Time-series sampling of trace metals in surface waters using osmotic sampler with air segmentation and preservative addition

Highlights: • An osmotic sampler was constructed for time-series sampling of trace metals in water. • The injected air segments could effectively separate time-series samples. • The addition of 30% HNO₃ could in situ preserve trace metals in collected samples. • Time-series concentrations of trace metals in water were measured using the sampler. High-resolution time-series concentrations (C_TS) are very important for the investigation of the biogeochemical processes of trace metals in the aquatic environment. However, the acquisition of C_TS of trace metals in water is still challenging because of the lack of suitable samplers. In this study, an osmotic sampler coupled with air segment injection and preservative addition was employed for time-series sampling of trace metals in surface waters. In the sampler, water sampling and preservative adding are both driven by osmotic pumps (OPs), while air segment injection is accomplished by a timer-controlled micro diaphragm pump. During deployment, the sampling OP continuously draws water through a filter and stores it in a narrow-bore coil. Simultaneously, a preservative OP slowly pushes 30% HNO₃ (v/v) into the collected sample for in situ preservation. Periodically, the micro diaphragm pump injects air into the continuous water stream to divide it into water segments, enabling accurate time-stamping. After retrieval, the time-series samples were pumped out from the coil and re-collected to analyze the C_TS of analytes. The sampler was deployed in river, reservoir, and marine waters for 26 h and one week to measure C_TS of trace metals at time resolutions of 2 h and 12 h. Results showed that the recoveries of a preloaded standard mixture (1.0 μg/L) in all samplers ranged from 93.1% to 117.8%. The measured C_TS of Cd, Co, Cr, Cu, Mn, and Ni in the waters only varied in small ranges. Accordingly, the measured C_TS data from the sampler were consistent with the obtained concentrations from grab sampling. The relative percent differences between the measurements from two samplers were less than 37.4%. These results demonstrate the reliability and accuracy of the sampler for time-series sampling of the chosen trace metals in surface waters.