## 3 Results and Discussion
### Short-Term Material Exposure  
For the first screening of estrogen sorption, six materials generally recommended for chemical handling were evaluated after a 30-min soak in a solution consisting of deionized water spiked by 100 ng/L of each estrogen. As using contaminated equipment can cause severe inaccuracies in the processing of samples, we also tested whether the estrogens were desorbed back from the material surface into the deionized water. The results of the sorption experiment are shown in Figure 2a, and the desorption experiments are shown in Figure 2b. As shown in Figure 2a, significant changes in concentrations were observed in the case of estrone having the highest \(logK_{0W}\) value (Table S1) and especially on SBR, where almost 50% of the initial concentration in solution was sorbed. Moreover, only 2% of the E1 initial concentration was desorbed back to the deionized water after 90 min, suggesting quite strong, probably hydrophobic, interactions. All estrogens in our study contain a phenolic hydroxyl group. Thus, they are strong OH donors and acceptors but weak \(\pi\) acceptors. Estrone, however, also includes a keto group. Therefore, another possible but less likely explanation could be the formation of an OH/\(\pi\) hydrogen bond between the enol-form of estrone and the unsaturated group of SBR. However, the enol form is generally less stable in aqueous media. Significant sorption (more than 20%) of all estrogens was observed on Tygon S3(tm), a DEHP-free PVC. This material contains a bio-based plasticizer. Generally, plasticizers increase the space between the polymer chains, thus increasing the material's permeability [37]. As plasticizers are small molecules, they can leach out of plastics. In the same way that deliberately added small molecules could drain out, the plastic can absorb small molecules from the environment. Following the results of desorption from this material, nearly half of the previously sorbed estrogens were desorbed after 90 min soaking with deionized water. This suggests that the estrogens did not interact with the material as we observed for estrone and SBR. They probably interact with common but weaker (2 kJ/mole) dispersion forces or could just reversibly permeate into it (be absorbed). The estrogens were released continuously from the surface throughout the time frame with maximal concentration (approx. 5% of initial concentration) during the first 30 min. After 90 min, the desorption of E1, E2 and EE2 reached nearly 10% of their initial concentration (Figure 2b). It should also be noted that Tygon S3(tm) has the highest water absorption values (Table S2). Due to the high sorption and subsequent continuous desorption of estrogens into the pure medium, Tygon S3(tm) was found to be an unsuitable material. A similar situation in \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Concentration** & **E1** & **E2** & **EE2** & **E3** \\ \hline
100 ng/L a & 97.06 & 100.35 & 99.03 & 123.84 \\
300 ng/L a & 73.15 & 83.34 & 88.52 & 94.11 \\
300 ng/L b & 88.47 & 92.43 & 92.91 & 95.17 \\
500 ng/L a & 114.33 & 104.61 & 111.48 & 83.62 \\
500 ng/L b & 96.80 & 107.75 & 103.07 & 113.09 \\ \hline \hline \end{tabular} a
[FOOTNOTE:a] a footnotetext: Deionized water; b  
\end{table} 
Table 4: Recoveries (%) of estrogens at individual concentration levels and matrices. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Compound** & **MDL** & **MQL** & **CV a (\%)** & **CV a (\%)** & **CV b (\%)** & **CV b (\%)** & **CV c (\%)** & **CV c (\%)** \\  & **(ng/L)** & **(ng/L)** & **Inter-Day** & **Intra-Day** & **Inter-Day** & **Intra-Day** & **Inter-Day** & **Intra-Day** \\ \hline E1 & 0.2 & 0.5 & 7.54 & 10.41 & 12.22 & 11.59 & 11.18 & 9.97 \\ E2 & 0.2 & 0.5 & 4.47 & 7.33 & 13.03 & 13.08 & 5.29 & 4.51 \\ EE2 & 0.1 & 0.4 & 3.69 & 7.65 & 8.88 & 8.76 & 5.62 & 4.50 \\ E3 & 0.2 & 0.6 & 6.43 & 3.84 & 13.25 & 13.28 & 13.94 & 15.09 \\ \hline \hline \end{tabular} a
[FOOTNOTE:a] a footnotetext: 100 ng/L—deionized water; b
[FOOTNOTE:a] a footnotetext: Deionized water; b  
\end{table}