Research into Heavy Metal Detection Techniques for Foodstuffs
Abstract: Heavy metal contamination represents one of the primary threats to food safety, potentially posing serious health risks when entering the human body via the food chain. Establishing efficient and accurate heavy metal detection techniques is therefore crucial for ensuring food safety. This paper provides a systematic review of current primary methods for detecting heavy metals in food, including atomic absorption spectroscopy, inductively coupled plasma mass spectrometry, and graphite furnace atomic emission spectroscopy. It also explores trends in combined detection techniques and emerging detection technologies, aiming to provide technical guidance for food safety testing. This work seeks to advance the innovation and optimisation of heavy metal detection methods, thereby better safeguarding public health.
The primary sources of heavy metal contamination in foodstuffs include the natural accumulation of heavy metals by plants and animals through environmental media such as soil and water during their growth processes. Owing to the non-biodegradability of heavy metal elements and their cumulative properties that intensify progressively along the food chain, a portion of the heavy metals accumulated in plants and animals is transferred to humans via edible parts. Given that heavy metal residues in food matrices typically exist at trace levels, detection and analytical techniques must possess ultra-trace detection capabilities. Currently, mainstream detection methods both domestically and internationally include ultraviolet spectrophotometry, atomic absorption/fluorescence/emission spectroscopy techniques, and inductively coupled plasma mass spectrometry. These methods each possess distinct advantages in terms of applicable scenarios, analytical throughput, and quantitative precision, forming a complementary technical matrix. Consequently, this study evaluates the characteristics, advantages, disadvantages, and applicable scenarios of multiple heavy metal detection technologies in foodstuffs through investigation and analysis, aiming to provide technical support for food safety regulation and quality control.
1. Traditional Heavy Metal Detection Techniques
1.1 Atomic Absorption Spectroscopy
Atomic Absorption Spectroscopy (AAS) is now extensively employed in food, pharmaceutical, and environmental monitoring sectors, establishing itself as a core detection methodology. This technique achieves quantitative elemental analysis by measuring the absorption intensity of characteristic spectral resonance lines when heavy metal ions are converted into ground-state atomic vapour. Depending on the specific operational method, AAS can be further subdivided into Flame Atomic Absorption Spectroscopy (FAAS), Graphite Furnace Atomic Absorption Spectroscopy (GFAAS), Hydride Generation Atomic Absorption Spectroscopy (HGAAS), and Cold Vapor Atomic Absorption Spectrometry (CVAAS). Among these, FAAS offers simplicity of operation and low cost, though with relatively lower sensitivity; GFAAS exhibits exceptionally high sensitivity, making it suitable for ultra-trace heavy metal detection, though it has a narrow analytical range and is time-consuming; HGAAS and CVAAS demonstrate outstanding performance in the precise determination of mercury, arsenic, selenium, and mercury, cadmium respectively. For instance, magnetic dispersion solid-phase microextraction enhanced lead detection sensitivity in Turkish black tea by 64.3-fold. Meanwhile, Liu Yi et al. employed GFAAS to precisely determine cadmium levels in river fish, demonstrating AAS techniques' exceptional capability in enhancing detection accuracy and efficiency.
1.2 Inductively Coupled Plasma Mass Spectrometry
Inductively Coupled Plasma Mass Spectrometry (ICP-MS) is a highly efficient analytical technique that converts samples into an ionic state within a high-temperature ion source. These ions are then directed into a mass spectrometer via an ion collection system for detection. This technique enables simultaneous, precise detection of multiple elements based on their mass-to-charge ratio, proving particularly suitable for analysing trace and ultra-trace elements. ICP-MS offers numerous advantages, including a broad linear range, rapid analysis speed, extremely low detection limits, minimal interference factors, and ease of interference elimination. For instance, Fu et al. employed ICP-MS for the rapid and accurate quantitative analysis of four heavy metals—arsenic (As), cadmium (Cd), mercury (Hg), and lead (Pb)—in medicinal plants with food applications. Results demonstrated linearity coefficients approaching 1, detection limits at the nanogram level, and low relative standard deviations. Furthermore, ZOU et al. successfully quantified arsenic in mushrooms using high-performance liquid chromatography coupled with inductively coupled plasma triple quadrupole mass spectrometry. This method demonstrated a broad linear range with low detection and quantification limits, and has been successfully applied to arsenic content detection and morphological analysis across multiple mushroom species.
1.3 Inductively Coupled Plasma Optical Emission Spectroscopy
Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) operates on the principles of atomic emission spectroscopy. When atoms of the target element are excited by thermal or electrical energy, they emit characteristic spectral lines at specific wavelengths. ICP-OES technology employs a high-temperature plasma generated by high-frequency current in argon gas as the excitation source, converting the components within the sample into atomic or ionic states. During this conversion, these atoms or ions emit light at specific wavelengths. As the wavelengths emitted by each element's atoms upon excitation or ionisation are unique, the types and concentrations of elements present in the sample can be determined by detecting the wavelength and intensity of this characteristic light. After the light source undergoes focusing, dispersion, and spectral separation through an optical system, a spectroscopic signal arranged in wavelength order is formed and received by a high-sensitivity detector. For instance, when determining cadmium content in rice, the sample undergoes digestion to convert it into a liquid form suitable for instrumental analysis. The digested sample is then introduced into the ICP-OES instrument. Under high-temperature excitation within the plasma, cadmium atoms in the sample become energised and emit light at specific wavelengths. Quantification of cadmium concentration is achieved by measuring the intensity of this characteristic light.
1.4 Graphite Induction Coupled Plasma Spectroscopy
Graphite Induction Coupled Plasma Spectroscopy employs cup-shaped or tubular electrodes fabricated from graphite material. These electrodes absorb atoms from the sample under analysis through current-induced heating. This method effectively detects metallic elements such as cadmium, chromium, and lead in foodstuffs. Its advantage lies in ensuring complete atomisation of all samples, offering higher accuracy and more comprehensive metal element analysis capabilities compared to traditional flame-based techniques. However, this method is not universally applicable to all testing conditions, as specific types of matrix modifiers are required for different metallic elements. This increases operational complexity when simultaneously detecting multiple heavy metals.
2. Coupled Detection Techniques
2.1 Inductively Coupled Plasma-Mass Spectrometry Coupled with High-Performance Liquid Chromatography
The principle of coupling ICP-MS with High-Performance Liquid Chromatography (HPLC) leverages HPLC's separation and purification capabilities alongside ICP-MS's high sensitivity and multi-element simultaneous detection properties. HPLC separates compounds within the sample, while ICP-MS subsequently performs elemental detection and quantitative analysis on the separated compounds. This coupled technique offers significant advantages in environmental analysis, enabling accurate determination of trace elements and their speciation in water samples, soils, and plants. It provides a powerful analytical tool for scientific research, facilitating deeper understanding of elemental composition and characteristics within samples. The ICP-MS/HPLC coupling technique demonstrates outstanding performance in food heavy metal detection [6]. For instance, when detecting multiple heavy metals such as lead, cadmium, and arsenic in cereals, HPLC employs reverse-phase columns to effectively separate complex components including organic and inorganic matter within the cereal sample. This ensures the heavy metals exist in isolation and enter the ICP-MS system for high-precision determination.
2.2 High Performance Liquid Chromatography-Atomic Fluorescence Spectroscopy Coupling Technique
High Performance Liquid Chromatography-Atomic Fluorescence Spectroscopy (HPLC-AFS) coupling technology employs HPLC to efficiently separate different components within a sample, utilising AFS for highly sensitive detection of the separated components. During detection, target elements undergo atomisation and excitation, emitting fluorescence signals at specific wavelengths. The intensity of these signals is directly proportional to the concentration of the target element, enabling quantitative analysis of specific elements within the sample. This method combines the high separation capability of HPLC with the high sensitivity of AFS, finding extensive application in environmental monitoring, bioanalysis, pharmaceutical research, and food testing. For instance, when determining lead content in tea leaves, a reversed-phase column effectively separates complex components such as tea polyphenols, caffeine, and amino acids, isolating lead for analysis. Subsequently, these lead ions enter the AFS detection system, where they transition from the ground state to an excited state under the influence of the excitation light source. Upon returning to the ground state, they emit a fluorescent signal at a specific wavelength. By measuring the intensity of this fluorescent signal, precise quantification of lead content in tea leaves can be achieved.
3. Rapid Detection Techniques
3.1 Colorimetric Methods
The principle of colorimetric methods in food testing relies on substances' light absorption properties. By observing colour changes resulting from specific reactions between sugars, certain additives, or contaminants with colour-developing agents, the concentration of the target substance can be inferred through comparison or measurement of these colour variations. In food analysis, the simplicity and speed of colourimetric techniques render them widely applicable where high precision is not paramount. Taking chromium ion detection in food as an example: when chromium ions react with diphenylcarbazide reagent, a purple-red complex forms. The intensity of this complex's colour correlates directly with chromium ion concentration—higher concentrations yield deeper hues. Leveraging this property enables rapid chromium ion detection via colorimetry. The food sample undergoes digestion and dilution. An appropriate amount of diphenylcarbazide reagent is added to the solution, which is then mixed and allowed to stand. Subsequently, a spectrophotometer measures the solution's absorbance at a specific wavelength. The measured absorbance value is converted into a chromium ion concentration value using a pre-established standard curve, thereby determining the chromium ion content.
3.2 Enzyme Inhibition Method
The enzyme inhibition method employed in food testing constitutes a classical technical approach, primarily leveraging the inhibitory effect of organophosphorus and carbamate pesticides on cholinesterase activity. This principle, however, also provides an innovative approach for heavy metal detection. Traditionally, these pesticides bind to cholinesterase, reducing its activity and thereby affecting the rate at which it breaks down specific substrates. By measuring changes in this degradation rate, the residual pesticide levels in food can be indirectly calculated. Within the field of food heavy metal detection, the application potential of the enzyme inhibition method is equally noteworthy. When food samples contain heavy metal ions, enzyme activity becomes inhibited, consequently reducing the rate at which substrates are broken down. For instance, employing an enzyme system sensitive to mercury ions would reveal a significant decrease in enzyme activity when applied to food samples containing mercury ions. By observing and measuring these changes in enzyme activity, the presence and concentration of mercury ions in food can be indirectly inferred. Although the direct application of enzyme inhibition methods in heavy metal detection remains exploratory, the adaptability of its principles and the technical feasibility of the approach undoubtedly open new avenues for research in the field of food heavy metal detection.
3.3 Electrochemical Methods
Electrochemical methods establish quantitative relationships between electrical signals—such as potential, current, or charge—generated by redox reactions between electrodes and target substances, and the concentration or properties of the measured material. This enables qualitative and quantitative analysis of heavy metals, harmful substances, microorganisms, and other components in food. Taking lead detection in food as an example, the square-wave anodic stripping voltammetry technique within electrochemical methods proves highly effective. In practice, the food sample must first be processed to extract lead ions. The sample solution is then placed in an electrochemical cell containing a working electrode, a reference electrode, and an auxiliary electrode. During the pre-electrolysis stage, a constant negative potential is applied, causing lead ions to be reduced to metallic lead and deposited onto the surface of the working electrode. During the square-wave anodic stripping phase, a series of square-wave potentials are applied. This re-oxidises the deposited metallic lead back into lead ions, which dissolve into the solution, generating a current signal proportional to the lead content. By measuring this current signal and correlating it with a standard calibration curve or samples of known concentration, the lead content in food can be accurately determined.
4. Conclusion
In today's rapidly advancing technological and industrial society, excessive heavy metal content in foodstuffs—whether originating from raw materials or processing procedures—poses potential threats to human health. Consequently, relevant research institutions and testing personnel should scientifically select appropriate heavy metal detection techniques based on practical circumstances. Rigorous and precise determination of heavy metal concentrations in food is essential to safeguard food safety and public health.
