Solid-Phase Extraction with Slurry Injection of the Resin Into ETAAS for Trace Determination of Thallium in Natural Water

Thallium in natural water samples was determined by electrothermal atomic absorption spectrometry after 1000-fold enrichment by mini solid-phase extraction from a 100-mL sample solution. A Tl-pyrrolidine-1-carbodithioate complex formed in a sample solution of pH 1.6 was extracted on fine particles of a cellulose nitrate resin dispersed in the sample solution. The cellulose nitrate resin was then collected on a membrane filter (25mm^ø) by filtration under suction using a glass funnel with an effective filtration area of 0.64cm². As a result, a circular thin layer of the resin phase with a diameter of 9mm was obtained. Then the resin phase was carved out by an acrylate resin puncher with a 10-mm^ø hole to put it into a sample cup containing 100µL of 10mM HNO₃ containing 0.5mM NaCl. The resin phase was suspended in the solution by ultrasonication. 1000-fold enrichment was thus attained within 15min, and the suspension was delivered to electrothermal atomic absorption spectrometry. The linear calibration graph was obtained in the range of 0–4ng of Tl in 100mL of a sample solution. The detection limit obtained by 3 method was 0.19ng. The proposed method was applied to the determination of Tl in natural water samples. The results showed the concentration of Tl in seawater was 12.1±1.8pgmL^–1 for the calibration graph method and 12.6±1.4pgmL^–1 for the standard addition method. A snowmelt sample contained 20.7±1.0pgmL^–1 of Tl.     

Adsorptive Catalytic Voltammetry of Physcion in the Presence of Dissolved Oxygen at a Carbon Paste Electrode

A novel electroanalytical method for the determination of physcion is described for the first time. Physcion yields an adsorption catalytic voltammetric peak at –0.74V (vs. SCE) in 0.4molL^–1 NH₄Cl–NH₃·H₂O buffer solution (pH 10.5) at a carbon paste electrode (CPE). The experimental results indicated that physcion is efficiently accumulated at a CPE by adsorption. In the subsequent potential scan, physcion was reduced to a homologous anthrahydroquinone compound. The compound was then immediately oxidized to physcion by the dissolved oxygen in the solution, and then physcion was again reduced at the CPE. As a result, a cyclic catalytic reaction was established. The second-order derivative peak current is proportional to the physcion concentration in the ranges of 2.0×10^–104.0×10^–9molL^–1 (accumulation 90s) and 4.0×10^–92.0×10^–8molL^–1 (accumulation 60s). The limit of detection is 8×10^–11molL^–1 (S/N=3) for a 120s accumulation time. The method was applied to the direct determination of physcion in the medicinal plant polygonum multiflorum Thumb with satisfactory results.     

Energy Transfer Electrogenerated Chemiluminescence for the Determination of Sulfite

A simple and novel electrogenerated chemiluminescence (ECL) method for the determination of sulfite has been developed based on the energy transfer ECL process. It was found that a weak ECL signal of sulfite was electrochemically generated on a platinum electrode in neutral aqueous solution. The signal was strongly enhanced by rhodamine B as an energy receptor and further enhanced by the neutral surfactant Tween 80. In 0.10M phosphate buffer solution (pH=7.5) containing 2.0×10^–6gmL^–1 rhodamine B and 0.4% (v/v) Tween 80, the ECL response to the concentration of sulfite at a potential of 0.82V was linear over a range of 1.0×10^–7gmL^–1 to 8.0×10^–6gmL^–1, and the detection limit was 5×10^–8gmL^–1. The relative standard deviation (n=11, 1.0×10^–6gmL^–1) was 3.8%. The proposed method has been successfully applied to the determination of sulfite in pharmaceutical injections and white sugar samples.     

Application of Functionalized Ag Nanoparticles for the Determination of Proteins at Nanogram Levels Using the Resonance Light Scattering Method

A novel method for the determination of proteins in aqueous solutions has been developed based on the enhancement of resonance light scattering (RLS) of Ag nanoparticles in the presence of proteins. Factors including acidity of the media, concentration of Ag hydrosol, reaction time, temperature, and interference of non-protein substances were investigated. Under the optimal conditions, with the enhanced RLS signals at 452nm, the linear ranges of calibration curves were 0–0.8µgmL^–1 for bovine serum albumin (BSA), 0–1.2µgmL^–1 for human serum albumin (HSA), and 0–2.5µgmL^–1 for human -IgG (-IgG), respectively. The detection limits were 1.3ngmL^–1 for BSA, 10ngmL^–1 for HAS, and 5.7ngmL^–1 for -IgG.This method has been applied to the analysis of synthetic samples and real human serum samples, and the results were in good agreement with those reported by the hospital, indicating that the method presented here is not only sensitive and simple, but also reliable and suitable for practical applications.     

Determination of Attogram Quantities of Silver via Quenching of the Solid Substrate Room-Temperature Phosphorescence of Rhodamine B Based on the Catalysis of its Oxidation by Potassium Persulfate

A new method of SS-RTP for the determination of trace silver has been established. This method is based on the fact that Ag⁺, when activated by ,-bipyridyl (bipy) in a media of HAc–NaAc (pH=4.9), can catalyze the reaction of Rhodamine B (RhoB) oxidized by K₂S₂O₈, thus causing the Solid Substrate Room-Temperature Phosphorescence (SS-RTP) of RhoB to be quenched. The activating efficiency of bipy is 6.7 times higher than that of o-phenanthroline (phen). The reduction of the phosphorescence intensity (I_p) of RhoB is directly proportional to the concentration of Ag⁺ ions in the range of 1.6016.0agspot^–1 (0.40µLspot^–1). The regression equation of the working curve can be expressed as I_p=18.78+5.100m_Ag+ (agspot^–1) (r=0.9994, n=6), the detection limit is 0.28agspot^–1. This rapid, accurate and sensitive method has been successfully applied to the determination of trace silver in tea and human hair samples, and the results agree well with the Atomic Absorption Spectroscopy (AAS) method. The mechanism of the catalyzing reaction is also discussed.     

Adsorptive Voltammetric Study of Thorium–Alizarin Complexon Complex at a Carbon Paste Electrode

A new sensitive adsorptive voltammetric procedure is described for trace measurement of thorium. It is based on the cathodic stripping peak of the thorium–alizarin complexon (ALC) complex at a carbon paste electrode (CPE). The complex of Th(IV) with alizarin is adsorbed at a CPE in a mixed buffer solution (pH 5.0) which consists of 0.1mol·L^–1 sodium acetate and 0.04mol·L^–1 potassium biphthalate, yielding a sensitive cathodic voltammetric peak corresponding to the reduction of alizarin in the complex at –0.57V (vs. SCE). The second-order derivative peak current of the complex is linearly dependent upon the concentration of Th(IV) over the range of 3.0×10^–9 8.0×10^–7mol·L^–1. The detection limit is 1.0×10^–9mol·L^–1 for 180s accumulation. The molar ratio of each component in the complex was estimated as n_Th(IV):n_ALC=1:1 by a continuous variation method. The electrode processes of the Th(IV)–alizarin complex at a CPE were investigated. The procedure was successfully applied to the trace determination of thorium in ore and clay samples.     

Determination of Picogram-Levels of Acetylspiramycin in Human Urine and Serum by Flow Injection Chemiluminescence

A sensitive chemiluminescence method for the determination of acetylspiramycin is presented. It is based on the greatly enhancive effect of acetylspiramycin on the chemiluminescence reaction between luminol and hydrogen peroxide in the flow system. The increase in chemiluminescence intensity was linearly proportional to the acetylspiramycin concentration in the range from 10pg·mL^–1 to 2.0ng·mL^–1 (r²=0.9979). The detection limit was 3pg·mL^–1 (3). At a flow rate of 2.0mL·min^–1, the process of determination, including sampling and washing, could be performed in 0.5 min, and the relative standard deviations of seven replicates are less than 5.0%. The proposed method was applied successfully to the determination of acetylspiramycin in pharmaceutical preparations, human urine and serum without pre-treatment. It was found that the excretive ratio of acetylspiramycin reached its maximum 2.0 hours after having been administered orally, and the excretive ratio in 12.0 hours was 8.4.     

Metal Sorption,Solid Phase Extraction and Preconcentration Properties of Two Silica Gel Phases Chemically Modified with 2-Hydroxy-1-Naphthaldehyde

Active silica gel phase (I) was chemically modified to the corresponding amino- (SiNH₂) and chloro- (SiCl) derivatives via silylation reactions. These were used to synthesize two newly modified silica gel phases (II, III) by direct chemical reaction with 2-hydroxynaphthaldehyde (2-HNA). The surface coverage values are 370, 432µmolg^–1 and 320, 355µmolg^–1 for (II) and (III), on the basis of thermal desorption and metal probe testing method, respectively. The metal sorption properties of silica gel phases (II, III) were studied and compared with active silica gel phase (I). The maximum determined metal capacity values were found to be 10–110, 20–290 and 20–370µmolg^–1 for phases I, II and III, respectively. The distribution coefficient values (K_d) were also determined for a series of metal ions, and the results showed that the two new chemically modified phases (II and III) were highly selective for Cu²⁺, Zn²⁺, Cd²⁺, Hg²⁺ and Pb²⁺. The potential applications of silica gel phases (II, III) as solid phase extractors for the same five metal ions spiked in drinking tap water (1.000µgmL^–1) were found to give percentage recovery values in the range of 90.2–96.3±4.1–6.3%, while pre-concentration of the same five metal ions spiked in drinking tap water (50.0ngmL^–1) was successfully accomplished with a percentage recovery range of 92.6–95.8±4.8–5.7%.Received December 16, 2002; accepted May 14, 2003 published online September 1, 2003     

Kinetic Spectrophotometric Determination of Chromium (VI) by Oxidation of Sodium Pyrogallol-5-sulphonate by Hydrogen Peroxide

Summary. A new selective, sensitive, and simple kinetic method is developed for the determination of trace amounts of chromium (VI). The method is based on the catalytic effect of Cr(VI) on the reaction of sodium pyrogallol-5-sulphonate (PS) with hydrogen peroxide. The reaction is followed spectrophotometrically by tracing the oxidation product at 437nm within 1min after addition of H₂O₂. The optimum reaction conditions are PS (1.32·10^–3mol·dm^–3), H₂O₂ (0.32mol·dm^–3), HClO₄ (2.6·10^–3mol·dm^–3) at 25°C. Following this procedure, chromium (VI) can be determined with a linear calibration graph up to 0.25ng·cm^–3 and a detection limit of 0.024ng·cm^–3, based on the 3 criterion. The interference effect of several species was also investigated and it was found that the most common cations and anions do not interfere with the determination. The developed procedure was successfully applied to the determination of Cr(VI) and total Cr in river waters and total Cr in herbal samples.     

Electrocatalytic Reduction of Nitrite Using a Carbon Nanotube Electrode in the Presence of Cupric Ions

A simple, inexpensive method for determining nitrite is presented. With a carbon nanotube modified glass carbon electrode (GC), the overpotential for reduction of nitrite decreased, and direct reduction could be achieved in acid solution. Sensitivity, however, was not very high. When cupric ions were added to the solution, the reduction peak current increased significantly, and in particular the presence of multiple nitrate did not interfere. Experimental conditions were optimized, and preliminary studies were performed on the electrochemical mechanism of nitrite reduction in the presence of cupric ions on the carbon nanotube modified electrode. Under optimized conditions, the peak current of reduction achieved with the differential pulse voltammetric method was proportional to the concentration of nitrite in the ranges of 2.0×10^–6–1.0×thinsp;10^–5molL^–1 and 2.0×10^–5–1.0×10^–3molL^–1. The detection limit reached 5.0×10^–7molL^–1, and most of the inorganic ions did not interfere. The determination of nitrite in samples of rain water and river water was satisfactory.     

Voltammetric Behavior of Dopamine at ct-DNA Modified Carbon Fiber Microelectrode

A sub-micrometer thin-layer DNA modified carbon fiber microcylinder electrode was prepared by electrodeposition of ct-DNA at 1.5V (vs. Ag/AgCl). The voltammetric behavior of dopamine (3-hydroxytyramine) was investigated at the modified electrode. It was found that the modified electrode exhibits a highly electrocatalytic activity toward dopamine oxidation. Differential pulse voltammetry was used for determination of dopamine in pH 7.4 phosphate buffer solution. A linear response of the peak current versus the concentration was found in the range of 4×10^–6 to 10^–4molL^–1 at 10^–4molL^–1 AA (ascorbic acid) coexistence (R=0.9959) and the range of 6×10^–5 to 10^–3molL^–1 at 10^–3molL^–1 AA (R=0.9960). The presence of a high concentration of ascorbic acid did not interfere with the determination. The proposed method exhibited good recovery and reproducibility. This method can be applied to the detection of DA in real samples.     

Flame Atomic Absorption Spectrometric Determination of Gold and Palladium Using Microcolumn On-line Preconcentration and Separation

A microcolumn on-line preconcentration and separation system was developed for the flame atomic absorption spectrometric (FAAS) determination of trace levels of gold and palladium. The analytes were selectively adsorbed onto the microcolumn packed with 2-mercaptothiazole immobilized silica gel (MBTSG) in an acidity range of 0.1 to 6.0M HCl at a sampling flow rate of 4.0mLmin^–1. The analytes adsorbed could be desorbed by a thiourea solution with a flow rate of 2.0mLmin^–1. Most of the common coexisting metal ions at a concentration of 25.0mgmL^–1 and anions at a concentration of 50.0mgmL^–1 did not interfere with the preconcentration and determination of Au and Pd. The limits of detection (LOD), defined as three times the standard deviation of the blank (3), of Au and Pd are 10ngmL^–1 and 26ngmL^–1, respectively, with a preconcentration time of 60s. The relative standard deviation (RSD) is about 2.0% for 0.20µgmL^–1 Au and 0.30µgmL^–1 Pd. With a sample loading time of 30min, 6.7ngmL^–1 Au and 10ngmL^–1 Pd can be preconcentrated quantitatively. A geological sample, an anode slime and a secondary nickel alloy were successfully determined with the proposed method, and the results obtained showed good agreement with the certified values.Received December 23, 2002; accepted May 14, 2003 Published online August 8, 2003     

Electrochemical Behavior of Epinephrine at a Penicillamine Self-Assembled Gold Electrode,and its Analytical Application

The fabrication and electrochemical characteristics of a penicillamine (PCA) self-assembled monolayer modified gold electrode were investigated. The self-assembled electrode shows obvious electrocatalytic activity for the oxidation of epinephrine (EP). In phosphate buffer (pH 7.73), a sensitive oxidation peak was observed at 0.190V with the PCA modified Au electrode. The peak current is proportional to the concentration of EP in the range of 2.0×10^–56.0×10^–4molL^–1 and 5.0×10^–6 2.0×10^–4molL^–1 for cyclic voltammetry (CV) and differential pulse voltammetry (DPV) with the detection limits of 1.8×10^–7 and 1.3×10^–7molL^–1, respectively. The possible reaction mechanism is also discussed. The PCA self-assembled monolayer modified gold electrode is highly stable and can be applied to the determination of EP in practical injection samples. Application is simple, rapid and produces accurate results.     

Determination of Iron in Tap and Waste Water Using Liquid–Liquid Extraction in a Flow-Injection System

A flow-injection procedure for the determination of iron(III) in water is described. The procedure is based on the formation of an ion pair between the tetraphenylarsonium (Ph₄As⁺) (TPA) or tetrabutylammonium (But₄N⁺) (TBA) cations and the tetrathiocyanatoferrate(III) complex (TTF). This ion pair is extracted with chloroform, and the absorbance of the organic phase is measured at 503nm (for Ph₄As⁺) or 475nm (for But₄N⁺). Iron concentrations higher than 0.9×10^–6molL^–1 (50µgL^–1) can be detected in the first case, with a relative standard deviation of 1.9% (n=12), a linear application rangeof between 1.34 and 54.0×10^–6molL^–1 (75–3015µgL^–1), and a sampling frequency of 30h^–1. For the ion pair with But₄N⁺, the detection limit is 0.52×10^–6molL^–1 (29µgL^–1), with a relative standard deviation of 1.6% and a linear application range between 0.73 and 54.0×10^–6molL^–1. Under the proposed working conditions, only Pd(IV), Cu(II) and Bi(III) interfere. With the application of the merging zones technique, considerable amounts of organic reagent can be saved. The TBA method was applied to the analysis of iron(III) in tap and industrial waste waters.     

Determination of Heavy Metal Ions in Chinese Herbal Medicine by Microwave Digestion and RP-HPLC with UV-Vis Detection

A new method for the simultaneous determination of heavy metal ions in Chinese herbal medicine by microwave digestion and reversed-phase high-performance liquid chromatography (RP-HPLC) has been developed. The Chinese herbal medicine samples were digested by microwave digestion. Lead, cadmium, mercury, nickel, copper, zinc, and tin ions in the digested samples were pre-column derivatized with tetra-(4-chlorophenyl)-porphyrin (T₄-CPP) to form the colored chelates which were then enriched by solid phase extraction with C₁₈ cartridge and eluted from the cartridge with tetrahydrofuran (THF). The chelates were separated on a Waters Xterra RP₁₈ column by gradient elution with methanol (containing 0.05molL^–1 pyrrolidine-acetic acid buffer salt, pH=10.0) and THF (containing 0.05molL^–1 pyrrolidine-acetic acid buffer salt, pH=10.0) as mobile phase at a flow rate of 0.5mLmin^–1 and detected with a photodiode array detector in the range of 350–600nm. In the original samples the detection limits of lead, cadmium, mercury, nickel, copper, zinc and tin are 4ngL^–1, 3ngL^–1, 6ngL^–1, 5ngL^–1, 2ngL^–1, 6ngL^–1, and 4ngL^–1, respectively. This method was applied to the determination of lead, cadmium, mercury, nickel, copper, zinc and tin in Chinese herbal medicine samples with good results.     

Electrocatalytic Properties and Voltammetric Determination of Melatonin at a Nanostructured Film Electrode

A multi-wall carbon nanotube (MWNT) film coated glassy carbon electrode (GCE) was fabricated, and the electrochemical behavior of melatonin on the MWNT film coated GCE was investigated. The oxidation peak current of melatonin increases significantly and the oxidation peak position shifts positively at the MWNT film modified GCE compared to that at a bare GCE. This indicates that MWNTs feature highly effective catalysis to the electrochemical oxidation of melatonin. A simple and sensitive electroanalytical method was developed for the determination of melatonin. The oxidation peak current is proportional to the concentration of melatonin from 8×10^–8 to 1×10^–5molL^–1. The detection limit is about 2×10^–8molL^–1 for 3min accumulation. The proposed method was demonstrated to work satisfactorily with commercial capsules.     

Preparation and Application of a Novel Core-Shell Organic Nanoparticle as a Fluorescence Probe in the Determination of Nucleic Acids

A novel type of core-shell organic nanoparticles (pyrene/poly-acrylic acid; PAA) has been prepared by precipitation polymerization under ultrasonic irradiation. It was characterized by transmission electron microscopy (TEM) and fluorescence lifetime. The lifetime is about 5µs, which is much longer than conventional organic dyes fluorescence lifetime. As the surface of the core-shell organic nanoparticles is covered with abundant carboxylic groups, the nanoparticles are water-soluble, stable and biocompatible. At pH 6.4, maximum fluorescence is produced, with the maximum excitation and emission wavelengths of 338nm and 397nm, respectively. Under optimum conditions, the calibration graphs are linear over the range of 4.0×10^–51.0×10^–2mgmL^–1 for calf thymus DNA (ct-DNA) and 2.0×10^–41.0×10^–2mgmL^–1 for fish sperm DNA (fs-DNA). The detection limits were 3.6×10^–5mgmL^–1 and 1.4×10^–4mgmL^–1 for ct-DNA and fs-DNA, respectively. The method was applied to the determination of ct-DNA and fs-DNA in synthetic samples and the results were satisfactory. The method is sensitive, stable, rapid and tolerant towards most interfering substances.     

Fluorescence Quenching Studies of Curcumin by Hydrogen Peroxide in Acetonitrile Solution

Summary. Steady state quenching studies of curcumin, 1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione, fluorescence by hydrogen peroxide were conducted in acetonitrile solution. A quenching rate constant, k_q, of 1.05×10¹⁰M^–1·s^–1 was obtained with a short fluorescence lifetime of 347ps. The reaction rate constant, which is within the diffusion-limited regime, is activation-controlled. The rate constant of deactivation of the thermally excited curcumin was 1.2 orders of magnitude more nonradiative (2.67×10⁹s^–1) than radiative (2.16×10⁸s^–1). The reaction was exothermic with a G° of –1.97eV and solvent reorganization energy of 1.37eV. These values indicate that the electron transfer reaction is solvent-mediated with electron transfer rate constant, k_ET, of 2.16×10¹⁰s^–1.     

Theoretical Study of Chemical Binding of Noble Gas Atom and Transition Metal Complexes: Ng–NiCO, Ng–NiN2, Ng–CoCO (Ng = He–Xe)

Summary. Ab initio multireference and coupled cluster methods (MR-SDCI(+Q), CASPT2, CCSD(T)) and density functional theory methods (B3LYP, MPWPW91) have been applied to examine geometrical structures and vibrational frequencies of noble gas (Ng) – transition metal compounds, Ng–NiCO, Ng–NiN₂, and Ng–CoCO (Ng = He, Ne, Ar, Kr, Xe). It is shown that the respective compounds can have a larger binding energy than a typical van der Waals interaction energy. The binding mechanism is explained by a partial electron transfer from a noble gas atom to the low-lying 4s and 3d vacant orbitals of the transition metal atom. Theoretical calculations show that the binding of noble gas atom results in a large shift of the bending frequency: 361.1cm^–1 (NiCO) to 403.5cm^–1 (Ar–NiCO); 308.5cm^–1 (NiN₂) to 354.8cm^–1 (Ar–NiN₂); 373.0cm^–1 (CoCO) to 422.6cm^–1 (Ar–CoCO). The corresponding experimental frequencies determined in solid argon are 409.1cm^–1 (NiCO), 357.0cm^–1 (NiN₂), and 424.9cm^–1 (CoCO), which are much closer to the corresponding frequency of Ar–NiCO, Ar–NiN₂, and Ar–CoCO, respectively.     

Voltammetric Determination of Trace Amounts of Fenitrothion on a Novel Nano-TiO2 Polymer Film Electrode

A novel nano-TiO₂ polymer modified glassy carbon (GC) electrode was developed for the determination of an organophosphorous pesticide, fenitrothion (-NO₂), in citrate buffer solution. The electrochemical behavior of fenitrothion was characterized by using cyclic voltammetry. An irreversible form, -NO₂, was transformed into a reversible redox couple (-NHOH/-NO), and it can be used to determine trace fenitrothion by square wave voltammetry. The experimental parameters, such as film thickness, pH value, accumulation potential and time were optimized. Interestingly, a cyclic voltammetric scan was observed to be more effective than a constant potential for the accumulation of fenitrothion. A linear response over a fenitrothion concentration of 2.5×10^–8 to 1.0×10^–5M was exhibited, with a detection limit of 1.0×10^–8M (S/N=3). The high sensitivity and selectivity of this film electrode was demonstrated by its practical application to the determination of trace amounts of fenitrothion in lake water and apple samples.