Melamine powder, a white monoclinic prismatic crystal with chemical stability under normal conditions, is a key chemical raw material widely used in industrial production. However, its illegal addition to food will pose a potential threat to human health. As melamine is not allowed to be added to food, establishing accurate and efficient quantitative analysis methods has become the core of food safety testing and industrial quality control.
This article will detail the common melamine quantitative analysis methods, their principles, operation characteristics, and the latest research progress, providing a comprehensive reference for relevant testing personnel and researchers.
To accurately detect melamine, it is necessary to first master its basic physical and chemical properties, which lay the foundation for selecting and optimizing detection methods. Melamine is odorless, with a density of 1.573g/cm³ (16℃) and an atmospheric melting point of 354℃ (decomposition); it sublimes rapidly at 300℃. In terms of solubility, it is soluble in hot water, slightly soluble in cold water, extremely slightly soluble in hot ethanol, and insoluble in ether, benzene, and carbon tetrachloride, but can be dissolved in methanol, formaldehyde, acetic acid, and other solvents.
Melamine is weakly alkaline (pKb=8), can form melamine salts with acids, and undergoes condensation and polycondensation reactions with formaldehyde under different pH conditions to form resin products. In a strong acid or strong alkaline aqueous solution, it will hydrolyze stepwise, with the amino group replaced by a hydroxyl group, finally generating cyanuric acid. In addition, melamine decomposes at high temperatures, releasing cyanide and nitrogen, which is also why it can be used as a flame retardant. These properties determine the extraction conditions, reaction systems, and separation methods in the detection process.
The picric acid method (GB9567-1997) is a common method for determining the content of industrial melamine. Its core principle is to form an insoluble melamine picrate precipitate by the reaction of melamine and picric acid, and calculate the purity of melamine by weighing the mass of the precipitate. The operation steps include sample dissolution, acid adjustment, filtration, constant-volume addition, low-temperature standing, filtration and washing, and drying and weighing.
The sublimation method is another traditional method for determining melamine purity. It uses the characteristic of melamine’s rapid sublimation at high temperature. The sample is heated under negative pressure in a sublimation device until complete sublimation, and the purity is determined by weighing the residual mass after sublimation. The key steps are sample weighing, vacuum sealing, temperature-controlled sublimation, cooling, and weighing.
Both methods are the classic detection technologies for industrial melamine, but they have obvious limitations: the operation process is complex, the analysis time is long (about 6~8h), and the detection results cannot be fed back in time, which is not suitable for the rapid detection of food samples with high timeliness requirements.
The potentiometric titration method is an improved, rapid detection method for melamine in solution. The method takes advantage of melamine’s weak alkalinity, uses a sulfuric acid standard solution to titrate the melamine solution, and records the pH change during titration to determine the equivalence point and calculate the melamine content.
The specific operation is to dissolve the sample by heating, titrate the hot solution to pH 5 with sulfuric acid standard solution, cool to room temperature, continue titration, add 0.1mL sulfuric acid standard solution each time, and record the accurate pH value until the pH is about 3. This method has the advantages of simple operation, fast detection, and high accuracy, with a relative standard deviation (RSD) of no more than 0.50%, fully meeting the needs of process control analysis in industrial production.
Gas chromatography-mass spectrometry (GC-MS) is a key method for quantitative detection of melamine in animal food, offering high sensitivity and strong selectivity. The core process includes sample extraction, derivatization, and instrumental detection: use a methanol/water/triethylamine mixed solution to extract melamine from animal food, blow-dry with nitrogen for silanization-derivatization, then detect with gas chromatography-mass spectrometry, and quantify using a melamine phenyl derivative internal standard method.
The experimental data show that the spiked recovery rate of this method in feed and animal food is 82.0%~105.6%, the RSD is not more than 5.8%, and it shows a good linear relationship over 0.1~50.0 mg/L, with a minimum detection limit of 0.1 μg/g. It can effectively eliminate interference from complex matrices in animal feed and is an effective confirmation method for detecting melamine and related organic compounds in common samples.
High-performance liquid chromatography (HPLC) is widely used for detecting melamine in plant raw materials and pet food, and the method is continually optimized for different sample matrices.
For plant raw materials, the sample is extracted by vortex mixing and ultrasonication with water, and the protein is precipitated and purified by adding potassium ferricyanide and zinc acetate. The Sunfire C18 chromatographic column (4.6×250mm, 5 μm) is used; the mobile phase is a water-ammonia-perchloric acid buffer solution, and the detection wavelength is 240nm, with a detection limit of 0.5mg/kg. In the range of 1.0~4.0mg/kg spiked addition, the recovery rate is 70.2%~80.2%, and RSD is less than 10%.
For pet food, the combination of ultrasonic extraction and HPLC greatly shortens the detection time, with extraction time nearly 3.5h shorter than that of nuclear magnetic oscillation extraction, thereby improving detection efficiency while ensuring accuracy.
Liquid chromatography-tandem mass spectrometry is a high-sensitivity analytical method for detecting melamine in beverages and food additives. The method uses Kromasil C18 column, with the mobile phase of acetonitrile-formic acid aqueous solution (volume ratio 5:95), adopts electrospray mass spectrometry in positive ion mode for detection, takes the quasi-molecular ion m/z127 obtained by primary mass spectrometry as the parent ion for collision-induced dissociation (CID) secondary mass spectrometry (MS²) analysis, and uses the parent ion and MS² fragment ions for qualitative confirmation, and the chromatographic peak area of three ion masses for quantitative analysis.
The optimized method has a linear range of 0.01~0.5 mg/L, a high recovery rate (80%~99%), and higher sensitivity than single HPLC, making it suitable for the trace detection of melamine in liquid foods and food additives with low matrix complexity.
Given the characteristics of dairy products with high protein content and complex matrices, an ion-exchange chromatography-ultraviolet detection method has been developed and established for the quantitative analysis of melamine in dairy products. The sample is extracted with water and acetonitrile, separated on an LC-SCX ion-exchange chromatographic column using a mobile phase of 0.05 mol/L potassium dihydrogen phosphate solution (pH 3.0) and acetonitrile (70:30), and detected at 240nm.
This method has excellent detection performance: melamine shows a good linear relationship (r=0.9999) over the range of 0.5~100.0mg/L, the minimum detection limit is 1.0mg/kg, the spiked recovery rate is 93.8%~102.5%, and the RSD is low. It offers the advantages of simplicity, rapidity, and accuracy, and can fully meet the routine detection requirements for melamine in dairy products.
At present, chromatographic separation methods combined with spectral analysis, such as HPLC, HPLC-MS, GC-MS, and LC-MS, are the main methods for quantitative analysis of melamine. These methods have high accuracy and sensitivity and are suitable for the accurate detection of melamine in various food matrices and industrial samples, but they also have their own shortcomings: some methods take a long time to detect, some require complex sample pretreatment, and they place high demands on experimental equipment and operators.
With the increasing demand for food safety and industrial production efficiency, the development trend of melamine detection methods is toward short detection time, simple operation, low cost, and high sensitivity.
The selection of melamine quantitative analysis methods should be based on actual detection needs, including sample type, matrix complexity, detection limit requirements, and detection timeliness. Traditional methods are suitable for industrial melamine quality control with low timeliness requirements; chromatography-mass spectrometry coupling methods are suitable for accurate quantitative detection of melamine in food samples with complex matrices; new rapid detection methods are suitable for on-site preliminary screening and rapid detection of a large number of samples. With the continuous innovation of detection technology, melamine quantitative analysis will be more efficient, accurate, and convenient, providing a stronger technical guarantee for food safety and industrial production quality control.
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