Graphene quantum dot-β-cyclodextrin modified glassy carbon electrode was used as a new nanosensor for determination of L-tyrosine (L-Tyr). It was found that graphene quantum dot-β-cyclodextrin has been stably electrodeposited on glassy carbon electrode modified by simple technique. The cyclic voltammograms of the modified electrode in an aqueous solution displayed a pair of well-defined, stable and irreversible reductive/oxidation redox systems. The apparent electron transfer rate constant (k s) and transfer coefficient (α) determined by cyclic voltammetry were approximately equal to 8.0 s–1 and 0.7, respectively. The modified electrode showed excellent catalytic activity towards the oxidation of L-Tyr at positive potential in buffer solution. The nanosensor also displayed fast response time, high sensitivity, low detection limit and a remarkably positive potential oxidation of L-Tyr that decreased the effect of interferences in analysis.
The main achievements of high-resolution NMR spectroscopy in combination with different chemometric methods in the analysis of food products in the last 40 years are reviewed. The essence of chemometric methods used for the analysis and interpretation of NMR spectra is briefly described. Sample preparation for NMR-spectroscopic analysis is characterized. Methods for the mathematical treatment of NMR spectra (smoothing, Fourier transformation, bucketing, normalization, and selection of spectral ranges) are considered. Currently available methods for the suppression of the signals of macrocomponents, including those for the simultaneous suppression of several signals, are described. The results are illustrated based on examples of analysis of different classes of foodstuffs and beverages with the use of NMR spectroscopy and chemometric methods for classification and discrimination (geographical and botanical origin as well as validating checking the brand authenticity).
Molecularly imprinted polymer (MIP) for malathion and its major metabolite malaoxon was prepared with sol‒gel polymerization method using malathion as template molecule, (3-aminopropyl)triethoxysilane (APTES) as functional monomer, tetraethyl orthosilicate as crosslinker, TiO2 nanowires as support and methanol as solvent. Interaction of APTES with malathion by hydrogen bond with a ratio of 2 : 1 was evaluated by the molecular recognition mechanism. The properties of MIP were characterized by Fourier transform infrared spectroscopy, scanning electron microscope, transmission electron microscope, X-ray diffractometry, surface area and porosimeter analysis, elemental analysis and thermogravimetric/differential thermal analysis. Molecularly imprinted polymers showed high affinity, recognition specificity and efficient adsorption performance for malathion. The residues of malathion and malaoxon in tap water were cleaned up by MIP-solid-phase extraction cartridge, determined by gas chromatography with flame photometric detector. The quantification limits of malathion and malaoxon were both 0.01 mg/L. The spiked recoveries of malathion and malaoxon in environmental water were 62.1‒76.7% (RSD 4.2‒6.5%) and 82.1‒95.4% (RSD 7.0‒7.7%), respectively. Gas chromatography‒tandem mass spectrometry analysis for confirmation was also performed.
We considered the physicochemical regularities and analytical capabilities of separation methods based on the manifestation of capillary effects in hydrophobic microporous media, namely, liquid–gas chromatography with a stationary gas phase in the pores of a solid-phase substrate and a mass-exchange process in liquid–gas and liquid–liquid systems, carried out in a biporous matrix. The advantages of these methods compared to conventional mass-exchange processes in the same systems in terms of higher efficiency and a possibility of the separation of substances in a continuous mode are shown.
The separation of a long peptide chain without derivatization, without amide columns and by using routine reversed-phase HPLC columns is itself a big task. Increase in the basic nature of the mobile phase as well as in column temperature and low flow rate of the mobile phase were demonstrated to permit the separation of polar compounds containing long peptide chains. The aim of this study was to develop a procedure for the determination of an antithrombing agent Bivalirudin containing a long peptide chain. This method was developed on a Princeton SPHER-C18 100 Å (4.6 × 250 mm, 5 µm) column and a mobile phase consisting of water-acetonitrile (pH 7, adjusted with triethylamine) in the ratio of 95 : 5. The flow rate was 0.7 mL/min at the thermostat temperature of 45°C. Validation of the method was performed as per the International Conference on Harmonization. The method was applied to the analysis of a pharmaceutical formulation. The developed method was accurate, fast, easy for implementation and economy as it did not require sophisticated equipment or tedious derivatization process.
Problems of the development of potentiometric sensors and multisensor systems for the determination of rare-earth elements in aqueous solutions are considered. The efficiency of the approach to the fabrication of sensors based on the application of substances and regularities used in solvent extraction is shown. Possibilities of multisensor systems for the selective determination of individual lanthanides in complex mixtures are demonstrated.
Methyl benzoate-based vortex-assisted surfactant-enhanced emulsification microextraction (VASEME) has been developed and applied for the preconcentration of fungicides (i.e., carbendazim, thiabendazole and fluberidazole) and their subsequent determination by high performance liquid chromatography. Methyl benzoate (as extraction solvent) and sodium dodecyl sulfate (as emulsifier) were used in VASEME. Parameters affecting the extraction performance were investigated and optimized. Under the optimum conditions, linearity was obtained in the range of 0.1–200 μg/L with the coefficient of determination exceeding 0.998. Limits of detection and quantitation were 0.01–0.5 and 0.1–3 μg/L, respectively. Recoveries of the spiked analytes (10–100 μg/L) from fruit juice samples were between 72 and 116% with the relative standard deviation below 10%. The proposed method is simple, rapid, consuming less solvent, sensitive and reliable for the determination of fungicide residues in fruit juice samples.
The concept of equilibrium flow-through methods and their capabilities and limitations are considered. Special attention is paid to cyclic injection analysis (CIA). General diagrams for the automation of cyclic injection analysis of liquid, gaseous, and solid phase samples are presented. Analytical performance of CIA is illustrated by examples of automated chemical analyses of food products, biological fluids, pharmaceuticals, and environmental samples.
In this study, cloud point extraction‒scanometry as a new, simple and inexpensive method was developed for the separation, preconcentration and determination of trace amounts of brilliant green (BG) and basic fuchsin (BF) in mixture. The method is based on the extraction of analytes simultaneously from aqueous solution using a non-ionic surfactant Triton X-114 as a cloud point extractor. After phase separation, the surfactant-rich phase was diluted with ethanol, digital image of the solution taken with a flatbed scanner and RGB parameters of the enriched analytes were calculated by special software written in visual basic (VB 6). The influence of analytical parameters including pH of system, the concentration of the surfactant, equilibration temperature and time were optimized. The method was linear in the concentration ranges of 0.025‒1.00 and 0.05‒2.50 mg/L with limits of detection of 0.008 and 0.019 mg/L for BG and BF, respectively. The preconcentration factors were 29 for BG and 28 for BF. The enrichment factors were found to be 34 and 16, precision (RSD, %) of the method, 1.9 and 1.3% for BG and BF, respectively. The effects of interfering ions and dyes were investigated. Finally, the proposed method was applied for the determination of BG and BF in real water samples with satisfactory results.
Electrochemical methods of analysis are usually characterized by high sensitivity, ease of automation, and a wide range of analytes and test samples. The development of electrochemical methods of analysis at the present stage is mostly determined by the creation of new nanostructured electrode materials with electrocatalytic properties. The use of such materials ensures an increase in the sensitivity and selectivity of the determination of a number of analytes. Another conventional way to decreasing the limit of detection for electrochemically active substances is the development of new measurement methods for improving the signal-to-noise ratio in nonequilibrium electrochemical methods of analysis. This article is devoted to the consideration of some new electrode materials and methods of electrochemical measurements.
History of the development of a hollow cathode as a version of a discharge cell for optical emission spectroscopy and glow-discharge mass spectrometry is discussed. Special attention was paid to the contribution of colleagues from the Department of Analytical Chemistry at the St. Petersburg State University to research, development, and implication of analytical systems with glow discharge, including discharge in a hollow cathode. New analytical methods developed on its basis and versions of their application are described, covering direct elemental and isotopic analysis, depth profiling, determination of hardly ionized elements in solid-phase samples of different conductivity, and the determination of volatile organic compounds in the air.
We analyzed the products of alkylphenol (ortho-, meta-, and para-cresols and 4-isopropylphenol) oxidation by iron(III) chloride in aqueous solutions by HPLC with electrospray ionization and mass-spectrometric detection in the negative ion mode. It was found that the main products formed by the nucleophilic addition of the initial alkylphenols to reactive intermediates, quinone methides. The resulting adducts can be further oxidized, which, after several oxidation–addition cycles, leads to the appearance of a series of compounds with molecular weights belonging to sequences of values of 108, 214, 320, 426, 536, 638, … for cresols and 136, 270, 404, 538, 672, … for 4-isopropylphenol. Such processes are similar to the oxidation of alkylphenols by air oxygen in aqueous solutions and to their electrochemical oxidation yielding dimeric and oligomeric products.
In this work, Fe3O4@ZnO core/shell magnetic nanoparticles (MNPs) were prepared by a facile method and used as an excellent electrocatalyst to construct an electrochemical sensor for simultaneous determination of quercetin (Qu) and rutin (Ru) in some pharmaceutical herbs including Borage, Chamomile, Asparagus, Teucrium, Tarragon and Pennyroyal. The morphology and structure of the nanoparticles were characterized by transmission electron microscopy, scanning electron microscopy, X-ray diffraction and Fourier transform infrared spectroscopy. Compared with the bare glassy carbon electrode, Fe3O4@ZnO MNPs modified carbon paste/glassy carbon electrode exhibited much higher electrocatalytic activity toward the oxidation of Qu and Ru with increasing of peak currents and decreasing of oxidation overpotentials. Under the optimized condition, the electrochemical parameters such as the effective surface area, charge transfer coefficient, electron transfer rate constant and diffusion coefficient were calculated. A good linearity was obtained in the concentration range of 2.9 × 10–7–6.47 × 10–5 M for Qu and 9.9 × 10–8–9.9 × 10–5 M for Ru. The limits of detection (S/N = 3) of Qu and Ru were 1.4 × 10–7 M and 7 × 10–8 M, respectively. The experimental results demonstrated that the proposed method shows a promising prospect in determination of Qu and Ru in pharmaceutical herbs.
The composition and properties of heparin preparations are studied by NMR spectrometry. Approaches to the determination of the most important heparin quality indicators (biological origin and manufacturing company) based on multivariate modeling of the NMR profile of a sample are considered. Chemometric modeling of two-dimensional (2D) NMR spectra (DOSY, HSQC, HMBC) by the principal component and discriminant analysis is performed to increase the efficiency of analysis. Multivariate regression models (partial least squares) are built for the first time to determine the average molecular weight of heparin. Quantitative NMR spectrometry is applied to the determination of water, calcium and sodium cations, and also chloride and acetate anions in heparin preparations. The determination of all characteristics requires five consecutive measurements by NMR (1H, 2D, 35Cl, 23Na, and 2D DOSY) spectrometry and one sample preparation. The duration of analysis of one sample, including measurements and signal processing, takes no more than 20 min. The proposed approach is universal and can be used in the analysis of other medicinal preparations.
In this study, a novel sorbent based on phenylsulfonic acid functionalized multiwall carbon nanotubes (MWCNTs-Ph-SO 3 H) was used for preconcentration and separation of trace Ni(II) ions in petrochemical wastewater samples by ultrasound-assisted solid-liquid trap phase extraction. In this work, a mixture of MWCNTs-Ph-SO3H as a nanosorbent and ionic liquid (1-hexyl-3-methylimidazolium hexafluorophosphate, IL) dispersed in acetone was rapidly injected by a syringe into 50 mL of a sample containing Ni(II). The metal was extracted by sulfonate group as (RSO3)2-Ni at optimized pH. After shaking and centrifuging, the nanoparticles of sorbent were trapped in IL phase and settled down in a conical centrifuge tube (IL/MWCNTs-Ph-SO3H). Hydrophobic IL was used to fast trap the Ni(II) loaded sorbent from the sample solution. Under the optimized conditions, the linear range (peak area), limit of detection and enrichment factor were obtained as 1.5–165 μg/L, 0.35 μg/L and 50, respectively.
The state-of-the-art of metrological support of chemical analysis is considered. The specificity of this field of metrology is discussed, that is, the absence of a primary material standard of the fundamental physical quantity, mole, and the dependence of the results of measurements on the chemical composition of a sample. Advances in the development of the metrology of chemical analysis are noted. The focus of the review is on the problems that remain unresolved or incompletely resolved. These are the metrology of sampling, the control of the accuracy of analysis, the assurance of metrological traceability, the nomenclature of the existing standard samples of composition, and the estimation of the budget of uncertainty. Particular emphasis is placed on metrology regarding new types of chemical analysis, for example, surface analysis, local analysis, and analysis of nanodimensional samples. The priority of improving the metrology of electron-probe analysis, Auger electron spectroscopy, electron spectroscopy for chemical analysis, and secondary-ion mass spectrometry is noted. A conclusion is made about the need in intensifying research for decreasing the number of unsolved problems cardinally.
A procedure for the determination of AICAR (5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside, Acadesine), an AMP-activated protein kinase agonist prohibited by the World Anti-Doping Agency since 2009, in urine has been proposed including sample clean-up from matrix components by means of solid-phase extraction and detection of the analyte by ultra-high performance liquid chromatography–tandem mass-spectrometry with heated electrospray ionization ion source. Due to the endogenous nature of AICAR, standard additions approach was applied to quantify AICAR and evaluate matrix effects. The limit of detection was 5 ng/mL, calibration curves were linear in the concentration range of 50–5000 ng/mL. The proposed procedure was applied to the analysis of real samples.
We developed an original gas chromatographic procedure for determining the products and reagents for the catalytic synthesis of dimethyl ether (DME) from synthesis gas, which enables the simultaneous detection of СН3ОН, DME, СО, СО2, Н2, Н2О, N2, and hydrocarbons to С6. The gas circuit includes four detectors, three packed columns, two precolumns (to prevent water and organic compounds from entering molecular sieve columns) in combination with two relief valves and two pressure regulators for the implementation of the back purging of precolumns. The system is assembled based on a Khromatek-Kristall 5000 chromatograph. The determination is carried out at a constant temperature of 140°C. The duration of analysis is not more than 12 min. Because of the presence of a flame ionization detector, it is possible to detect trace amounts of hydrocarbons, as well as ethane in the presence of large amounts of CO2.
The problem of using octanometers for the rapid control of the octane number of compounds in the production of commercial gasoline is considered, and the factors forming it are indicated. Error spectra of a multidimensional absorption spectral analytic signal of commercial gasolines and compounds of their individual fractions (components) were studied in measurements in the mid-IR region. The effect of a change of the proportion of the main constituents (aromatic hydrocarbons, benzene catalysate, isomerizate, methyl tert-butyl ether, and isopentane) in the compound on the spectra of commercial gasolines was evaluated. Multidimensional measurements of light absorption by gasolines in a wide spectral region and difference spectra were used. It was shown that spectral noise does not form limitations in spectroscopic measurements, while uncontrollable variations in the process conditions can form significant deviations in the absorption spectra, leading to problems in using calibration characteristics based on commercial gasoline samples.
Isomeric compounds are characterized, on the one hand, by the proximity of the main physicochemical characteristics, including mass, NMR and IR spectra, and on the other hand, by significant differences in properties essential for a human being, for example, reactivity and toxicity. Virtually all known physicochemical methods of analysis are used to study and determine isomers; chromatography–mass spectrometry is among the most effective ones. Mass spectrometric identification includes library search, building the structure of a molecule based on fragmentation patterns for various classes of organic substances, and the use of special methods of ionization, yielding information about the structure of molecules. Chromatographic separation enables the identification of isomeric compounds with identical mass spectra. Information about the presence of specific isomers in their complex mixtures is necessary for making serious arbitral decisions in forensic science, toxicology, or doping analysis. The review discusses examples of using chromatography–mass spectrometry for isomer-specific analysis in antidoping control.