Post-column terbium complexation and sensitized fluorescence detection for the determination of norepinephrine, epinephrine and dopamine using high-performance liquid chromatography

Terbium sensitized fluorescence was used as a post-column detection system to develop a simple, sensitive and rapid high-performance liquid chromatographic method for the simultaneous determination of catecholamines norepinephrine (NE), epinephrine (E) and dopamine (DA).Catecholamines were separated by an ion-pair reversed-phase chromatography on a BDS-Hypersil analytical column with a mobile phase of methanol and 50 mmol l⁻¹ acetate buffer (pH 4.7) containing 1.1 mmol l⁻¹ SOS and 0.11 mmol l⁻¹ EDTA (15+85 v/v).Catecholamines and the internal standard (3,4-dihydroxybenzylamine, DHBA) were post-column derivatized by the addition to the eluent of an alkaline solution containing a stoichiometric mixture of terbium(III) chloride and EDTA. Fluorescence detection (λ_ex=300 nm, λ_em=545 nm) is based on the sensitization of terbium ion fluorescence after complexation with catecholamines.The chemical compatibility between the eluent and the post-column reagent was studied and the analytical characteristics of the method were established. Detection limits found were 1.0×10⁻⁸, 4.0×10⁻⁸ and 7.0×10⁻⁸ mol l⁻¹ for NE, E and DA, respectively. The method has been successfully applied to the determination of catecholamines in urine samples after solid-phase extraction (SPE) pre-treatment. Recoveries from urine spiked with NE (4.0×10⁻⁷, 2.0×10⁻⁶ and 4.0×10⁻⁶ mol l⁻¹), E (8.2×10⁻⁸, 4.1×10⁻⁷ and 8.2×10⁻⁷ mol l⁻¹) and DA (1.0×10⁻⁶, 5.0×10⁻⁶ and 1.0×10⁻⁵ mol l⁻¹) varied from 98 to 100% (mean=99.3%), from 106 to 107% (mean=106.3%) and from 98 to 101% (mean=99.3%), respectively. The between-run precision (relative standard deviation, R.S.D.) for the method for three urine samples at different concentration levels of each catecholamine varied from 3.6 to 7.0%.     

Highly sensitive catalytic determination of molybdenum

A novel, highly sensitive, selective, and simple kinetic method was developed for the determination of Mo(VI) based on its catalytic effect on the oxidation of 1-amino-2-naphthol-4-sulfonic acid (ANSA) with H₂O₂. The reaction was followed spectrophotometrically by tracing the oxidized product at 465 nm after 30 min of mixing the reagents. The optimum reaction conditions were: 10 mmol l⁻¹ ANSA, 50 mmol l⁻¹ H₂O₂, 100 mmol l⁻¹ acetate buffer of pH 5.0 ± 0.05 and at 40 °C. Addition of 200 μg ml⁻¹ diethylenetriaminepentaacetic acid (DTPA) conferred high selectivity for the proposed method. Following the recommended procedure, Mo(VI) could be determined with a linear calibration graph up to 2.5 ng ml⁻¹ and a detection limit, based on the 3S_b-criterion, of 0.027 ng ml⁻¹. The unique sensitivity and selectivity of the implemented method allowed its direct application to the determination of Mo(VI) in natural and industrial waste water. The method was validated by comparison with the standard ETAAS method. Moreover, published catalytic-spectrophotometric methods for the determination of molybdenum were reviewed.     

Determination of amino acids in Sargassum fusiforme by high performance capillary electrophoresis

High performance capillary electrophoresis (HPCE) was developed for quantitative determination of 18 phenylthiohydantoin (PTH)-amino acids. The influence of electrolyte concentration, pH, organic modifier and applied voltage on HPCE performance was investigated. The HPCE separation of a PTH-amino acids mixture was much improved by adding organic modifier and Tris-boric acid buffer to the run buffer. After optimization of the method, 17 PTH-amino acids in a solution containing 18 PTH-amino acids could be separated using 400 mmol l⁻¹ Tris-boric acid, 1.0 mmol l⁻¹ diethylamine at pH 9.5 adjusted with 0.1 mol l⁻¹ NaOH as a run buffer, voltage of 25 kV was applied, temperature was maintained at 25 °C, detection wavelength was 254 nm. The precision (n = 7) of this method is less than 3.2% (peak area) and 1.1% (migration time) of relative standard deviation (R.S.D.). Linearity was established over the concentration range 50-1000 μM of each derivative, with correlation coefficients (r) ranging between 0.9904 and 0.9993. The detection limits (S/N = 3) range from 2 to 48 μmol l⁻¹. The method was applied to determine amino acids in Sargassum fusiforme, a marine algae collected from Tongtou County of Zhejiang Province in China with satisfactory results.     

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.     

Electrochemical determination of maltol in beverages with glassy carbon electrode and its silica sol-gel modified electrode

The electrochemical behavior of maltol on a glassy carbon (GC) electrode was investigated. The results were applied to differential pulse voltammetric determination of maltol in beverages pretreated by ultrafiltration. Under the optimum experimental conditions, the linear range is 1×10⁻⁵ to 6×10⁻⁴ mol l⁻¹ maltol and the relative standard deviation for 0.4 mmol l⁻¹ maltol is 0.6% (n=9). The detection limit was 5 μmol l⁻¹. Furthermore, silica sol-gel film on GC electrode could be used as suitable selective membrane, which integrated selective membrane on the electrode and substituted for the pretreatment of ultrafiltration. Under the above conditions, maltol was determined by semi-differential linear sweep voltammetry at a silica sol-gel modified GC electrode in the concentration range of 5×10⁻⁶ to 5×10⁻⁴ mol l⁻¹. The detection limit was 2 μmol l⁻¹ and the relative standard deviation for 0.1 mmol l⁻¹ maltol was 0.7% (n=7). The proposed method is of sensitivity, simplicity, rapidness and no contamination. It had been applied to the direct determination of maltol in beverages such as grape wines, drinks and beers without any pretreatment. The results obtained with the present method were satisfactory with those obtained by spectrophotometry. It could be used as a simple and practical method for the determination of the flavor enhancer maltol in beverages.     

A novel kinetic determination of dissolved chromium species in natural and industrial waste water

A highly sensitive, selective and simple kinetic method was developed for the determination of dissolved chromium species based on the catalytic effect of Cr(III) and/or Cr(VI) on the oxidation of 2-amino-5-methylphenol (AMP) with H₂O₂. The fixed time and initial rate variants were used for kinetic spectrophotometric measurements by tracing the oxidized product at 400 nm for 10 min after starting the reaction. Boric acid and Tween-40 exerted pronounced activating and micellar sensitizing effects on the studied redox reaction, respectively. The optimum reaction conditions were: 3.0 mmol l⁻¹ AMP, 0.45 mol l⁻¹ H₂O₂, 0.50 mol l⁻¹ boric acid, 4 v/v% Tween-40, 10 mmol l⁻¹ phosphate buffer and pH 6.45 ± 0.02 at 35 °C. Both Cr(III) and Cr(VI) ions exerted the same catalytic effect on the studied reaction. Linear calibration graphs were obtained for the determination of up to 6.0 ng ml⁻¹ Cr with detection limits of 0.054 and 0.10 ng ml⁻¹ Cr; following the fixed time and initial rate methods, respectively. The proposed method was successfully applied to the speciation and determination of trace levels of dissolved Cr(III) and Cr(VI) in natural and effluents of industrial waste water. The total dissolved Cr(III) and Cr(VI) species was determined first. In a second run, Cr(VI) was determined alone after precipitation of Cr(III) ions in presence of Al(OH)₃ collector, where Cr(III) is then determined by difference. Moreover, published catalytic-spectrophotometric methods for chromium determination were reviewed.     

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.     

Determination of atrazine using square wave voltammetry with the Hanging Mercury Drop Electrode (HMDE)

This paper presents the optimization of instrumental and solution parameters for determination of atrazine in river waters and formulation by square wave voltammetry (SWV) using a hanging mercury drop electrode. The best sensitivity (35.2±0.4 μA ml μg⁻¹) was achieved using a frequency of 400 Hz and a medium composed of 40 mmol l⁻¹ Britton-Robinson (BR) buffer at pH 1.9. The detection limit was 2 μg l⁻¹ with a linear dynamic range between 10 and 250 μg l⁻¹. Application of the method to real samples of river waters fortified with 10 μg l⁻¹ of atrazine resulted recoveries between 92 and 116%. Additionally, good agreement was observed between results obtained by the proposed method and by HPLC for river water samples spiked with 25 μg l⁻¹ of atrazine. The determination was not affected by the presence of humic acid at concentration of 5 mg l⁻¹, indicating that interactions of the herbicide with this class of compounds are fully labile. The stability of the voltammetric signal for samples spiked with 250 μg l⁻¹ atrazine was evaluated over a period of 14 days in four samples. For two samples, no systematic variation was observed, while for the other two, a decrease of peak current between 3 and 15% occurred, suggesting that the stability is dependent on the sample nature. HPLC analyses suggest formation of deethylatrazine during the second week of storage in the samples for which the SWV peak current had the more intense decrease.     

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.     

Separation and simultaneous determination of niobium and tantalum in steel by reversed-phase high-performance liquid chromatography using 2-(2-pyridylazo)-5-diethylamino phenol as a pre-column derivatizing reagent

A method for the simultaneous determination of Nb and Ta in steel and alloy by reversed-phase high-performance liquid chromatography (RP-HPLC) was proposed. 2-(5-Bromo-2-pyridylazo)-5-diethylamino phenol (5-Br-PADAP) was used as a pre-column chelating agent to form a ternary complex with Nb(V) and Ta(V) and tartaric acid. The ternary complexes of Nb(V) and Ta(V) were eluted within 8 min on a C₁₈ column with a mobile phase of methanol-water (55:45, v/v) containing 10 mmol l⁻¹ acetate buffer (pH3.5) and determined with spectrophotometric detection at 598 nm. The detection limits for Nb(V) and Ta(V) were 0.60 and 0.72 μg l⁻¹, respectively, when the ratio of signal-to-noise is 3. The proposed method was used to analyze Nb and Ta in cast iron, alloy and stainless steel.     

A permeation liquid membrane system for determination of nickel in seawater

The use of a permeation liquid membrane system for the preconcentration and separation of nickel in natural and sea waters and subsequent determination by atomic absorption spectroscopy is presented. 2-Hydroxybenzaldehyde N-ethylthiosemi-carbazone (2-HBET) in toluene is used as the active component of the liquid membrane. A study strategy based on a simplex design has been followed. Several chemical and physical parameters were optimized. Maximum permeation coefficient was obtained at a feed solution pH of 9.4, 0.3 mol l⁻¹ of HNO₃ in the stripping solution and 1.66 mmol l⁻¹ of 2-HBTE in toluene as carrier. The precision of the method was 4.7% at 95% significance level and a detection limit of 0.012 μg l⁻¹ of nickel was achieved. The preconcentration procedure showed a linear response within the studied concentration range from 3 to 500 μg l⁻¹ of Ni in the feed solution. The method was validated with different spiked synthetic seawater and certified reference water samples: TMDA-62 and LGC 6016, without matrix interferences and showing good concordance with the certified values, being the relative errors −5.9% and −2.2%, respectively. Under optimal conditions, the average preconcentration yield for real seawater samples was 98 ± 5%, with a nickel preconcentration factor of 20.83 and metal concentrations ranging between 2.8 and 5.4 μg l⁻¹.     

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.     

Sensitive determination of pipecolic acid in serum by high-performance liquid chromatography using 4-(5,6-dimethoxy-2-phthalimidinyl)-2-methoxyphenylsulfonyl chloride as a fluorescent labelling reagent

A simple and highly sensitive high-performance liquid chromatography (HPLC) for the determination of pipecolic acid (PA) in serum was developed. Pipecolic acid and nipecotic acid (internal standard (IS)) were derivatised with 4-(5,6-dimethoxy-2-phthalimidinyl)-2-methoxyphenylsulfonyl chloride (DMS-Cl) to produce fluorescent sulfonamides. The labelling reaction was carried out at 70 °C for 15 min at pH 9.0. The fluorescent derivatives were separated on a reversed-phase column (45 °C) with a stepwise elution using methanol/acetonitrile/10 mmol l⁻¹ acetic acid (42:5:53) and methanol at a flow rate of 1.0 ml/min and detected at excitation and emission wavelengths of 316 and 403 nm, respectively. The labelling yield was 100.8%. The detection limit of pipecolic acid was 4 fmol at signal-to-noise ratio of 3. The within-day and day-to-day relative standard deviations were 3.3-8.1 and 1.4-6.4%, respectively. The concentration of pipecolic acid in normal human serum was 1.09±0.37 μmol l⁻¹.     

Cold vapour atomic fluorescence determination of mercury in milk by slurry sampling using multicommutation

A highly sensitive mechanized method has been developed for the determination of mercury in milk by atomic fluorescence spectrometry (AFS). Samples were sonicated for 10 min in an ultrasound water bath in the presence of 8% (v/v) aqua regia, 2% (v/v) antifoam A and 1% (m/v) hydroxilamine hydrochloride, and after that, they were treated with 8 mmol l⁻¹ KBr and 1.6 mmol l⁻¹ KBrO₃ in an hydrochloric medium. Atomic fluorescence measurements were made by multicommutation, which provides a fast alternative in quality control analysis, due to the easy treatment of a large number of samples (approximately 70 h⁻¹), and is an environmentally friendly procedure, which involves a waste generation of only 94.5 ml h⁻¹ as compared with the 605 ml h⁻¹ obtained by using continuous AFS measurements. The limit of detection found was 0.011 ng g⁻¹ Hg in the original sample. The method provided a relative standard deviation of 3.4% for five independent analysis of a sample containing 0.30 ng g⁻¹ Hg. To validate the accuracy of the method, a certified reference material NIST-1459 (non-fat milk powder) containing 0.3±0.2 ng g⁻¹ Hg was analysed and a value of 0.27±0.06 ng g⁻¹ Hg was found. A comparison made between data found by the developed procedure and those obtained by microwave-assisted digestion and continuous AFS measurements evidenced a good comparability between these two strategies. Results obtained for commercially available milk samples varied between 0.09 and 0.61 ng g⁻¹ Hg depending on the type of sample and its origin. The confluence of the analytical waste with a 6 mol l⁻¹ NaOH allowed us to reduce the waste generation in a working session from 1 l to 5 g solid residue with a matrix of Fe(OH)₃ which contributes to the deactivation of traces of heavy metals presents in the samples that does not form volatile hydrides.     

Determination of diphenhydramine by capillary electrophoresis with tris(2,2'-bipyridyl)ruthenium(II) electrochemiluminescence detection

Tris(2,2′-bipyridyl)ruthenium(II) electrochemiluminescence detection in a capillary electrophoresis separation system was used for the determination of diphenhydramine. In this study, platinum disk electrode (300 μm in diameter) was used as a working electrode and the influence of applied potential and buffer conditions were investigated. Under optimal conditions: 1.2 V applied potential, pH 8.50, 15 kV separation voltage and 10 mmol l⁻¹ running buffer, the calibration curve of diphenhydramine was linear over the range of 4×10⁻⁸ to 1×10⁻⁵ mol l⁻¹. This technique gave satisfactory precision, and relative standard deviations of migration times and chemiluminescence peak intensities were less than 1 and 6%, respectively. The technique was applied to animal studies for determination of diphenhydramine extracted from rabbit plasma and urine samples, and the extraction efficiency were between 92 and 98.5%.     

Chemiluminescence behavior based on oxidation reaction of rhodamine B with cerium(IV) in sulfuric acid medium

The chemiluminescence (CL) of the rhodamine B (RhB)-cerium(IV) system was investigated by flow-injection. Rhodamine B was suggested to be a suitable chemiluminescent reagent in acidic conditions. When the concentration of rhodamine B was 100 mg l⁻¹ and cerium sulfate was 1.6 mmol l⁻¹ in sulfuric acid, the chemiluminescent intensity was found to be highest by using 0.3 mol l⁻¹ sulfuric acid as a carrier solution. The particular chemiluminescent system could tolerate such distinct acidic environments that it was utilized for detecting many compounds that are stable in acidic solutions. Furthermore, by virtue of IR, UV-Vis and luminescence spectroscopic measurements, the chemiluminescent behavior of rhodamine B was studied and a possible mechanism for this chemiluminescent reaction was proposed. The emitter was affirmed to be a radical species due to one of the oxidation products of RhB; the chemiluminescent emissive wavelength was about 425 nm.     

Synthesis of cross-linked chitosan possessing N-methyl-d-glucamine moiety (CCTS-NMDG) for adsorption/concentration of boron in water samples and its accurate measurement by ICP-MS and ICP-AES

A chitosan resin derivatized with N-methyl-d-glucamine (CCTS-NMDG) was synthesized by using a cross-linked chitosan (CCTS) as base material. The N-methyl-d-glucamine (NMDG) moiety was attached to the amino group of CCTS through the arm of chloromethyloxirane. The adsorption behavior of 59 elements on the synthesized resin was systematically examined by using the resin packed in a mini-column, passing water samples through it and measuring the adsorbed elements in eluates by ICP-MS. The CCTS-NMDG resin shows high ability in boron sorption with the capacity of 0.61 mmol ml⁻¹ (= 2.1 mmol g⁻¹). The sorption kinetics of this resin was faster than that of the commercially available resins. Other advantages of the synthesized resin are: (1) quantitative collection of boron at neutral pH regions; (2) complete removal of large amounts of matrices; (3) no loss of efficiency over prolonged usage; (4) effective collection of boron in wide range concentration using a mini column containing 1 ml resin; (5) complete elution of boron with 1 mol l⁻¹ nitric acid. The resin was applied to the collection/concentration of boron in water samples. Boron in tap water and river water was found to be in the range of 6-8 μg l⁻¹. The limit of detection (LOD) of boron after pretreatment with CCTS-NMDG resin and measurement by ICP-MS was 0.07 μg l⁻¹ and the limit of quantification (LOQ) was 0.14 μg l⁻¹ when the volume of each sample and eluent was 10 ml.     

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%.     

Flow injection system with in-line Winkler's procedure using 16-way valve and spectrofluorimetric determination of dissolved oxygen in environmental waters

Flow injection spectrofluorimetry with in-line Winklers procedure was developed for the dissolved oxygen (DO) determination. 2-Thionaphthol reacted with iodine produced by Winkler’s method to form fluorescence inactive disulfide compound. To automate the process completely, a 5-channel flow system with a newly designed 16-way valve was assembled. The system consisted of a dispersion coil (DC), a precipitate formation coil (PFC), a precipitate dissolving coil (PDC), and extraction coil (EC). A calibration can be constructed by using a standard iodine solution for dissolved oxygen. The calibration graph was linear over the range 1.2×10⁻⁴∼6.0×10⁻⁴ mol l⁻¹ iodine (1.96∼9.80 mg O l⁻¹)). The relative standard deviation (n=6) was below 0.3% for the 4×10⁻⁴ mol l⁻¹ iodine (6.27 mg O l⁻¹) determination. The sample throughput was 12/h.     

Simple and fast spectrophotometric determination of H(2)O(2) in photo-Fenton reactions using metavanadate

This work proposes a spectrophotometric method for the determination of hydrogen peroxide during photodegradation reactions. The method is based on the reaction of H₂O₂ with amonium metavanadate in acidic medium, which results in the formation of a red-orange color peroxovanadium cation, with maximum absorbance at 450 nm. The method was optimized using the multivariate analysis providing the minimum concentration of vanadate (6.2 mmol L⁻¹) for the maximum absorbance signal. Under these conditions, the detection limit is 143 μmol L⁻¹. The reaction product showed to be very stable for samples of peroxide concentrations up to 3 mmol L⁻¹ at room temperature during 180 h. For higher concentrations however, samples must be kept refrigerated (4 °C) or diluted. The method showed no interference of Cl⁻ (0.2-1.3 mmol L⁻¹), NO₃⁻ (0.3-1.0 mmol L⁻¹), Fe³⁺ (0.2-1.2 mmol L⁻¹) and 2,4-dichlorophenol (DCP) (0.2-1.0 mmol L⁻¹). When compared to iodometric titration, the vanadate method showed a good agreament. The method was applied for the evaluation of peroxide consumption during photo-Fenton degradation of 2,4-dichlorophenol using blacklight irradiation.