Microanalytical Determination of Formaldehyde by Direct Titration with Hydroxylamine Using Interdigitated Microelectrode Array Biamperometric End-Point Indicator

 An interdigitated microelectrode array (IDA) biamperometric titration is presented for the microdetermination of formaldehyde. It is based on a direct titration of formaldehyde with hydroxylamine under the formation of formaldoxime. The released hydrogen cations react with hydroxyl anions in 0.1 M NaOH. The interdigitated pair of two individually polarizable microelectrodes serves as a biamperometric end-point indicator. Hydroxylamine together with dissolved free oxygen behave as a quasi reversible mixed redox system, therefore a significant increase of current flowing between the IDA segments polarized by 100 mV is observed when the first excess of titrant appears in the titrated solution. Because of the small dimension of the IDA sensor this principle can be extended for a microanalytical variant of this method. Formaldehyde contents down to 10⁻³ M in small volume waste water samples could be determined by this technique. These samples usually contain carboxylic acids which interfere in iodimetric determinations, but do not influence the titration of formaldehyde with hydroxylamine. Received January 21, 2002; accepted August 7, 2002     

Calibrationless determination of electroactive species using chronoamperograms at collector segment of interdigitated microelectrode array

The possibility of calibrationless chronoamperometric determination is described using a pair of individually addressable and diffusion layers interacting segments of interdigitated microelectrode array (IDA). It utilizes dual voltammetric mode where the first segment is polarized with potential corresponding to the limiting current of determined species electrode reaction and the second segment is polarized with potential corresponding to the opposite electrode reaction limiting current. Time at which the current of the collector segment reaches one half of the steady state is hyperbolically dependent on the diffusion coefficient of analyte. The determination of diffusion coefficient allows direct calculation of bulk concentration avoiding calibration with a standard solution. The equipment for measuring of fast response of IDA arrays in dual mode has been developed using a bipotentiostat connected with A/D transducer. It allows less than 1 ms sampling period for ultrafast registration of chronoamperogram. The method was tested and validated with [Fe(CN)₆]⁴⁻, [Ru(NH₃)₆]Cl₃, and ferrocene model samples using various types of IDA arrays.     

Novel surface acoustic wave-interdigitated array electrode gas sensor for dissolved ammonia

A novel surface acoustic wave-interdigitated array electrode (SAW-IDA) ammonia gas sensor is proposed. A gas-permeable membrane is employed to separate the buffer solution in the inner cell of the gas-sensing probe from the sample solution in the detection cell. The response of the IDA conductive electrodes is based on the impedance change of the buffer solution during ammonia adsorption. Therefore, this gas sensor overcomes the influence of water vapour in the conventional film-coated SAW gas sensor and can be used for the detection of gases in aqueous solutions. The ammonia sensor exhibits a favourable frequency response to 5 × 10⁻⁷−1 × 10⁻³ mol/1 ammonia. The optimal buffer composition and probe parameters have been determined. Dynamic range, response time, selectivity, and temperature drift are discussed. The ammonia sensor was also applied to the determination of serum ammonia. Results were in good agreement with those from the conventional enzymatic-spectro-photometric method.     

Adsorptive stripping voltammetric determination of netilmicin in the presence of formaldehyde

A linear sweep adsorptive stripping voltammetric method for the determination of netilmicin in the presence of formaldehyde has been proposed for the first time. In the presence of 3.0×10⁻³ g ml⁻¹ formaldehyde, netilmicin exhibits a sensitive cathodic peak at −1.30 V (vs. the saturated calomel electrode, SCE) in a medium of Britton–Robinson buffer (pH 8.7) with a scan rate of 100 mV s⁻¹ after a preconcentration period of 120 s at −1.10 V (vs. SCE). The peak current showed a linear dependence on the netilmicin concentration over the range 4.2×10⁻⁹–1.0×10⁻⁷ g ml⁻¹. The achieved limits of detection and quantitation were 1.0×10⁻¹⁰ and 3.3×10⁻¹⁰ g ml⁻¹ netilmicin, respectively. It was deduced from the experiments that the amine–aldehyde condensation product formed between netilmicin and formaldehyde is mainly responsible for the appearance of the peak. The electrochemical behavior of netilmicin in the presence of formaldehyde has been studied. The method was applied to the direct determination of netilmicin in injectable formulations and spiked human urine and serum samples.      

Oxidation of Cu(II) aminopolycarboxylates by carbonate radical: A flash photolysis study

Reactions of carbonate radical (Co₃ ⁻) generated by photolysis or by radiolysis of a carbonate solution, with Cu(II) complexes of aminopolycarboxylic acids viz., Cu(II)ethylenediamine tetraacetate [Cu^IIEDTA]²⁻ and Cu(II)-iminodiacetate [Cu^IIIDA] were studied at pH 10. 5 and ionic strength 0.2 mol·dm⁻³. Time-resolved spectroscopy and kinetics for the transients were studied using flash photolysis and stable products arising from the ligand degradation of the complex were ascertained by steady-state radiolysis experiments. From the kinetic data it is observed that CO₃ ⁻, radical reacts initially with Cu^II-complex to form a transient intermediate having maximum absorption at 335 nm and 430 nm. From the subsequent reactions of this intermediate it was assigned to be Cu^III. species. This Cu(III) species undergoes intermolecular electron transfer with the Cu^II-complex to give a radical intermediate which again slowly reacts with Cu^II-complex to give a long lived species containing Cu−C bond. This long lived species, however, slowly decomposed to give glyoxalic reaction between Cu^III-complex and a suitable donor, the one electron reduction potential for [Cu^IIIEDTA]¹⁻/[Cu^IIEDTA]²⁻ and [Cu^IIIIDA]⁺¹/Cu^IIIDA was determined.     

Electrochemical behavior of uric acid at a penicillamine self-assembled gold electrode

The fabrication and electrochemical characteristics of a penicillamine (PCA) self-assembled monolayer modified gold electrode were investigated. The electrode can enhance the electrochemical response of uric acid (UA), and the electrochemical reaction of UA on the PCA electrode has been studied by cyclic voltammetry and differential pulse voltammetry. Some electrochemical parameters, such as diffusion coefficient, standard rate constant, electron transfer coefficient and proton transfer number have been determined for the electrochemical behavior on the PCA self-assembled monolayer electrode. The electrode reaction of UA is an irreversible process, which is controlled by the diffusion of UA with two electrons and two protons transfer at the PCA/Au electrode. In phosphate buffer (pH 5.0), the peak current is proportional to the concentration of UA in the range of 6.0 × 10⁻⁵–7.0 × 10⁻⁴ mol L⁻¹ and 2.0 × 10⁻⁵–7.0 × 10⁻⁴ mol L⁻¹ for the cyclic voltammetry and differential pulse voltammetry methods with the detection limits of 5.0 × 10⁻⁶ and 3.0 × 10⁻⁶ mol L⁻¹, respectively. The method can be applied to determine UA concentration in real samples.     

Simple LC Method with Chemiluminescence Detection for Simultaneous Determination of Arbutin and l-Ascorbic Acid in Whitening Cosmetics

Simultaneous determination of arbutin (ART) and l-ascorbic acid (AA) by HPLC with chemiluminescence detection is proposed for the first time. This method is based on the CL reaction of acidic potassium permanganate with ART and AA in the presence of formaldehyde as enhancer. The separation was performed on a C₁₈ column with a 90:10 (v/v) mixture of 0.02 M phosphate buffer and methanol as mobile phase. The effects of several conditions on HPLC resolution and CL emission were studied systematically. The linear ranges were 0.5–50 and 1–200 μg mL⁻¹ for ART and AA, respectively. The detection limits were 0.2 and 0.3 μg mL⁻¹, respectively. The method was successfully applied to the determination of ART and AA in whitening cosmetics.     

Sequential spectrofluorimetric determination of free and total glycerol in biodiesel in a multicommuted flow system

A new procedure for spectrofluorimetric determination of free and total glycerol in biodiesel samples is presented. It is based on the oxidation of glycerol by periodate, forming formaldehyde, which reacts with acetylacetone, producing the luminescent 3,5-diacetyl-1,4-dihydrolutidine. A flow system with solenoid micro-pumps is proposed for solution handling. Free glycerol was extracted off-line from biodiesel samples with water, and total glycerol was converted to free glycerol by saponification with sodium ethylate under sonication. For free glycerol, a linear response was observed from 5 to 70 mg L⁻¹ with a detection limit of 0.5 mg L⁻¹, which corresponds to 2 mg kg⁻¹ in biodiesel. The coefficient of variation was 0.9% (20 mg L⁻¹, n = 10). For total glycerol, samples were diluted on-line, and the linear response range was 25 to 300 mg L⁻¹. The detection limit was 1.4 mg L⁻¹ (2.8 mg kg⁻¹ in biodiesel) with a coefficient of variation of 1.4% (200 mg L⁻¹, n = 10). The sampling rate was ca. 35 samples h⁻¹ and the procedure was applied to determination of free and total glycerol in biodiesel samples from soybean, cottonseed, and castor beans.     

Determination of formaldehyde by staircase voltammetry based on its electrocatalytic oxidation at a nickel electrode

In this paper, a novel method for detection of formaldehyde (HCHO), based on its electrocatalytic oxidation of HCHO at a nickel electrode, is reported. The mechanism of electrocatalytic oxidation and quantification of HCHO have been investigated by cyclic and staircase voltammetry, respectively. The electrocatalytic oxidation peak potential of HCHO is at about 475 mV vs. Ag/AgCl electrode; the peak current responds proportionally to concentrations of HCHO in alkaline solution. The linear range of detection is from 46.8 to 1640 μg/L (1.56 × 10⁻⁶ to 5.46 × 10⁻⁵ M) with a correlation coefficient of 0.996 and a detection limit of 23.4 μg/L (7.80 × 10⁻⁷ M). The relative standard deviation (RSD) is less than 6% (n = 5), and the recovery is in the range 98–106% for real samples. The result is consistent with that from the spectrophotometry. The text was submitted by the authors in English.     

Radiolysis of N,N-dimethylhydroxylamine and its radiolytic liquid organics

N,N-dimethylhydroxylamine (DMHA) is a novel salt-free reducing reagent used in the separation U from Pu and Np in the reprocessing of power spent fuel. This paper reports on the radiolysis of aqueous DMHA solution and its radiolytic liquid organics. Results show that the main organics in irradiated DMHA solution are N-methyl hydroxylamine, formaldehyde and formic acid. The analysis of DMHA and N-methyl hydroxylamine were performed by gas chromatography, and that of formaldehyde was performed by ultraviolet–visible spectrophotometry. The analysis of formic acid was performed by ion chromatography. For 0.1–0.5 mol L⁻¹ DMHA irradiated to 5–25 kGy, the residual DMHA concentration is (0.07–0.47) mol L⁻¹, the degradation rate of DMHA at 25 kGy is 10.1–30.1%. The concentrations of N-methylhydroxylamine, formaldehyde and formic acid are (8.25–19.36) × 10⁻³, (4.20–36.36) × 10⁻³ and (1.35–10.9) × 10⁻⁴ mol L⁻¹, respectively. The residual DMHA concentration decreases with the increasing dose. The concentrations of N-methylhydroxylamine and formaldehyde increase with the dose and initial DMHA concentration, and that of formic acid increases with the dose, but the relationship between the concentration of formic acid and initial DMHA concentration is not obvious.     

Isotope dilution quantification of ultratrace gamma-glutamyl-Se-methylselenocysteine species using HPLC with enhanced ICP–MS detection by ultrasonic nebulisation or carbon-loaded plasma

A method for the accurate determination of ultratrace selenium species of relevance to cancer research, such as gamma-glutamyl-Se-methylselenocysteine (γ-glutamyl-SeMC), using species-specific double isotope dilution analysis (IDA) with HPLC–ICP–MS is reported for the first time. The ⁷⁷Se-enriched γ-glutamyl-SeMC spike was produced in-house by collecting the fraction at the retention time of the γ-glutamyl-SeMC peak from a chromatographed aqueous extract of ⁷⁷Se-enriched yeast, pooling the collected fractions and freeze-drying the homogenate. The Se content of this spike was characterised using reverse isotope dilution mass spectrometry (IDMS) and the isotopic composition of this spike was checked prior to quantification of the natural abundance dipeptide species in garlic using speciated IDA. The extraction of the γ-glutamyl-SeMC species in water was performed in a sonication bath for 2 h after adding an appropriate quantity of ⁷⁷Se-enriched γ-glutamyl-SeMC to 50 mg of garlic to give optimal ⁷⁸Se/⁷⁷Se and ⁸²Se/⁷⁷Se ratios of 1.5 and 0.6, respectively. The effect of ultrasonic nebulisation, in comparison with the loading of the ICP with carbon (through the addition of methane gas on-line), on the detection of Se associated with γ-glutamyl-SeMC using collision/reaction cell ICP–MS with hydrogen as collision gas was investigated. Sensitivity enhancements of approximately fourfold and twofold were achieved using USN and methane mixed plasma, respectively, in comparison with conventional nebulisation and conventional Ar ICP–MS. However, an approximately twofold improvement in the detection limit was achieved using both approaches (42 ng kg⁻¹ for ⁷⁸Se using peak height measurements). The use of species-specific IDMS enabled quantification of the dipeptide species at ng g⁻¹ levels (603 ng g⁻¹ Se) in the complex food matrix with a relative standard deviation (RSD, n = 4) of 4.5%, which was approximately half that obtained using standard addition as a confirmatory technique. Furthermore, good agreement was found between the γ-glutamyl-SeMC species concentrations obtained using both calibration methods.     

Electrochemistry at cobalt(II)tetrasulfophthalocyanine-multi-walled carbon nanotubes modified glassy carbon electrode: a sensing platform for efficient suppression of ascorbic acid in the presence of epinephrine

Electrochemistry of water-soluble cobalt(II) tetrasulfophthalocyanine (CoTSPc) electrodeposited on glassy carbon nanotube pre-modified with acid-functionalized multi-walled carbon nanotubes (MWCNT) is described. Both charge transfer resistances toward [Fe(CN)₆]^3−/4− redox probe and electrocatalytic responses toward epinephrine (EP) detection follow the trend: bare GCE < GCE-MWCNT < GCE-CoTSPc < GCE-MWCNT-CoTSPc. EP analysis was then carried out in details using GCE-MWCNT-CoTSPc. The catalytic rate constant value k _ch = 2.2 × 10⁷ (mol cm⁻³)⁻¹ s⁻¹ was obtained from rotating disk electrode experiment. Interestingly, GCE-MWCNT-CoTSPc efficiently suppressed the detection of ascorbic acid (the natural interference of neurotransmitters in physiological conditions) showing good sensitivity (0.132 ± 0.003 A l mol⁻¹), limit of detection (4.517 × 10⁻⁷ mol l⁻¹), and quantification (15.056 × 10⁻⁷ mol l⁻¹). In addition, GCE-MWCNT-CoTSPc was conveniently used to determine EP in epinephrine hydrochloric acid injection with recovery of 101.1 ± 2.2%.     

Reagentless biosensor for phenolic compounds based on tyrosinase entrapped within gelatine film

A simple and new reagentless phenolic compound biosensor was constructed with tyrosinase immobilized in the gelatine matrix cross-linked with formaldehyde. The morphologies of gelatine and gelatine/tryosinase were characterized by SEM. The tyrosinase retains its bioactivity when being immobilized by the gelatine film. Phenolic compounds were determined by the direct reduction of biocatalytically liberated quinone at -0.1 V vs SCE. The process parameters for the fabrication of the enzyme electrode were studied. Optimization of the experimental parameters has been performed with regard to pH, operating potential, temperature and storage stability. This biosensor exhibits a fast amperometric response to phenolic compounds. The linear range for catechol, phenol, and p-Cresol determination was from 5×10⁻⁸ to 1.4×10⁻⁴ M, 5×10⁻⁸ to 7.1×10⁻⁵ M, and 1×10⁻⁷ to 3.6×10⁻⁵ M, with a detection limit of 2.1×10⁻⁸ M, 1.5×10⁻⁸ M, and 7.1×10⁻⁸ M, respectively. The enzyme electrode retained ca.77% of its activity after 7 days of storage at 4°C in a dry state. The proposed sensor presented good repeatability, evaluated in terms of relative standard deviation (R.S.D.=8.6%) for eight different biosensors and was applied for determination in water sample. The recovery for the sample was from 99.0% to 99.8%.     

Determination of total iron in fresh waters using flow injection with potassium permanganate chemiluminescence detection

A flow injection method is described for the determination of iron in fresh water based on potassium permanganate chemiluminescence detection via oxidation of formaldehyde in aqueous hydrochloric acid. Total iron concentrations are determined after reducing Fe(III) to Fe(II) using hydroxylamine hydrochloride. The detection limit (three standard deviations of blank) is 1.0 nM, with a sample throughput of 120 h⁻¹. The calibration graph was linear over the range (2–10) × 10⁻⁷ M (r ² = 0.9985) with relative standard deviations (n = 5) in the range 1.0–2.3%. The effect of interfering cations (Ca(II), Mg(II), Zn(II), Ni(II), Co(II), Fe(III), Mn(II), Pb(II), and Cu(II)) and common anions (Cl⁻, SO ₄ ²⁻ , PO ₄ ³⁻ , NO ₃ ⁻ , NO ₂ ⁻ , I⁻, F⁻, and SO ₃ ²⁻ ) was studied at their maximum admissible concentrations in fresh water. The method was applied to fresh-water samples from the Quetta Valley, and the results obtained (0.04 ± 0.001–0.11 ± 0.01 mg/L Fe(II)) were in reasonable agreement with those obtained using the spectrophotometric reference method (0.05 ± 0.01–0.12 ± 0.02 mg/L Fe(II)). The text was submitted by the authors in English.     

A reusable liposome array and its application to assay of growth-hormone-related peptides

We describe a reusable liposome array based on the formation of cleavable disulfide cross-links between liposomes and the surface of a glass slip. The N-succinimidyl 3-(2-pyridyldithio)-propionate (SPDP)-modified liposomes encapsulating a pH-sensitive fluorescence dye were immobilized on a 3-mercaptopropyltrimethoxysilane (MTS)-modified glass slip through the formation of disulfide bonds. The regeneration of a used slip was performed by the lysis of immobilized liposomes with Triton X-100 and the cleavage of disulfide bonds by reduction with TCEP, followed by immobilization of SPDP-modified liposomes. The regeneration steps did not affect the fluorescence intensity of re-immobilized liposomes. The liposome array was applied to simultaneous quantification of growth hormone related peptides, i.e., GHRF and somatostatin, in a mixture. After optimizing the assay condition, the method allowed quantification of GHRF and somatostatin in concentration ranges from 0.5 × 10⁻⁹ to 0.5 × 10⁻⁷ g/mL with detection limits of 2 × 10⁻¹⁰ and 3 × 10⁻¹⁰ g/mL, respectively.     

HPLC quantification of formaldehyde, as formaldemethone, in plants and plant-like organisms

Summary Formaldehyde, as its dimedone adduct formaldemethone, has been detected and quantified in all the tested species of angiosperms, gymnosperms, pteridophytes, lichens and fungi, as well as in the two species tested of cyanobacteria and the one species of charophyte. Yields ranged from<10μg g⁻¹ to 6940 μg g⁻¹ fresh weight, calculated as formaldemethone (equivalent to <1 μg g⁻¹ to 713 μg g⁻¹ fresh weight, calculated as formaldehyde). An HPLC procedure was used for quantification of formaldemethone. A linear relationship was found between 20 and 2160 μg g⁻¹ and the statistical limit of detection was calculated as 48 μg g⁻¹.     

Direct electron transfer of Mb on chitosan/single-wall carbon nanotubes film modified Au electrode and its interaction with cimetidine

Three methods were used to immobilize myoglobin (Mb) on chitosan/single-wall carbon nanotubes (SWNTs) film, and direct electrochemistry of the immobilized Mb was extensively investigated. Immobilized Mb displayed a couple of stable and well-defined redox peaks with the formal potential (E’) is at about −0.27 V (vs. SCE) in 0.1 M phosphate buffer solution (pH 7.0). The E′ was shifted linearly with pH in the range of 3.0 to 9.0 with a slope of −54.1 mV pH⁻¹, denoting that one-electron accompanies with one-proton transfer in electrode reaction process. The FT-IR spectroscopy and UV-vis spectroscopy showed that Mb on the film retained its secondary structure similar to its native state. The experimental results demonstrated that the immobilized Mb exhibited excellent electrocatalytic activity to reduction of cimetidine with a significant lowering of overpotential. The electrocatalytic current was proportional to the concentration of cimetidine over the range from 9.80 × 10⁻⁶ to 1.1 × 10⁻⁴ M; the detection limit is 8.40 × 10⁻⁶ M (signal-to-noise ratio of 3). The proposed method exhibits good sensitivity, stability and reproducibility. Published in Russian in Elektrokhimiya, 2008, Vol. 44, No. 2, pp. 235–243. The text was submitted by the authors in English     

Isotope dilution GC-MS routine method for the determination of butyltin compounds in water

A standard GC-MS instrument with electron impact ionisation has been used to develop a fast, simple and reliable method for the simultaneous determination of monobutyltin (MBT), dibutyltin (DBT) and tributyltin (TBT) in water samples. Isotope dilution analysis (IDA) is used for the determination of species, taking advantage of a commercially available spike solution containing a mixture of MBT, DBT and TBT enriched in ¹¹⁹Sn. Method detection limits for 100-mL samples were between 0.18 and 0.25 ng L⁻¹ for the three butyltin compounds with typical RSD between 2 and 4% at levels between 100 and 10 ng L⁻¹, respectively. Recovery of tin species in spiked samples (natural water, wastewater and seawater) was quantitative. The stability of butyltin compounds in collected seawater samples was also studied. The addition of a 1% (v/v) glacial acetic acid preserved tin species in the samples for at least 5 days at room temperature. The IDA method was finally implemented in a routine testing laboratory and it was subsequently accredited by the Spanish National Accreditation Body according to the requirements of UNE-EN ISO/IEC 17025.     

Kinetic-potentiometric assay of formaldehyde in pharmaceutical and industrial samples, monitored by copper solid ion selective electrode, after its reaction with the copper(II)-bis-(ethylenediamine) complex

A kinetic-potentiometric method is described for the quantitative assay of formaldehyde (HCHO) in pharmaceutical and industrial preparations. It is based on the reaction of HCHO with (ethylenediamine)-Cu(II)-sulfate [Cu(CH₂NH₂)₂(H₂O)₂] · SO₄. The changes in potential, resulting from the release of the Cu(II) cations, are monitored with a Cu(II)-ion selective electrode. The calibration curve for the HCHO is linear in the concentration range 50–250 mg L⁻¹, with a limit of detection of 8.5 mg L⁻¹. The method shows very good reproducibility with an RSD of 2.6% for successive injections (n = 5) of 150 mg L⁻¹ HCHO primary solution, while it is interference free. The method was successfully tested in various industrial and pharmaceutical preparations.     

Ionic strength dependence of stability constants,complexation of W(VI) with iminodiacetic acid

The equilibria between tungsten(VI) and iminodiacetic acid (IDA) have been studied in aqueous solution. The stoichiometry and stability constants of the complexes formed are determined from a combination of potentiometric and Uv spectroscopic measurements. All measurements are carried out at 25°C, pH 7.5 and different ionic strengths ranging from (0.1 to 1.0) mol dm⁻³ (NaClO₄). According to these results, tungsten(VI) forms a mononuclear complex with IDA of the type (WO₃L²⁻). By introducing two empirical parameters C and D in the complex-formation reaction between tungsten(VI) and IDA, the dependence of the dissociation and stability constants on ionic strength is described by a modified Debye-Huckel-type equation. Finally, a pattern for the ionic strength dependence is obtained.