## 3 Influence Factors of \(R_{s}\)
### Influence of Pollutant Nature on \(R_{s}\)  
In previous studies, more efforts have been made to explore the correlation between \(R_{s}\) values of the POCIS and \(log\)\(K_{ow}\) or logarithmic octanol-water distribution coefficient (\(log\)\(D\)) of analytes; unfortunately, no consistent results were obtained. A poor correlation between the \(R_{s}\) and \(log\)\(K_{ow}\) or \(log\)\(D\) values was observed by [PERSON] et al. [96] and a large scattering was found by [PERSON] et al. [97]. Meanwhile, several researchers suggested the non-linear or linear relationship between the \(R_{s}\) of POCIS and \(log\)\(K_{ow}\) or \(log\)\(D\) values [53; 57; 71] Those different results [51; 98; 99; 100] were more likely due to the diverse functional groups of chemicals that were linked to variable trend of their hydrophobicity with alteration of solution pH values. As expressed in Equations (15) and (16) [101], the \(D\) value (representing the effective distribution of the neutral and ionic parts of compounds between octanol and water) could be adjusted according to the pH value of the medium, the \(pK_{a}\) of the given chemical and the \(K_{ow}\) values [51].  
\[D_{Acid}=K_{ow}/\left(1+10^{pH-pKa}\right) \tag{15}\]  
\[D_{Base}=K_{ow}/\left(1+10^{pKa-pH}\right) \tag{16}\]  
Therefore, [PERSON] et al. [51] found that in terms of pH values, the \(log\)\(D\) values decreased for acidic pharmaceuticals and increased for basic pharmaceuticals. This phenomenon revealed a positive relationship between the \(R_{s}\) and \(log\)\(D\) over the pH range of 3-9 and the higher \(R_{s}\) for neutral substances than those for ionic substances by POCIS. Using 88 polar organic micropollutants as targets to examine the correlation between in situ \(R_{s}\) value and \(log\)\(D\), researchers found a slight linear correlation between the \(R_{s}\) and \(log\)\(D\) in spite of the low regression coefficient (\(R^{2}\) = 0.37) and existing uncertainty. In general, polar ionic compounds with low \(log\)\(D\) had lower \(R_{s}\) values than neutral ones with higher \(log\)\(D\)[100]. It was also reported that for some of pharmaceuticals and personal care products (PPCPs) and endocrine-disrupting substances (EDS) with the \(log\)\(K_{ow}\) range from 0 to 4, there was a positive correlation between the \(R_{s}\) and \(log\)\(K_{ow}\), whereas for those with \(log\)\(K_{ow}\) values higher than 4, the upward trend of the \(R_{s}\) was not significant [50]. This was in agreement with [PERSON]'s research, where, using a second-order polynomial function to perform a non-linear regression between the \(R_{s}\) and \(log\)\(K_{ow}\) value, the author observed an increased \(R_{s}\) value with the hydrophobicity for compounds with \(log\)\(K_{ow}\) range from 1.2 to 3.7, reaching a plateau afterwards [25].  
The uptake of pollutant in the integrative phase is governed by its diffusion across WBL and membrane in the POCIS [18]. The diffusion coefficient of a chemical is inversely proportional to molecular mass; thus, a negative correlation of the \(R_{s}\) values with molecular mass would be expected. However, [PERSON] et al. [26] performed a regression analysis to determine the relationship between molecular mass of compounds and the \(R_{s}\) for the neutral compounds; unfortunately, no clear linear correlation was observed between both variables. Additionally, the data summary of the \(R_{s}\) values of POCIS determined in 23 previous studies [13; 21; 26; 27; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59] for different chemicals with various molecular mass was presented in Figure 4. As expected, a large scattering between \(R_{s}\) values and molecular mass was obtained with a relative weak relationship (\(R^{2}\) = 0.011), which was mainly due to a combined effect of solute-sorbent-solution on the \(R_{s}\) value, such as the interaction between chemical function groups of analytes and sorbents.