## 1 Introduction
The DGT device is mainly composed of a fixed layer (fixed membrane) and a diffusion layer (diffusion membrane and filter membrane). The target ions pass through the diffusion layer and are then captured by the fixed layer. At the same time, a linear gradient distribution of target ions is formed in the diffusion layer (Figure 1). The diffusive flux of the DGT device to the target (\(F_{\mathrm{DGT}}\)) can be calculated using the following formula:  
\[F_{\mathrm{DGT}}=\frac{M}{At} \tag{1}\]  
Figure 1: Schematic of concentration gradient in DGT technique. (\(\Delta\)r, \(\Delta\)g, \(\delta\) represent the thicknesses of the binding gel, diffusion gel, and diffusion boundary layer, respectively). \[F_{\text{DGT}}=\frac{DC_{\text{DGT}}}{\Delta g} \tag{2}\]  
where \(t\) is the placement time of the DGT device; \(M\) the accumulation of target ions by the fixed membrane during the placement time of the DGT device; \(A\) the exposed window area of the DGT device; \(\Delta g\) the thickness of the diffusion layer, cm, and the diffusion coefficient of the target ion in the diffusion layer, D. \(C_{\text{DGT}}\) represents the concentration of the linear gradient of the diffusion layer near the end of the ambient medium. From the above formula, the calculation formula of \(C_{\text{DGT}}\) can be obtained as  
\[C_{\text{DGT}}=\frac{M\Delta g}{DAt} \tag{3}\]  
Solvent extraction was generally used to determine the mass (\(M\)) of target ions accumulated in the fixed membrane. \[M=\frac{C_{e}(V_{e}+V_{g})}{f_{e}} \tag{4}\]  
where \(C_{e}\) represents the concentration of the extract; \(V_{e}\) is the volume of the extractant; \(V_{g}\) is the volume of the fixed membrane; and the extraction rate of the target ions on the fixed membrane by the extractant,\(f_{e}\). In the early days of DGT devices, a solid binding phase was used (as shown in Figure 2a), and the device was mainly composed of a plastic shell, a cellulose film, and a diffusion layer. The main function of the cellulose film was to limit the size of the particles passing through, while protecting the internal diffusion layer and binding phase from contamination. The function of the diffusion layer was to limit the passing speed of the particles, which was proportional to the time, the external solution, and the internal ion concentration. The main role of the binding phase is to combine the diffused ions, so that the concentration difference between the diffusion phase and the binding phase is reduced to a minimum. With the development of DGT technology, DGT devices with a liquid binding phase had appeared, which solved the disadvantages of the poor mechanical strength and elution efficiency of solid binding phase DGT devices [17]. The DGT device of the liquid combined phase innovatively used the dialysis membrane as the diffusion layer, which was the biggest difference from the solid DGT device. Moreover, the basic part of the liquid binding phase DGT device, which played the role of holding the binding phase and fixing the diffusion layer, was made of polyethylene. There was a 2 cm diameter front opening window in front of the device, which was the only channel for the exchange of internal and external substances. A DGT device using a liquid-bound phase is shown in Figure 2b. Although the liquid binding phase had the advantages of simple operation, easy handling, no elution,  
Figure 2: Solid binding phase DGT device (**a**) and liquid binding phase DGT device (**b**). and high accuracy, which had been further studied in recent years, it was still difficult to prepare compared to the solid binding phase. Therefore, most of the current research was focused on the solid binding phase. The DGT technique had achieved a rapid development since it is widely used in evaluating metal bioavailability in natural water bodies, sediment and soil, etc. [18; 19].