High-sensitivity detection of low-concentration heavy metal ions in solution by multiple reflection enhanced absorption (MREA) spectroscopy†

Heavy metal ions (Cr⁶⁺, Co²⁺, Ni²⁺, and Cu²⁺) in the electroplating and electrolysis industries are significantly related to process parameters and product quality, even at lower concentrations. Absorption spectroscopy is widely used for substance qualitative and quantitative analysis, which is an analytical method with the potential for real-time monitoring of heavy metal ions concentration in industrial processes. In this paper, a low-concentration heavy metal ion analysis method based on multiple reflection enhanced absorption (MREA) is proposed. Compared with traditional absorption, MREA has the advantages of low concentration detection limit and high-sensitivity. First, a reflective film (Al–SiO₂) was prepared and a multiple reflection optical structure was designed to realize multiple parallel reflections of light in the solution medium. Then absorption spectra of low-concentration Cr⁶⁺, Co²⁺, Ni²⁺ and Cu²⁺ solutions were measured by MREA and traditional absorption methods. Finally, spectral bandwidth and incident light spots were optimized to obtain a superior absorption enhancement effect. The results showed that MREA could effectively increase the substance absorbance compared with traditional absorption. At the same time, with the optimal spectral bandwidth (0.4 nm) and incident light spot (1 mm), the detection limit of Cr⁶⁺, Co²⁺, Ni²⁺ and Cu²⁺ was reduced by 81.48%, 82.52%, 80.92% and 82.93%, respectively. The sensitivity was improved by 5–6 times, which was more obvious for low-concentration detection. In addition, the MREA method can achieve ion concentration analysis when Cr⁶⁺, Co²⁺, Ni²⁺, and Cu²⁺ coexist, and the linear correlative coefficients of the C–A curves were all greater than 0.999. Moreover, by adjusting reflectivity of the reflective film and the number of reflections in the optical structure, the results of the MREA method can be further optimized for the low-concentration heavy metal ion analysis. The MREA method has the advantages of simplicity, rapidity and versatility, which can provide the technical foundation for real-time monitoring method development of low-concentration heavy metal ions in industrial processes.