## 3 Description of the Treatment Procedure
The typical curves corresponding to the comparison of the buffer solution with \(c_{0}=0\) and \(c_{1}=1\) for C- and Pt-electrodes are given by Figure 2a,b for the normalized data (d\(//\)d\(U\) (derivative) and \(J(U)\)-usual VAG) in data space. Figure 3a,b demonstrate the behavior of the ranges (Rg(\(Y_{m}\)) and Rg(\(JY_{m}\)) in measurement space for the same concentrations \(c_{0,1,9}\) (C-, Pt-electrodes), calculated for the derivatives (Figure 3a) and usual VAGs (Figure 3b). We remind the reader here that the range of a function \(f(x)\), defined in the region of a variable \(x\) from the interval [min(\(x\)), max(\(x\))] and representing itself the maximal deviation is defined as Rg(\(f\)) = max(\(f\)) \(-\) min(\(f\)). This preliminary analysis, supported by Figures 2b and 3b, prompts the choice of the integral curves (usual VAGs) for comparison of the influence of electrodes and the derivation of the desired calibration curves that change monotonically with respect to the applied concentration. We start from the analysis of the parameters that form the invariants of the second order. As shown in the Appendix A, we demonstrate the concentration dependence of the following parameters: the ratio of the gravity centers (\(Rt(c)=<\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!We want to stress here that the standard deviation of each curve, on average, gave 8-10% deviations from the mean curve. We decided _not_ to show these error bars for each deviation \"tube\" in order not to overcharge Figures 4-7 with this excess and inessential information. In the calculation of the invariants of the second and the fourth orders, we chose the mean curves corresponding to the massive dataset compared. Therefore, in all presented figures, the corresponding statistical parameters were obtained for the mean curves. They were calculated with the help of Equation (1), where \(M=100\)  
Figure 2: **(a)** VAG derivatives (dJ/d\(U\)) for two types of electrodes (Pt and C). Minimal concentration (\(c=1\)) corresponds to 6.54 \(\times\) 10\({}^{-5}\) mol\(\cdot\)L\({}^{-1}\); see Table 2 for details. **(b)** Normalized and integrated VAGs corresponding to the curves shown in the previous figure. These curves are more preferable for the detection of additives. The integration procedure was realized with the help of Equation (2), which eliminates the additional fluctuations and possible deviations evoked by the presence of an additive. Concentration \(c=1\) corresponds to the minimal concentration of 6.54 \(\times\) 10\({}^{-5}\) mol\(\cdot\)L\({}^{-1}\); see Table 2 for details. Figure 3: (**a**) Distribution of the ranges for differential (dJ/d\(U\)) VAGs in the measurement space. One hundred measurements for \(c_{0,1,9}\) were repeated in the same experimental (temperature, pressure, humidity, pH) conditions. The period of one VAG registration was 3 s. The duration between measurements was 10 s. One can notice that the chosen minimal concentration \(c_{1}\) was very close to the curve, corresponding to the \(c_{0}\) concentration for both types of electrodes. All measurements were reduced to the same interval [1,100]. (**b**) Distribution of the ranges for the usual VAGs for the same values of concentrations \(c_{0,1,9}\). In comparison with the previous figure, these curves become more sensitive to the presence of d-tryptophan in the buffer solute and, hence, more preferable for further analysis. Figure 4: (**a**) Behavior of parameters \(\mathrm{Rt}(c)\) = \(<\)\(>\)/\(<\)\(<\)\(>\) and \(\mathrm{Dsp}(c)\) = \(<\)DY\({}^{2}\)\(>\)/\(<\)DX\({}^{2}\)\(>\) for two types of electrodes in data space. All these curves changed monotonically with respect to the increase in d-tryptophan concentration. The true c-values are given in Table 2. (**b**) Behavior of the same parameters \(\mathrm{Rt}(c)\) = \(<\)\(>\)/\(<\)\(>\) and \(\mathrm{Dsp}(c)\) = \(<\)DY\({}^{2}\)\(>\)/\(<\)DX\({}^{2}\)\(>\) is shown for two types of electrodes in the measurement space.