## 3 Results
### Chemiluminescent EIA of NP in Natural Waters  
Natural and artificial sources of drinking water differ in chemical composition and biological environment. Therefore, despite the apparent simplicity of this object, changes in the course of the curve or the analytical characteristics of the method should be taken into account. Despite the fact that the free valences in wells of a microplate are blocked using a blocking buffer (PBS with the addition of 0.1% gelatin) during chemiluminescence EIA before the competition, the effect of the sample matrix is able to be detected. Typically, two main approaches were used for enzyme immunoassay to obtain a calibration curve for the determination of analytes in complex matrices. The first approach is to adapt the calibration to the matrix, and the second one is to eliminate the influence of the matrix (processing, extraction, solvent change, protein precipitation, etc.). To evaluate the possible influence of the sample matrix, calibration curves were obtained in various media. The tested media were distilled water, PBS of various molarities (from 1 to 50 mM), PBST, and PBS with the addition of 0.1% gelatin. Table 3 shows the values of the IC\({}_{10}\) parameter, as well as other data estimated from the obtained curves. It can be seen that when the medium is replaced, there is a significant change in both the detection limits and the boundaries of the working ranges of NP concentrations; therefore, all these parameters should be taken into account when analyzing water samples in the future. It is noteworthy that in terms of analytical parameters, distilled water turned out to be closest to the water samples. Therefore, when carrying out the experiment, the values of the chemiluminescent signal were extrapolated to the curve obtained using distilled water as a medium at the competition stage (Figure 7). The advantage of using this environment is, on the one hand, universality, and, on the other hand, the absence of the influence of the quality of salts used in the preparation of a buffer solution. In addition, in such a matrix, we can analyze samples without dilution, which would help to avoid false negative results of the analysis, when the low content of the NP is even more reduced by diluting the sample. The \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**No.** & **Medium** & **IC\({}_{\textbf{10}}\), ng/mL** & **IC\({}_{\textbf{20}}\), ng/mL** & **IC\({}_{\textbf{50}}\), ng/mL** & **IC\({}_{\textbf{50}}\), ng/mL** & **Responses** & **Error *, \%** \\ \hline
1 & Distilled water & 15.0 & 60.97 & 5300 & 669.80 & 765/70 & 5.3 \\
2 & PBS 1 mM & 8.9 & 32.27 & 2633.1 & 291.51 & 574/153 & 6.8 \\
3 & PBS 10 mM & 28.4 & 85.67 & 3731.93 & 565.44 & 608/140 & 9.2 \\
4 & PBS 50 mM & 22.0 & 51.8 & 970.13 & 224.10 & 550/101 & 8.4 \\
5 & PBSST & 4.06 & 17.80 & 2778.7 & 222.34 & 580/92 & 4.2 \\
6 & 50 mM PBS + 0.1\% gelatin & 24.1 & 73.11 & 3251.55 & 487.56 & 640/95 & 6.0 \\ \hline \hline \end{tabular}
* The average error of detection in the range of IC\({}_{10}\)-IC\({}_{\textbf{80}}\).  
\end{table} 
Table 3: Analytical parameters of competitive curves for the NP determination obtained using various media. resulting curve is characterized by a detection limit of nonylphenol of 15 ng/mL and the working range of determined concentrations of 61-5300 ng/mL. Analysis of the literature, which provides data on the detection of NP in natural waters, demonstrates different levels of content--from 0.3 ng/mL to 6 \(\mu\)g/mL [46, 47]. The obtained analytical data satisfy the requirements of environmental monitoring. The samples used in this work were confirmed as negative and free of nonylphenol. The test samples and distilled water showed identical analytical signal values at the zero point. Therefore, spiked samples were prepared for analysis by introducing known amounts of NP. As can be seen from Table 4, when nonylphenol was added in various concentrations to water samples that did not contain nonylphenol, this method allowed detection from 82.7 to 122%.