## 2 Materials and Methods  
All voltammetric measurements were conducted with the use of potentiostat/galvanostat Elins-P30S (Chernogolovka, Russia) and a three-electrode cell, where the glassy carbon rod was used as a counter electrode and an Ag/AgCl (3.5 M KCl) electrode was used as the reference electrode. The C (graphite) and Pt electrodes were used as the working electrodes. The added d-tryptophan solution at a concentration of 10\({}^{-3}\) M (SIGMA-ALDRICH, assay \(\geq\)98.0% (HPLC)) was prepared by dissolving an accurately weighed portion in the background electrolyte. As a background electrolyte, the standard phosphate buffer solution (pH 6.86, a mixture of Na\({}_{2}\)HPO\({}_{4}\) and KH\({}_{2}\)PO\({}_{4}\)) was used. In total, 1000 measurements were continuously registered with constant stirring. An amino-acid additive was added after every 100 measurements to the analyzed solution. The concentrations of the analyzed solution after each addition of amino acid are presented in Table 2. \begin{table}
\begin{tabular}{c c c} \hline
**Method** & **Basic Limitations** & **Comments** \\ \hline Principal component analysis [10] & In single-valued decomposition (SVD), only 3-4 basic components are used. The influence of other components is not evaluated and explained properly. & This is a widely used method in chemometrics for the detection of possible pair correlations between two random sequences. \\ \hline Wavelet decomposition [11] & The criterion of the selection of the proper wavelet from the wide wavelet family is absent. Each chosen application of continuous wavelets to discrete data. \\ Wavelet decomposition [11] & wavelet has us own treatment error, and, in many cases, it cannot be evaluated and applied for the analysis of nanomoises in the interval 10\({}^{-9}\)-10\({}^{-6}\) A. & This method is important in the detection of \\ \hline [PERSON]’s method (flicker-noise spectroscopy) [12]. & The basic supposition is related to a _continuous_ representation of an initial sequence (with imposed definitions of intermittency, spikes, outliers, etc.) & The final expressions are derived from the apparatus of continuous mathematics and integral F-transform. These treatment errors are not properly evaluated in analysis of the chosen discrete data. \\ \hline Artificial neural network (ANN) [13] & There is a limited possibility for the physical interpretation of the model parameters. & Analytical possibilities of advice of different transfer functions, and by a wide variation of the number of ”neurons” in the intermediate layer, which limits the possibilities of the _optimal_ (well-tocated) selection of the desired transfer function. \\ \hline Projection on latent structures, partial least squares (PLS) [14] & The PLS method does not allow deriving the reliable calibration model based on the measured voltammograms (VAGs) at multiple sensors because of the appearance of ellipse-like clusters in the score plots. \\ \hline \end{tabular}
\end{table} 
Table 1: The basic methods used in electrochemistry for the detection of small additives. \begin{table}
\begin{tabular}{c c c} \hline
**Notation of the Solution Affected by** & **Volume of Additive (mL)** & **D-Typtophan Concentration** \\
**D-Typtophan Additive** & **Volume of Additive (mL)** & **(10\({}^{-5}\) mol-L\({}^{-1}\))** \\ \hline “0”-buffer solution & 0 & 0 \\ “1” & 0.7 & 6.54 \\ “2” & 1.4 & 12.3 \\ “3” & 2.1 & 17.4 \\ “4” & 2.8 & 21.9 \\ “5” & 3.5 & 25.9 \\ “6” & 4.2 & 29.6 \\ “7” & 4.9 & 32.9 \\ “8” & 5.6 & 35.9 \\ “9” & 6.3 & 38.7 \\ \hline \end{tabular}
\end{table} 
Table 2: The values of d-tryptophan added in the given buffer solute (the phosphate buffer solution (Na\({}_{2}\)HPO\({}_{4}\) + KH\({}_{2}\)PO\({}_{4}\)) with pH 6.86). Each measurement cycle included the following two stages:  
1. Electrochemical regeneration (precycling)--five successive cycles with a potential scan rate of 2.5 V/s in the range of potentials from 0-2.5 V.
2.