Fig. 1. Groundwater heavy metal in situ detection device. (a) Photo of optical module of underwater detector and spectrometer ; (b) schematic diagram of components of device

Fig. 2. Underwater detector. (a) Output spectrum curve of selected ultraviolet (UV) LED; (b) schematic diagram of optical path of underwater detector

Fig. 3. Spectrum curve of UV LED after passing through filter and focusing

Fig. 4. Schematic of detection on Thermo Scientific fluorescence spectrophotometer

Fig. 5. Hg²⁺ mass fraction gradient titration experiments on fluorescence spectrophotometer. (a) Effect of adding different mass fractions of Hg²⁺ on fluorescence emission spectra; (b) mass fraction-peak fluorescence intensity standard curve

Fig. 6. Device repeatability test results

Fig. 7. Calibration results of groundwater heavy metal in situ detection device. (a) Fluorescence spectra; (b) mass fraction-peak fluorescence intensity standard curve