Determination of carbon dioxide in gaseous samples by gas diffusion-flow injection

A gas diffusion-flow injection system was developed for the determination of carbon dioxide in gaseous samples. The calibration was based on the use of either gaseous carbon dioxide or aqueous sodium carbonate standards. Gaseous carbon dioxide samples and gaseous or aqueous standards were injected directly into a donor stream of 1.0×10⁻⁴ M H₂SO₄. In the gas diffusion unit, carbon dioxide diffused through a PTFE membrane into an acceptor stream containing a mixed acid/base indicator. The absorbance of the acceptor stream was monitored spectrophotometrically at 554 nm. The calibration plot was linear over the range of 5.00×10² to 1.27×10⁴ μl l⁻¹ with a sample throughput of 28 h⁻¹ and 3.2% R.S.D. ([CO₂]=2.37×10³ μl l⁻¹, n=12). The detection limit was determined as 2.50×10² μl l⁻¹. The flow system was successfully applied to the analysis of several natural gaseous samples and the headspace of milk containers during storage. The flow injection results were found to be statistically indistinguishable at the 95% confidence level from those obtained by gas chromatography using thermal conductivity detection.     

Low pressure solubility and thermodynamics of solvation of oxygen,carbon dioxide,and carbon monoxide in fluorinated liquids

The solubility of oxygen, carbon dioxide, and carbon monoxide in three fluorinated liquids – perfluorohexylethane, perfluorooctane and bromoperfluorooctane – is presented. Mole fraction solubilities were calculated from new experimental Ostwald coefficient data for CO₂ and CO, and from previously published values for O₂, associated with original values of density and vapour pressure for the pure solvents. Carbon dioxide is the most soluble gas with mole fraction solubilities of the order of 10⁻². Oxygen and carbon monoxide are one order of magnitude less soluble. The measurements were done as a function of temperature between (288 and 313) K and from the variation of the calculated Henry’s law constants with temperature, the thermodynamic properties of solvation such as the Gibbs free energy, the enthalpy and the entropy were calculated. The precision of the experimental data, considered as the average absolute deviation of the Henry’s law constants from appropriate smoothing equations is of 1% for carbon dioxide and oxygen and of 3% for carbon monoxide. The data obtained here are judged accurate to within ±5%.     

An environmentally friendly multicommutated alternative to the reference method for anionic surfactant determination in water

It has been developed a fully mechanized procedure for the spectrophotometric determination of anionic surfactants in water expressed in terms of SDS concentration. The reference method, based on the reaction of SDS with methylene blue (MB) followed by extraction in chloroform, was mechanized in order to reduce the consumption of organic solvents. The system was based on the multicommutation approach and provided a 35 times reduction of the waste production without sacrificing the figures of merit of the method in terms of sensitivity and repeatability, for a dynamic linear range from 0.2 to 1.7 mg l⁻¹. Results obtained for washing water samples were comparable with those obtained using the reference method and no significant differences, at 95% confidence level, were observed. Other useful characteristics are a solvent consumption of 0.7 ml per determination, a sampling throughput of 40 determinations per hour, a relative standard deviation of 5.9% (n = 10) for a sample containing 2 × 10⁻⁶ mol l⁻¹ (576 μg l⁻¹) surfactant and a limit of detection of 6.1 × 10⁻⁹ mol l⁻¹ (1.7 μg l⁻¹).     

A highly sensitive amperometric sensor for oxygen based on iron(II) tetrasulfonated phthalocyanine and iron(III) tetra-(N-methyl-pyridyl)-porphyrin multilayers

The development of a highly sensitive sensor for oxygen is proposed using a glassy carbon (GC) electrode modified with alternated layers of iron(II) tetrasulfonated phthalocyanine (FeTsPc) and iron(III) tetra-(N-methyl-pyridyl)-porphyrin (FeT4MPyP). The modified electrode showed excellent catalytic activity for the oxygen reduction. The reduction potential of the oxygen was shifted about 330 mV toward less negative values with this modified electrode, presenting a peak current much higher than those observed on a bare GC electrode. Cyclic voltammetry and rotating disk electrode (RDE) experiments indicated that the oxygen reduction reaction involves 4 electrons with a heterogenous rate constant (k_obs) of 3 × 10⁵ mol⁻¹ L s⁻¹. A linear response range from 0.2 up to 6.4 mg L⁻¹, with a sensitivity of 4.12 μA L mg⁻¹ (or 20.65 μA cm⁻² L mg⁻¹) and a detection limit of 0.06 mg L⁻¹ were obtained with this sensor. The repeatability of the proposed sensor, evaluated in terms of relative standard deviation (R.S.D.) was 2.0% for 10 measurements of a solution of 6.4 mg L⁻¹ oxygen. The sensor was applied to determine oxygen in pond and tap water samples showing to be a promising tool for this purpose.     

Potentiometric stripping analysis of methyl and ethyl parathion employing carbon nanoparticles and halloysite nanoclay modified carbon paste electrode

Carbon nanoparticles (CNPs) and halloysite nanoclay (HNC) modified carbon paste electrode (HNC–CNP–CPE) was developed for the determination of methyl parathion (MP) and ethyl parathion (EP). The electrochemical behavior of these molecules was investigated employing cyclic voltammetry (CV), chronocoulometry (CC), electrochemical impedance spectroscopy (EIS) and potentiometric stripping analysis (PSA). After optimization of analytical conditions employing this electrode at pH 5.0 in acetate buffer (0.1 M), the peak currents were found to vary linearly with its concentration in the range of 1.55 × 10⁻⁹ to 3.67 × 10⁻⁶ M and 1.21 × 10⁻⁹ to 4.92 × 10⁻⁶ M for MP and EP, respectively. The detection limits (S/N = 3) of 4.70 × 10⁻¹⁰ M and 3.67 × 10⁻¹⁰ M were obtained for MP and EP, respectively, using PSA. The prepared modified electrode showed several advantages such as simple preparation method, high sensitivity, very low detection limits and excellent reproducibility. The proposed method was employed for the determination of MP and EP in fruits, vegetables, water and soil samples.     

Preparation of silver hexacyanoferrate nanoparticles and its application for the simultaneous determination of ascorbic acid, dopamine and uric acid

A silver hexacyanoferrate nanoparticles/carbon nanotubes modified glassy carbon electrode was fabricated and then successfully used for the simultaneous determination of ascorbic acid, dopamine and uric acid by cyclic voltammetry. A detailed investigation by transmission electron microscopy (TEM) and electrochemistry was performed in order to elucidate the preparation process and properties of the nanocomposites. The size of silver hexacyanoferrate nanoparticles was examined by TEM around 27 nm. Linear calibration plots were obtained over the range of 4.0 × 10⁻⁶-7.8 × 10⁻⁵, 2.4 × 10⁻⁶-1.3 × 10⁻⁴ and 2.0 × 10⁻⁶-1.5 × 10⁻⁴ mol L⁻¹ with detection limits of 4.2 × 10⁻⁷,1.4 × 10⁻⁷ and 6.0 × 10⁻⁸ mol L⁻¹ for ascorbic acid, dopamine and uric acid, respectively. The practical analytical utilities of the modified electrode were demonstrated by the determination of ascorbic acid, dopamine and uric acid in urine and human blood serum samples.     

A novel high selective and sensitive para-nitrophenol voltammetric sensor, based on a molecularly imprinted polymer-carbon paste electrode

By using a molecularly imprinted polymer (MIP) as a recognition element, the design and construction of a high selective voltammetric sensor for para-nitrophenol was formed. Para-nitrophenol selective MIP and a non-imprinted polymer (NIP) were synthesized, and then used for carbon paste (CP) electrode preparation. The MIP-CP electrode showed greater recognition ability in comparison to the NIP-CP. It was shown that electrode washing after para-nitrophenol extraction led to enhanced selectivity, without noticeably decreasing the sensitivity. Some parameters affecting sensor response were optimized and a calibration curve was plotted. A dynamic linear range of 8 × 10⁻⁹ to 5 × 10⁻⁶ mol L⁻¹ was obtained. The detection limit of the sensor was calculated as 3 × 10⁻⁹ mol L⁻¹. Thus, this sensor was used successfully for the para-nitrophenol determination in different water samples.     

Anodic stripping voltammetry at a new type of disposable bismuth-plated carbon paste mini-electrodes

A new type of disposable carbon paste mini-electrodes (CPmEs), with dimensions in the 50-300 μm range, have been fabricated by heat-shrinking the end-tip of plastic micropipette tips and filling them with carbon paste. The CPmEs have been characterized by microscopic and electrochemical means and tested as substrates for in situ plated Bi film electrodes (BiF-CPmEs), used in the determination of heavy metals by square wave anodic stripping voltammetry (SWASV). It was found that this new class of CPmEs combines the advantages of carbon paste electrodes (readily renewable surface and high surface area) with those of near-microelectrode behaviour (no stirring or electrolyte excess needed). During SWASV experiments in unstirred Pb(II) and Cd(II) solutions well-shaped stripping peaks were obtained whose height varied linearly with analyte concentration in the wide 1 × 10⁻⁸ to 10⁻⁶ M range, both in acetate buffer and unbuffered solutions. Under optimal conditions detection limits of 8 × 10⁻¹⁰ and 1.3 × 10⁻⁹ M were achieved for Pb(II) and Cd(II), respectively and in a trial application, these metal ions have been determined in a spiked tap water sample using a BiF-CPmE.     

Determination of indomethacin in urine using molecule imprinting-chemiluminescence method

A soluble Mn (IV)-formaldehyde-indomethacin chemiluminescence system was found. Using a synthesized indomethacin MIP as recognition material and soluble Mn (IV)-formaldehyde-indomethacin as detection system, a new molecule imprinted-chemiluminesence method of determination of indomethacin was established. The response range of this method was between 1.0 × 10⁻⁷ and 1.0 × 10⁻⁵ g/mL with a linear correlation coefficient of 0.994. The detection limit was 4 × 10⁻⁸ g/mL. The relative standard deviation for 5.0 × 10⁻⁷ g/mL of indomethacin solution was 3.1% (n = 7).     

A novel high selective and sensitive metronidazole voltammetric sensor based on a molecularly imprinted polymer-carbon paste electrode

The design and construction of a highly selective voltammetric sensor for metronidazole by using a molecularly imprinted polymer (MIP) as recognition element were introduced. A metronidazole selective MIP and a nonimprinted polymer (NIP) were synthesized and then incorporated in the carbon paste electrodes (CPEs). The sensor was applied for metronidazole determination using cathodic stripping voltammetric method. The MIP-CP electrode showed very high recognition ability in comparison to NIP-CPE. Some parameters affecting the sensor response were optimized and then the calibration curve was plotted. Two dynamic linear ranges of 5.64 × 10⁻⁵ to 2.63 × 10⁻³ mg L⁻¹ and 2.63 × 10⁻³ to 7.69 × 10⁻² mg L⁻¹ were obtained. The detection limit of the sensor was calculated as 3.59 × 10⁻⁵ mg L⁻¹. This sensor was used successfully for metronidazole determination in biological fluids.     

Development of a sensitive and selective Riboflavin sensor based on carbon ionic liquid electrode

The electrochemical properties of Riboflavin adsorbed on carbon ionic liquid electrode (CILE) were studied by cyclic voltammetry. A film with a surface coverage of up to 3.3 × 10⁻⁹ mol cm⁻² was formed after 10 min exposure time. Electron transfer coefficient and rate constant of electron transfer across the modified electrode were found to be 0.43 and 3.03 s⁻¹, respectively. Differential pulse voltammetry was used for the determination of Riboflavin. Two linear working ranges of 0.8-110 nM and 0.11-1.0 μM were obtained with correlation coefficients of 0.998 and 0.996, respectively. The experimental detection limit was obtained as 0.1 nM. The relative standard deviation for five replicate analyses was 4.7%. Other soluble vitamins had no significant interferences and the electrode was used for the determination of Riboflavin in pharmaceutical products, nutrition and beverages.     

Sensitive and selective determination of molybdenum by flow injection chemiluminescence method combined with controlled potential electrolysis technique

A sensitive and selective flow injection chemiluminescence (CL) method combined with controlled potential electrolysis technique was described for the determination of molybdenum. The method is based on the chemiluminescence reaction of luminol with unstable molybdenum(III) in alkaline solution. The molybdenum(III) was on-line reduced from molybdenum(VI) via controlled potential electrolysis technique using a homemade flow-through carbon electrolytic cell at the potential of −0.6 V (versus Ag/AgCl). The method allows the determination of molybdenum in the 5.0×10⁻¹⁰ to 5.0×10⁻⁷ g ml⁻¹ range with a limit of detection (3σ) of 5×10⁻¹¹ g ml⁻¹ molybdenum. The relative standard deviation is 2.6% for the 1.0×10⁻⁹ g ml⁻¹ molybdenum solution in 11 repeated measurements. This method was successfully applied to the determination of molybdenum in water samples.     

Simultaneous determination of butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT) in food samples using a carbon composite electrode modified with Cu3(PO4)2 immobilized in polyester resin

A simple electrochemical method was developed for the single and simultaneous determination of butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT) in food samples using square-wave voltammetry (SWV). A carbon composite electrode modified (MCCE) with copper (II) phosphate immobilized in a polyester resin was proposed. The modified electrode allowed the detection of BHA and BHT at potentials lower than those observed at unmodified electrodes. A separation of about 430 mV between the peak oxidation potentials of BHA and BHT in binary mixtures was obtained. The calibration curves for the simultaneous determination of BHA and BHT demonstrated an excellent linear response in the range from 3.4 × 10⁻⁷ to 4.1 × 10⁻⁵ mol L⁻¹ for both compounds. The detection limits for the simultaneous determination of BHA and BHT were 7.2 × 10⁻⁸ and 9.3 × 10⁻⁸ mol L⁻¹, respectively. In addition, the stability and repeatability of the electrode were determined. The proposed method was successfully applied in the simultaneous determination of BHA and BHT in several food samples, and the results obtained were found to be similar to those obtained using the high performance liquid chromatography method with agreement at 95% confidence level.     

A novel protocol for ultra-trace detection of pesticides: Combined electrochemical reduction of Ellman's reagent with acetylcholinesterase inhibition

This paper proposed a novel method for ultra-trace detection of pesticides combining electrochemical reduction of Ellman's reagent with acetylcholinesterase (AChE) inhibition. The amperometric biosensor, fabricated by immobilizing AChE on multi-walled carbon nanotubes-chitosan (MWCNTs-Chi) nanocomposites modified glassy carbon electrode, enjoyed high sensitivity owing to the excellent conductivity and favourable biocompatibility of MWCNTs-Chi nanocomposites. Meanwhile, the sensitivity of the biosensor was further enhanced using the electrochemical reduction signal of DTNB for determination. Under optimum conditions, methyl parathion was detected based on its inhibition effect on AChE activity and the subsequent change in electrochemical reduction response of DTNB. Good relationship was obtained between the reduction current and pesticide concentration in the ranges of 5.0 × 10⁻⁷ to 1.0 × 10⁻¹² M with a detection limit of 7.5 × 10⁻¹³ M (S/N = 3). Moreover, the proposed protocol was successfully employed for the determination of methyl parathion in water and soil samples.     

Stripping voltammetric determination of silymarin in formulations and human blood utilizing bare and modified carbon paste electrodes

Silymarin is one of the most powerful natural substances that have the ability to protect and rebuild the liver cells damaged by alcohol and other toxic substances. Silymarin showed two irreversible anodic peaks in buffered solutions (pH 2.5-8.0) at either the bare carbon paste electrode or the montmorillonite-Ca modified carbon paste one. These two peaks have been attributed to oxidation of the two phenolic OH groups at positions C-20 and C-7 of silymarin molecule. A square-wave adsorptive anodic stripping voltammetry method was optimized for determination of silymarin utilizing the bare and the modified carbon paste electrodes. The method was fully validated and successfully applied for the determination of silymarin in commercial formulations and human serum without prior extraction utilizing both carbon paste electrodes. Limits of quantitation of 1 × 10⁻⁷ and 7 × 10⁻⁹ mol L⁻¹ silymarin have been achieved in bulk form or in formulations while 2 × 10⁻⁷ and 8 × 10⁻⁹ mol L⁻¹ silymarin were achieved in spiked human serum utilizing the bare carbon paste electrode and the modified one, respectively. The two electrodes exhibited excellent selectivity towards silymarin even in the presence of 10²to 10³-fold excess of its co-formulated drugs, common excipients, and common metal ions. The pharmacokinetic parameters of silymarin in plasma of healthy human volunteers were estimated following the administration of a single oral dose of 120 mg silymarin utilizing the modified carbon paste electrode. The estimated pharmacokinetic parameters were favorably compared with those reported in literature.     

An ultrasensitive post chemiluminescence reaction of ammonium in NBS-dichlorofluorescein system and its application

A strong post chemiluminescence (PCL) phenomenon was observed when ammonium was injected into the reaction mixture after the finish of CL reaction of N-bromosuccinimide (NBS) and dichlorofluorescein. Based on this, a sensitive flow injection PCL method was established for the determination of ammonium. The possible CL mechanism of the reaction was proposed based on a series of experiments. The PCL intensity responded linearly to the concentration of ammonium in the range 3.0 × 10⁻¹¹-1.0 × 10⁻⁷ g mL⁻¹ with a detection limit of 1 × 10⁻¹¹ g mL⁻¹. The relative standard deviation (R.S.D.) was 1.4% for 1.0 × 10⁻⁹ g mL⁻¹ ammonium (n = 11). This method had been applied to the determination of ammonium in samples of mineral water, tap water and river water.     

Bia-amperometric quantification of salbutamol in pharmaceutical products

This paper presents a simple, rapid and reproducible method of analysis of salbutamol in pharmaceutical products, utilizing batch injection analysis (BIA) associated with amperometric detection. A study of salbutamol oxidation demonstrated a strong dependence between electrode fouling and pH. All determinations were done utilizing a glassy carbon electrode in presence of 3.0 mol l⁻¹ NaOH. A large linear dynamic range from 8×10⁻⁷ to 2×10⁻⁴ mol l⁻¹ was obtained by using an injected volume of 100 μl with a detection limit of 2.5×10⁻⁷ mol l⁻¹. R.S.D. of 0.92% for 50 successive injections of 4×10⁻⁶ mol l⁻¹ of salbutamol and a sample throughput of 60 samples per hour were achieved. The method was applied for salbutamol quantification in syrups.     

Analysis of parabens in cosmetics by low pressure liquid chromatography with monolithic column and chemiluminescent detection

This paper presents an application of chromatographic separation based on an ultra-short monolithic column and chemiluminescent detection in an FIA type instrument manifold for the determination of four paraben mixtures: methylparaben (MP), ethylparaben (EP), propylparaben (PP) and butylparaben (BP). The separation is achieved in 150 s using two consecutive carriers: first 12% ACN:water that changes 75 s after injection to 27% ACN:water. The detection is based on the oxidation of the hydrolysis product of parabens, p-hydroxybenzoic acid, with Ce(IV) in the presence of Rhodamine 6G which evokes chemiluminescence of sufficient intensity to enable a sensitive determination of these species. After optimization of the variables involved, the analytical method is characterized, displaying the following values for concentration ranges, detection limits and precision, as relative standard deviation at low concentration (0.15 mg l⁻¹)—MP: from 9.9 × 10⁻⁷ to 3.3 × 10⁻⁴ M; 1.9 × 10⁻⁸; 5.6%; EP: from 9.0 × 10⁻⁷ to 3.3 × 10⁻⁴ M; 2.8 × 10⁻⁸; 3.5%; PP: from 8.3 × 10⁻⁷ to 9.9 × 10⁻⁵ M; 2.3 × 10⁻⁸; 4.2%; and BP: from 7.7 × 10⁻⁷ to 9.9 × 10⁻⁵ M; 4.2 × 10⁻⁸ M; 6.2%. The method was applied and validated satisfactorily for the determination of these parabens in cosmetic samples, comparing the results against a liquid chromatography reference method.     

Application of manganese (II) phthalocyanine synthesized in situ in the SiO2/SnO2 mixed oxide matrix for determination of dissolved oxygen by electrochemical techniques

This work describes the in situ immobilization of Mn(II) phthalocyanine (MnPc) in a porous SiO₂/SnO₂ mixed oxide matrix obtained by the sol gel processing method. The chemically modified matrix SiO₂/SnO₂/MnPc, possessing an estimated amount of 8 × 10⁻¹⁰ mol cm⁻² of MnPc on the surface, was used to prepare an electrode to analyze dissolved oxygen in water by an electrochemical technique. The electrode was prepared by mixing the material with ultrapure graphite and evaluated using differential pulse voltammetry. Dissolved O₂ was reduced at −0.31 V with a limit of detection (LOD) equal to 7.0 × 10⁻⁴ mmol L⁻¹. A mechanism involving four electrons in O₂ reduction was determined by the rotating disk electrode technique.     

Simultaneous spectrofluorimetric determination of levodopa and propranolol in urine using feed-forward neural networks assisted by principal component analysis

The simultaneous determination of levodopa (LD) and propranolol (PRO) using fluorescence spectrometric technique is described. The method involves measuring the natural fluorescence of these drugs in the micellar media of sodium dodecyl sulfate (SDS) using principal component analysis-feed-forward neural networks (PC-FFNNs). Experimental conditions such as effect of pH and SDS concentration were optimized. Under the optimum conditions, the linear determination ranges of LD and PRO are 2.0 × 10⁻⁸ to 1.0 × 10⁻⁵ mol L⁻¹ and 3.6 × 10⁻⁹ to 1.8 × 10⁻⁶ mol L⁻¹, respectively. A set of synthetic binary mixtures of LD and PRO was prepared and their concentrations were predicted by the proposed method. Satisfactory results were obtained by the combination of fluorescence technique with chemometrics methods. The method was successfully applied to the determination of LD and PRO in tap water and in urine samples.