Vol 74, No 3 (2019)

Actively developing “unconventional” methods for the solid-phase extraction (SPE) of organic substances, for example, solid-phase microextraction, solid-phase dynamic extraction, dispersive solid-phase extraction, matrix solid-phase dispersion, microextraction in a packed syringe, etc., are characterized. The main distinctive features of the methods and the fields and prospects for their application are considered. Solid-phase microextraction is most popular, mainly because of a possibility of the full automation of the separation and determination of analytes and the ease of combination with gas and liquid chromatography, as well as with other analytical methods.

A procedure is developed for the direct determination of all rare-earth and a number of accompanying elements in niobium−rare-earth ores by inductively coupled plasma atomic emission spectrometry after sample decomposition by fusion and further disolution. Because of the complex elemental composition of the samples, numerous mutual overlapping of spectral lines of matrix and rare-earth elements were observed, which required corrections in the calculations of concentrations. The procedure is based on a calibration, modeling spectral and matrix interferences; compilation of the set (mathematical matrix) of mutual interference coefficients; and use of the matrix obtained and the results of sample measurement for the computation of element concentrations. A precise method is used to solve a system of linear equations; it implies the resolution of interferences for all lines. Software utilizing a regularization algorithm ensuring stable solutions of the system of linear equations and also a method of evaluation of the uncertainty of the results of measurements are developed. The method ensures correct results and is rather stable to changes in the composition of the analyzed sample. This was confirmed by the results of analyses of certified reference materials (CRMs) of the composition of rare-earth ores of different origin.

The capabilities of countercurrent chromatography (CCC) in the elemental analysis of samples with different chemical and phase compositions are demonstrated. The combination of the characteristic features of extraction chromatography and multistage extraction with separation and preconcentration processes performed in a closed system (a Teflon coiled column) opens up prospects for applying this method to the analysis of a wide range of materials from high-purity substances to oil and rocks. The use of CCC at the stage of sample preparation makes it possible to decrease the detection limits of impurity elements in high-purity materials to a pg/g level, i.e., by two or three orders of magnitude, as compared with those in the direct determination by inductively coupled plasma mass spectrometry. In the analysis of oil samples, CCC allows one to determine their complete trace element composition (including platinum group, rare, and rare earth elements) due to the concentration of trace elements in a small volume of a stationary aqueous phase retained in the column.

Some specific features of the valence FeLα_1,2 line are investigated: a correlation between the line intensity and the oxidation state of iron is shown; the dependence of the intensity of the FeLα_1,2 line on the presence of other elements in the sample is found.

Electrochemical methods for heavy metals detection have attracted considerable attention due to their simplicity, rapidity and high sensitivity. Voltammetric methods are a valid and very effective alternative for the simultaneous determination of heavy metals. An independent atomic absorption spectroscopic analysis of the seaweed sample was carried out and the results were compared with the World Health Organization permissible limits. The sample was clearly observed in the atomic force microscopic images, characterized by Fourier transform infrared spectroscopy and X-ray diffraction analysis. The electrochemical behavior of heavy metals on a bare glassy carbon electrode and a multiwalled carbon nanotubes modified glassy carbon electrode was studied by cyclic voltammetry, linear sweep anodic voltammetry, square wave anodic voltammetry and differential pulse anodic voltammetry. Very good responses were observed for all the metals when the modified electrode was employed.

A new voltammetric approach was developed for the determination of paracetamol in tablet and syrup samples using chemometric optimization. The method is based on the use of the gold nanoparticles–polyaniline (AuNPs−PA) modified pencil graphite electrode (PGE). Electrode system was fabricated by using the electrochemical reduction of gold on aniline modified pencil electrode to give gold nanoparticles. A 3³ full factorial design was used to find optimal oxidation conditions of paracetamol on the AuNPs−PA−PGE. The optimal oxidation conditions of drug substance were found as –524 mV for the accumulation potential, 295 s for the accumulation time and pH 8.69 (for the phosphate buffer solution, 0.1 M). The determination of paracetamol was linear in the range of 2.0 × 10^–7–6.0 × 10^–6 M with a detection limit of 5 × 10^–8 M. As a result, the proposed voltammetric approach gave satisfactory results for the determination of the related drug substance in tablets and syrup.

A portable instrument for analysis of solutions by atomic emission spectrometry is described based on recording the emission spectrum of a drop-spark discharge using a Maya 2000 Pro spectrometer (Ocean Optics, United States).