Analytical methods and recent developments in the detection of melamine
Introduction
The detection of melamine (2, 4, 6-triamino-s-triazin, MEL) in food products has been the subject of much recent research. MEL is a synthetic compound, commonly used as an industrial chemical in the production of MEL-formaldehyde polymer resins for laminates, coatings, commercial filters, glues or adhesives, and plastics and flame-retardants [1]. Because it contains 66% nitrogen, MEL was added to cattle feed as a non-protein nitrogen (NPN) source in 1958, but its use was discontinued in 1978, because it is incompletely hydrolyzed in ruminants [2].
However, because the Kjeldahl method that measures total nitrogen content as an indication of protein levels is non-scientific, unethical manufacturers deliberately added MEL to food and food-related products (e.g., milk, infant formula, frozen yogurt, biscuits, candy, coffee drinks, and pet food) to boost the nitrogen level and to reduce costs [3], [4]. Although MEL has low toxicity, it may lead to kidney stones, eventual renal failure, and ultimately death, when it forms an insoluble compound (see Fig. 1) with the analog cyanuric acid (CYA) [5], [6], [7]. Pathogenesis may be through formation of high-molecular-weight network complexes by self-association of MEL-CYA, which has poor aqueous solubility and precipitates in renal tubules, causing urinary system damage and ultimately death [8], [9].
In 2004, an outbreak of food adulteration with MEL led to renal failure in dogs and cats in Asia [10]. In the spring of 2007, pet food adulterated with MEL was blamed for the illness or death of thousands of dogs and cats in the USA [11].
In September 2008, the occurrence of kidney stones in thousands of infants across China captured the attention of the world. According to a report from the Ministry of Health (MOH), in the People’s Republic of China, more than 54,000 infants and young children were hospitalized, and at least six children died in this incident [4], [12]. An investigation revealed that the illnesses resulted directly from consumption of milk, infant formula, or related dairy products adulterated with MEL, so rapid, widely available, cost-effective methods for detecting MEL in various substances are of paramount importance.
In this review, we present an overview of the advanced analytical methods for measuring MEL and analog contaminants in various foods. We try to be comprehensive and highlight new developments, and discuss theoretical and technical aspects of methods for MEL-contaminant screening and confirmation. We also preview trends and new detection methods based on new materials.
Section snippets
Sample preparation
Food samples are typically complex matrices that are difficult to analyze because of the abundance of proteins and carbohydrates. Effective isolation and extraction of MEL and analogs from complex matrices is necessary prior to MEL determination. The main objectives of sample treatment, including extraction, preconcentration, and derivatization, are to achieve lower limits of detection (LODs) by removing matrix constituents that may affect detection or enrichment of analytes [13]. However,
Background contamination and analysis of melamine
In 2007, MEL was found in pet-food products, and led to kidney toxicity in dogs and cats in the USA. Later, MEL contamination was found in milk-based products in China. Because China is a major exporter of milk products and ingredients, the events created a widespread food-safety scare. Reports of MEL-contaminated foods manufactured in the USA and other countries occurred in subsequent months [26].
These MEL-contamination incidents prompted the US Food and Drug Administration (FDA), the European
Modern instrument analytical methods
To date, CE, HPLC, LC-MS, LC-MS2, GC-MS, MALDI-MS, NMR spectroscopy and vibrational spectroscopy techniques have been the most important published methods.
Immunoassay
Immunoassays and related immunochemical analytical procedures [e.g., ELISA, immunochromatographic assay (ICA), and fluorescence polarization immunoassay (FPIA)] have been widely used to detect various residues in foods and environment. However, ICA and FPIA have not been reported so far for MEL.
The use of ELISA for the determination of MEL has been reported in only a few papers. A comparison was made between three commercial ELISA test kits [i.e. Abraxis Melamine Plate kit (5005B), Abraxis
Novel detection methods
Recently, sensor technology has been widely used for the detection of residues in foods because of its sensitivity, rapidity, simplicity and cost effectiveness. Sensors have also been developed for the analysis of MEL and its analogs including MIP-based and NP-based sensors.
Conclusion and trends
The recent reports summarized in this review show that many methods have been developed for detecting MEL residues in various matrices. The different methods each have their advantages, disadvantages, and requirements for sample preprocessing of complex samples.
HPLC will continue to be widely used for MEL detection, and it will certainly be combined with other techniques.
GC-MS is an important and ideal method for detecting trace samples. GC-MS generally purifies and resolves samples, and
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2022, Ecotoxicology and Environmental SafetyCitation Excerpt :Melamine (MEL, 2,4,6-triamino-1,3,5-triazine), a synthetically produced chemical, is widely used for the manufacture of plastics, laminates, glues, fertilizers, kitchenware, adhesives and other products (Sun et al., 2010).