## 1 Introduction  
The development and application of a fast, efficient, and low-cost integrated sampling approach is of great significance for monitoring the concentration and fate of trace pollutants in the water environment, and furthermore for accurately assessing their toxicity and risk. Passive sampling is the developing technique that is based on the flow of pollutants from environmental medium to uptake phase due to chemical potential difference between pollutants [1; 2; 3; 4]. On the basis of the principle of permeation or molecular diffusion, pollutants could enter the uptake phase by passive diffusion through a well-defined diffusion barrier or permeation through a membrane, thus achieving the enrichment of target substances in the environment media [5]. Compared to active sampling, the uptake rate of compounds by passive sampling is relatively low, and thus a longer exposure time is needed for sampling in a low-concentration environment, which has a significant advantage,allowing the measurement of time-weighted average (TWA) concentration of target pollutants and more comprehensively reflecting the long-term effects of these pollutants [6; 7; 8; 9]. Nowadays, many configurations of water passive sampling devices have been proposed for inorganic pollutants, such as diffusive gradient in thin film technique (DGT), peeper (a kind of balanced permeate device), and Chemcatcher [6; 10; 11], and for organic pollutants, such as polar organic chemical integrative sampler (POCIS), semi-permeable membrane devices (SPMDs), membrane enclosed sorptive coating (MESCO), Chemcatcher, and others [12; 13; 14; 15; 16]. The development milestones in the field of passive sampling are shown in Figure 1[10; 17]. These passive sampling devices can be performed in two different accumulation regimes during field exposure: the kinetic linear uptake mode with fast mass transfer rate to the uptake phase, and the equilibrium regime with slower stagnant uptake, described by a partition coefficient of contaminant between the uptake phase and the environmental media [18]. The POCIS has been identified as an effective passive sampler for qualitative analysis of pesticides, pharmaceuticals, and perfluorinated compounds (PFCs), among others, in surface water, groundwater, and wastewater [8; 19; 20; 21; 22]. It has the advantages of simple operation, low cost, no power demand, no maintenance requirement for isolation, and enrichment of organic compounds at low concentrations in a water environment. Before application of POCIS for the monitoring of target compounds, it is necessary to determine the sampling rate (\(R_{s}\), the equivalent volume of water drawn by the sampler per unit time) for each compound of interest, which is the key parameter to deduce the concentrations of pollutant in water from the amount of pollutants accumulated by the sampling device [23]. Obtained  
Figure 1: Milestones in the development of passive sampling in aquatic environment (DET: diffusive equilibrium in thin films technique; SLM: supported liquid membrane; PISCES: passive in situ concentration extraction sampler; PRCs: performance reference compounds; LDPE: low-density polyethylene; SPATT: solid phase adsorption toxin tracking bags.). values can be further optimized to determine the average concentration of analytes in the environment so as to decrease the uncertainty of in situ application [18; 24; 25; 26]. It has been reported that the \(R_{s}\) values would not be affected by analyte concentrations in water, however, this is totally dependent on a range of environment factors, such as water turbulence, temperature, salinity, biofouling, physicochemical parameters of water body, and so on [16; 27; 28; 29; 30]. For example, [PERSON] et al. [27] reported that the \(R_{s}\) values increased with temperature, flow rate, and pH, while [PERSON] et al. and others [21; 31; 32; 33] only observed a significant effect of the salinity on uptakes for basic pharmaceuticals.