Simultaneous voltammetric detection of ascorbic acid, dopamine and uric acid using a pyrolytic graphite electrode modified into dopamine solution

Pyrolytic graphite electrodes (PGE) were modified into dopamine solutions using phosphate buffer solutions, pH 10 and 6.5, as supporting electrolyte. The modification process involved a previous anodization of the working electrode at +1.5 V into 0.1 mol L⁻¹ NaOH followed by other anodization step, in the same experimental conditions, into dopamine (DA) solutions. pH of the supporting electrolyte performed an important role in the production of a superficial melanin polymeric film, which permitted the simultaneous detection of ascorbic acid (AA), (DA) and uric acid (UA), ΔE_AA-DA = 222 mV; ΔE_AA-UA = 360 mV and ΔE_DA-UA = 138 mV, avoiding the superficial poisoning effects. The calculated detection limits were: 1.4 × 10⁻⁶ mol L⁻¹ for uric acid, 1.3 × 10⁻⁵ mol L⁻¹ for ascorbic acid and 1.1 × 10⁻⁷ mol L⁻¹ for dopamine, with sensitivities of (7.7 ± 0.5), (0.061 ± 0.001) and (9.5 ± 0.05) A mol⁻¹ cm⁻², respectively, with no mutual interference. Uric acid was determined in urine, blood and serum human samples after dilution in phosphate buffer and no additional sample pre-treatment was necessary. The concentration of uric acid in urine was higher than the values found in blood and serum and the recovery tests (92-102%) indicated that no matrix effects were observed.     

Linear polyamines as carriers in thiocyanate-selective membrane electrodes

The polyamines, octyl-[2-(2-octylamino-ethylamino)-ethyl]-amine (L¹) and octyl-{2-[2-(2-octylamino-ethylamino)-ethylamino]-ethyl}-amine (L²), have been used as anion ionophores in PVC-based membrane ion-selective electrodes. Different electrodes were prepared containing L¹, or L², and o-nitrophenyl octyl ether (NPOE) or bis(2-ethylhexyl)sebacate (DOS) as plasticizers. The response of the electrodes was tested in two different buffers, HEPES-KOH (pH 7) and MES-KOH (pH 5.6). Electrodes containing L¹ and L² with NPOE (E1 and E2, respectively) showed a Nernstian response for thiocyanate with a good response time. The detection limit, linear range and slope for electrode E1 were 3.8 × 10⁻⁶ mol dm⁻³, 1 × 10⁻⁵ to 1 × 10⁻¹ mol dm⁻³ and −57.2 mV decade⁻¹ at pH 5.6 and 4.47 × 10⁻⁶ mol dm⁻³, 1.95 × 10⁻⁵ to 1 × 10⁻¹ mol dm⁻³ and −58.1 mV decade⁻¹ at pH 7.0. For electrode E2 the detection limit, linear range and slope found were 2.63 × 10⁻⁶ mol dm⁻³, 7.94 × 10⁻⁶ to 1 × 10⁻¹ mol dm⁻³ and −58.5 mV decade⁻¹ at pH 5.6 and 1.23 × 10⁻⁵ mol dm⁻³, 7.95 × 10⁻⁵ to 1 × 10⁻¹ mol dm⁻³ and −46.0 mV decade⁻¹ at pH 7. In contrast, electrodes containing DOS as plasticizers gave only response at pH 5.6 (detection limit, linear range and slope at pH 5.6 were 3.16 × 10⁻⁵ mol dm⁻³, 1 × 10⁻⁴ to 1 × 10⁻¹ mol dm⁻³ and −52.6 mV decade⁻¹). Selectivity coefficients for different anions with respect to thiocyanate were calculated. The electrode E2 at pH 5.6 was also used for the determination of SCN⁻ by potentiometric titrations with Ag⁺ ions with good results. The electrode E2 was also used to determine concentrations of thiocyanate in biological samples.     

Studying the distribution pattern of selenium in nut proteins with information obtained from SEC-UV-ICP-MS and CE-ICP-MS

In this work, size exclusion chromatography (SEC) with UV and inductively coupled plasma mass spectrometry (ICP-MS) detection was used to study the association of selenium to proteins present in Brazil nuts (Bertholletia excelsa) under five different extraction conditions. As expected, better solubilization of proteins was observed using 0.05 mol L⁻¹ sodium hydroxide and 1% sodium dodecylsulfate (SDS) in Tris/HCl buffer (0.05 mol L⁻¹, pH 8) as compared to 0.05 mol L⁻¹ HCl, 0.05 mol L⁻¹ Tris/HCl or hot water (60 °C). Due to non-destructive character of Tris-SDS treatment, this was applied for studying molecular weight (MW) distribution patterns of selenium-containing nut proteins. Three different SEC columns were used for obtaining complete MW distribution of selenium: Superdex 75, Superdex Peptide, and Superdex 200 were tested with 50 mmol L⁻¹ Tris buffer (pH 8), 150 mmol L⁻¹ ammonium bicarbonate buffer (pH 7.8), phosphate (pH 7.5), and CAPS (pH 10.0) mobile phases. Using Superdex 200 column, the elution of at least three MW fractions was observed with UV detection (200-10 kDa) and ICP-MS chromatogram showed the co-elution of selenium with the two earlier fractions. The apparent MWs of these selenium-containing fractions were respectively about 107 and 50 kDa, as evaluated from the column calibration. For further characterization of individual selenium species, the defatted nuts were hydrolyzed with proteinase K and analyzed by capillary electrophoresis (CE) with ICP-MS detection. The suitability of CE for the separation of selenite, selenate, selenocystine and selenomethionine in the presence of the nut sample matrix is demonstrated. Complete separation of the above mentioned selenium species was obtained within a migration time of 7 min. In the analysis of nut extracts with CE-ICP-MS, selenium was found to be present mainly as selenomethionine.     

Determination of brominated phenols in water samples by on-line coupled isotachophoresis with capillary zone electrophoresis

A method for determination of nine brominated phenols as environmental risk compounds was developed by on-line coupled capillary isotachophoresis and capillary zone electrophoresis (ITP–CZE). For ITP step, 1 × 10⁻² mol L⁻¹ hydrochloric acid with 3 × 10⁻² mol L⁻¹ ammediol pH 9.1 was used as the leading electrolyte, and 3 × 10⁻² mol L⁻¹ β-alanine with 2 × 10⁻² mol L⁻¹ sodium hydroxide pH 10.05 was used as the terminating electrolyte. As the background electrolyte for CZE separation, 2.5 × 10⁻² mol L⁻¹ β-alanine with 2.5 × 10⁻² mol L⁻¹ lysine pH 9.6 was used. All electrolytes contained 0.05% or 0.1% (m/v) hydroxyethylcellulose to suppress the electroosmotic flow. UV detection at wavelength 220 nm was used. Detection limits in order of tens of nmol L⁻¹ were achieved. Good repeatability of migration times (less than 0.33% RSD) and good repeatability of peak areas (less than 7.19% RSD) at concentration level 5 × 10⁻⁸ mol L⁻¹ were observed. Developed ITP–CZE method was applied to determination of brominated phenols in spiked tap and river water samples.     

Zwitterionic hydrophilic interaction chromatography coupled with post-column derivatization for the analysis of glutathione in wine samples

In this study the development, validation and application of a new chromatographic method for the determination of glutathione (GSH) in wine samples is presented. The separation of the GSH was carried out using a sulfobetaine-based hydrophilic interaction chromatography (HILIC) analytical column whereas its detection was carried out spectrofluorimetrically (λ_ext/λ_em = 340/455 nm) after post-column derivatization with o-phthalaldehyde. GSH was separated efficiently from matrix endogenous compounds of wines by using a mobile phase of 15 mmol L⁻¹ CH₃COONH₄ (pH = 2.5)/CH₃CN, 35/65% (v/v). The parameters of the post-column reaction (pH, amount concentration of the reagent and buffer solution, flow rate, length of the reaction coil) were investigated. The linear determination range for GSH was 0.25–5.0 μmol L⁻¹ and the LOD was 19 nmol L⁻¹. No matrix effect was observed, while the accuracy was evaluated with recovery experiments and was ranged between 89% and 108%.     

Analysis of cytokinin nucleotides by capillary zone electrophoresis with diode array and mass spectrometric detection in a recombinant enzyme in vitro reaction

A capillary zone electrophoresis (CZE) method for separation of adenosine and N⁶-isopentenyladenosine (cytokinin) nucleotides was developed, optimized and validated. Aqueous solutions of several amino acids were evaluated as the background electrolyte constituents. Separation of six nucleotides in less than 20 min with high theoretical plate number (up to 400 000 for isopentenyladenosine triphosphate) was achieved using a 100 mM sarcosine/ammonia buffer at pH 10.0. The detection limits of the CZE-UV method are in the low micromolar range (0.69–1.27 μmol L⁻¹). Good repeatability of migration times (within 1.3%), peak areas (within 1.8%) and linearity (R² > 0.999) was achieved over the concentration range 5–1000 μmol L⁻¹. The method was used to assay the activity of the recombinant Arabidopsis thaliana isopentenyltransferase 1 (AtIPT1). Baseline separation of isopentenylated nucleotides by CE–ESI-MS using a volatile buffer (30 mM ammonium formate; pH 10.0) was accomplished. The identities of the reaction products – isopentenyladenosine di- and triphosphate were confirmed by HPLC-QqTOF-MS. Dephosphorylation of ATP was observed as a parallel reaction.     

Optimization of the simultaneous determination of imatinib and its major metabolite, CGP74588, in human plasma by a rapid HPLC method using D-optimal experimental design

A simple, rapid and specific HPLC method has been developed and validated for the simultaneous determination of imatinib, a tyrosine kinase inhibitor, and its major metabolite, CGP74588, in human plasma. The optimization of the HPLC procedure involved several variables, of which the influences of each was studied. After a series of preliminary-screening experiments, the composition of the mobile phase and the pH of the added buffer solution were set as the investigated variables, while the resolution between imatinib and CGP74588 peaks, the retention time and the imatinib peak width were chosen as the dependent variables. Applying D-optimal design, the optimal chromatographic conditions for the separation were defined. The method proved to show good agreement between the experimental data and predictive values throughout the studied parameter range.The optimum assay conditions were achieved with a Chromolith™ Performance RP-8e 100 mm × 4.6 mm column and a mixture of methanol/acetonitrile/triethylamine/diammonium hydrogen phosphate (pH 6.25, 0.048 mol L⁻¹) (20:20:0.1:59.9, v/v/v/v) as the mobile phase at a flow rate of 2 mL min⁻¹ and detection wavelength of 261 nm. The run time was less than 5 min, which is much shorter than the previously optimized methods. The optimized method was validated according to FDA guidelines to confirm specificity, linearity, accuracy and precision.     

Determination of melamine and related triazine by-products ammeline, ammelide, and cyanuric acid by micellar electrokinetic chromatography

In this study, micellar electrokinetic chromatographic (MEKC) methods were developed for the detection of traces of melamine and its related by-products (ammeline, ammelide, and cyanuric acid). Two on-line sample concentration steps namely reversed electrode polarity stacking mode (REPSM) and cation-selective injection (CSI) were used for improving the detection sensitivity. For REPSM, a borate-NaOH buffer (pH 10, 35 mM) composed of 60 mM SDS and 10% (v/v) methanol, was used as carrier electrolyte, and samples were prepared in an aqueous solution of 10 mM NaOH. In CSI, a phosphate buffer (pH 2, 50 mM) containing 41 mM SDS was used as the carrier electrolyte, and samples were prepared with an aqueous solution of 10 mM NaOH and a phosphate buffer (pH 2.0, 25 mM) in a volume ratio of 1:9. The results indicated that REPSM enhanced all analyte signals except for melamine, which could be concentrated only by the CSI. The detection limit was reduced from 1.7 mg L⁻¹ to 2.8 μg L⁻¹ for melamine by the optimal CSI step, and from 0.23-1.2 mg L⁻¹ to 2.4-5.0 μg L⁻¹ for the other three analytes by the optimal REPSM step. Tableware made of melamine and samples of flour were used as test samples, and the results indicated that the proposed MEKC methods can successfully determine contaminations from melamine. The study also indicated that when the plastic made of melamine was exposed only once to an acidic solution (acetic or phosphoric acid) at 80 °C for 30 min, melamine continuously leached out from the test sample even without any further treatment with an acidic solution.     

Retention of ionisable compounds on high-performance liquid chromatography: XIX. pH variation in mobile phases containing formic acid,piperazine and tris as buffering systems and methanol as organic modifier

In previous works a model to estimate the pH of methanol–aqueous buffer mobile phases from the aqueous pH and concentration of the buffer and the fraction of organic modifier was developed. This model was successfully applied and validated for buffers prepared from ammonia, acetic, phosphoric and citric acids. In the present communication this model has been extended to formic acid, piperazine and tris(hydroxymethyl)aminomethane buffers. Prior to the modelling work, the pK_a values of the studied buffers at several methanol–water compositions were determined.     

Simultaneous voltammetric determination of paracetamol and caffeine in pharmaceutical formulations using a boron-doped diamond electrode

A simple and highly selective electrochemical method was developed for the single or simultaneous determination of paracetamol (N-acetyl-p-aminophenol, acetaminophen) and caffeine (3,7-dihydro-1,3,7-trimethyl-1H-purine-2,6-dione) in aqueous media (acetate buffer, pH 4.5) on a boron-doped diamond (BDD) electrode using square wave voltammetry (SWV) or differential pulse voltammetry (DPV). Using DPV with the cathodically pre-treated BDD electrode, a separation of about 550 mV between the peak oxidation potentials of paracetamol and caffeine present in binary mixtures was obtained. The calibration curves for the simultaneous determination of paracetamol and caffeine showed an excellent linear response, ranging from 5.0 × 10⁻⁷ mol L⁻¹ to 8.3 × 10⁻⁵ mol L⁻¹ for both compounds. The detection limits for the simultaneous determination of paracetamol and caffeine were 4.9 × 10⁻⁷ mol L⁻¹ and 3.5 × 10⁻⁸ mol L⁻¹, respectively. The proposed method was successfully applied in the simultaneous determination of paracetamol and caffeine in several pharmaceutical formulations (tablets), with results similar to those obtained using a high-performance liquid chromatography method (at 95% confidence level).     

Development and validation of a hydrophilic interaction liquid chromatographic method for determination of aromatic amines in environmental water

A simple, precise, and accurate hydrophilic interaction liquid chromatographic (HILIC) method has been developed for the determination of five aromatic amines in environmental water samples. Chromatography was carried out on a bare silica column, using a mixture of acetonitrile and a buffer of NaH₂PO₄–H₃PO₄ (pH 1.5, containing 10 mM NaH₂PO₄) (85:15, v/v) as a mobile phase at a flow rate of 1 mL min⁻¹. Aromatic amines were detected by UV absorbance at 254 nm. The linear range of amines was good (r² > 0.998) and limit of detection (LOD) within 0.02–0.2 mg L⁻¹ (S/N = 3). The retention mechanism for the analytes under the optimum conditions was determined to be a combination of adsorption, partition and ionic interactions. The proposed method was applied to the environmental water samples. Aromatic amines were isolated from aqueous samples using solid-phase extraction (SPE) with Oasis HLB cartridges. Recoveries of greater than 75% with precision (RSD) less than 12% were obtained at amine concentrations of 5–50 μg L⁻¹ from 100 mL river water and influents from a wastewater treatment plant (WWTP). The present HILIC technique proved to be a viable method for the analysis of aromatic amines in the environmental water samples.     

Sensitive detection of clozapine using a gold electrode modified with 16-mercaptohexadecanoic acid self-assembled monolayer

Clozapine, an effective antipsychotic drug, was found generating a pair of redox peaks at about 0.33-0.4 V (versus SCE) at 16-mercaptohexadecanoic acid (i.e. MHA) self-assembled monolayer (SAM) modified gold electrode (i.e. MHA/Au) in 0.05 mol L⁻¹ Tris-HCl (pH 8.1) buffer solution. Sensitive and quantitative measurement of clozapine based on anodic peak was established under optimum conditions. The anodic peak current was linear to clozapine concentration in the range from 1 × 10⁻⁶ to 5 × 10⁻⁵ mol L⁻¹ with the detection limit of 7 × 10⁻⁹ mol L⁻¹. This method was successfully applied to the detection of clozapine in drug tablets and proved to be reliable compared with ultraviolet spectrophotometry (UV). The MHA SAM was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), contact angle goniometry, electrochemical impedance spectroscopy (EIS) and electrochemical probe.     

High-performance liquid chromatographic method for determination of amino acids by precolumn derivatization with 4-chloro-3,5-dinitrobenzotrifluoride

This work presents an high-performance liquid chromatography method for the determination of amino acids after precolumn derivatization with 4-chloro-3,5-dinitrobenzotrifluoride (CNBF) which can readily react with both primary and secondary amines. The precolumn derivatization conditions, including the CNBF concentration, reaction pH, temperature and reaction time were investigated for method optimization. In pH 9.0 borate buffer, the reaction of amino acids with CNBF was carried out at 60 °C for 30 min, the optimized concentration of CNBF was 70 mmol L⁻¹ and the molar ratio of amino acids to CNBF was 1:5.25. The chromatographic separation of 19 amino acids derivatives was performed on a Kromasil ODS C₁₈ column (250 mm × 4.6 mm, 5 μm) with good reproducibility, and ultraviolet detection was applied at 260 nm. The mobile phase was a mixture of phase A (acetonitrile) and phase B (acetate buffer, acetonitrile, triethylamine; 82.8:17:0.2, pH 4.9), and the flow rate was 0.4 mL min⁻¹. The separation of all the labeled amino acids was achieved within 45 min at room temperature by gradient elution mode. The method linearity, calculated for each amino acid, had a correlation coefficient higher than 0.9979, in concentrations ranging from 9.60 to 3330.00 μmol L⁻¹. The detection limits of amino acids were 2.40-6.50 μmol L⁻¹, at a signal-to-noise ratio of 3. The proposed method was applied for the determination of amino acids in beer with recoveries of 97.0-103.9% and relative standard deviations of 2.62-4.22%, respectively. This method showed good accuracy and repeatability that can be used for the quantification of amino acids in real samples.     

Rosmarinic acid determination using biomimetic sensor based on purple acid phosphatase mimetic

A new heterodinuclear Fe(III)Zn(II) complex which mimics the active site of the hydrolytic enzyme red kidney bean purple acid phosphatase was synthesized and characterized by IR, CHN and X-ray crystallographic analyses. This complex, [Fe^IIIZn^II(μ-OH)bpbpmp-CH₃](ClO₄)₂, containing the ligand (H₂bpbpmp-CH₃ = {2-[bis(2-pyridylmethyl)aminomethyl]-6-[(2-hydroxy-5-methylbenzyl) (2-pyridyl-methyl) aminomethyl]-4-methyl-phenol}) was employed in the construction of a biomimetic sensor and used in the determination of rosmarinic acid in plant extract samples. The response parameters and optimization of the biomimetic sensor design were evaluated. The best performance of this sensor was obtained for 75:15:10% (w/w/w) of the graphite powder:nujol:Fe(III)Zn(II) complex, 0.1 mol L⁻¹ phosphate buffer solution (pH 7.5), 1.19 × 10⁻⁴ mol L⁻¹ hydrogen peroxide with frequency, pulse amplitude, and scan increment at 30 Hz, 100 mV, and 0.6 mV, respectively. The rosmarinic acid concentration was linear in the range of 2.98 × 10⁻⁵ to 3.83 × 10⁻⁴ mol L⁻¹ (r = 0.9991) with a detection limit of 2.30 × 10⁻⁶ mol L⁻¹. This biomimetic sensor demonstrated long-term stability (300 days; 900 determinations) and reproducibility, with a relative standard deviation of 12.0%. The recovery study of rosmarinic acid in plant extract samples gave values from 90.3 to 98.3% and the concentrations determined showed agreement when compared with those obtained using capillary electrophoresis at the 95% confidence level.     

Chromatographic analysis of serotonin, 5-hydroxyindolacetic acid and homovanillic acid in dried blood spots and platelet poor and rich plasma samples

An isocratic high-performance liquid chromatographic method has been developed for the measurement of serotonin, 5-hydroxyindolacetic and homovanillic acids in dried blood spots and in platelet poor and rich plasma samples. Analyses were carried out on a C18 reversed-phase column using a mobile phase composed of 13% methanol and 87% aqueous citrate buffer, containing octanesulfonic and ethylendiaminotetracetic acids. Coulometric detection was used, setting the guard cell at +0.100 V, the first analytical cell at −0.200 V and the second analytical cell at +0.400 V. For the pre-treatment of biological samples a novel solid-phase extraction procedure, based on mixed-mode reversed-phase – strong anion exchange Oasis cartridges, was implemented. Extraction yields of the analytes from all these matrices were satisfactory, being always higher than 89.0%. The calibration curve was linear over the on-column concentration range of 0.1–22.5 ng mL⁻¹ for serotonin and 5-hydroxyindolacetic acid and of 0.25–22.5 ng mL⁻¹ for homovanillic acid. The sensitivity was good with a limit of detection of 0.05 ng mL⁻¹ for serotonin and 5-hydroxyindolacetic acid and 0.12 ng mL⁻¹ for homovanillic acid. Results were also satisfactory in terms of precision, selectivity and accuracy. The analytical method was successfully applied to human platelet poor and rich plasma samples and to dried blood spots from volunteers and psychiatric patients.     

Determination of artificial sweeteners by capillary electrophoresis with contactless conductivity detection optimized by hydrodynamic pumping

The common sweeteners aspartame, cyclamate, saccharin and acesulfame K were determined by capillary electrophoresis with contactless conductivity detection. In order to obtain the best compromise between separation efficiency and analysis time hydrodynamic pumping was imposed during the electrophoresis run employing a sequential injection manifold based on a syringe pump. Band broadening was avoided by using capillaries of a narrow 10 μm internal diameter. The analyses were carried out in an aqueous running buffer consisting of 150 mM 2-(cyclohexylamino)ethanesulfonic acid and 400 mM tris(hydroxymethyl)aminomethane at pH 9.1 in order to render all analytes in the fully deprotonated anionic form. The use of surface modification to eliminate or reverse the electroosmotic flow was not necessary due to the superimposed bulk flow. The use of hydrodynamic pumping allowed easy optimization, either for fast separations (80 s) or low detection limits (6.5 μmol L⁻¹, 5.0 μmol L⁻¹, 4.0 μmol L⁻¹ and 3.8 μmol L⁻¹ for aspartame, cyclamate, saccharin and acesulfame K respectively, at a separation time of 190 s). The conditions for fast separations not only led to higher limits of detection but also to a narrower dynamic range. However, the settings can be changed readily between separations if needed. The four compounds were determined successfully in food samples.     

Rapid simultaneous analysis of 1-hydroxypyrene, 2-hydroxyfluorene, 9-hydroxyphenanthrene, 1- and 2-naphthol in urine by first derivative synchronous fluorescence spectrometry using Tween-20 as a sensitizer

A novel method of first derivative synchronous fluorescence was developed for the rapid simultaneous analysis of trace 1-hydroxypyrene (1-OHP), 1-naphthol (1-NAP), 2-naphthol (2-NAP), 9-hydroxyphenanthrene (9-OHPe) and 2-hydroxyfluorene (2-OHFlu) in human urine. Only one single scan was needed for quantitative determination of five compounds simultaneously when Δλ = 10 nm was chosen. In the optimal experimental conditions, there was a linear relationship between the fluorescence intensity and the concentration of 1-OHP, 1-NAP, 2-NAP, 9-OHPe and 2-OHFlu in the range of 1.75 × 10⁻⁹ to 4.50 × 10⁻⁶ mol L⁻¹, 3.64 × 10⁻⁸ to 2.20 × 10⁻⁴ mol L⁻¹, 8.18 × 10⁻⁹ to 1.20 × 10⁻⁴ mol L⁻¹, 3.26 × 10⁻⁹ to 8.50 × 10⁻⁵ mol L⁻¹ and 4.88 × 10⁻⁹ to 5.50 × 10⁻⁶ mol L⁻¹, respectively. The limits of detection (LOD) were found to be 5.25 × 10⁻¹⁰ mol L⁻¹ for 1-OHP, 1.10 × 10⁻⁸ mol L⁻¹ for 1-NAP, 2.46 × 10⁻⁹ mol L⁻¹ for 2-NAP, 9.77 × 10⁻¹⁰ mol L⁻¹ for 9-OHPe and 1.46 × 10⁻⁹ mol L⁻¹ for 2-OHFlu. The proposed method is reliable, selective and sensitive, and has been used successfully in the determination of traces of 1-OHP, 1-NAP, 2-NAP, 9-OHPe and 2-OHFlu in human urine samples, whose results were in good agreement with those gained by the HPLC method.     

Determination of uranium by adsorptive stripping voltammetry at a lead film electrode

An adsorptive stripping voltammetric procedure for the determination of U(VI) at an in situ plated lead film electrode is described. The U(VI) complex with cupferron was accumulated from an acetate buffer solution of pH 4.2 at the potential −0.65 V. The measurements were carried out from undeaerated solutions. The calibration graph for an accumulation time of 180 s was linear from 5 × 10⁻¹⁰ to 2 × 10⁻⁸ mol L⁻¹. The detection limit was 2 × 10⁻¹⁰ mol L⁻¹, the relative standard deviation for 2 × 10⁻⁸ mol L⁻¹ U(VI) was 4.3%. The proposed procedure was validated in the course of U(VI) determination in water certified reference materials.     

Simultaneous determination of tiopronin and d-penicillamine in human urine by liquid chromatography with ultraviolet detection

d-Penicillamine and tiopronin are drugs widely used for the treatment of many diseases. Because of the relatively high frequency of side effects to these compounds, some of which are dose-related, drug monitoring in urine samples during treatment is advisable. In this paper, we describe a simple method for the determination of tiopronin and d-penicillamine in human urine. The method was based on derivatization with 2-chloro-1-methylquinolinium tetrafluoroborate followed by ion-pairing reversed-phase liquid chromatography separation and ultraviolet-absorbance detection. 2-S-quinolinium derivatives of thiols were detected at 355 nm. The derivatization was optimized in terms of pH and time of the reaction. Baseline separation was achieved on an analytical Zorbax SB C-18 (5 μm, 150 mm × 4.6 mm) column with a mobile phase consisting of pH 2.0 0.09 mol L⁻¹ trichloroacetic acid buffer (component A) and acetonitrile (component B) pumped at 1.0 mL min⁻¹. Gradient elution was used: 0-4 min, 12% B; 4-8 min, 12-40% B; 8-12 min, 40-12% B. The d-penicillamine and tiopronin standards added to the urine show that the response of the detector is linear within the range studied, from 1 to 200 μmol L⁻¹ urine. The imprecision ranges for tiopronin and d-penicillamine were within 1.61-8.24% and 2.92-10.60%, respectively. The analytical accuracy for determined compounds was from 97.24 to 109.39%. The lower limits of detection and quantitation were 0.5 μmol L⁻¹ and 1.0 μmol L⁻¹ urine, respectively. This method can be used for routine clinical monitoring of the title thiol-drugs. Cysteine can be measured concurrently, if needed.     

Poly(methylene blue) functionalized graphene modified carbon ionic liquid electrode for the electrochemical detection of dopamine

An ionic liquid 1-butylpyridinium hexafluorophosphate based carbon ionic liquid electrode (CILE) was used as the substrate electrode and a poly(methylene blue) (PMB) functionalized graphene (GR) composite film was co-electrodeposited on CILE surface by cyclic voltammetry. The PMB–GR/CILE exhibited better electrochemical performances with higher conductivity and lower electron transfer resistance. Electrochemical behavior of dopamine (DA) was further investigated by cyclic voltammetry and a pair of well-defined redox peaks appeared with the peak-to-peak separation (ΔE_p) as 0.058 V in 0.1 mol L⁻¹ pH 6.0 phosphate buffer solution, which proved a fast quasi-reversible electron transfer process on the modified electrode. Electrochemical parameters of DA on PMB–GR/CILE were calculated with the electron transfer number as 1.83, the charge transfer coefficients as 0.70, the apparent heterogeneous electron transfer rate constant as 1.72 s⁻¹ and the diffusional coefficient (D) as 3.45 × 10⁻⁴ cm² s⁻¹, respectively. Under the optimal conditions with differential pulse voltammetric measurement, the linear relationship between the oxidation peak current of DA and its concentration was obtained in the range from 0.02 to 800.0 μmol L⁻¹ with the detection limit as 5.6 nmol L⁻¹ (3σ). The coexisting substances exhibited no interference and PMB–GR/CILE was applied to the detection of DA injection samples and human urine samples with satisfactory results.