Separation of chelating agents as copper complexes by capillary zone electrophoresis using quaternary ammonium bromides as additives in N-methylformamide

This study presents the use of quaternary ammonium bromides as additives in N-methylformamide (NMF) for the separation and quantification of chelating agents as copper complexes by capillary zone electrophoresis (CZE). The new quaternary ammonium bromides were synthesized in our laboratory and used for the first time for CZE applications performed in NMF media. The methods were developed and optimized for determination of six chelating agents (trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid (CDTA), diethylenetriaminepentaacetic acid (DTPA), ethylenediaminetetraacetic acid (EDTA), N-(2-hydroxyethyl)ethylenediamine-N,N′,N′-triacetic acid (HEDTA), nitrilotriacetic acid (NTA) and triethylenetetraaminehexaacetic acid (TTHA)) as copper complexes. Among the tested electrolyte additives in NMF media (pH_app 10.2) dimethyldioctylammonium bromide (DMDOAB), dimethyldinonylammonium bromide (DMDNAB) and dimethyldidecylammonium bromide (DMDDAB), at a concentration of 20 mmol L⁻¹ improved the separation of the copper complexes. The optimized methods require only 12 min for one analysis, and the detection limits for copper complexes of DMDNAB, the best-performing additive, were ≤24 μmol L⁻¹. Relative standard deviations (R.S.D.) for migration times were ≤2.5, ≤2.1, ≤3.1% and for peak areas, ≤3.1, ≤3.0, ≤3.2% for DMDOAB, DMDNAB and DMDDAB used as additives, respectively. All three methods were successfully applied to the analysis of natural and wastewater samples. No matrix effects from these samples were observed. The interaction between quaternary ammonium bromides and copper complexes is discussed.     

A sensitive post-column photochemical derivatization/fluorimetric detection system for HPLC determination of bisphosphonates

A new reversed-phase ion-pair high-performance liquid chromatographic (HPLC) method has been developed for the determination of the following bisphosphonic acids: alendronic acid (ALEN), etidronic acid (ETID), ibandronic acid (IBAN) and risedronic acid (RISE). Separation was achieved on a C₁₈ column using a mixture of 50 mmol L⁻¹ borate buffer pH 9.0 containing 0.25 mmol L⁻¹ tetrabutylammonium chloride and 0.5 mmol L⁻¹ EDTA and acetonitrile (97:3) as the mobile phase. The sensitive detection of the above bisphosphonic acids was based on their oxidation to orthophosphate by the on-line peroxydisulfate-assisted photolysis followed by post-column reaction with molybdate to yield phosphomolybdate. This subsequently reacted with thiamine to generate thiochrome and, finally, the fluorescence of thiochrome was measured at 440 nm with excitation at 375 nm. The developed method is precise with a mean relative standard deviation of 1.3%, sensitive (with a detection limit at the nmol L⁻¹ level), accurate, specific, rapid (analysis time approximately 13 min) and inexpensive because to the low cost of the reagents. The assay was applied to the analysis of the four bisphosphonic acids in commercial dosage formulations, in which the excipients did not interfere with the determination. The method was also applied to the determination of etidronate, risedronate and ibandronate in human urine. Sample preparation involves precipitation of the analytes from urine along with endogenous phosphates such as calcium salts by addition of calcium chloride at alkaline pH and dissolution of the precipitate in 0.05 mol L⁻¹ ethylene glycol-bis(β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid.     

Confirmation of vanadium complex formation using electrospray mass spectrometry and determination of vanadium speciation by sample stacking capillary electrophoresis

Capillary zone electrophoresis (CZE) with UV detection was used to determine vanadium species. Nitrilotriacetic acid (NTA), hydroxyethylethylenediaminetriacetic acid (HEDTA), ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), ethylene glycol-bis(2-aminoethylether)-tetraacetic acid (EGTA) and 2,6-pyridinedicarboxylic acid (PDCA) were investigated to determine whether these ligands formed stable anionic complexes with vanadium. Of all the ligands studied HEDTA was the most suitable ligand because it gave the largest UV response with reasonable migration time. Electrospray mass spectrometry (ES-MS) was used to confirm the formation of [VO₂(HEDTA)]²⁻ and [VO(HEDTA)]¹⁻ in solution. An electrolyte containing 25 mM phosphate, 0.25 mM tetradecyltrimethylammonium bromide (TTAB) at pH 5.5 was optimum for the separation of these anionic vanadium complexes. Sample stacking techniques, including large-volume sample stacking (LVSS) and field-amplified sample injection (FASI), were tested to improve the sensitivity. Best sensitivity was obtained using FASI, with detection limits of 0.001 μM, equivalent to 0.4 μg L⁻¹, for [VO₂(HEDTA)]²⁻ and 0.01 μM, equivalent to 3.4 μg L⁻¹ for [VO(HEDTA)]¹⁻. The utility of the method for the speciation of V(IV) and V(V) was demonstrated using ground water samples.     

Sulfonylcalix[4]arenetetrasulfonate as pre-column chelating reagent for selective determination of aluminum(III), iron(III), and titanium(IV) by ion-pair reversed-phase high-performance liquid chromatography with spectrophotometric detection

Sulfonylcalix[4]arenetetrasulfonate (SO₂CAS) has been examined as a pre-column chelating reagent for ultratrace determination of metal ions by ion-pair reversed-phase high-performance liquid chromatography with spectrophotometric detection. Metal ions were converted into the SO₂CAS chelates in an acetic buffer solution (pH 4.7). The chelates were injected onto a n-octadecylsilanized silica-type Chromolith™ Performance RP-18e column and were eluted using a methanol (50 wt.%)-water eluent (pH 5.6) containing tetra-n-butylammonium bromide (7.0 mmol kg⁻¹), acetate buffer (5.0 mmol kg⁻¹), and disodium ethylendiamine-N,N,N′,N′-tetraacetate (0.10 mmol kg⁻¹). Under the conditions used, Al(III), Fe(III), and Ti(IV) were selectively detected among 21 kinds of metal ions [Al(III), Ba(II), Be(II), Ca(II), Cd(II), Co(II), Cr(III), Cu(II), Fe(III), Ga(III), Hf(IV), In(III), Mg(II), Mn(II), Mo(VI), Ni(II), Pb(II), Ti(IV), V(V), Zn(II), and Zr(IV)]. The detection limits on a 3σ blank basis were 8.8 nmol dm⁻³ (0.24 ng cm⁻³) for Al(III), 7.6 nmol dm⁻³ (0.42 ng cm⁻³) for Fe(III), and 17 nmol dm⁻³ (0.80 ng cm⁻³) for Ti(IV). The practical applicability of the proposed method was checked using river and tap water samples.     

Headspace liquid-phase microextraction using ionic liquid as extractant for the preconcentration of dichlorodiphenyltrichloroethane and its metabolites at trace levels in water samples

A novel technique, high temperature headspace liquid-phase microextraction (HS-LPME) with room temperature ionic liquid (RTIL), 1-butyl-3-methylimidazolium hexafluorophosphate ([C₄MIM][PF₆]) as extractant, was developed for the analysis of dichlorodiphenyltrichloroethane (p,p′-DDT and o,p′-DDT) and its metabolites including 4,4′-dichlorodiphenyldichloroethylene (p,p′-DDE) and 4,4′-dichlorodiphenyldichloroethane (p,p′-DDD) in water samples by high performance liquid chromatography with ultraviolet detection. The parameters such as salt content, sample pH and temperature, stirring rate, extraction time, microdrop volume, and sample volume, were found to have significant influence on the HS-LPME. The conditions optimized for extraction of target compounds were as follows: 35% NaCl (w/v), neutral pH condition, 70 °C, 800 rpm, 30 min, 10 μL [C₄MIM][PF₆], and 25 mL sample solutions. Under the optimized conditions, the linear range, detection limit (S/N = 3), and precision (R.S.D., n = 6) were 0.3-30 μg L⁻¹, 0.07 μg L⁻¹, and 8.0% for p,p′-DDD, 0.3-30 μg L⁻¹, 0.08 μg L⁻¹, and 7.1% for p,p′-DDT, 0.3-30 μg L⁻¹, 0.08 μg L⁻¹, and 7.2% for o,p′-DDT, and 0.2-30 μg L⁻¹, 0.05 μg L⁻¹, and 6.8% for p,p′-DDE, respectively. Water samples including tap water, well water, snow water, reservoir water, and wastewater were analyzed by the proposed procedure and the recoveries at 5 μg L⁻¹ spiked level were in the range of 86.8-102.6%.     

Implementing stepwise solvent elution in sequential injection chromatography for fluorimetric determination of intracellular free amino acids in the microalgae Tetraselmis gracilis

The concept of sequential injection chromatography (SIC) was exploited to automate the fluorimetric determination of amino acids after pre-column derivatization with o-phthaldialdehyde (OPA) in presence of 2-mercaptoethanol (2MCE) using a reverse phase monolithic C₁₈ stationary phase. The method is low-priced and based on five steps of isocratic elutions. The first step employs the mixture methanol: tetrahydrofuran: 10 mmol L⁻¹ phosphate buffer (pH 7.2) at the volumetric ratio of 8:1:91; the other steps use methanol: 10 mmol L⁻¹ phosphate buffer (pH 7.2) at volumetric ratios of 20:80, 35:65, 50:50 and 65:35. At a flow rate of 10 μL s⁻¹ a 25 mm long-column was able to separate aspartic acid (Asp), glutamic acid (Glu), asparagine (Asn), serine (Ser), glutamine (Gln), glycine (Gly), threonine (Thr), citruline (Ctr), arginine (Arg), alanine (Ala), tyrosine (Tyr), phenylalanine (Phe), ornithine (Orn) and lysine (Lys) with resolution >1.2 as well as methionine (Met) and valine (Val) with resolution of 0.6. Under these conditions isoleucine (Ile) and leucine (Leu) co-eluted. The entire cycle of amino acids derivatization, chromatographic separation and column conditioning at the end of separation lasted 25 min. At a flow rate of 40 μL s⁻¹ such time was reduced to 10 min at the cost of resolution worsening for the pairs Ctr/Arg and Orn/Lys. The detection limits varied from 0.092 μmol L⁻¹ for Tyr to 0.51 μmol L⁻¹ for Orn. The method was successfully applied to the determination of intracellular free amino acids in the green alga Tetraselmis gracilis during a period of seven days of cultivation. Samples spiked with known amounts of amino acids resulted in recoveries between 94 and 112%.     

Development of a D-amino acids electrochemical sensor based on immobilization of thermostable D-proline dehydrogenase within agar gel membrane

A novel biosensor for determination of d-amino acids (DAAs) in biological samples by using an electrode based on immobilization of a thermostable d-Proline dehydrogenase (d-Pro DH) within an agar gel membrane was developed. The electrode was simply prepared by spin-coating the agar solution with the d-Pro DH on a glassy carbon (GC) electrode.An electrocatalytic oxidation current of 2,6-dichloroindophenol (DCIP) was observed at −100 mV vs. Ag/AgCl with the addition of 5 and 20 mmol L⁻¹d-proline. The current response and its relative standard deviation were 0.15 μA and 7.6% (n = 3), respectively, when it was measured in a pH 8.0 phosphate buffer solution containing 10 mmol L⁻¹d-proline and 0.5 mmol L⁻¹ DCIP at 50 °C. The current response of d-proline increased with increase of the temperature of the sample solution up to 70 °C. The electrocatalytic response at the d-Pro DH/agar immobilized electrode subsequently maintained for 80 days. Finally, the d-Pro DH/agar immobilized electrode was applied to determination of DAAs in a human urine sample. The determined value of DAAs in the human urine and its R.S.D. were 1.39 ± 0.12 mmol L⁻¹ and 8.9% (n = 3), respectively.     

A rapid quantitative method for humic substances determination in natural waters

A simple and rapid method was proposed for humic substances (HS) determination at microgram levels in natural waters. This assay method is based on the binding of a dye, Toluidine Blue (TB), to HS molecules to produce a dye-HS complex, which causes a decrease in absorbance at 630 nm. This method was calibrated with HS samples with up to a concentration of 40 mg L⁻¹, which covered the range of dissolved HS concentrations present in natural waters. The detection limit was 0.8 mg L⁻¹ of HS, and the relative standard deviation of 10 replicate measurements for a 20-mg L⁻¹ standard sample was 3.5%. From the Langmuir adsorption isotherm theory, the binding equilibrium constant and total number of binding sites at neutral pH were calculated to be (8.17 ± 0.42) × 10⁵ L mol⁻¹ and N of 1.45 ± 0.04 mmol g⁻¹ HS, respectively. The determination results with five water samples from lake, river and pond were consistent with those measured with the reference methods, demonstrating that this quantification method for HS determination was rapid, sensitive and feasible.     

Simultaneous determination of first-line anti-tuberculosis drugs by capillary zone electrophoresis using direct UV detection

An alternative methodology for simultaneous analysis of ethambutol, isoniazid, rifampicin and pyrazinamide in pharmaceutical formulations by capillary zone electrophoresis under UV direct detection with an analysis time of 8.0 min is proposed. Background running was based on the effective mobility curve of the analytes and an optimum separation condition was achieved using a 3³ Box-Behnken design, with Brij 35, Cu²⁺ and acetic acid/sodium acetate buffer as factors. An electrolyte consisting of 50.0 mmol L⁻¹ of acetic acid/sodium acetate buffer, 12.5 mmol L⁻¹ of CuSO₄, and standard and sample solutions prepared in 2.00 mmol L⁻¹ of Brij 35 and 12.5 mmol L⁻¹ of CuSO₄ were optimized. After evaluating validation parameters, the method was successfully applied to the analysis of samples in the form of tablets and sachets.     

Use of molybdate as novel complex-forming selector in the analysis of polyhydric phenols by capillary zone electrophoresis

Molybdate was examined as a complex-forming additive to the CE background electrolytes (BGE) to affect the selectivity of separation of polyhydric phenols such as flavonoids (apigenin, hyperoside, luteolin, quercetin and rutin) and hydroxyphenylcarboxylic acids (ferulic, caffeic, p-coumaric and chlorogenic acid). Effects of the buffer concentrations and pH and the influence of molybdate concentration on the migration times of the analytes were investigated. In contrast to borate (which is a buffering and complex-forming agent generally used in CE at pH ≥9) molybdate forms more stable complexes with aromatic o-dihydroxy compounds and hence the complex-formation effect is observed at considerably lower pH. Model mixtures of cinnamic acid, ferulic acid, caffeic acid and 3-hydroxycinnamic acid were separated with 25 mM morpholinoethanesulfonic acid of pH 5.4 (adjusted with Tris) containing 0.15 mM sodium molybdate as the BGE (25 kV, silica capillary effective length 45 cm × 0.1 mm I.D., UV-vis detection at 280 nm). With 25 mM 2-hydroxy-3-[4-(2-hydroxyethyl)-1-piperazinyl]propanesulphonic acid/Tris of pH^* 7.4 containing 2 mM sodium molybdate in aqueous 25% (v/v) methanol as the BGE mixtures of all the above mentioned flavonoids, p-coumaric acid and chlorogenic acid could be separated (the same capillary as above, UV-vis detection at 263 nm). The calibration curves (analyte peak area versus concentration) were rectilinear (r > 0.998) for ≈8-35 μg/ml of an analyte (with 1-nitroso-2-naphthol as internal standard). The limit of quantification values ranged between 1.1 mg l⁻¹ for p-coumaric acid and 2.8 mg l⁻¹ for quercetin. The CE method was employed for the assay of flavonoids in medicinal plant extracts. The R.S.D. values ranged between 0.9 and 4.7% (n = 3) when determining luteolin (0.08%) and apigenin (0.92%) in dry Matricaria recutita flowers and rutin (1.03%) and hyperoside (0.82%) in dry Hypericum perforatum haulm. The recoveries were >96%.     

Isotachophoretic determination of hydrosulfite and metabisulfite in technical samples

A method for determination of metabisulfite and hydrosulfite in poultice and decolorant by isotachophoresis was developed. Metabisulfite and hydrosulfite are ionizable oxoanions of sulfur of similar character that can easily be oxidized to sulfates. To protect the analytes from oxidation the solid samples were dissolved in a 1% (w/v) solution of formaldehyde. Hydrosulfite and metabisulfite present in the samples were transformed by the reaction with formaldehyde to stable compounds, hydroxymethanesulfinate and hydroxymethanesulfonate that were determined isotachophoretically without any pretreatment except for sample filtering and degassing. A capillary of 0.4 mm i.d. and 100 mm effective length made of fluorinated ethylene-propylene copolymer was filled with an electrolyte system consisting of 10 mmol L⁻¹ HCl + 11 mmol L⁻¹ imidazole, 0.15% (w/v) hydroxyethylcellulose, pH 6.0 (leading electrolyte) and 5 mmol L⁻¹ benzoic acid + 6 mmol L⁻¹ imidazole, pH 6.5 (terminating electrolyte). Separation was performed at a driving current of 80 μA and for detection current was decreased to 30 μA. Using contactless conductivity detection, the calibration curves in the tested concentration range up to 2.5 mmol L⁻¹ were linear for both metabisulfite and hydrosulfite complexes. The concentration detection limits for metabisulfite and hydrosulfite were 2.9 and 3.4 μmol L⁻¹, respectively. For 1 mmol L⁻¹ concentration, values of R.S.D. (n = 6) were 2.6% for hydrosulfite and 0.8% for metabisulfite. Isotachophoretic determination took about 20 min. The elaborated isotachophoretic procedure is simple to perform, sufficiently sensitive and accurate. In addition to this, low cost of analyses makes the method an alternative procedure to methods used so far for the determination of oxoanions of sulfur.     

A renewable copper electrode as an amperometric flow detector for nitrate determination in mineral water and soft drink samples

A novel approach was developed for nitrate analysis in a FIA configuration with amperometric detection (E = −0.48 V). Sensitive and reproducible current measurements were achieved by using a copper electrode activated with a controlled potential protocol. The response of the FIA amperometric method was linear over the range from 0.1 to 2.5 mmol L⁻¹ nitrate with a detection limit of 4.2 μmol L⁻¹ (S/N = 3). The repeatability of measurements was determined as 4.7% (n = 9) at the best conditions (flow rate: 3.0 mL min⁻¹, sample volume: 150 μL and nitrate concentration: 0.5 mmol L⁻¹) with a sampling rate of 60 samples h⁻¹. The method was employed for the determination of nitrate in mineral water and soft drink samples and the results were in agreement with those obtained by using a recommended procedure. Studies towards a selective monitoring of nitrite were also performed in samples containing nitrate by carrying out measurements at a less negative potential (−0.20 V).     

Simultaneous determination of Mn(II), Cu(II) and Fe(III) as 2-(5′-bromo-2′-pyridylazo)-5-diethylaminophenol complexes by adsorptive cathodic stripping voltammetry at a carbon paste electrode

A simple and precise square-wave adsorptive cathodic stripping voltammetry (SW-AdCSV) method has been described for simultaneous determination of Mn(II), Cu(II) and Fe(III) in water samples using a carbon paste electrode. In 0.1 mol L⁻¹ acetate buffer (pH 5) containing 50 μmol L⁻¹ of 2-(5′-bromo-2′-pyridylazo)-5-diethylaminophenol (5-Br-PADAP), Mn(II), Cu(II) and Fe(III) were simultaneously determined as metal-complexes with 5-Br-PADAP following preconcentration onto the carbon paste electrode by adsorptive accumulation at +1.0 V (vs. Ag/AgCl/3 M KCl). Insignificant interference from various cations (K⁺, Na⁺, Mg²⁺, Ca²⁺, Al³⁺, Bi³⁺, Sb³⁺, Se⁴⁺, Zn²⁺, Ni²⁺, Co²⁺, Cd²⁺, Pb²⁺, V⁵⁺, Ti⁴⁺ and NH₄⁺), anions (HCO₃⁻, Cl⁻, NO³⁻, SO₄²⁻ and PO₄³⁻) and ascorbic acid was noticed. Limits of detection of 0.066, 0.108 and 0.093 μg L⁻¹ and limits of quantitation of 0.22, 0.36 and 0.31 μg L⁻¹ Mn(II), Cu(II) and Fe(III), respectively, were achieved by the described method. The described stripping voltammetry method was successfully applied for simultaneous determination of Mn(II), Cu(II) and Fe(III) in ground, tap and bottled natural water samples.     

Ultrasensitive detection of pepsinogen I and pepsinogen II by a time-resolved fluoroimmunoassay and its preliminary clinical applications

A fast and highly sensitive assay for pepsinogen I (PG I) and pepsinogen II (PG II) by using time-resolved fluoroimmunoassay (TRFIA) detection technique has been developed for the determination of serum PG I and PG II against gastrointestinal diseases. On the noncompetitive assay, one monoclonal antibody (McAb) coated on wells was directed against a specific antigenic site on the PG I or PG II. The McAb, called as labelling McAb, was prepared with the europium-chelate of N-(p-isothiocyanatobenzyl)-diethylenetriamine-N,N,N,N-tetraacetic acid and directed against a different antigenic site on the PG I or PG II molecule. After bound/free separation by washing, the fluorescence counts of bound Eu³⁺-McAb were measured. The levels of PG in sera from patients or healthy volunteers were determined by PG I and PG II TRFIA using the autoDELFIA₁₂₃₅ system. The measurement ranges of PG I-TRFIA were 3.5-328.0 μg L⁻¹ and those of PG II-TRFIA were 2.0-55.0 μg L⁻¹. The within-run and between-run CVs of the PG I-TRFIA were 1.9% and 4.7%, respectively, and those of PG II-TRFIA were 2.1% and 3.8%, respectively. The recovery rates of PG I-TRFIA and PG II-TRFIA were 102.7% and 104.6%, respectively. The detection limitations of PG I and PG II were 0.05 μg L⁻¹ and 0.02 μg L⁻¹, respectively. The dilution experiments showed the percentage of expected value of PG I-TRFIA was 93.2-102.3% and of PG II-TRFIA was 97.3-110.6%. The cross-reacting rate between PG I and PG II was negligible. The linear correlation of radioimmunoassay (RIA) and TRFIA measurements resulted in a correlation coefficient as 0.926 of PG I and as 0.959 of PG II. The europium-labelling McAbs were stable for at least one year at −20 °C, and the results of the TRFIA with same reagents were reproducible over one year as well. The means of 1600 healthy volunteers were 162.4 ± 52.1 μg L⁻¹ for serum PG I, 11.7 ± 6.8 μg L⁻¹ for serum PG II, and 13.8 ± 7.4 for the PG I/PG II ratio. The normal ranges of Serum PG I levels for healthy volunteers were 58.2-266.6 μg L⁻¹, and those of serum PG II levels were less than 25.3 μg L⁻¹. The availability of a highly sensitive, reliable, and convenient PG-TRFIA method for quantifying PG will allow investigations into the possible diagnostic value of this analysis in various clinical conditions, including gastric carcinoma, duodenal ulcer, gastric ulcer and gastritis. The sensitivity and reproducibility of the assay were satisfactory for clinical applications.     

Determination of traces of cobalt in the presence of nioxime and cetyltrimethylammonium bromide by adsorptive stripping voltammetry

A sensitive method of Co(II) determination by adsorptive stripping voltammetry is presented. The method exploits the enhancement of cobalt peak current observed in the system Co(II)-nioxime-cetyltrimethylammonium bromide-piperazine-N,N′-bis(2-ethanesulfonic acid). The calibration plot for an accumulation time of 60 s is linear from 5 × 10⁻¹¹ to 3 × 10⁻⁹ mol L⁻¹. The relative standard deviation is 3.8% for Co(II) determination at concentration 1 × 10⁻⁹ mol L⁻¹. The detection limit is 1.7 × 10⁻¹¹ mol L⁻¹. The validation of the method is performed by the analyses of certified reference materials and comparing the result of Co(II) determination in river water sample by the proposed method with those obtained by ET AAS. The main advantage of this new system is the micro-trace Co(II) determination by adsorptive stripping voltammetry, as compared to those described before, a low concentration of the supporting electrolyte used, and so commercially available reagents without additional purification can be used.     

Use of 1-(2-pyridylazo)-2-naphthol as the post column reagent for ion exchange chromatography of heavy metals in environmental samples

Ion exchange chromatography (IEC) using a bi-functional column (quaternary ammonium and sulfonate groups), followed by post-column reaction (PCR) with 1-(2-pyridylazo)-2-naphthol (PAN), was used to separate and quantitate Cu(II), Ni(II), Zn(II), Co(II), Cd(II), Mn(II) and Hg(II) at low concentration levels. IEC-PCR separation was achieved within 14 min using the mobile phase containing 3 mmol L^− 1 2,6-pyridinedicarboxylic acid (PDCA) and 3 mmol L^− 1 oxalate at pH 12.5. Effects of pH as well as PAN, detergent and chloride ion concentrations during post-column reaction on detector response were examined. Detection limits were less than 4.5 μg L^− 1 for all metals except Hg(II) (19 μg L^− 1) using spectrophotometric measurements at 550 nm. Analytical validations showed good linearity for detection up to 6.0 mg L^− 1, with R² higher than 0.99. Precisions based on retention time evaluation for intra-day and inter-day measurements with the relative standard deviation (RSD) were less than 2.9% and 3.6%, respectively. The method gave good accuracy with the recoveries ranged from 80.5 to 105% for all metal ions studied. The proposed method was applied to the analysis of metal ions in environmental samples (leachate, soil and sediment) in Northeastern Thailand. The results were in good agreement with atomic spectroscopic measurements on the same samples.     

A novel fused-silica capillary dropping mercury electrode with long drop-time and its application on determination of critical micelle concentration

A novel long drop time mercury electrode has been constructed from common fused-silica capillary (50 μm I.D., 360 μm E.D.). Proposed device provides reproducible mercury drops with typical lifetime 40-120 s. The electrode was used for a set of electrocapillary measurements aimed at determination of critical micelle concentration of anionic surfactants by a convection controlled drop-time technique. A critical micelle concentration of sodium dodecyl sulfate 5.6 ± 0.4 mmol L⁻¹ and 4.3 ± 0.4 mmol L⁻¹ were obtained in 1 mmol L⁻¹ and 5 mmol L⁻¹ phosphate buffer (pH 7.0), respectively. The values were comparable to those obtained from conductometric measurements under the same conditions (7.0 ± 0.1 mmol L⁻¹ and 5.2 ± 0.1 mmol L⁻¹, respectively) and the difference was explained in accordance with theory of hemi-micelle formation.     

Comparative studies of ONNO-based ligands as ionophores for palladium ion-selective membrane sensors

Palladium sensors based on two neutral ionophores, N,N′-bis(acetylacetone) cyclohexanediamine (L₁) and N,N′-bis(o-hydroxyacetophenone)-1,2-cyclohexanediamine (L₂) for quantification of palladium ions are described. Effect of various plasticizers (o-NPOE, DBP, DEP, DOP, TBP, and CN) and anion excluder, sodium tetra phenyl borate (NaTPB) has been studied. The best performance is obtained with a membrane composition of PVC:o-NPOE:ionophore (L₁):NaTPB of 150:300:5:5 (%, w/w). The sensor exhibits significantly enhanced selectivity towards palladium ion over the concentration range 1.0 × 10⁻⁸ to 1.0 × 10⁻¹ M with a lower detection limit of 4.0 × 10⁻⁹ M and a Nernstian compliance (29.1 ± 0.3 mV decade⁻¹ of activity) within pH range 2.0-6.0 and fast response time of 10 s. Influence of the membrane composition and possible interfering ions has also been investigated on the response properties of the electrode. Fast and stable response, good reproducibility and long-term stability of the sensor are demonstrated. The sensor has been found to work satisfactorily in partially non-aqueous media up to 20% (v/v) content of methanol, ethanol and acetonitrile and could be used for a period of 4 months. Selectivity coefficients determined with fixed interference method (FIM) indicate high selectivity for palladium. The proposed electrode shows fairly good discrimination of palladium from other cations. The application of prepared sensor has been demonstrated in determination of palladium ions in spiked water sample.     

A quick method for the simultaneous determination of ascorbic acid and sorbic acid in fruit juices by capillary zone electrophoresis

A method of quickly determining ascorbic acid and sorbic acid by capillary zone electrophoresis with ultraviolet detection was developed. The choice of background electrolyte, wavelength, injection time and applied voltage were discussed. Ascorbic acid and sorbic acid were well separated in 80 mmol L⁻¹ boric acid-5 mmol L⁻¹borax (pH = 8.0) in 5 min at the detecting wavelength of 270 nm. Under the optimum condition, the method has linear ranges of 2.54-352.00 mg L⁻¹ for ascorbic acid and 1.08-336.39 mg L⁻¹ for sorbic acid with the detection limit of 1.70 mg L⁻¹ for ascorbic acid and 0.54 mg L⁻¹ for sorbic acid, respectively. Other organic acids in fruit juices have no effect on the detection. This method is very feasible and simple and can be used to detect ascorbic acid and sorbic acid in fruit juices.     

New microdialysis-electrochemical device for simultaneous determination of ascorbic acid and 5-hydroxyindole-3-acetic acid in rat striatum

A new device combining microdialysis with electrochemical microsensor was developed. It can be applied to monitor the biomolecules in the brain for biological and pharmaceutical research. In this paper, the device was applied to simultaneously determine ascorbic acid (AA) and 5-hydroxyindole-3-acetic acid (5-HIAA) in rat striatum. The microsensor used for the device was poly (sulphosalicylic acid) microsensor, which exhibited a good electrocatalytic effect on oxidization of AA and 5-HIAA. The oxidation currents measured by differential pulse voltammetry (DPV) were linear for AA in the range of 0.02-1.0 mmol l⁻¹, and for 5-HIAA from 0.5 to 10.0 μmol l⁻¹ (r=0.9998 and 0.9991, respectively). The detection limits were calculated to be 0.01 mmol l⁻¹ for AA and 0.25 μmol l⁻¹for 5-HIAA (S/N=3). Studies also showed that co-existing substances in biological fluids did not interfere with AA and 5-HIAA determination when using this microsensor. Since, the substances in the microdialysate are easily oxidized by air, the microdialysate in this device was under the protection of N₂. It was found that the concentrations of AA and 5-HIAA in rat striatum were 215±5 and 6.21±0.61 μmol l⁻¹ (mean±S.E.M., n=7), respectively with this device under the protection of N₂. In addition, the effect of the nitric oxide donor, sodium nitroprusside (SNP), on 5-HIAA in the rat striatum was investigated. It was found that a high concentration of SNP (1.0 mmol l⁻¹) resulted in a 34% increase in 5-HIAA level.