In a 0.125 mol/L phosphate (pH 6.6)/2.5 × 10-4 mol/L 2-iodoacetamide solution, lomefloxacin yields a response of a polarographic catalytic current. The second-order derivative peak current of the catalytic wave of lomefloxacin is proportional to its concentration in the range of 1.0 × 10-8 - 1.0 × 10-6 mol/L (r = 0.998). The sensitivity of the catalytic wave is 25-times higher than that of the corresponding reduction wave for 5.0 × 10-7 mol/L lomefloxacin. The proposed method was applied to the determination of lomefloxacin in pharmaceutical preparations. The polarographic reduction wave is ascribed to a one-electron reduction of the C=C bond of lomefloxacin zwitterion accompanied by an acid-base equilibrium. The catalytic wave should be caused by regeneration of the lomefloxacin molecule at electrode surface due to the one-electron reduction product being further oxidized by electroreductive intermediate products of 2-iodoacetamide.
A novel biotinylated molecular beacon (MB) probe was developed to prepare a DNA biosensor using a bridge structure. MB was biotinylated at the quencher side of the stem and linked on a biotinylated glass cover slip through streptavidin, which acted as a bridge between MB and glass matrix. An efficient fluorescence microscope system was constructed to detect the fluorescence change caused by the conformation change of MB in the presence of complementary DNA target. The proposed biosensor was used to directly detect, in real-time, the target DNA molecules. The bridge immobilization method caused the proposed DNA biosensor to have a faster and more stable response. Under the optimal conditions, the newly developed DNA biosensor showed a linear response toward ssDNA in the range of 5 - 100 nM with a detection limit of 2 nM. It was interesting to note that the described biosensor was reproducible after being regenerated by urea.
We evaluated a new restricted access media (RAM) precolumn for direct analysis of drugs in plasma using a column switching HPLC system. The new RAM material was prepared by the modification of the external surface of porous silica with hydrophilic methylcellulose (MC), followed by modification of the internal surface with octadecylsilane (ODS). The external surface of the MC-immobilized ODS silica material (MC-ODS) suppressed the adsorption of proteins, while the internal surface of MC-ODS retained various types of drugs, such as ketoprofen, propranolol, caffeine and atenolol in plasma samples. In addition, MC-ODS allowed direct analysis of drugs in a 1000-µL plasma sample to monitor trace amounts of analytes contained. Reduced efficiency and clogging of the MC-ODS precolumn and/or the analytical column were not observed even after the repetitive injection of plasma sample up to 40 mL. Our results indicated that the MC-ODS precolumn could be used in pharmacodynamic and clinical studies.
A flow sensor with immobilized oxidases is proposed for the determination of histamine in fish meat. Chemiluminometric measurement of histamine was based on the luminol reaction with hydrogen peroxide produced by immobilized histamine oxidase (EC 1.4.3.-.) and peroxidase (EC 188.8.131.52.) within a flow cell. Histamine oxidase was found in cells of Arthrobacter crystallopoietes KAIT-B-007 isolated from soil. The oxidase and peroxidase were coimmobilized covalently on tresylated hydrophilic vinyl polymer beads and packed into transparent PTFE; the tubing was used as the flow cell. One assay for histamine was done at intervals of 2 min without carryover. The calibration curve for histamine was linear from 0.1 µM to 50 µM. The response was reproducible within 1.25% of the relative standard deviation for 115-replicate injections of 50 µM histamine. The sensor system was applied to the determination of histamine in fish meat extracts.
The preparation and electrochemical properties of a glassy carbon (GC) electrode modified with cobaloxime complex were investigated. The complex of the type [CoIII(DO)(DOH)pn)Cl2] where (DO)(DOH)pn = N2,N2′-propanediylbis-2,3-butanedione2-imine-,3-oxime) was adsorbed irreversibly and strongly on the surface of preanodized glassy carbon electrode. Electrochemical behavior and stability of modified GC electrode were investigated by cyclic voltammetry. The electrocatalytic reduction of dioxygen has been studied using this modified glassy carbon electrode by cyclic voltammetry, chronoamperometry and rotating disk electrode voltammetry as diagnostic techniques. The modified electrode showed excellent eletrocatalytic ability for the reduction of dioxygen to hydrogen peroxide in acetate buffer (pH 4.0) with overpotential 1.0 V lower than the plain glassy carbon electrode. The formal potential for this modified electrode is not shifted to more negative potentials by repeated reduction-oxidation cycles in oxygen-saturated supporting electrolyte solution. The apparent electron transfer rate constant (ks), the transfer coefficent (α) and the catalytic rate constant of O2 reduction at a GC modified electrode were determined by cyclic voltammetry and rotating disk electrode voltammetry and were found to be around 2.6 s-1, 0.33 and 2.25 × 104 M-1 s-1. Based on the results, a catalytic mechanism is proposed and discussed.
Ferricyanide anion has usually been used as a marker of ion-channel sensors. In this work we first found that ferricyanide, itself, can act as a stimulus to regulate the permeability of sBLM prepared from didodecyldimethylammonium bromide (a kind of synthetic lipid) on a GC electrode. We used cyclic voltammetry and a.c. impedance to investigate this phenomenon. The interaction between sBLM and ferricyanide concerns time. Furthermore, we developed a sensor for ferricyanide anion. The ion-channel sensor is highly sensitive. It can detect ferricyanide concentration as low as 5 µM.
It has been demonstrated that a glass-like silicone ladder-type polymer permits one to homogeneously incorporate high amounts of ionophores into the covalently-bonded double chain structure. Furthermore, by making use of this feature, we have successfully fabricated an iodide ion-sensitive field-effect transistor based on two kinds of ionophores and silicone ladder polymer matrix. As ionophores, 5,10,15,20-tetraphenyl-21H,23H-porphyrin and dimethyloctadecyl-3-trimethoxylsilylpropylammonium chloride were homogeneously incorporated into the matrix. The ion-sensitive field-effect transistor showed a linear potential response ranging in the I- concentration between 1.0 × 10-5 and 1.0 × 10-1 M. The selectivity coefficients for I- towards interferences of ClO4- and NO3- were estimated to be KpotI-,ClO4- ≈ 6.2 × 10-4 and KpotI-,NO3- ≈ 4.9 × 10-4. The matrix has proved to be so stable that the selectivity coefficients have not been altered over six months.
Matrix or impurities remaining in a DNA sample solution after various sample treatment procedures may influence a subsequent DNA analysis. In this work, several matrices were investigated concerning their effects on the analysis of oligonucleotide by using an ion-trap mass spectrometer equipped with a sonic spray ionization source. Inorganic salts of sodium chloride and magnesium chloride depressed the signal intensity by about 50% when the content of the salts was about 10 µM. dNTPs and Taq showed more severe depression on the oligonucleotide. However, Tris, or (hydroxymethyl)aminomethane, intensified the signal intensity, if its content was within an appropriate range. When the content of Tris was about 500 µM, the signal intensity was enhanced by factors of 3 and 5 for the 6-mer and the 20-mer oligonucleotides, respectively. With the existence of Tris, matrix effects from the inorganic salts, dNTPs and Taq were reduced.
Enantiomer separation of mandelates and their analogs, which are important intermediates in asymmetric synthetic and pharmaceutical chemistry, was investigated by capillary gas chromatography using different cyclodextrin derivative chiral stationary phases (CD CSPs). The used cyclodextrin derivatives included permethylated β-CD (PMBCD), permethylated γ-CD, heptakis(2,6-di-O-butyl-3-O-butyryl)-β-CD, heptakis(2,6-di-O-pentyl-3-O-acetyl)-β-CD and heptakis(2,6-di-O-nonyl-3-O-trifluoroacetyl)-β-CD (DNTBCD), respectively. Among all the CSPs used, PMBCD and DNTBCD exhibited the broadest and best enantioselectivity for all the racemates investigated. Some thermodynamic parameters were evaluated and an enthalpy-entropy compensation effect was observed in enantiomer separation processes of mandelates and their analogs. Based on thermodynamic data and molecular mechanics calculations, the chiral recognition mechanism of mandelate derivatives on CD CSPs is discussed.
A simple, rapid and selective procedure for the indirect spectrophotometric determination of Se(IV) and As(V) has been developed. It is based on the reduction of Se(IV) to Se(0) and As(V) to As(III) with hydroiodic acid (KI + HCl). The liberated iodine, equivalent to each analyte, is quantitatively extracted with oleic acid (HOL) surfactant. The iodine-HOL system exhibits its maximum absorbance at 435 nm. The different analytical parameters affecting the extraction and determination processes have been examined. The calibration graphs were found to be linear over the ranges 5 - 120 and 0.25 - 20 ppm of Se(IV) and As(V), with lower detection limits of 2.5 and 0.15 ppm and molar absorptivities of 1 × 104 and 0.5 × 104 dm3 mol-1 cm-1, respectively. Sandell’s sensitivity was calculated to be 0.0078 and 0.0149 µg/cm2 in the same order. The relative standard deviation for five replicate analyses of 40 ppm Se(IV) and 4 ppm As(V) were 1.0 and 0.9%, respectively. The proposed procedure in the presence of EDTA as a masking agent for foreign ions has been successfully applied to the determination of Se(IV) in a reference sample and As(V) in copper metal, in addition to their determination in spiked and polluted water samples.
A sensitive and selective kinetic-spectrophotometric method is proposed for the determination of µg mL-1 amounts of Cu2+ based on its catalytic effect on the oxidation of L-ascorbic acid by Methylene Blue in a strongly acidic medium. The reaction is monitored spectrophotometrically by measuring the decrease in color intensity of Methylene Blue at 665 nm. The analysis of Cu2+ ion is performed by a fixed-time method. At a given time of 2 min at pH 2.20 and 32°C, the detection limit is 10 ng mL-1 and the relative standard deviation for 0.4 µg mL-1 Cu2+ is 3.60% (n = 6). The method is free from most of the interferences and the effect of diverse ions on the determination of Cu2+ is also reported. The proposed method is virtually specific to copper and has been satisfactorily applied to its determination in electric copper wire samples and pharmaceutical products. Results were also verified by the atomic absorption spectrometry technique (AAS).
A rapid, simple, precise and accurate method is proposed for the determination of ternary mixtures of periodate-iodate-bromate based on their reaction with iodide ion at different pH values. The absorbance was measured at 352 nm. Three sets of reaction conditions were developed. In the first set of conditions, only periodate reacted with iodide, but in the second set the periodate and iodate reacted with iodide and in the third set the three ions reacted with iodide during the first 3 min after initiation of the reaction. The method could be used for individual determinations of periodate, iodate and bromate in the concentration range of 0.05 - 8.0 µg/ml, 0.05 - 5.0 µg/ml and 0.2 - 12 µg/ml, respectively. The data were evaluated by simultaneous equations.
The acid-base properties of the antihistamine (H1 receptor antagonist) cetirizine have been studied by using a previously developed multiwavelength spectrophotometric titration method. A new computational procedure called “Two-Step-Divide-and-Conquer” (TSDC), which applied evolving factor analysis (EFA) and target factor analysis (TFA), has been derived to unravel the micro-equilibria of the triprotic zwitterionic compound from the spectral data. We have demonstrated that a single spectrophotometric titration experiment is sufficient to determine the 12 unknown microconstants and the distribution of microspecies, which are in good agreement with literature values, where available.
A flow-injection on-line photochemical spectrofluorometry (FI-PF) was developed for the determination of menadione sodium bisulfite (MSB) using acetone and sodium sulfite as sensitizing reagents. An injected sample band carried by a water stream was on-line merged with a mixed NaOH, Na2SO3 and acetone solution in a “T” connector. It was then driven to pass a knotted PTFE photochemical reactor (0.5 mm i.d. × 200 cm, KR) that was freely coiled around a 6-W low-pressure mercury lamp. While passing the KR, MSB was derived into an intensively fluorescent compound that was on-line delivered into a flow-through cell and detected therein at an emission wavelength of 459 nm and an excitation wavelength of 336 nm. Under optimized conditions a detection limit of 0.38 µg l-1 was achieved at a sampling rate of 90 h-1. Eleven determinations of 0.5 mg l-1 and 0.05 mg l-1 MSB standard solution gave RSDs of 0.75% and 1.3%, respectively. The calibration curve was linear in the MSB concentration range 0.005 - 1.5 mg l-1. The proposed method was successfully applied to assay the MSB content in MSB injection.