Journal of Analytical Chemistry最新文献

The oxidation of hydroquinone with hydrogen peroxide in the presence of catalytic amounts of FeSO₄ results in complex mixtures of oligomers. The average composition of these products is determined by the molar ratio of reagents and varies from (C₆H₄O₄)_n at the hydroquinone to hydrogen peroxide ratio 1 : 3 to (C₆H₄O₆)_n at the ratio 1 : 5. A characteristic feature of MALDI mass spectra for polymers is the periodicity of signals. However, no such periodicity is observed for the products of hydroquinone oxidation within the m/z range <1000 Da, indicating the absence of fixed sequences of structural fragments. This unique feature arises from the variable ratios and irregular positions of polyhydroxyphenylene and polyhydroxybenzoquinone fragments. Additionally, several factors contribute to the lack of mass-spectral periodicity, namely, the potential formation of peroxides, the presence of stable hydrates, various types of linkages, and possible interactions between hydroxylated phenylene and benzoquinone fragments. In contrast, for m/z values >1000 Da, MALDI mass spectra exhibit periodicity with a mass difference of Δ(m/z) = 74. This value likely corresponds to the fragment C₂H₂O₃, which is neither a hydroquinone nor a benzoquinone structural unit. Taking into account that an equal Δ(m/z) value is observed for fragment ions in the EI mass spectrum of tetrahydroxy-p-benzoquinone, this gives indirect evidence for the presence of polyhydroxybenzoquinone fragments within the oligomers. A key issue regarding the structure of hydroquinone oxidation products is the type of linkages between the polyhydroxyphenylene and/or polyhydroxybenzoquinone units, which could involve C–C or C–O–C bonds (or both). The available spectral data do not resolve this issue conclusively. However, the presence of a sample of the composition (C₆H₄O₆)_n, in which each unit contains six oxygen atoms, suggests that at least one oxygen atom forms a bond between the units, indicating a C–O–C connection. Overall, the unique nature of oligomeric products of hydroquinone oxidation explains their high structural variability and redox properties, which likely contribute to their unique pharmacological characteristics.

This work is devoted to the analysis of the results obtained during the separation of the mass spectra of apamin and its complex after the deutero-hydrogen substitution of these molecules when leaving the electrospray ion source. The goal of the work was to determine the origin of the structural components identified in apamin and its complex. The separation method assumes that the substitution of labile hydrogen atoms with deuterium ones occurs independently, while charge carriers are retained independently too by various amino acid residues of a polypeptide with a specific structural form. This method also includes an indirect assessment of the contribution of natural isotopes to the measured data, without requiring knowledge of the elemental composition or other structural features of the biomolecule under study. Previously, we performed similar calculations by directly accounting for the contribution of natural isotopes. For apamin electrosprayed ions, after introducing a gas flow of ND₃ into the molecular ion reactor of the mass spectrometer, three main components were identified, with contributions of approximately 57, 39, and 5%. The new method yielded three components with similar contributions, which was anticipated. More noteworthy, though, and particularly relevant in various data collection and computation scenarios, is the paper’s description of the reliable identification of minor components with contributions of no more than 4%. Some of these components have almost identical formally found H/D distributions across different charges, remaining within or near the natural isotopic distribution range of apamin. This suggests that these components correspond to the structures of apamin ions and their complex in a solution without labile hydrogen atoms, such as during the formation of hydrogen bonds, or with a relatively small number of D-substituted hydrogen atoms, where protons carrying an ion charge do not participate in the exchange, particularly when more than two ion charges are present. Such a result was not anticipated in advance. This paper demonstrates the probable presence of three forms of apamin ions and its complex, each with distinct capacities for H/D exchange directly in the analyzed apamin solution. The limitations of the developed approach in decreasing the number of sites accepted to be involved in the exchange of hydrogen for deuterium, and in retaining charge carriers, are also discussed.

Kraft (sulfate) lignin is a large-scale by-product of the pulp and paper industry and a promising renewable raw material for the production of a wide range of chemicals and materials with high added value. One of the main directions of valorization of technical lignin is conversion into low-molecular-weight phenolic products using various depolymerization methods. To solve the problems of operational control of ongoing processes and the molecular level characterization of complex mixtures of formed low-molecular-weight compounds, this study proposes the use of atmospheric pressure photoionization high-resolution mass spectrometry in combination with visualizing the resulting data sets based on elemental ratios (van Krevelen diagrams) and Kendrick mass defects. Using this methodology, the kraft lignin depolymerization products formed in a supercritical isopropanol medium were characterized, and the effect of additives of nitrogen-containing catalysts (hydroxylamine and diethylamine) on the ongoing processes and the composition of the resulting products were studied. The leading role of reduction processes in the transformation of lignin oligomers under the action of isopropanol was shown, and the characteristic differences between the products obtained in the absence and with the addition of nitrogen-containing catalysts were revealed.

A new approach to detecting dibenzothiophene-5,5-dioxides by soft mass spectrometry methods has been developed. This method involves the chemical modification of target compounds with phenols in the presence of trifluoroacetic anhydride to form organic salts. The cations of these salts are easily desorbed under matrix-assisted laser desorption/ionization (MALDI) conditions and extracted by electrospray ionization (ESI). Upon collisional activation, the derivatization products eliminate the aryl oxy substituent from the sulfur atom, enabling the detection of derivatization products through selected reaction monitoring.

Misuse of chloramphenicol (CAP) could lead to its residue in animal-sourced food and subsequently harm human health through the food chain. Therefore, it is urgently needed to effectively determine CAP residues in animal-sourced food. Here, an easily operated target recycling strategy for the determination of CAP was developed based on a CAP-specific aptamer. The CAP-specific aptamer was hybridized with a complementary short chain modified with fluorescent and quenching groups at both ends. Upon addition of CAP, the aptamer would be separated from its complementary probe and bind with CAP. The fluorescent and quenching groups labeled on the complementary probe were separated by exonuclease I (Exo I) digestion to release a fluorescent signal. Meanwhile, Exo I also digested the aptamer, and CAP was released from the complex, triggering the determination signal amplification based on CAP recycling. Factors (i.e., NaCl concentration and Exo I digestion time) that would affect the determination performance of this sensor system were optimized. Under the optimized conditions, the developed sensor system exhibited high determination sensitivity toward CAP with a limit of detection of 0.08 ng/mL (linear range from 0.1 to 5 ng/mL). Moreover, this sensor system was applied to determine CAP in real spiked milk, and the recovery rates ranged from 97.2 to 100.0%. Meanwhile, this sensor system also suggested cost efficiency with a determination cost of only $0.29 per sample. Results presented here and the high specificity of the aptamer suggested by the previous work indicated the application potential of this developed aptamer-based sensor system for the determination of CAP residue in animal-sourced food.

Several techniques are available to determine an analyte, but the most preferred technique is fluorescence-based sensing owing to its sensitivity and selectivity. One of the main components of fluorescent-based sensors is a filter that allows a specific range of wavelengths to pass through and reach the sensing element. Existing filters still lack certain aspects such as durability, interference, cost, integration with a sensing element, biocompatibility, etc. This research aims to design and develop a polyvinyl acetate (PVAc)-benzophenone-based emission filter by spin coating and integrating it with a complementary metal-oxide semiconductor imaging sensor for use as a portable biochemical sensing device. In addition, the fabricated filter is intended to allow the conjugation of recognition elements necessary for biosensing using plasma oxidation. In this study, PVAc and benzophenone concentrations were optimized to fabricate an emission filter (EF) with precise thickness and uniformity. Subsequently, the fabricated EF underwent analysis using a single-beam UV-Vis spectrophotometer. Additionally, plasma treatment was administered to the EF, enabling further examination using fluorescein isothiocyanate dye under a fluorescent microscope. These examinations collectively suggest that the prepared filter possesses dual properties, i.e., it not only serves as an emission filter but also facilitates surface modification through plasma oxidation for the conjugation of chemical groups that can selectively bind to target analytes for biosensing purposes.