Preconcentration and determination of β-carotene nanoparticles in fruit and juice samples based on a micelle mediated system

Abstract

This article presents a simple, green and low cost analytical method for preconcentration and determination of β-carotene nanoparticle content in fruit and juice samples. The method is based on the enrichment of β-carotene nanoparticles in a non-ionic surfactant phase using cloud point extraction (CPE) followed by UV-vis spectrophotometric determination at 468 nm. The influence of chemical variables such as pH of the sample solution, type and volume of the buffer, electrolyte type and concentration, surfactant concentration, temperature and incubation time on the CPE procedure was studied. Under optimum conditions the method yielded a linear calibration curve in the range of 2.80 × 10⁻⁸ to 2.52 × 10⁻⁶ mol L⁻¹ for β-carotene nanoparticles (r = 0.9978). The LOD and LOQ were found to be 1.12 × 10⁻⁸ mol L⁻¹ and 3.74 × 10⁻⁸ mol L⁻¹, respectively. The relative standard deviations (RSD) for eight replicate measurements of 1.68 × 10⁻⁶ mg L⁻¹ and 2.80 × 10⁻⁷ mol L⁻¹ of BC-NPs were 3.2% and 5.6%, respectively. The developed CPE method was applied to the analysis of watermelon, tomato, cherry and orange juice samples. The recoveries of the analytes in spiked samples were in the range of 96–104%.

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Introduction

Beta-carotene (BC) is among the group of red, orange and yellow pigments called carotenes or carotenoids, having a chemical formula of –C₄₀H₅₆– which was discovered in 1907. β-Carotene is a precursor of vitamin A and plays an important role in metabolism and human health and is mainly found in fresh fruits, vegetables, fish and other sea products.^1–5 This compound helps maintain healthy skin and also plays a vital role in eye health. Individuals who consume the necessary levels of BC can lower their risk for coronary artery disease, stroke, macular degeneration, and other age-related diseases.^4–7 BC like other carotenoids has an antioxidant property and protects the body against the free radicals which could damage cells through oxidation.⁸ Owing to various health-related properties of BC, it is among the significant food additives. However it seems that BC might have both beneficial and damaging effects for human health. At high level supplements of BC, it may turn the skin yellowish or orange. It has also been shown that the individuals who use high dose of supplemental BC have an increased relative risk (18–23%) for developing lung cancer compared to non-BC supplemented control subjects.⁹ High doses of BC have also been linked to an increased risk of prostate cancer, cardiovascular disease, mortality and presumably can be toxic to the liver.^10–15 Nevertheless, upper intake level (UL) as “safe range” for BC has not been determined yet.¹⁶

On the other hand, BC has poor solubility in aqueous solutions and low solubility in oil (only about 0.2 g L⁻¹),¹⁷ due to the high hydrophobicity, which leads to low delivery of BC, poor dissolution and absorption in the gastro-intestinal tract and limited bioavailability.^{5–7,18–21} Also, BC can be degraded in food processing at high temperature^6,21 which limits its application in food formulations.^2,22 For elimination of this problem, researchers have reported a simple approach for the preparation of BC nanoparticles (BC-NPs) using various emulsification and evaporation techniques.^2–5,17

In the last decades, nanotechnology has quickly attracted considerable attention in the food, pharmaceutical and health industries. This technology provides useful platforms to improve solubility and bioavailability of bioactive compounds and chemicals such as BC.^1,5 BC-NPs increase the total surface area of the poorly-water soluble BC and making it more bioavailable.^2,23 BC-NPs can protect BC against degradation during the most common industrial stabilization processes such as heat, light, sterilization, pasteurization and also increase its bioavailability during gastro-intestinal passage.

All these new properties might facilitate commercial applications of β-carotene nanoparticles in food, nutraceutical, cosmetic, pharmaceutical and chemical industries as well as environmental and agriculture engineering. Since β-carotene has direct application in food samples as the main source of vitamin A and also as an anti oxidant for decreasing cancer risk, BC-NPs properties such as stability, resistance to heat and better solubility⁶ can enhance its uses in the food and beverage industries. As a result attempts have been made for preparation and study of β-carotene carotene nanocapsules^6,24 and nanodispersions⁴ as potential active ingredients for food formulations. Therefore preconcentration and determination of BC-NPs in different food samples become an important task.

Cloud point extraction (CPE) has been successfully utilized for preconcentration, extraction and determination of organic compounds,^25,26 metal ions,^27,28 dyes,²⁹ drugs and other biological compounds^30,31 and different nanoparticles.³² This technique is based on use of non-ionic surfactants such as Triton X-100 and Triton X-114 which form micelles in aqueous solutions and when heated to a certain temperature called cloud point temperature; becomes turbid. The micellar solution separates into a surfactant-rich phase, with a small volume which include the analyte molecules, and an aqueous phase, with a large volume. When the analyte ions, which are primarily present in the aqueous solution, interact with the micelles, form hydrophobic compounds and are concentrated into the surfactant-rich phase, and therefore easily separated and preconcentrated.³³ There is a very limited report on the determination of BC in a variety of fruit and food samples.^34–37 However, to the best of our knowledge studies about the preconcentration and determination of BC-NPs in food samples have not been explored and there are only some articles presenting the preparation of BC-NPs.^2–5,7,21

The aim of the present work is to develop a green, simple, low cost, sensitive and reliable CPE method for the preconcentration of BC-NPs by enrichment into surfactant phase and determination by UV-vis spectrophotometric method.

Experimental section

Apparatus

The absorbance measurements and absorption spectra of the BC-NPs solution were performed using a GBC UV-visible spectrophotometer model Cintra101 (Sidney, Australia) operating at 468 nm using glass cells. pH measurements were carried out by a digital pH-Meter model 632, Metrohm (Herisau, Switzerland) with a combined glass electrode. A Colora thermostat bath (London, England) maintained at the desired temperature was used for the cloud point temperature experiments. A rotary evaporator (Laborota 4000 Heidolph, Germany) was used for solvent removing.

Reagents

All chemicals were of analytical grade and doubled distilled water was used throughout. Phosphate buffer solution pH 7 was prepared by adding 0.05 mol L⁻¹ of NaOH (Merck, Darmstadt, Germany) to 0.05 mol L⁻¹ of phosphoric acid (Merck) and adjusting the pH to 7 using a pH meter. β-Carotene was purchased from Sigma Aldrich (St. Louis, MO, USA).

Synthesis of β-carotene nanoparticles

The BC-NPs were fabricated by slight modification of previously reported method by Yin et al.¹⁷ A typical fabrication procedure was depicted as follows: the organic phase was prepared by dissolving 1.5 mg of BC in 100 mL of acetone. An aqueous phase containing 0.9 mL of Triton X-100, 18 mg of sodium azide in 90 mL of phosphate buffer (pH = 7) was prepared. Next, 10 mL of organic phase was added dropwise to the aqueous phase (about 10 drops per min), under magnetic stirring. Following that, the contents were stirred on a magnetic stirrer at room temperature for 20 min, until a yellow color was obtained. Eventually, this solution was placed in a rotary evaporator in order to ensure the complete removal of acetone. The colloidal solution was stored in a brown bottle. The total concentration of the BC-NPs solution was 2.80 × 10⁻⁵ mol L⁻¹. It was found that the prepared beta carotene has a storage time of over 4 weeks when kept at 4 °C.

Recommended procedure

An aliquot of BC-NPs solution, 3 mL of phosphate buffer (pH 7), 1 mL of 1 mol L⁻¹ of NaCl and 7 mL of Triton X-100 5% (v/v) as the nonionic surfactant were added to a 50 mL volumetric flask and diluted to the mark with water. In the case of food or fruit samples 500 mg L⁻¹ of EDTA is added as masking agent for cations. The solution was mixed and transferred to a 50 mL conical centrifuge tube and incubated in a thermostat water bath at 80 °C for 40 min. Subsequently, the solution was cooled in an ice-bath for 5 min and then the aqueous phase was decanted. The enriched surfactant phase was placed in a 2 mL volumetric flask and made up to the mark with deionized water. The absorbance of the solution was measured by UV-vis spectrophotometer at 468 nm. A blank solution was also run under the same conditions without adding any BC-NPs. The steps involved are presented in Fig. 1.

Fig. 1 Schematic presentation of the CPE process involved in the enrichment of BC-NPs into surfactant during.

Sample preparation

Fruit samples (watermelon and tomato) were purchased from a local market in Ahvaz, Iran. Non-meat constituents or inedible parts of the fruits e.g. skin, tendons and fruit stones were removed. The collected samples were cleaned, washed thoroughly with deionized water and let to dry in air. An amount of 1 kg of each sample was sliced and homogenized in a blender. Then, 20 g portion of this sample was weighed and placed in a tube and centrifuged to separate the supernatant. After that, the upper portion of the centrifuged solution was separated and diluted to 50 mL with de-ionized water. This solution was filtered through a filter paper, the pH of the solution was adjusted to 7 and transferred to a 500 mL volumetric flask and made up to the mark with water (resulting solution). Then known quantity of BC-NPs solution (which was within the linear range of calibration graph) was spiked to an aliquot of the resulting solution and subjected to the recommended procedure.

Cherry and orange juices were purchased from Zarrin Jam Marina Company Kaveh Industrial City (Saveh, Iran). 5 mL of each of the fruit juice was placed in a tube and centrifuged. Then, the supernatant solution was filtered through a filter paper, the pH was adjusted to 7 and diluted to 500 mL with deionized water. Known quantity of BC-NPs solution (which was within the linear range of calibration graph) was spiked to an aliquot of the resulting solution and the total BC-NPs solution was determined by applying the recommended procedure.

Results and discussion

Cloud point extraction procedure was applied to separate and pre-concentrate the BC-NPs from aqueous solution. The BC-NPs interact with nonionic surfactant (NIS) and separation of organic phase including the BC-NPs-NIS and the aqueous phase occurs after incubation of the solution.

Fig. 2 illustrates the UV-vis absorption spectra of and the corresponding TEM images of the BC-NPs before and after CPE. Comparing the TEM images (Fig. 2) of BC-NPs (A) before and (B) after CPE show that BC-NPs have been transferred and enriched into Triton X-100. The UV-vis spectra (Fig. 2C and D) indicate that λ_max of the enriched BC-NPs in Triton X-100 is at 468 nm and there is no significant change in the absorption spectra before and after CPE. This suggests lack of any decomposition of BC-NPs upon heating during CPE experiments and stability of the NPs in the enriched surfactant phase. Also, the disappearance of the absorption peak at 468 nm for the dilute surfactant-evacuated phase (Fig. 2D, b) shows that most of the BC-NPs are transferred into the enriched surfactant phase (Fig. 2D, a).

Fig. 2 TEM image of BC-NPs (A) before, and (B) after CPE, (C) UV-vis spectra of BC-NPs before CPE (concentration of 1.40 × 10⁻⁵ mol L⁻¹) and (D, a) enriched phase after CPE (initial concentration of 7.0 × 10⁻⁷ mol L⁻¹) (D, b) dilute surfactant-evacuated phase after CPE.

Effect of pH

It is known that for organic molecules, the pH of the sample solution is a critical factor for regulating the partitioning of the target analyte in the micellar phase. Maximum extraction efficiency is achieved at pH values where the uncharged form of the target analyte prevails.³⁸ Hence, the effect of pH on the CPE of BC-NPs was studied in the range of 4–9. The sample solutions were adjusted to the desired pH using 0.1–1.0 mol L⁻¹ of hydrochloric acid and/or sodium hydroxide. The result of this investigation demonstrated in Fig. 3, indicate that the highly effective preconcentration of BC-NPs is achieved at pH 7. The extraction efficiency was decreased below and above pH 7. Although the influence of pH on the extraction efficiency is not normally critical for the neutral or non-ionized analytes in most CPE procedures, a few exceptions have also been reported.^39,40 Since the BC-NPs are also synthesized at pH 7, it is possible that they are more stable in this pH.¹⁷

Fig. 3 Influence of pH on the cloud point extraction of 7.0 × 10⁻⁷ mol L⁻¹ of BC-NPs.

In order to stabilize the pH of the solution at 7, different buffer solutions with pH 7 such as carbonate, phosphate and Britton–Robinson were examined. Among them phosphate buffer was more suitable because it did not have a significant effect on the absorbance value of micellar phase and 3 mL of the phosphate buffer was adequate for maintaining pH at 7.

Effect of electrolyte

The effect of salt on the micellar aqueous solution has been verified in the previous studies. It has been shown that the addition of salts can affect the cloud point in the micellar solutions (salting-out effects)³⁸ leading to an increase in the extraction efficiency of nano materials. In order to investigate the effect of the electrolytes in this work, four salts NaCl, NaNO₃, KCl and KNO₃ were chosen and investigated. Different concentrations (0.01–0.05 mol L⁻¹) of aforesaid salts were added to the system. The results showed that no significant difference was distinguished upon examination of various salts, but NaCl slightly enhanced the phase separation. Thus, according to the obtained data, concentration of 2 × 10⁻² mol L⁻¹ of NaCl was selected as optimum in further experiments.

Surfactant selection and influence of its concentration

The choice of a suitable surfactant is a necessary parameter for a successful CPE analysis. Triton X-100 is a widely used non-ionic surfactant due to its commercial availability, low cost and low toxicological properties as well as producing a high-density surfactant-rich phase with low water content in the surfactant-rich phase. Furthermore in this study we found that a better phase separation occurs when using Triton X-100, thus it was selected as the non-ionic surfactant. Selecting an appropriate concentration of surfactant is also a critical parameter which enhances the efficiency of CPE. Therefore, Triton X-100 concentration was optimized by adding 3–8 mL of 5% (v/v) Triton X-100 to the sample solutions. The experiments showed that the extraction of BC-NPs increased by using up to 6 mL of 5% (v/v) Triton X-100 and remained constant above that (Fig. 4). At lower concentrations, the extraction efficiency was low, probability due to the inadequacy in entrapment of BC-NPs. On the other hand an increase in the Triton X-100 concentration enhanced the surfactant-rich phase volume and thus decreased the enrichment factor. Therefore, 7 mL of 5% (v/v) Triton X-100 was adopted for the subsequent experiments.

Fig. 4 Influence of the volume of Triton X-100 (5% v/v) on cloud point extraction of 7.0 × 10⁻⁷ mol L⁻¹ of BC-NPs.

Effect of equilibrium temperature and time

Incubation temperature and time have important and additional roles in a desirable CPE methodology. Using high temperatures cause the dehydration of the micelles, and hence increase the efficiency of the method.^38,40 Thus, the use of the adequate temperature and time is essential for completion of extraction and two phase separation. In the following studies, the effect of equilibration temperature and incubation time on CPE of BC-NPs was evaluated in the range of 70–85 °C and 30–55 min, respectively. The results indicated that the maximum analytical signal was obtained at 80 °C for 40 min. The results also showed that the standing time of surfactant enriched phase in the ice bath has no significant effect on the extraction efficiency. Thus, 5 min was chosen as the standing time in ice bath for all experiments.

Analytical performance

The developed analytical method was validated under the optimum experimental conditions. There was a linear relationship between absorbance (at 468 nm) and BC-NPs concentration in the range of 2.80 × 10⁻⁸ to 2.52 × 10⁻⁶ mol L⁻¹ with a correlation coefficient (r) of 0.9978. The linear equation was A = 397 430C + 0.0011, where C is the BC-NPs concentration (mol L⁻¹). The limit of detection (LOD) defined as LOD = 3S_b/m and the limit of quantification as LOQ = 10S_b/m where S_b is the standard deviation of the blank measurements (n = 8) and m is the slop of the calibration curve⁴¹ was found to be 1.12 × 10⁻⁸ mol L⁻¹ and 3.74 × 10⁻⁸ mol L⁻¹, respectively. The relative standard deviation (RSD) for eight replicate measurements of 1.68 × 10⁻⁶ mg L⁻¹ and 2.80 × 10⁻⁷ mol L⁻¹ of BC-NPs was calculated to be 3.2% and 5.6%, respectively.

Interferences study

In order to investigate the selectivity of the method for determination BC-NPs, the effect of some common compounds presents in fruit and juice samples, anions and cations were examined using a constant concentration of BC-NPs (7.0 × 10⁻⁷ mol L⁻¹) (Table 1). An ion or compound was suggested to interfere significantly when it showed an error in BC-NPs measurement more than ±5%. In this work, because of the presence of phosphate ion (used in the synthesis BC-NPs and as a buffer in the procedure), some of the metal ions such as Cu²⁺, Ca²⁺, Zn²⁺, Mg²⁺ when present in the samples could react with phosphate ion and form precipitates which interfere in the phase separation of the rich phase. Therefore, several complexing agents such as tartrate, F⁻, EDTA and citrate (at concentration of 500 mg L⁻¹) were examined and EDTA ligand was found to be more suitable for masking these metal ions. It should also be mentioned that BC-NPs extraction efficiency did not differ in the presence of EDTA alone (even at high concentrations). Therefore 500 mg L⁻¹ EDTA was added to the sample solution as masking agent for above mentioned ions. The results of this study demonstrate that the common coexisting ions did not interfere seriously with preconcentration of BC-NPs.

Table 1 Effect of interfering species on the determination of 7.0 × 10⁻⁷ mol L⁻¹ of BC-NPs

Interfering species	Concentration (mg L^−1)
a After addition of 500 mg L^−1 of EDTA.
K^+, Cl^−, SO3^2−, F^−, CO3^2−, C2O4^2, NO3^−	1000
Ascorbic acid, glucose, cystein, citrate, tartarate	—
Sn^2+, Fe^3+, Al^3+, (Ca^2+, Cu^2+, Zn^2+, Mg^2+)^a	100
Cd^2+, Co^2+, Mn^2+	30

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Application to food and juice samples

In order to evaluate the analytical applicability of the method, the recovery experiment was accomplished. Under optimized conditions, a known amount of BC-NPs was spiked to fruit and juice samples (prepared as explained in sample preparation section) and the total amount of the BC-NPs was determined. The results summarized in Table 2, clearly indicate that this preconcentration method was successful for the analysis of BC-NPs in these samples with mean recovery of 96% to 104%.

Table 2 Determination of BC-NPs in real samples

Sample	BC-NPs added (mol L^−1)	BC-NPs found^a (mol L^−1)	Recovery (%)
a x ± ts/√n at 95% confidence (n = 5).
Watermelon	5.60 × 10^−7	(5.43 ± 0.04) × 10^−7	97
	1.68 × 10^−6	(1.61 ± 0.09) × 10^−6	96
Tomato	5.60 × 10^−7	(5.71 ± 0.04) × 10^−7	102
	1.68 × 10^−6	(1.75 ± 0.07) × 10^−6	104
Cherry juice	5.60 × 10^−7	(5.60 ± 0.02) × 10^−7	100
	1.68 × 10^−6	(1.66 ± 0.03) × 10^−6	99
Orange juice	5.60 × 10^−7	(5.66 ± 0.02) × 10^−7	101
	1.68 × 10^−6	(1.65 ± 0.05) × 10^−6	98

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Conclusions

To the best of our knowledge, this is the first report on the preconcentration and colorimetric determination of BC-NPs. The new approach has proved to be an efficient and green route for extraction of BC-NPs from different samples. The proposed method is simple, inexpensive, environmental friendly and can be utilized for BC-NPs detection in various samples without any need for sophisticated instrumentation. The results demonstrated in Table 2, shows that the method has a great potential for successful determination of BC-NPs in the fruit and juice samples.

Acknowledgements

The authors are sincerely grateful to Shahid Chamran University, Ahvaz, Iran, for the financial support of this project (Grant 1394). The financial support of the Iranian Nanotechnology Initiative Council is also greatly appreciated.

References

1. M. J. Saiz-Abajo, C. Gonzalez-Ferrero, A. Moreno-Ruiz, A. Romo-Hualde and C. J. Gonzez-Navarro, Food Chem., 2013, 138, 1581.
2. H. D. Silva, M. A. Cerqueira, B. W. S. Souza, C. Ribeiro, M. C. Avides, M. A. C. Quintas, J. S. R. Coimbra, M. G. Carneiro-da-Cunha and A. A. Vicente, J. Food Eng., 2011, 102, 130.
3. H. S. Ribeiro, B. S. Chu, S. Ichikaw and M. Nakajim, Food Hydrocolloids, 2008, 22, 12.
4. Y. Yuan, Y. Gao, J. Zhao and L. Mao, Food Res. Int., 2008, 41, 61.
5. Y. Yuan, Y. Gao, L. Mao and J. Zhao, Food Chem., 2008, 107, 1300.
6. L. Chen, G. Bai, R. Yang, J. Zang, T. Zhou and G. Zhao, Food Chem., 2014, 149, 307.
7. X. Pan, P. Yao and M. Jiang, J. Colloid Interface Sci., 2007, 315, 456.
8. T. Helgason, T. S. Awad, K. Kristbergsson, E. A. Decker, D. J. Mcclements and J. Weiss, J. Agric. Food Chem., 2009, 57, 8033.
9. W. A. Pryor, W. Stahl and C. L. Rock, Nutr. Rev., 2000, 58, 39.
10. J. Yi, Y. Li, F. Zhong and W. Yokoyama, Food Hydrocolloids, 2014, 35, 19.
11. V. A. Kirsh, R. B. Hayes, S. T. Mayne, N. Chatterjee, A. F. Subar, L. B. Dixon, D. Albanes, G. L. Andriole, D. A. Urban and U. Peters, J. Natl. Cancer Inst., 2006, 98, 245.
12. K. A. Lawson, M. E. Wright, A. Subar, T. Mouw, A. Hollenbeck, A. Schatzkin and M. F. Leitzmann, J. Natl. Cancer Inst., 2007, 99, 754.
13. G. Bjelakovic, D. Nikolova, L. L. Gluud, R. G. Simonetti and C. Gluud, J. Am. Med. Assoc., 2007, 297, 842.
14. S. Hercberg, P. Galan, P. Preziosi, S. Bertrais, L. Mennen, D. Malvy, A. M. Roussel, A. Favier and S. Briançon, J. Am. Med. Assoc., 2004, 164, 2335.
15. U. C. Obermuller-Jevic, P. I. Francz, J. Frank, A. Flaccus and H. K. Biesalski, FEBS Lett., 1999, 460, 212.
16. H. K. Biesalski and U. C. Obermueller-Jevic, Arch. Biochem. Biophys., 2001, 389, 1.
17. L. J. Yin, B. S. Chu, I. Kobayashi and M. Nakajima, Food Hydrocolloids, 2009, 23, 1617.
18. E. de Paz, Á. Martín, A. Estrell, S. Rodríguez-Roj, A. A. Matia, C. M. M. Duart and M. J. Cocero, Food Hydrocolloids, 2012, 26, 17.
19. G. V. L. Gomes, I. A. S. Simplício, E. B. Souto, L. P. Cardoso and S. C. Pinho, Food Technol. Biotechnol., 2013, 51, 383.
20. A. Malaki Nik, S. Langmaid and A. J. Wright, J. Agric. Food Chem., 2012, 60, 4126.
21. C. Qian, E. A. Decker, H. Xiao and D. J. McClements, Food Chem., 2012, 132, 1221.
22. N. Anantachoke, M. Makha, C. L. Raston, V. Reutrakul, N. C. Smith and M. Saunders, J. Am. Chem. Soc., 2006, 128, 13847.
23. K. A. Lawson, M. E. Wright, A. Subar, T. Mouw, A. Hollenbeck, A. Schatzkin and M. F. Leitzmann, J. Natl. Cancer Inst., 2007, 99, 754.
24. R. M. Gonz-alez-Reza, D. Quintanar-Guerrero, J. J. Flores-Minutti, E. Gutierrez-Cortez and M. L. Zambrano-Zaragoza, LWT--Food Sci. Technol., 2015, 60, 124.
25. S. Xiea, M. C. Paaua, C. F. Li, D. Xiaob and M. F. Choia, J. Chromatogr. A, 2010, 1217, 2306.
26. E. Caballero-Díaz, B. Simonet and M. Valcárcel, Anal. Methods, 2013, 5, 3864.
27. N. Pourreza and T. Naghdi, Talanta, 2014, 128, 164.
28. J. Chao, J. Liu, S. Yu, Y. Feng, Z. Tan, R. Liu and Y. Yin, Anal. Chem., 2011, 83, 6875.
29. N. Pourreza and M. Ghomi, Talanta, 2011, 84, 240.
30. N. Pourreza, M. R. Fat'hi and A. Hatami, J. AOAC Int., 2014, 97, 1225.
31. G. Hartmann, C. Hutterer and M. Schuster, J. Anal. At. Spectrom., 2013, 28, 567.
32. J. Liu, R. Liu, Y. Yin and G. Jiang, Chem. Commun., 2009, 1514.
33. N. Pourreza and H. Golmohammadi, Anal. Methods, 2014, 6, 2150.
34. I. Brabcova, M. Hlavacková, D. Satínsky and P. Solich, Food Chem., 2013, 141, 1433.
35. G. Ziyatdinov, E. Ziganshina and H. Budnikov, Talanta, 2012, 99, 1024.
36. S. Luterottia, M. Frankob and D. Bicanic, J. Pharm. Biomed. Anal., 1999, 901.
37. V. Andres, M. J. Villanuev and M. D. Tenorio, LWT--Food Sci. Technol., 2014, 58, 557.
38. E. K. Paleologos, D. L. Giokas and M. I. Karayannis, Trends Anal. Chem., 2005, 24, 426.
39. G. F. Jia, C. G. Lv, W. T. Zhu, J. Qiu, X. Q. Wang and Z. Q. Zhou, J. Hazard. Mater., 2008, 159, 300.
40. S. Xie, M. C. Paau, Ch. F. Li, D. Xiao and M. M. F. Choi, J. Chromatogr. A, 2010, 1217, 2306.
41. J. D. Ingles and S. R. Crouch, Spectrochemical Analysis, Prentice Hall, Inc., New Jersey, USA, 1988, pp. 173–176.

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