Ultratrace Xanthine Determination in Diluted Alkaline Electrolyte by Adsorptive Stripping Voltammetry at the Mercury Film Electrode

Abstract

A stripping method for the determination of xanthine at the submicromolar concentration level is described. The method is based on controlled adsorptive accumulation of xanthine at a thin-film mercury electrode followed by a linear scan voltammetry measurement of the surface species. Optimum experimental conditions were found to be the use of a 5.0 × 10^?3 M NaOH solution, an accumulation potential of 0.00 V, and a scan rate of 20 mV s^?1. The response of xanthine is linear over the concentration range 20–140 ppb. For an accumulation time of 30 min, the detection limit was found to be 36 ppt (2.3 × 10^?10 M). The more convenient relations for measuring xanthine in the presence of the metals, hypoxanthine, amino acids, and other nitrogenated bases were also investigated. The utility of the method is demonstrated by the presence of xanthine in adenosine-5′-triphosphate or DNA.     

Histidine Determination in the Presence of Copper in Diluted Alkaline Electrolyte by Adsorptive Stripping Voltammetry at the Mercury Film Electrode

Abstract

A stripping method for the determination of histidine at the submicromolar concentration levels is described. The method is based on controlled adsorptive accumulation of histidine-copper complex at mercury film electrode followed by linear cyclic scan voltammetry measurement of the surface species. Optimum experimental conditions were found to be the use of a 1.0 × 10^?3 M NaOH solution, an accumulation potential of ? 0.20 V, and a linear scan rate of 200 mV sec^?1. The response of histidine is linear over the concentration range 0.02–0.12 ppm. For an accumulation time of 20 min, the detection limit was found to be 0.5 ppb (3.2 × 10^?9 M). The more convenient relation to measuring the histidine in the presence of metals, cysteine, tyrosine, and other amino acids was also investigated. The utility of the method is demonstrated by presence of casein, ATP, and ss DNA.     

DFT and ab initio potential energy scan and hydrogen bond analysis of N-substituted hydrazino acetamides: Characterization of the “hydrazinoturn” hydrogen bonding pattern

We studied a series of model primary amides in gas phase at the DFT (B3LYP) and HF at 6-31+G/6-31+G** levels of theory in order to shed light on their conformation, structure, and intramolecular hydrogen bonding network. A potential energy scan was performed by rotating around the appropriate bond for each molecule studied in this paper. In this manner, it was possible to show that the amidic group of these model compounds acts as H-bond donor and interacts with two different H-bond acceptors which stabilizes a C₈ pseudocycle, the so called “hydrazinoturn”. This study was addressed theoretically in order to understand the conformation adopted by hydrazino acetamides as model compounds for aza-β³-peptides. We thus investigated the conformational analysis of hydrazinoturns computationally and showed that these systems represent a very stabilizing folding driving force, provided that the neighboring molecular functional groups do not imply other competing hydrogen bonding patterns.     

Electrochemical Deposition of Gold Nanoparticles on Pencil Graphite by Fast Scan Cyclic Voltammetry

Gold nanoparticles (AuNPs) were electrodeposited on the surface of pencil graphite (PG) by fast scan cyclic voltammetry without using any additives in acidic medium. The effect of deposition time on the size of electrodeposited AuNPs was investigated in sulfuric acid as a supporting electrolyte. The deposition time was varied by varying the scan rate, number of cycles and applied potential range. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X‐ray diffraction (XRD) were used for characterization of PG and electrodeposited AuNPs. The results confirmed that nanosized gold particles (20 ± 8 nm) were deposited on the PG substrate with almost spherical geometry.     

A liquid chromatography–mass spectrometry method based on class characteristic fragmentation pathways to detect the class of indole-derivative synthetic cannabinoids in biological samples

This article describes a liquid chromatographic/tandem mass spectrometric method, based on the use of precursor ion scan as the acquisition mode, specifically developed to detect indole-derived cannabinoids (phenylacetylindoles, naphthoylindoles and benzoylindoles) in biological fluids (saliva, urine and blood). The method is designed to recognize one or more common “structural markers”, corresponding to mass spectral fragments originating from the specific portion of the molecular structure that is common to the aminoalkylindole analogues and that is fundamental for their pharmacological classification. As such, the method is also suitable for detecting unknown substances, provided they contain the targeted portion of the molecular structure.     

Improved Sensitivity for Imaging Spin Trapped Hydroxyl Radical at 250 MHz

Radicals, including hydroxyl, superoxide, and nitric oxide, play key signaling roles in vivo. Reaction of these free radicals with a spin trap affords more stable paramagnetic nitroxides, but concentrations in vivo still are so low that detection by electron paramagnetic resonance (EPR) is challenging. Three innovative enabling technologies have been combined to substantially improve sensitivity for imaging spin‐trapped radicals at 250 MHz. 1) Spin‐trapped adducts of BMPO have lifetimes that are long enough to make imaging by EPR at 250 MHz feasible. 2) The signal‐to‐noise ratio of rapid‐scan EPR is substantially higher than for conventional continuous‐wave EPR. 3) An improved algorithm permits image reconstruction with a spectral dimension that encompasses the full 50 G spectrum of the BMPO–OH spin adduct without requiring the wide sweeps that would be needed for filtered backprojection. A 2D spectral–spatial image is shown for a phantom containing ca. 5 μM BMPO–OH.     

Versatility of tandem mass spectrometry for focused analysis of oxylipids

Liquid chromatography‐tandem mass spectrometry (LC‐MS/MS) in the multiple reaction monitoring (MRM) scan mode has been the primary MS method applied for the target identification of specific and minor oxylipids in complex matrices, such as eicosanoids and docosanoids, which are potent lipid mediators derived from polyunsaturated fatty acid oxygenation. However, the high specificity of MRM can limit the detection of species with m/z MRM transitions not covered by the method. In addition to MRM, tandem‐quadrupole mass analyzers enable other experiments to be conducted, by fragmenting ions via collision‐induced dissociation process (CID). This paper presents the potential of tandem mass spectrometry for the focused analysis of oxylipids. We have successfully developed an LC‐MS/MS method for the identification of precursor ions of m/z 115, a diagnostic product ion of 5‐hydroxy‐ and 5‐epoxy‐fatty acids. As a proof of concept, the developed method was used to discover several oxylipids oxidized at C5 derived from arachidonic acid (C20 : 4) oxygenation in a hypothalamus rat extract that were not identified using the target MRM methodology. The proposed focused MS/MS‐based approach in a tandem mass analyzer has proven to be a powerful strategy to accelerate the identification of oxylipids with structural similarities and assist the field of lipidomic research. Copyright © 2015 John Wiley & Sons, Ltd.     

Evaluation of different scan methods for the urinary detection of corticosteroid metabolites by liquid chromatography tandem mass spectrometry

Different approaches for the non‐target detection of corticosteroids in urine have been evaluated. As a result of previous studies about the ionization (positive/negative) and fragmentation of corticosteroids, several methods based on both precursor ion (PI) and neutral loss (NL) scans are proposed. The applicability of these methods was checked by the injection of a standard solution containing 19 model compounds. Five of the studied methods (NL of 76 Da; PI of 77, 91 and 105; PI of 237; PI of 121, 147 and 171; and NL of 38 Da) exhibited satisfactory results at the concentration level checked (corresponding to 20 ng/ml in sample). Some other methods in negative ionization mode such as the NL of 104 Da were found to lack sufficient sensitivity. Some of the applied methods were found to be specific for a concrete structure (NL of 38 Da for fluorine containing corticosteroids) while others showed a wide range applicability (PI of 77, 91 and 105 showed response in all model compounds). Interference by endogenous compounds was also tested by the analysis of negative urines and urines spiked with different corticosteroids. The suitability of these methods for the detection of corticosteroid metabolites was checked by the analysis of urine samples collected after the administration of methylprednisolone and triamcinolone. A combination of the reported methods seems to be the approach of choice in order to have a global overview about the excreted corticosteroid metabolites. Copyright © 2009 John Wiley & Sons, Ltd.     

Phase Behavior of Microemulsions Formulated with Sodium Alkyl Polypropylene Oxide Sulfate and a Cationic Hydrotrope

The partial ternary phase diagram of anionic extended surfactant of alkyl polypropylene oxide sulfate C₁₂(PO)₄SO₄ alone and combined with the cationic hydrotrope, tetrabutyl ammonium bromide with water and decane were determined under ambient conditions. Middle phase microemulsion was formulated using salinity scans in the dilute region of surfactant/brine/decane. Visual inspection as well as cross polarizer and optical microscopy were used to detect anisotropy. Spinning drop tensiometer was used to measure interfacial tension (IFT). The first ternary phase diagram using the extended surfactant alone showed three one phase regions, the anisotropic lamellar liquid crystalline phase, L _α and the isotropic L₁ micellar liquid and L₃ sponge phase. In the second ternary phase diagram using the extended surfactant combined with tetra butyl ammonium bromide, an isotropic micellar region, L ₁, appeared in the diluted area of the phase diagram. Meanwhile the L _α phase disappeared completely and the three phase region has a bluish transparent middle phase. Interfacial tension measurements between middle phase and brine, and between decane and brine yielded ultra low values. Calculated IFT values using the characteristic length obtained using De Gennes approximation gave almost half the measured values. The interfacial rigidity was also calculated and compared to values obtained from the literature.     

Voltammetric Studies on the Electrochemical Reduction of Methylmercury in HCl Aqueous Medium at a Carbon Microelectrode

Fast scan voltammetry applied to methylmercury in chloride medium at a carbon fiber microelectrode has shown two cathodic peaks located at ?0.45 and ?1.1 V and a single anodic peak at ?0.33 V (vs. Ag|AgCl). It was concluded that the reoxidation process, at high sweep rates, in acidic media behaves as a reversible one‐electron transfer process coupled to a chemical reaction, with the reaction product weakly adsorbed. Good linear calibration plots for the methylmercury determinations in the concentration range from 75 to 300 μM, have been obtained using the currents of the anodic peak measured on the CVs recorded, at 10 V s^?1, at a carbon fiber microelectrode, in a stationary solution of methylmercury chloride. Data have shown that the carbon microelectrode, cycled continuously in the analyte, can replace the dropping mercury electrode (DME), with the advantages that the carbon microelectrode is a promising tool for further studies in high resistive media, such as in natural waters.