Microextraction of melamine from dairy products by thymol-nonanoic acid deep eutectic solvent for high-performance liquid chromatography-ultraviolet determination

doi:10.1016/j.jfca.2022.105083

Journal of Food Composition and Analysis

Volume 116, March 2023, 105083

https://doi.org/10.1016/j.jfca.2022.105083 Get rights and content

Highlights

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First DES-based microextraction procedure for the determination of melamine.
•
Green DES for separation of melamine from dairy products.
•
Role of DES precursors in melamine separation was studied.
•
Simple and fast sample pretreatment for the determination of melamine in dairy products.
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No derivatization of melamine for its extraction from aqueous phase.

Abstract

In this work, it was shown for the first time that deep eutectic solvents provide effective liquid-phase microextraction of melamine from dairy products. In accordance with the developed procedure, the milk sample is mixed with salt (sodium sulfate) to precipitate proteins and, after that, melamine is extracted into the phase of a hydrophobic eutectic solution based on thymol and nonanoic acid. After phase separation, the analyte is detected in the eutectic solvent phase. In the developed analytical procedure, a medium-chain fatty acid as a precursor of the extraction solvent promotes mass-transfer of melamine due to strong interactions between carboxyl and amino groups. The terpenoid as a precursor of the extraction solvent has a synergistic effect in melamine extraction due to possible π–π interaction. The estimated phenomenon was applied to the effective separation of highly hydrophilic analyte from aqueous sample phase followed by its determination by high-performance liquid chromatography. Under optimal conditions, a range of determined concentration was from 0.1 to 30 mg L⁻¹ with a limit of detection of 0.03 mg L⁻¹. The procedure is fast (7 min), miniaturized (100 μL of the extraction solvent) and repeatable (RSD % < 5 %). The developed procedure was optimized and used for the determination of melamine in real samples of powdered and whole milk.

Graphical Abstract

Introduction

Milk and dairy products are among the most common food products. They are rich in various useful organic (fats, proteins, carbohydrates, vitamins) and inorganic (calcium, magnesium, iron) substances necessary for the normal functioning of the body (Nemati et al., 2022). Thousands of tons of milk-based products are produced every day. Such a wide distribution of this food product has led to the need for strict control of its quality, in order to determine both useful and prohibited substances in it. Among prohibited substances are additives that lead to a distortion of the real properties of the milk products. To increase the proteins content in dairy products some dairy producers can illegally add melamine (1.3.5-triazin-2.4.6-triamino-1), (ESM Fig. 1) in milk. Melamine is a cheap and available additive with a high nitrogen content (67 %). It is known, that the Kjeldahl method is almost always used for the protein’s determination in dairy products (Gao et al., 2015). The Kjeldahl method is the industry standard test based on the proteins digestion to obtain ammonium ions followed by their distillation and determination by acid-base titration (Wojciechowski and Barbano, 2015). Melamine like proteins is digested into ammonium ions during dairy product pretreatment by the Kjeldahl method. Thus, the nitrogen from nonprotein additives or contaminants in the food, such as melamine, is also measured. Meanwhile the World Health Organization (2008) has imposed restrictions on the addition of melamine to food (Altunay et al., 2020) because melamine can cause various diseases including cancer and uric acid stone accumulation (Wang et al., 2017). The tolerable daily intake for melamine according to the World Health Organization is 2.5 mg kg⁻¹ body weight per day. Potentially melamine can be consumed daily with dairy products and foods prepared with the use of milk falsified with additives. Thus, despite the prohibition on this additive, there is a need for strict control of its content in milk and dairy products.

Various analytical procedures based on high-performance liquid chromatography with UV-detection (HPLC-UV) (Akbari-adergani et al., 2017), liquid chromatography-electrospray tandem mass spectrometry (Ibáñez et al., 2009), gas chromatography mass spectrometry (Li et al., 2009), UV-Visible spectrophotometry (Hashemi and Nazari, 2018), flame atomic absorption spectrometry (melamine as complex with Mn(II) and Eosin Y) (Gürkan and Kartal Temel, 2020) and capillary electrophoresis with ultraviolet UV-detection (Zhang et al., 2016) have been developed for the determination of melamine in milk and dairy products (Table 1). Milk and dairy products have high protein and fat contents, and they can interfere with the melamine determination. Therefore, sample pretreatment procedures including defatting, protein precipitation and finally melamine separation and preconcentration are typically used in analysis of milk and dairy products (Ritota and Manzi, 2018). Different separation and preconcentration procedures based on solid-phase (SPE) (Sun et al., 2010) and liquid-phase (LPE) extraction (Table 1) are available for the melamine determination.

For SPE of melamine various molecularly imprinted polymers (Anirudhan et al., 2017), hollow fibers (Li et al., 2009), SPE cartridges (Sun et al., 2010) have been developed. These procedures are time-consuming due to slow sorption/desorption kinetic and require significant amounts of organic solvents (methanol, acetonitrile) as eluents.

Liquid-phase microextraction (LPME) has been recognized as a green and miniaturized separation and preconcentration approach for the pretreatment of complex matrices due to fast mass-transfer kinetic and less organic solvents consumption. Meanwhile, for the LPME of melamine the extraction solvents set is limited due to high hydrophilicity of analyte. Ionic liquids ([C₆mim][BF₄] (Altunay et al., 2020) and ([C₆mim][Tf₂N] (Altunay et al., 2017) and Triton X-114 (Gürkan and Kartal Temel, 2020) have been applied to the LPME of melamine based on analyte complexes formation with metal cations (Fe(II), Mn(II)) or anionic surfactant. However, it is a fact that the high cost of ionic liquids limits their application in analytical laboratories. The limitation of the Triton X-114-based procedure is high viscosity of the surfactant-rich phase, which is unacceptable for direct injection into liquid or gas chromatography equipment. The search for new effective extraction systems has interest in the determination of melamine in milk and dairy products.

To solve this problem, new, environmentally friendly solvents – deep eutectic solvents (Aydin et al., 2018, Bazzaz Dilmaghani et al., 2022, Kanberoglu et al., 2019a) can be effectively used. Recently, deep eutectic solvents have been increasingly used in chemical analysis (Alimoradi et al., 2022, Soylak and Koksal, 2019) (Shishov et al., 2021), including food products (Elik et al., 2022, Kanberoglu et al., 2019b; Shishov et al., 2020, Shishov et al., 2020) and, in particular, dairy products (Nemati et al., 2022).

Deep eutectic solvents are a mixture of a hydrogen bond donor and acceptor, the formation of which bond leads to a significant decrease in the melting temperature of such a mixture (Zhang et al., 2023). Initially, these solvents were made from water-soluble compounds such as choline chloride (Shishov et al., 2020) or tetrabutylammonium bromide. However, such eutectic solvents cannot be used in the analysis of aqueous samples, since they are decomposed upon contact with water (Shishov et al., 2022). Therefore, lately, deep eutectic solvents based on natural terpenoids such as thymol and menthol (Sportiello et al., 2023) have been widely used. They provide water stability and efficient recovery of various analytes. However, these solvents have not yet been used to extract melamine from food products, which is done for the first time in this work.

In this work, it was established that deep eutectic solvents (DESs) based on terpenoids and medium-chain fatty acids (Lanjwani et al., 2023) provide effective separation of melamine from aqueous phase. To the best of our knowledge, the application of DES-based extraction systems for melamine microextraction has not been presented in literature.

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Section snippets

Reagents and materials

All chemicals used in this work were of analytical reagent grade and were used without further purification. Ultra-pure water from Millipore Milli-Q RG system (Millipore, USA) was used throughout the work. Thymol, menthol, hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, melamine, methanol, hydrochloric acid, formic acid (85 %), sodium hydroxide, sulfate, nitrate, acetate, hydrophosphate and dihydrophosphate were purchased from Vekton, Russia. To prepare a melamine

Primary studies

Melamine (ESM Fig. 1) is a highly water-soluble compound and has limited solubility in organic solvents used in LPME. At the same time, melamine has a system of conjugated bonds, which can interact by Van der Waals forces with terpenoids. Moreover, amino groups of melamine can interact with fatty acids by an acid-base mechanism and with the hydroxyl group of the terpenoid, since it is also capable of exhibiting acidic properties. Therefore, the hydrophobic DESs based on natural terpenoids such

Conclusions

It was shown that DESs based on terpenoids and medium-chain fatty acids provide effective separation of highly hydrophilic melamine from aqueous phase without additional analyte derivatization. The established phenomenon was used in the development of fast and simple microextraction procedure for the determination of melamine in dairy products by HPLC-UV. It is first DES-based microextraction procedure in literature for the determination of melamine. Compared to the procedures proposed in

CRediT authorship contribution statement

Andrey Shishov: Conceptualization. Egor Nizov: Investigation. Andrey Bulatov: Writing – original draft.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Andrey Shishov gratefully acknowledges financial support of Russian Science Foundation (research projects No. 22–73-10039). Scientific research was performed using the equipment of the Research Park of St. Petersburg State University (Chemical Analysis and Materials Research Centre).

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Microextraction of melamine from dairy products by thymol-nonanoic acid deep eutectic solvent for high-performance liquid chromatography-ultraviolet determination

Highlights

Abstract

Graphical Abstract

Introduction

Access through your organization

Section snippets

Reagents and materials

Primary studies

Conclusions

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgements

LWT

Food Chem.

Food Chem.

Microchem. J.

J. Dairy Sci.

J. Food Compos. Anal.

Anal. Chim. Acta

J. Mol. Liq.

Microchem. J.

Food Chem.

J. Chromatogr. A

J. Food Compos. Anal.

Food Control

Talanta

J. Mol. Liq.

J. Chromatogr. A

Microchem. J.

Anal. Chim. Acta

J. Mol. Liq.