Determination of Trace Proteins by Rayleigh Light Scattering Technique with Indophenol Blue

The interaction of indophenol blue (IPB) with proteins in aqueous solution has been studied by optical absorption and Rayleigh light scattering (RLS) spectroscopy. At pH 3.8, the weak RLS of IPB is enhanced by proteins. Based on this phenomenon, a novel method for the determination of proteins at nanogram levels using the RLS technique is developed. The method is simple, practical and sensitive. The linear range is 0.25–20.9µgmL^–1 for BSA, and 0.25–17.6µgmL^–1 for HSA. The detection limits (S/N=3) are 23ngmL^–1 for BSA and 22ngmL^–1 for HAS. The results for the determination of proteins in human serum samples are very close to those obtained by an established clinical method. There is very little interference from amino acids, metal ions or other coexisting compounds.     

Determination of Caffeine Using a Simple Graphite Pencil Electrode with Square-Wave Anodic Stripping Voltammetry

A simple commercial graphite pencil electrode (GPE) was utilized for monitoring caffeine using the square-wave anodic stripping voltammetry (SWASV) method. This method was applied to determine the caffeine levels in several tea samples, which yielded a relative error of 1% in the concentrations. Caffeine was deposited at 0.0V (vs. Ag/AgCl), then reduced at +1.40V to strip it on the GPE. Optimal experimental conditions for the analysis were found to be as follows: pH value of 9 for the medium; deposition potential of 0.0V; deposition time of 120s; SW frequency of 25Hz; SW amplitude of 45mV, and step potential of 6mV. Given these optimum conditions, a linear range was observed within the concentration of 0500mgL^–1. At caffeine concentrations of 50.0, 250.0, and 500.0mgL^–1, the relative standard deviations in measured concentrations (n=12) were 0.19, 0.09, and 0.11%, respectively. The detection limit was found to be 9.2mgL^–1, which is comparable with the result obtained using a carbon paste electrode, equivalent to 8.2mgL^–1.     

Electrochemical Determination of 4-Nitrophenol Using a Single-Wall Carbon Nanotube Film-Coated Glassy Carbon Electrode

A single-wall carbon nanotubes (SWNT) film coated glassy carbon electrode (GCE) was fabricated for the direct determination of 4-nitrophenol (4-NP). The electrochemical behaviors of 4-NP at the SWNT-film coated GCE were examined. In 0.1M phosphate buffer with a pH of 5.0, 4-NP yields a very sensitive and well-defined reduction peak at the SWNT-modified GCE. It is found that the SWNT film exhibits obvious electrocatalytic activity towards the reduction of 4-NP since it not only increases the reduction peak current but also lowers the reduction overpotential. Based on this, an electrochemical method was proposed for the direct determination of 4-NP. The reduction peak current varies linearly with the concentration of 4-NP ranging from 1×10^–8 to 5×10^–6M, and the detection limit is 2.5×10^–9M after 3min of open-circuit accumulation. The relative standard deviation at 2×10^–7M 4-NP was about 6% (n=10), suggesting excellent reproducibility. This new method was successfully employed to determine 4-NP in several lake water samples.     

Multi-Syringe Flow Injection System with In-Line Pre-Concentration for the Determination of Total Phenolic Compounds

A multi-syringe flow injection system for the determination of total phenolics based on the 4-aminoantipyrine reaction is proposed. In-line preconcentration using solid-phase extraction was implemented, offering an environmentally friendly alternative to organic solvent extraction performed in the batch procedure and in previously described flow methodologies. Therefore, phenolic compounds were retained in Amberlite XAD4 resin using acidic conditions. The elution was carried out using 0.10molL^–1 NaOH solution, promoting the ionisation of the retained phenolic compounds, and thus their desorption.Since different elution profiles were obtained for several phenol derivatives, the calibration curves were established using the peak area as analytical signal. Moreover, a dynamic calibration range was applied by using a variable sample volume (0.500–6.00mL) and calibration curves based on the mass of phenol retained in the pre-concentration column. A methodology based on a fixed sample volume (24.0mL) was also proposed for determination at concentration levels below 50µgL^–1. A limit of detection of 52ng of phenol (9 or 2µgL^–1) was achieved with a sampling frequency of 10–16h^–1 for the variable volume method and 4h^–1 for the fixed volume method. The application of the proposed methodology to reference material provided results that were within the certified acceptance limits.     

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.     

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.     

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     

Tishchenko Reaction Over Solid Base Catalysts

Catalytic behavior of solid bases for mixed Tishchenko reaction in which an equimolar mixture of two different aldehydes is allowed to react was investigated employing the combinations of benzaldehyde and pivalaldehyde, pivalaldehyde and cyclopropanecarbaldehyde, and cyclopropanecarbaldehyde and benzaldehyde. The reactions were performed at 353K for 4h in vacuo without solvent using 5mmol of each aldehyde and 100mg of solid base catalyst. For all the combinations, the catalytic activity of alkaline earth oxides increased in the order of BaOMgO2O₃, ZrO₂, ZnO, -alumina, hydrotalcite, KF/alumina, and KOH/alumina produced either no amount or very small amounts of cross-esterification and self-esterification products. Quantum chemical calculations carried out at the PM3-MO level for the positive charges on the carbonyl carbon atoms of aldehydes and the structure parameters of the active species for the ester formations indicated that the selectivities to four Tishchenko dimers over MgO and CaO are determined primarily in the step of the nucleophilic addition of the active species (PhCH₂O^-, ^tBuCH₂O^-, and C₃H₅CH₂O^-) to the carbonyl carbon atoms of aldehydes. The reaction of the aldehyde having more positively charged and sterically less hindered carbonyl carbon atom with the active species having less hindered oxygen atom proceeds faster.We also attempted the application of solid base catalysts to the challenging Tishchenko reaction of furfural, and excellent results were obtained with CaO and SrO. For instance, when furfural (10mmol) was treated with SrO (100mg) without solvent at 353K for 6h in vacuo, almost quantitative conversion to the corresponding ester was accomplished. Furthermore, application of SrO to the Tishchenko reaction of 3-furaldehyde was carried out successfully. The catalytic systems were also successfully applicable to the intramolecular Tishchenko reaction (lactonization) of o-phthalaldehyde to phthalide. For example, treatment of o-phthalaldehyde (1mmol) with CaO (50mg) in benzene (1mL) solvent at 313K under N₂ produced phthalide quantitatively in a short time of 15min. We finally refer to the perspective of application of solid base catalysts to Tishchenko reaction.     

Solid-Phase Extraction and Spectrophotometric Determination of Trace Amounts of Mercury in Natural Samples

A sensitive and selective solid phase spectrophotometric method for the determination of trace amounts of inorganic mercury is described. Hg²⁺ was sorbed on a silica gel-packed column as an Hg²⁺–N,N-bis(2-mercaptophenyl)ethanediamide (H₂L) complex. The Hg²⁺ complex was eluted from the column using 7mL of acetone. Various parameters including pH, column flow rate, and ligand concentration were optimized. The complex was found to obey Beers law from 2.3 to 73.7µgmL^–1 within the optimum range when the preconcentration factor was two. The effective molar absorption coefficient at 523nm was 1.17×10³Lmol^–1cm^–1 at 523nm. The concentration limits in Beers law dropped from 0.09 to 2.95µgmL^–1 within the optimum range when the preconcentration factor was 50. The relative standard deviation at a concentration level of 5µgmL^–1 Hg²⁺ (9 repetitive determinations) was 1.6%. The detection limits are 0.34µgmL^–1 and 0.015µgmL^–1 when the preconcentration factors are 2 and 50, respectively. The method has been used for routine determination of trace levels of Hg²⁺ in natural waters. The potential application of this method for the removal of Hg²⁺ from natural samples (sea water and lake water) spiked with 100ngmL^–1 of Hg²⁺ was studied. In order to validate the proposed method, LGC 6156 (harbour sediment – extractable metals) was analysed by this method. The results proved that excellent extraction of Hg²⁺ from both natural water samples was obtained by solid phase extraction using N,N-bis(2-mercaptophenyl) ethanediamide.     

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.     

Non-Aqueous Capillary Electrophoresis with Indirect Photometric Detection for Direct Analysis of Pb2+, Zn2+ and Cd2+ Ions

A new approach, based on non-aqueous capillary electrophoresis separation and indirect photometric detection, was established for the determination of the transition metal ions Pb²⁺, Zn²⁺ and Cd²⁺. Under optimized conditions, the method produced baseline separation of these three metal ions. The linear range and detection limits were 1050µM, 1.9µM for Cd²⁺; 1050µM, 2.1µM for Zn²⁺; and 20100µM, 3.8µM for Pb²⁺, respectively.     

Selective Spectrophotometric Determination of Aluminium in the Presence of Beryllium and Lanthanide Cations

A spectrophotometric method for the selective determination of Al(III) in the presence of Be and Ln (lanthanide) cations is proposed. It is based on the selective reaction of SXO (Semi-Xylenol Orange) with Al(III) at pH 2.6. The presence of 8%v/v of 1,2-ethanediol serves to stabilize the chelate formed by heating at 100°C for 5min. 0.5mg each of Be(II), Ce(III), La(III), 4mg of Mn(II), 1.2mg of Pb(II), 1mg of Tl(I), 40mg each of Ca(II) and Mg(II) and 1.9mg of sodium dihydrogen phosphate are tolerable. A ligand buffer of HEDTA-Pb is incorporated to further enhance the selectivity of the color reaction. Under the specified conditions SXO reacts with Al(III) to form a 1:1 chelate. Its molar absorptivity at 526nm was found to be 3.3×10⁴Lmol^–1cm^–1. The linear regression equation for 2–20µg of Al(III) is A=0.04458C+0.0112 (where C stands for the concentration of Al(III), µg per 25mL) and correlation coefficient =0.9988. The RSD at the level of 10µg (n=10) and LOQ were found to be 3.5% and 2µg, respectively.     

Structural studies and diffusion measurements of water-swollen cellophane by NMR imaging

NMR imaging and spatially resolved diffusometry have been used to study the distribution of water within swollen cellophane and measure its diffusion coefficient. Water concentration and diffusion coefficient were found to be essentially constant across most of the film thickness. However, significantly slower diffusion was indicated for water near the film surface (D=0.5×10^–9m²s^–1) compared with water in the centre of the film (D=0.88×10^–9m²s^–1). This was also reflected in lower T ₂ values at the edge of the film indicating water with more restricted motion. These observations were interpreted in terms of dense surface regions of cellulose (skin) over a more porous interior (core).     

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.     

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.     

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.     

Electrochemical Determination of Ascorbic Acid at γ-MnO<Subscript>2</Subscript> Modified Carbon Black Microelectrodes

A carbon black microelectrode modified by -MnO₂ has been prepared. The electrocatalytical oxidation of ascorbic acid (AA) at this microelectrode was investigated. The 2nd-order linear scan voltammograms of AA are recorded from –0.5 to 0.5V (vs. SCE). The relationship between the oxidation peak current of AA and its concentration in the range of 1.0×10^–64.0×10^–3molL^–1 is linear. The detection limit (3) was found to be 6.0×10^–7molL^–1. Also, the determination of AA in samples is evaluated, and the results are satisfactory.     

Photoluminescence of osmocene and decamethylosmocene

Osmocene (and decamethylosmocene) is characterized by a lowest-energy ligand field triplet which occurs at _max=372nm (374nm) in absorption and 567nm (572nm) in emission. This orange–yellow phosphorescence is rather intense at 77K but is also visible at r.t.     

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.