Potentiometric determination of methionine in alkaline solutions by used Nafion and MF-4SC membranes treated and modified
Abstract
Methionine is an indispensable sulfur-containing amino acid used in the treatment of liver diseases and intoxication, as well as therapy for protein deficiency, atherosclerosis and diabetes mellitus. In industry methionine is produced by microbiological synthesis, isolation from natural raw materials (for example, from casein hydrolyzate) and chemical synthesis from 3-methylthiopropionic aldehyde, the last stage of which is alkaline hydrolysis. Therefore, the development of methods for the determination of methionine in an alkaline medium at the presence of alkali metal ions is actual. The important problem is the influence of medium pH on the accuracy of determination of amino acids due to their buffer properties, which determine the presence of several ionic (including zwitterion) forms of analyte simultaneously in solution. For determination of amino acids in media with variable pH without the use of reagents, it is necessary to take into account its effect on the analytical signal. For this, promising is the use of a multisensory approach, in which the influence of all components of the test solution on the responses of an array of cross-sensitive sensors is accounted for by multidimensional calibration. The aim of the work was the development of cross-sensitive DP-sensors (the analytical signal is the Donnan potential) for the determination of methionine anions and zwitterions together with potassium cations in alkaline solutions by using Nafion and MF-4SC membranes, modified by oxides nanoparticles EO2 (E=Ce, Zr, Si) and / or treated at different temperature and relative humidity.
The characteristics of DP-sensors were determined in the aqueous solutions containing methionine (Met) and KOH with different concentrations of components in the range from 1.0·10‑4 to 5.0·10‑2 M (pH 8-11). In the test solutions, methionine is predominantly in the form of anions (Met-) and partly in the form of zwitterions (Met±). The initial Nafion and MF-4SC membranes, hybrid materials based on them, with hydrated oxides nanoparticles EO2 (E=Ce, Zr, Si), and also unmodified and modified samples treated at the different temperature and relative humidity were used in DP-sensors. Nafion 115 membranes (Aldrich) were used, their modification was carried out in situ (sequential treatment by precursor solutions and ammonia solution to form oxides nanoparticles). Two types of MF-4SC membranes were used: obtained by extrusion from the polymer melt and obtained by casting from the polymer solution (Plastpolymer). MF-4SC + EO2 membranes were obtained by casting a polymer solution containing a precursor, followed by treatment with ammonia to produce oxides. Unmodified samples and hybrid materials based on them were treated. The treatment was carried out at 90-95°C and relative humidity of 60-95%, and at the 100-140°C under hydrothermal conditions in contact with aqua.
The membranes pairs that provided a high sensitivity of DP-sensors to ions of opposite sign, response stability and minimal correlation between responses of sensor pairs were chosen for the simultaneously determination of methionine anions and zwitterions and potassium cations in alkaline solutions. The sensitivity of the DP-sensors to K+ cations in Met + KOH solutions is high (from 39.42±0.18 to 72.5±0.7 mV/pC) for all test samples. The sensitivity of DP-sensors to Met-, Met± ions increases with the introduction into the Nafion(extrusion) and MF-4SC(casting)membranes of ZrO2 and SiO2 oxides exhibiting acid properties at pH> 7. This is due to an increase in the concentration of bipolar ions and co-ions in the membrane due to the electrostatic repulsion of the pore walls and the surface of the deprotonated dopant. The influence of the volume fraction of oxides in the Nafion(extrusion) and MF-4SC(casting) membranes on the sensitivity of sensors differed due to the structural features of these membranes. Treatment of MF-4SC(extrusion) membranes in hydrothermal conditions at a temperature of 100-120°C provides an increase in the sensitivity of DP-sensors to Met-, Met± ions in proportion to the diffusion permeability of membranes (with a maximum of 23.4±0.8 mV/pC at 16.2·10‑8 cm2/s). This is due to an increase of analyte concentration in membrane. Besides, the probability of interaction of charged amino groups of methionine zwitterions with the sulfo groups of the membrane increases by increasing the volume of the intra porous space and hydration of the membrane.
Arrays of cross-sensitive DP-sensors was developed to determine of methionine anions and zwitterions together with potassium cations in the alkaline solutions. Selected membrane samples provide sufficiently low values of the relative error and relative standard deviation of the analytes determination, despite variable pH values of solutions varying over a wide range (pH from 8 to 11).
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