13.8
0.02-0.2 mg/kg*
Neutron capture
Anderson,
prompt gamma-ray
Cunningham and
activation analysis
Lindstrom (1994)
UV-visible
Naghii, Wall and
spectrophotometry
Samman (1996)
Date, Prune, Raisin,
10.8, 18.8, 45.1,
Almond &
28.2 & 27.7
-
Hazelnut Şimşek, Korkmaz, Hazelnuts
13.8-22.2
-
ICP-OES
Velioğlu and Ataman (2003)
Hazelnuts
13.6-23.9
0.1 μg/L**
19
ICP-MS
Simsek and Aykut (2007)
Özkutlu, Doğru, Hazelnuts
14.6-29.9
-
ICP-OES
Özenç, Yazici, Turan and Akçay (2013)
Almond, Date, Hazelnut, Peanut, Prune &
80, 12.8-26.0
100,
Voltammetry
125
This study
μg/L***
Raisin * LOD increases with increasing chloride content. ** It is based on the technique, not matrix-matched. *** LODs are for p-XO/PGE, CuNP/CNT/GCE and AuNP/CNT/GCE, respectively.
20
Table 2. The boron contents of dried fruit and nut samples obtained from the proposed methods and ICP-MS method (𝑥 ± 𝑠, 𝑁 = 9)
Sample
Almond Date Hazelnut Peanut Prune Raisin
AuNP/CNT/GCE
CuNP/CNT/GCE
p-XO/PGE
ICP-MS
Boron found
Boron found
Boron found
Boron found
(mg/kg)
(mg/kg)
(mg/kg)
(mg/kg)
21.34 ± 0.23 13.08 ± 0.34 17.21 ± 0.26 25.77 ± 0.29 17.13 ± 0.24 13.73 ± 0.20
21.05 ± 0.38 12.76 ± 0.52 17.40 ± 0.55 26.03 ± 0.63 16.73 ± 0.48 13.87 ± 0.70
21.42 ± 0.33 13.05 ± 0.33 17.32 ± 0.71 25.76 ± 0.70 17.10 ± 0.60 13.87 ± 0.43
21.26 ± 0.16 12.88 ± 0.37 17.33 ± 1.00 25.96 ± 0.38 16.92 ± 0.26 13.84 ± 1.17
21
A
B
Figure 1. (A) DP voltammograms obtained at AuNP/CNT/GCE (a: 0.07 mM ARS, 0.1 M KCl in 0.5 M NH4+/NH3 buffer solution (pH 8.5), b: a + 200 μg/L B and c: a + 300 μg/L B). Other conditions: scan rate: 3.6 mV/s, pulse amplitude: 70 mV, accumulation potential: - 900 mV and accumulation time.: 40 s, (B) DP voltammograms obtained at p-XO/PGE (a: 0.3 M KCl, 4 mM tiron and 0.055 M phosphate buffer (pH 8), b: a + 200 μg/L B and c: a + 300 μg/L B). Other conditions: scan rate: 13.3 mV/s and pulse amplitude: 130 mV
22
Figure 2. Schematic representation of the proposed methods
23
Table 3. ANOVA test results for all electrodes and ICP-MS in real samples
Hazelnut
Peanut
Almond
Raisin
Prune
Date
Source of variance Between groups Within groups Source of variance Between groups Within groups Source of variance Between groups Within groups Source of variance Between groups Within groups Source of variance Between groups Within groups Source of variance Between groups Within groups
SS
df
MS
P value
0.919
0.171
3
0.057
Fcritical
2.975
9.021
26
0.347
Fexperimental
0.165
SS
df
MS
P value
0.700
0.443
3
0.148
Fcritical
2.975
8.033
26
0.309
Fexperimental
0.478
SS
df
MS
P value
0.085
0.711
3
0.237
Fcritical
2.975
2.506
26
0.096
Fexperimental
2.459
SS
df
MS
P value
0.945
0.121
3
0.040
Fcritical
2.975
8.476
26
0.326
Fexperimental
0.124
SS
df
MS
P value
0.236
0.920
3
0.307
Fcritical
2.975
5.291
26
0.203
Fexperimental
1.507
SS
df
MS
P value
0.333
0.581
3
0.194
Fcritical
2.975
4.232
26
0.163
Fexperimental
1.191
24
Table 4. Boron determination in UME CRM 1202 Certified value
CuNP/CNT/GCE
AuNP/CNT/GCE
p-XO/PGE
Boron (mg/kg)
Boron (mg/kg)
Boron (mg/kg)
Boron (mg/kg)
16.80 ± 2.20
17.02 ± 0.31
16.77 ± 0.29
16.46 ± 0.60
25
This is the first study for voltammetric detection of boron in dried fruits & nuts
The cheapest electrode material was used
The methods are simple, cheap, accurate & alternative to other existing methods
The uncertainties of the methods are low in dried fruit & nut matrix
Declaration of interests
☒ 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.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
26
S0308-8146(19)32156-9 https://doi.org/10.1016/j.foodchem.2019.126013 FOCH 126013
To appear in:
Food Chemistry
Received Date: Revised Date: Accepted Date:
8 July 2019 2 December 2019 2 December 2019
Please cite this article as: Liv, L., Nakiboğlu, N., Cost-effective voltammetric determination of boron in dried fruits and nuts using modified electrodes, Food Chemistry (2019), doi: https://doi.org/10.1016/j.foodchem.2019.126013
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Cost-effective voltammetric determination of boron in dried fruits and nuts using modified electrodes Lokman Liva* and Nuri Nakiboğlub aElectrochemistry
Laboratory, Chemistry Group, National Metrology Institute, (TUBITAK UME), Kocaeli, Turkey
Barış Dist. Dr. Zeki Acar St. No:1, P. Box: 41470, TUBITAK UME, Gebze, Kocaeli, Turkey. Phone: +90 262 679 50 00-6504. Fax: +90 262 679 50 01. Email: [email protected] bDepartment
of Chemistry, Faculty of Arts and Sciences, Balıkesir University, Balıkesir, Turkey
Balıkesir University Çağış Campus, Bigadiç Road 17th km, P. Box: 10145, Balıkesir, Turkey. Phone: +90 266 612 10 00-1113. Fax: +90 266 612 15 15. Email: [email protected] ABSTRACT Cost effective, simple and accurate two voltammetric methods for determination of boron in hazelnut, peanut, almond, raisin, prune and date samples were described. Metal nanoparticles-carbon nanotube modified glassy carbon electrode (MNP/CNT/GCE, M= Au or Cu) and poly xylenol orange modified pencil graphite electrode (p-XO/PGE) were used as working electrodes. The oxidation of alizarin red s (ARS) in the boron-ARS complex at MNP/CNT/GCE and the oxidation of tiron in the B-tiron complex at p-XO/PGE were monitored as response. The limit of determination values (based on visual evaluation) for CuNP/CNT/GCE, AuNP/CNT/GCE and p-XO/PGE were calculated as 100 µg/L, 125 µg/L and 80 µg/L, respectively. The results were compared with the results obtained by inductively coupled plasma mass spectrometric method and no significant difference between the results was observed. The accuracy experiments of the methods and uncertainty calculations 1
were also performed using a certified reference material (UME CRM 1202 Elements in Hazelnut). Keywords: Boron; Alizarin red s; Tiron; Dried fruits and nuts; Modified electrode; Validation; Voltammetry. 1. Introduction Boron is a significant element for human health and some of the properties of boron can be listed as follows; preventing arthritis, reducing the allergenic and inflammatory conditions, increasing testosterone levels in men, estrogen production in menopausal women, embryonic development, cancer treatment, providing appropriate cell membrane function, preventing blood clots, alleviating the effect on congestive heart failure, decreasing lipid levels and fungal infections, improving brain functions and mental performance and eliminating harmful enzymes (Organicfacts, 2017). Consuming boron over 500 mg/day can lead to vomiting, diarrhoea, nausea, skin rash and breathing problems (Von Burg, 1992). Exposure to large amounts of boron (about 30 g of boric acid) over short periods can affect the stomach intestines, liver, kidney, and brain and can eventually lead to death (Agency for Toxic Substances and Disease Registry, 2010). An acceptable safe oral boron intake for adults could be between 1 and 20 mg/day. Tolerable upper intake levels (UL) for boron vary from 3 to 20 mg/day according to age (Drugs.com, 2017). Dried fruits, nuts, avocados and legumes are the excellence boron sources (Kuoppala, 2017). Dried fruits and nuts contain crucial bioactive compounds which are minerals (calcium, magnesium, sodium, potassium, copper and/or phosphorus), vitamins (vitamin A, 2
vitamin E, choline, niacin, and/or folic acid), carotenoids, phenolic compounds, fibers, phytosterols and/or lutein-zeaxanthin. Although nuts contain mostly unsaturated fat, dried fruits contain mainly carbohydrates. It is thought that dried fruits and nuts have the synergic effect of treating cardiovascular diseases (CVD), hypertension, hypercholesterolemia and type 2 diabetes (T2D) (Hernández-Alonso, Camacho-Barcia, Bulló, & Salas-Salvadó, 2017). Moreover, it was reported that nuts help to control body weight and dried fruits, especially prunes, improve the gastrointestinal function (INC-International Nut & Dried Fruit, 2016). Because of these features of the boron, simple, cheap, sufficiently sensitive and accurate methods for determination of boron in real samples are important. Techniques used in the existing literature for determination of boron in dried fruits and nuts are volumetric titrimetry, UV–visible spectrophotometry, neutron capture prompt gamma-ray activation analysis and plasma-derived atomic spectrometry. The methods using these techniques are summarized in Table 1. Of these, volumetric titrimetry is not sensitive enough, and spectrophotometric methods require complex procedures based on the formation of coloured compounds in concentrated acidic medium. Neutron capture prompt gamma-ray activation analysis is a non-destructive nuclear method that requires special expertise, has a high investment cost and is not easy to provide a source of neutrons and is therefore not widely used. Plasma-derived atomic spectroscopic techniques, namely ICP-OES and ICP-MS, consist of cumbersome devices with high investment and operating costs. However, voltammetry is a technique at which a time-dependent potential is applied to between working electrode and reference electrode, and the resulting current is measured as an analytical signal. This technique has some advantages such as simplicity, cheapness, rapidity, sensitivity and accuracy over the aforementioned techniques. In addition, the measurement system is portable and can be moved if desired. For these reasons, 3
voltammetry has become a preferred technique in recent years. Since the boric acid is electrochemically inactive in the potential range of the electrode used, electroactive complexing agent is required such as alizarin red s (ARS) (Şahin & Nakiboglu, 2006a; Tunay, Şahin, & Nakiboglu, 2011), beryllon(III) (Jin, Jiao, & Metzner, 1993; Lu, Li, & Deng, 1994; Thunus, 1996; Tanaka, Nishu, Nabekawa, & Hayashi, 2006), mannitol (Lewis, 1956; Boyd, 1965; Şahin & Nakiboglu, 2006b), azomethine-H (Isbir, 2006) and tiron (Fujimori, Akimoto, Tsuji, Takehara, & Yoshimura, 2010; Liv & Nakiboglu, 2016; 2018) for its voltammetric determination. In our recent studies, we reported two different voltammetric methods for the determination of boron in various water samples and eye lotion samples using tiron and ARS as an electroactive ligand (Liv & Nakiboglu, 2018; Liv, Dursun, & Nakiboglu, 2018). In the first method, the oxidation current of the tiron in the boron-tiron complex in phosphate buffered medium (pH 8) was measured as a signal at the poly xylenol orange modified pencil graphite electrode (p-XO/PGE). In the second method, ARS was used as the ligand, and the oxidation current of the ARS in the boron-ARS complex at metal nanoparticles (Au or Cu)/carbon nanotube modified glassy carbon electrode (MNP/CNT/GCE) in the ammonium/ammonia buffered medium (pH 8.5) was measured. Depending on our best knowledge, there is no voltammetric method for determination of boron in dried fruits and nuts. Therefore, it is aimed to develop an accurate, precise, fast and cost effective voltammetric method for determination of boron. The results were compared with the ICP-MS technique, and no statistically significant difference between the methods was observed for 95% confidence level. Besides the accuracy of the methods were approved with UME CRM 1202 (Elements in Hazelnut).
4
2. Materials and methods 2.1. Chemicals and apparatus HAuCl4.3H₂O in 12.7% HCl (Titrisol Standard) standard solution was used for gold modification. NIST (National Institute of Standards and Technology) SRM 3107 boron standard reference solution was used in ICP-MS measurements. NIST SRM 2387 (peanut butter) was used to detect the limit of determination values. The other chemicals were analytical reagent grade. Ultrapure water (18.2 MΩ) provided from ELGA PureLab Flex 2 water purification system was used for all preparation of solutions. High-density polyethylene (HDPE) falcon tubes and bottles were used for storage of all solutions. Metrohm Autolab PGSTAT 128N instrument with FRA32M impedance module and BASi C3 stand was used for voltammetric measurements. The instrument is controlled by a computer. A three-electrode system consisting of CuNP/CNT/GCE, AuNP/CNT/GCE (supporting surface: BASi MF-2012, 3.0 mm dia.) and p-XO/PGE as working electrodes, Ag/AgCl/3 M NaCl (BASi MF-2052 RE-5B) as reference electrode and platinum wire electrode (BASi MW-1032 (7.5 cm)) as counter electrode was used. 0.5 mm H type pencil graphite lead was supplied from Tombow brand, and MP 775 model graphite lead holder was supplied from ONAS. pH measurements were performed using Mettler Toledo Seven Compact pH meter with temperature controlled circulating bath (Thermo Haake DC 10 K20) for stable and accurate readings. ISOLAB 3 L Ultrasonic Bath was used to clean the surfaces of platinum wire and glassy carbon electrodes. Milestone Ethos SEL (Solvent extraction labstation) microwave system was used for digestion of the dried fruits and nuts.
5
Thermo brand Finnigan Element 2 model High-Resolution ICP-MS was used as a comparison technique for determination of boron in dried fruits and nuts. 2.2. Preparation of modified electrodes 2.2.1. Preparation of MNP/CNT/GCE 100 mg of multi-walled carbon nanotube (MWCNT) was weighed in a small beaker, and 3 mL H2SO4 and 1 mL HNO3 (3:1, 98%, 65%, v/v) were added into this beaker. The suspension was boiled for 3 hours at 90 °C. The nanotubes were washed many times with ultrapure water until obtaining neutral supernatant. Lastly, MWCNTs were dispersed in dimethylformamide and ultrasonicated for 30 minutes. GCE surface was polished with 0.05 µm alumina on synthetic cloth and rinsed many times with ultrapure water. Finally, it was ultrasonicated in 1:1 ethanol: ultrapure water and ultrapure water, respectively. The activation of GCE was performed in 0.5 M pH 8.5 ammonium/ammonia buffer by waiting at + 1.2 V for 120 s in stirring media then cyclic voltammetry was used by sweeping the potential between - 1 V and + 1 V with 50 mV.s-1 until obtaining a stable background. GCE surface was dried with infrared light, and 10 µL of MWCNTs suspension was dropped on the GCE, and the solvent was evaporated with infrared light for 5 minutes. Notation of this electrode was expressed as CNT/GCE. The gold modification was made in a solution consisting of 5x10-5 M HAuCl4 and 0.25 M H2SO4 using cyclic voltammetry with a potential sweeping from + 0.25 V to + 1.30 V with 50 mV.s-1 scan rate and 4 cycles. Notation of this electrode was expressed as AuNP/CNT/GCE. Copper modification was made in a solution consisting of 5x10-5 M CuSO4 and 0.15 M H2SO4 using cyclic voltammetry with a potential sweeping from - 0.80 V to + 0.70 V with 50 mV.s-1 scan rate and 4 cycles. Notation of this electrode
was
expressed
as
CuNP/CNT/GCE. 6
Characterization
of
the
prepared
MNP/CNT/GCEs were performed by scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), electrochemical impedance spectroscopy (EIS) and x-ray photoelectron spectroscopy (XPS) techniques and the results obtained were given in our previous study (Liv, Dursun, & Nakiboglu, 2018). 2.2.2. Preparation of p-XO/PGE modified electrode The bare pencil graphite electrode was activated in 0.1 M pH 7 phosphate buffer solution at + 2.0 V for 180 s. The electrode is stated as oxidized pencil graphite electrode. Oxidized pencil graphite electrode was modified with poly xylenol orange in 0.1 M pH 10 phosphate and 10-4 M xylenol orange solution using cyclic voltammetry via 7 scans and 150 mV.s-1 scan rate between - 0.4 V, and + 2.0 V. The current increase was observed between + 1.2 and + 2.0 V, indicating the electro-polymerization. The modified electrode is expressed as poly xylenol orange modified and oxidized pencil graphite electrode (p-XO/PGE). The modified electrode was prepared daily. The results related to the characterization of the electrode by SEM, EDX and EIS techniques were presented in our previous study (Liv &
Nakiboglu, 2018). 2.3. Measurement procedure All voltammetric measurements were performed in a quartz voltammetric cell, which has 5/4 oz. volume. GCE and PGE were activated and modified before the measurements. For CuNP/CNT/GCE and AuNP/CNT/GCE, the solution consisting of a decomposed sample, 0.1 M KCl and 0.07 mM ARS in 0.5 M pH 8.5 ammonium/ammonia buffer solution was set to 10 mL with ultrapure water and purged with argon gas for 5 minutes. The potential was swept from - 0.9 V to - 0.4 V using differential pulse scanning mode. The optimal values of
7
interval time, step and pulse amplitude, pulse time, deposition potential and time were used as 0.275 s, 1 mV, 70 mV, 0.05 s, - 900 mV and 40 s, respectively. For p-XO/PGE, the solution consisting of decomposed sample, 0.3 M KCl and 4 mM tiron in 0.055 M pH 8 phosphate buffer was set to 10 mL with ultrapure water and purged with argon gas for 5 minutes. The optimal values of interval time, step amplitude, pulse amplitude and pulse time were used as 0.3 s, 4 mV, 130 mV, 0.03 s, respectively. The potential was swept from 0 V to + 1.3 V using differential pulse scanning mode. All measurements were carried out at 21 ± 3°C. ICP-MS measurements were performed according to the US EPA (United States Environmental Protection Agency) 6020B method (U.S. Environmental Protection Agency, 2014). The measurement conditions of ICP-MS can be seen in Table S1. The samples diluted approximately 30-fold by microwave digestion system were analyzed by ICP-MS by further diluting as 100-fold with 2% of nitric acid. Matrix-matched standard addition method was used as a calibration method and each sample was measured 3 times. Each measurement includes 10 repetitive results from the instrument and 50 repetitive mass scans for each repetitive result. The calibration curves for B10 and B11 and the signals of the samples and the standard additions were shown in Figure S1 and Figure S2, respectively. 2.4. Sample preparation procedure Different microwave digestion methods involving different acid mixtures such as HNO3 or HNO3 and HF (U.S. Environmental Protection Agency, 1996), HNO3 and H2O2 (Hosseinzadeh, Abbasi & Ahmadi, 2007; Dündar, Altundag & Tunca, 2018) and HNO3, HClO4 and HF (Gonzalez & Watters, 2015) can be found in the literature. Only nitric acid solution, which is the digestion method recommended for UME CRM 1202, has been used for 8
digestion of the samples. Almond, date hazelnut, peanut, prune, raisin and hazelnut CRM (UME CRM 1202) are separately chopped with a food processor, and approximately 1 g of each sample was weighed. All samples were transferred into the microwave digestion cells, and 10 mL HNO3 (65%, v/v) was added into each cell. It was waited for 2 hours to provide complete penetration of the acid into the samples then the microwave digestion process was performed. The program consists of 4 steps. The first step is to reach 170 °C in 6 minutes. The second step is to hold at 170 °C for 4 minutes. The third step is to reach 210 °C in 10 minutes, and lastly, the fourth step is to hold at 210 °C for 25 minutes. The power used for microwave digestion was 1000 watt for all steps. The decomposed samples were diluted with some ultrapure water, and pH values of these samples were set to 7.5 with the addition of solid sodium hydroxide, and the final volume of the solutions was set to 25 mL (20 mL for UME CRM 1202) with ultrapure water. These samples were diluted with ultrapure water as 1:1 and 1:100 ratio and analysed with voltammetry using all modified electrodes and ICP-MS (except UME CRM 1202, the certificated value traceable to SI was used for accuracy evaluation), respectively. 3. Results and discussion 3.1. Principles of the methods Figure 1A shows the differential pulse adsorptive stripping voltammograms of free ARS and ARS in the B-ARS complex for CuNP/CNT/GCE and AuNP/CNT/GCE electrodes. This figure shows that free ARS in the solution gives an oxidation peak at - 0.73 V. When boron is added to the solution, a new peak at - 0.63 V emerges. This new peak increases with boron concentration and, both of the electrodes exhibited similar behaviour in the same solutions.
9
The principle of the other voltammetric method based on the oxidation of tiron in boron-tiron complex at p-XO/PGE is shown in Figure 1B. In solution containing 0.3 M KCl, 4 mM tiron and 0.055 M phosphate buffer, the free tiron gives an oxidation peak of about + 0.82 V (Figure 1B-a). When boron is added to this solution, the peak potential remains unchanged while the peak height increases (Figure 1B-b, c). This increase in peak height is directly proportional to the boron concentration, allowing for the voltammetric determination of boron. The detailed information about the voltammetric methods used in this study can be seen in our previous studies (Liv & Nakiboglu, 2018; Liv, Dursun, & Nakiboglu, 2018) and in Figure 2. Herein, only the studies related to the application of these methods for the determination of boron in dried fruits and nuts were evaluated. 3.2. Application of the methods to dried fruits and nuts samples and validation The boron contents in hazelnut, peanut, almond, raisin, prune and date were determined with the voltammetric methods described above (N= 9). Voltammograms related to the oxidation of ARS in the B-ARS complex at AuNP/CNT/GCE were shown as an example in Figure 1A. The voltammograms belong to the dried fruits and nuts can be seen in Figure S3 for CuNP/CNT/GCE and Figure S4 for AuNP/CNT/GCE, respectively. In this method, the results were calculated by plotting the external calibration graphs which are 𝐼(𝜇𝐴) = 0.229𝐶𝐵𝑜𝑟𝑜𝑛(𝜇𝑔/𝐿) ― 21.421 (𝑅2 = 0.998) for AuNP/CNT/GCE and 𝐼(𝜇𝐴) = 0.124 𝐶𝐵𝑜𝑟𝑜𝑛(𝜇𝑔/𝐿) + 3.467 (𝑅2 = 0.991) for CuNP/CNT/GCE, respectively. However, in the Btiron system where p-XO/PGE was used, the standard addition method was used because the external calibration graph did not yield good results. The voltammograms of the standard addition method used for the determination of boron in UME CRM 1202 were 10
based on the oxidation of tiron in the B-tiron complex at p-XO/PGE (Figure 1B). The voltammograms and standard addition equations belong to the dried fruits and nuts can be seen in Figure S5 and Table S2 for p-XO/PGE, respectively. ICP-MS method was also used as a comparison method. The results obtained from each method were given in Table 2. These results were compared with the ICP-MS method using F and t-tests. No significant difference between the methods for the 95% confidence level was found. Moreover, one way ANOVA test was performed for four groups and the averages of the results were the same for the 95% confidence level, as seen in Table 3. Besides the accuracy experiments of the methods (Table 4) and uncertainty calculations were performed using CRM named as Elements in Hazelnut (UME CRM 1202). For accuracy evaluation, F and t-tests were applied, and no significant difference for the 95% confidence level was found. For the estimation of the LOD values of the methods, boron-free NIST SRM 2387 was used as a matrix. For this purpose, the voltammogram of the sample was recorded and it was ensured that there was no signal related to boron. Then, standard boron solutions were added to the sample and estimated as LOD, where a significant boron signal was observed. The LOD values thus obtained were 80 μg/L, 100 μg/L and 125 μg/L for p-XO/PGE, AuNP/CNT/GCE and CuNP/CNT/GCE electrodes, respectively. In our previous studies, LOD values were estimated as 28, 55 and 83 μg/L for p-XO/PGE, AuNP/CNT/GCE and CuNP/CNT/GCE, respectively. It is seen that LOD values of the methods using the same electrodes are slightly increased due to the matrix effect. Obtaining the best LOD value with p-XO/PGE was attributed that there were more heteroatoms in the p-XO molecule, such as O and N, and therefore more boron complexes could be adsorbed on the electrode surface.
11
Additionally, uncertainty calculations were applied according to equation (1), where U is expanded uncertainty, T is trueness and RSD is relative standard deviation. The first part of the equation (100-T%), indicates the total systematic error and the second part of the equation (2×RSD%), indicates the total random error (Menditto, Patriarca, & Magnusson, 2007). UME CRM 1202 was used to calculate T% and RSD% values of each electrode. For this purpose, the average of nine independent measurements was divided by the value specified in UME CRM 1202 and then multiplied by 100. RSD% values were found by dividing the standard deviation of these five measurements by the mean value and then multiplying by 100. The uncertainties for CuNP/CNT/GCE, AuNP/CNT/GCE and p-XO/PGE were obtained as 0.38 mg/kg, 0.32 mg/kg and 0.50 mg/kg, respectively.
U%
100 T % 2 xRSD%
(1)
4. Conclusions The voltammetric methods proposed here are the first studies using voltammetry for the determination of boron in dried fruits and nuts. The results of voltammetric methods were compared with those obtained from the ICP-MS method, and no statistically significant difference between the methods was found for 95% confidence level. The accuracy of the proposed methods was approved, and uncertainty calculations were performed using CRM named as Elements in Hazelnut (UME CRM 1202). The lowest uncertainty and bias values were obtained with AuNP/CNT/GCE. Moreover LOD values were investigated successfully in NIST 2387 SRM, “Peanut Butter”, which has a similar matrix with dried nuts. These reference material studies prove that the methods were not affected by the interferences in the dried fruit and nut matrices.
12
On the other hand, it is crucial to indicate that the method using p-XO/PGE is more sensitive to boron than bare PGE and the electrode material is very cheap compared to other solid electrodes. The boron contents of the dried fruits and nuts from the literature (between 10–50 mg/L) are comparable with the results of proposed methods (between 12– 26 mg/L). Finally, the voltammetric methods given in this study can be suggested as a simple, inexpensive and accurate method as an alternative to other methods in the literature for the determination of boron in dried fruits and nuts. Conflicts of interest The authors declare no conflict of interest. Acknowledgements This article is a part of the PhD thesis of Lokman Liv. This study was performed in the Electrochemistry Laboratory of TUBITAK UME. ICP-MS measurements were performed in the Inorganic Chemistry Laboratory of TUBITAK UME. The proposed study was supported by the scientific research project division of Balıkesir University (Contract number: 2015/130) and TUBITAK UME. References Agency for Toxic Substances and Disease Registry. Toxicological profile for boron. (2010). https://www.atsdr.cdc.gov/ToxProfiles/tp26.pdf Accessed 02.07.2019. Al-Warthan, A. A., Al-Showiman, S. S., Al-Tamrah, S. A. & Baosman, A. A. (1993). Spectrophotometric determination of boron in dates of some cultivars grown in Saudi Arabia. Journal of AOAC International, 76, 601-603.
13
Anderson, D. L., Cunningham, W. C. & Lindstrom, T. R. (1994). Concentrations and intakes of H, B, S, K, Na, Cl, and NaCl in foods. Journal of food composition and analysis, 7, 59-82. Boyd, C. (1965). Indirect polarographic determination of boron. Analytical Chemistry, 37, 1587-1588. doi: 10.1021/ac60231a038 Brown, W. B. (1936). The determination of boric acid in dried fruit. Analyst, 61, 671680. doi: 10.1039/AN9366100671 Drugs.com.
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(2017).
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Table 1. The methods in the literature for the determination of boron in dried fruits and nuts
Sample
Raisins Peanut
Boron content (mg/kg) 100-220 24.9
Limit of detection
Technique
Reference
Volumetry
Brown (1936)
UV-visible
Salazar and Young
spectrophotometry
(1984)
(LOD) -
Al-Warthan, AlDates
32.6-56.4
-
UV-visible
Showiman, Al-
spectrophotometry
Tamrah and Baosman (1993)
Raisin, Prune & Peanut
19.0, 21.5 & 13.8
0.02-0.2 mg/kg*
Neutron capture
Anderson,
prompt gamma-ray
Cunningham and
activation analysis
Lindstrom (1994)
UV-visible
Naghii, Wall and
spectrophotometry
Samman (1996)
Date, Prune, Raisin,
10.8, 18.8, 45.1,
Almond &
28.2 & 27.7
-
Hazelnut Şimşek, Korkmaz, Hazelnuts
13.8-22.2
-
ICP-OES
Velioğlu and Ataman (2003)
Hazelnuts
13.6-23.9
0.1 μg/L**
19
ICP-MS
Simsek and Aykut (2007)
Özkutlu, Doğru, Hazelnuts
14.6-29.9
-
ICP-OES
Özenç, Yazici, Turan and Akçay (2013)
Almond, Date, Hazelnut, Peanut, Prune &
80, 12.8-26.0
100,
Voltammetry
125
This study
μg/L***
Raisin * LOD increases with increasing chloride content. ** It is based on the technique, not matrix-matched. *** LODs are for p-XO/PGE, CuNP/CNT/GCE and AuNP/CNT/GCE, respectively.
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Table 2. The boron contents of dried fruit and nut samples obtained from the proposed methods and ICP-MS method (𝑥 ± 𝑠, 𝑁 = 9)
Sample
Almond Date Hazelnut Peanut Prune Raisin
AuNP/CNT/GCE
CuNP/CNT/GCE
p-XO/PGE
ICP-MS
Boron found
Boron found
Boron found
Boron found
(mg/kg)
(mg/kg)
(mg/kg)
(mg/kg)
21.34 ± 0.23 13.08 ± 0.34 17.21 ± 0.26 25.77 ± 0.29 17.13 ± 0.24 13.73 ± 0.20
21.05 ± 0.38 12.76 ± 0.52 17.40 ± 0.55 26.03 ± 0.63 16.73 ± 0.48 13.87 ± 0.70
21.42 ± 0.33 13.05 ± 0.33 17.32 ± 0.71 25.76 ± 0.70 17.10 ± 0.60 13.87 ± 0.43
21.26 ± 0.16 12.88 ± 0.37 17.33 ± 1.00 25.96 ± 0.38 16.92 ± 0.26 13.84 ± 1.17
21
A
B
Figure 1. (A) DP voltammograms obtained at AuNP/CNT/GCE (a: 0.07 mM ARS, 0.1 M KCl in 0.5 M NH4+/NH3 buffer solution (pH 8.5), b: a + 200 μg/L B and c: a + 300 μg/L B). Other conditions: scan rate: 3.6 mV/s, pulse amplitude: 70 mV, accumulation potential: - 900 mV and accumulation time.: 40 s, (B) DP voltammograms obtained at p-XO/PGE (a: 0.3 M KCl, 4 mM tiron and 0.055 M phosphate buffer (pH 8), b: a + 200 μg/L B and c: a + 300 μg/L B). Other conditions: scan rate: 13.3 mV/s and pulse amplitude: 130 mV
22
Figure 2. Schematic representation of the proposed methods
23
Table 3. ANOVA test results for all electrodes and ICP-MS in real samples
Hazelnut
Peanut
Almond
Raisin
Prune
Date
Source of variance Between groups Within groups Source of variance Between groups Within groups Source of variance Between groups Within groups Source of variance Between groups Within groups Source of variance Between groups Within groups Source of variance Between groups Within groups
SS
df
MS
P value
0.919
0.171
3
0.057
Fcritical
2.975
9.021
26
0.347
Fexperimental
0.165
SS
df
MS
P value
0.700
0.443
3
0.148
Fcritical
2.975
8.033
26
0.309
Fexperimental
0.478
SS
df
MS
P value
0.085
0.711
3
0.237
Fcritical
2.975
2.506
26
0.096
Fexperimental
2.459
SS
df
MS
P value
0.945
0.121
3
0.040
Fcritical
2.975
8.476
26
0.326
Fexperimental
0.124
SS
df
MS
P value
0.236
0.920
3
0.307
Fcritical
2.975
5.291
26
0.203
Fexperimental
1.507
SS
df
MS
P value
0.333
0.581
3
0.194
Fcritical
2.975
4.232
26
0.163
Fexperimental
1.191
24
Table 4. Boron determination in UME CRM 1202 Certified value
CuNP/CNT/GCE
AuNP/CNT/GCE
p-XO/PGE
Boron (mg/kg)
Boron (mg/kg)
Boron (mg/kg)
Boron (mg/kg)
16.80 ± 2.20
17.02 ± 0.31
16.77 ± 0.29
16.46 ± 0.60
25
This is the first study for voltammetric detection of boron in dried fruits & nuts
The cheapest electrode material was used
The methods are simple, cheap, accurate & alternative to other existing methods
The uncertainties of the methods are low in dried fruit & nut matrix
Declaration of interests
☒ 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.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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