**2.1 Construction of the sequential ionization device**

A sequential ionization device was home-built (**Figure 1**), which consists of eight systems: (1) quantitative addition and mixing system of the chemical reagents, which was used to prepare the corresponding reagents for extraction of the defined speciation from geological samples; (2) microscale liquid sample transportation system, which was used for precision transfer of the reagents and the extracted speciation; (3) microstructure morphology imaging system, which was used to monitor the morphology change in real time; (4) micro-electrolytic cell and interface system, which was used to load the sample; (5) field energy (temperature, ultrasonic, microwave) coupling and regulation system, which was used to promote the extraction of speciation from samples; (6) high-efficiency ionization system, which was used to ionize the extracted speciation; (7) timing trigger and intelligent control system, which was used to control the device; and (8) power supply and support connection system. This device can be directly coupled to the commercial mass spectrometer for direct MS analysis of geological samples.
