Exploiting kinetic spectrophotometric determination of captopril, an angiotensin-converting enzyme inhibitor, in a multi-pumping flow system

The implementation of a differential kinetic spectrophotometric method for the determination of angiotensin-converting-enzyme inhibitors in pharmaceutical formulations is described. The determination method was based on the monitoring (350 nm) of the reaction between captopril and iodate, in the presence of iodide, versus time and was fully automated by exploiting the multi-pumping flow concept. The developed multi-pumping flow system included four discretely actuated solenoid micro-pumps as unique flow manifold active components. The automatic control of the solenoid micro-pumps, under time-based and pulse-counting routines, allowed the implementation of a reliable and versatile analytical determination, with the additional advantage of permitting a runtime access to important analytical parameters, such as flow rate, sample insertion and reagent addition synchronisation, facilitating this way the establishment of an approach for kinetic measurements, directly due to the efficient solution handling and accurate timing control.

A linear range of determination was verified for captopril concentrations between 10.0 and 60.0 μg mL⁻¹ with a sample throughput of about 100 determinations per hour. The results were in agreement with those obtained by the reference procedure with relative deviations between 1.81 and 4.48%.     

An improved procedure for flow-based turbidimetric sulphate determination based on a liquid core waveguide and pulsed flows

An improved flow-based procedure is proposed for turbidimetric sulphate determination in waters. The flow system was designed with solenoid micro-pumps in order to improve mixing conditions and minimize reagent consumption as well as waste generation. Stable baselines were observed in view of the pulsed flow characteristic of the systems designed with solenoid micro-pumps, thus making the use of washing solutions unnecessary. The nucleation process was improved by stopping the flow prior to the measurement, thus avoiding the need of sulphate addition. When a 1-cm optical path flow cell was employed, linear response was achieved within 20–200 mg L⁻¹, described by the equation S = −0.0767 + 0.00438C (mg L⁻¹), r = 0.999. The detection limit was estimated as 3 mg L⁻¹ at the 99.7% confidence level and the coefficient of variation was 2.4% (n = 20). The sampling rate was estimated as 33 determinations per hour. A long pathlength (100-cm) flow cell based on a liquid core waveguide was exploited to increase sensitivity in turbidimetry. Baseline drifts were avoided by a periodical washing step with EDTA in alkaline medium. Linear response was observed within 7–16 mg L⁻¹, described by the equation S = −0.865 + 0.132C (mg L⁻¹), r = 0.999. The detection limit was estimated as 150 μg L⁻¹ at the 99.7% confidence level and the coefficient of variation was 3.0% (n = 20). The sampling rate was estimated as 25 determinations per hour. The results obtained for freshwater and rain water samples were in agreement with those achieved by batch turbidimetry at the 95% confidence level.     

Multi-pumping flow systems: the potential of simplicity

Nowadays, the trend towards more compact, smarter and simpler devices is generally recognized as one of the most challenging aspects in the development of analytical instrumentation. Modern flow-based procedures do not escape this tendency. The level of integration and automation and the operational functionality of Multi-pumping flow systems (MPFS) would, in most of the situations, meet this requirement. The essential elements of MPFS are multiple solenoid actuated micro-pumps strategically positioned in the flow manifold, which are accountable for solutions insertion, propelling and commutation, conditioning the establishment and subsequent detection of the reaction zone. Being the only active components of the flow manifold they provide a great operational simplicity and assure a straightforward run-time control of important analytical variables. Moreover, the reduction of active components minimizes the probability of occurrence of equipment failures, malfunctions or errors. The low size and low cost of solenoid micro-pumps make them ideal tools to build up compact environmentally friendly analytical systems, which are characterized by low solutions consumptions and the minimisation of hazardous waste generation. Furthermore, the reproducible pulsed flowing stream produced by micro-pumps actuation has proven to be a valuable feature regarding sample/reagent mixing and reaction zone homogenisation.     

Flow injection determination of nitrite by fluorescence quenching

A simple, sensitive and selective fluorimetric method for the determination of nitrite ion in waters using a merging zones flow injection system is described. The fluorimetric determination is based on the measurement of the quenching effect produced by nitrite on proflavine (3,6-diaminoacridine) fluorescence (λ_ex/λ_em=290/519 nm).

The optimum experimental conditions were investigated by merging 0.5 ml of the sample and 0.5 ml of a solution of 5 mg l⁻¹ of proflavine (in 0.1 M HCl) in a flow injection system, on-line connected to a flow-cell placed in the conventional sample compartment of a spectrofluorimeter. The selected carrier solution and final flow rate were 0.1 M HCl and 0.5 ml min⁻¹, respectively. A reaction coil of 2 ml was used. As a result of the simplicity of this system, a sample throughput of about 50 samples h⁻¹ can be achieved with the proposed methodology.

The detection limit was 1.1 ng ml⁻¹ (3σ criterion) of nitrite. The repeatability for five sample injections containing 100 ng ml⁻¹ of nitrite was ±0.3% and the observed linear range extended up to 400 ng ml⁻¹. Also, the effect of interferences from various metals and anions commonly present in waters was also studied.

The method was successfully applied to the determination of low levels of nitrite in different water samples (river, fountain, tap and commercial drinking waters).     

A multicommuted stop-flow system employing LEDs-based photometer for the sequential determination of anionic and cationic surfactants in water

It has been developed an automatic stop-flow procedure for sequential photometric determination of anionic and cationic surfactants in a same sample of water. The flow system was based on multicommutation process that was designed employing two solenoid micro-pumps and six solenoid pinch valves, which under microcomputer control carry out fluid propelling and reagent solutions handling. A homemade photometer using a photodiode as detector and two light emitting diodes (LEDs) with emission at 470 nm (blue) and 650 nm (red) as radiation sources, which was tailored to allow the determination of anionic and cationic surfactants in waters. The procedure for anionic surfactant determination was based on the substitution reaction of methyl orange (MO) by the anionic surfactant sodium dodecylbenzene sulfonate (DBS) to form an ion-pair with the cetyl pyridine chloride (CPC). Features such as a linear response ranging from 0.35 to 10.5 mg L⁻¹ DBS (R = 0.999), a detection limit of 0.06 mg L⁻¹ DBS and a relative standard deviation of 0.6% (n = 11) were achieved. For cationic surfactant determination, the procedure was based on the ternary complex formation between cationic surfactant, Fe(III) and chromazurol S (CAS) using CPC as reference standard solution. A linear response range between 0.34 and 10.2 mg L⁻¹ CPC (R = 0.999), a detection limit of 0.05 mg L⁻¹ CPC and a relative standard deviation of 0.5% (n = 11) were obtained. In both cases, the sampling throughput was 60 determinations per hour. Reagents consumption of 7.8 μg MO, 8.2 μg CPC, 37.2 μg CAS and 21.6 μg Fe(III) per determination were achieved. Analyzing river water samples and applying t-test between the results found and those obtained using reference procedures for both surfactant types provide no significant differences at 95% confidence level.     

A novel homocystine-agarose adsorbent for separation and preconcentration of nickel in table salt and baking soda using factorial design optimization of the experimental conditions

Homocystine was for the first time, chemically linked to a highly cross-linked agarose support (Novarose) to be employed as a chelating adsorbent for preconcentration and AAS determination of nickel in table salt and baking soda. Nickel is quantitatively adsorbed on a small column packed with 0.25 ml of the adsorbent, in a pH range of 5.5–6.5 and simply eluted with 5 ml of a 1 mol l⁻¹ hydrochloric acid solution.

A factorial design was used for optimization of the effects of five different variables on the recovery of nickel. The results indicated that the factors of flow rate and column length, and the interactions between pH and sample volume are significant.

In the optimized conditions, the column could tolerate salt concentrations up to 0.5 mol l⁻¹ and sample volumes beyond 500 ml. Matrix ions of Mg²⁺ and Ca²⁺, with a concentration of 200 mg l⁻¹, and potentially interfering ions of Cd²⁺, Cu²⁺, Zn²⁺ and Mn²⁺, with a concentration of 10 mg l⁻¹, did not have significant effect on the analyte's signal. Preconcentration factors up to 100 and a detection limit of 0.49 μg l⁻¹, corresponding to an enrichment volume of 500 ml, were obtained for the determination of the analyte by flame AAS. Application of the method to the determination of natural and spiked nickel in table salt and baking soda solutions resulted in quantitative recoveries. Direct ETAAS determination of nickel in the same samples was not possible because of a high background observed.     

A multicommuted flow system with solenoid micro-pumps for paraquat determination in natural waters

A flow system designed with solenoid micro-pumps is proposed for the determination of paraquat in natural waters. The procedure involves the reaction of paraquat with dehydroascorbic acid followed by spectrophotometric measurements. The proposed procedure minimizes the main drawbacks related to the standard chromatographic procedure and to flow analysis and manual methods with spectrophotometric detection based on the reaction with sodium dithionite, i.e. high solvent consumption and waste generation and low sampling rate for chromatography and high instability of the reagent in the spectrophotometric procedures. A home-made 10-cm optical-path flow cell was employed for improving sensitivity and detection limit. Linear response was observed for paraquat concentrations in the range 0.10–5.0 mg L⁻¹. The detection limit (99.7% confidence level), sampling rate and coefficient of variation (n = 10) were estimated as 22 μg L⁻¹, 63 measurements per hour and 1.0%, respectively. Results of determination of paraquat in natural water samples were in agreement with those achieved by the chromatographic reference procedure at the 95% confidence level.     

An intelligent flow analyser for the in-line concentration, speciation and monitoring of metals at trace levels

An intelligent and versatile flow system is proposed for the in-line speciation and/or concentration of metal ions at a wide range of concentrations without requiring manifold reconfiguration. On one hand, sample enrichment strategies are accomplished using packed-bed reactors, on the other hand speciation procedures are readily performed exploiting the selective complexation of the different oxidation states with the appropriate chromogenic reagents.

The potentials of the automated methodology were evaluated using the spectrophotometric monitoring of iron as a model of chemistry. Under the optimised physical and chemical variables, linear analytical curves over the ranges 0.025–0.5 or 2.0–40 mg l⁻¹ Fe were attained. The 3σ detection limit, the repeatability at the 0.5 mg l⁻¹ level, the enrichment factor for a sampling volume of 10 ml, and the maximum injection throughput were 8.4 ng ml⁻¹ Fe, 2.5%, 58.6 and 22 h⁻¹, respectively. The flowing system was applied to the speciation analysis of iron in waters, pharmaceutical formulations and agricultural products, using ICP-OES detection as an external reference method for total iron determination.

A remarkable feature of the expert system hereby presented is the ability to decide by itself if the pre-concentration and/or oxidation of the sample zone is required.     

Reverse flow injection spectrophotometric determination of iron(III) using chlortetracycline reagent

A simple reversed flow injection colourimetric procedure for determining iron(III) was proposed. It is based on the reaction between iron(III) with chlortetracycline, resulting in an intense yellow complex with a suitable absorption at 435 nm. A 200 μl chlortetracycline reagent solution was injected into the phosphate buffer stream (flow rate 2.0 ml min⁻¹) which was then merged with iron(III) standard or sample in dilute nitric acid stream (flow rate 1.5 ml min⁻¹). Optimum conditions for determining iron(III) were investigated by univariate method. Under the optimum conditions, a linear calibration graph was obtained over the range 0.5–20.0 μg ml⁻¹. The detection limit (3σ) and the quantification limit (10σ) were 0.10 and 0.82 μg ml⁻¹, respectively. The relatives standard deviation of the proposed method calculated from 12 replicate injections of 2.0 and 10.0 μg ml⁻¹ iron(III) were 0.43 and 0.59%, respectively. The sample throughput was 60 h⁻¹. The proposed method has been satisfactorily applied to the determination of iron(III) in natural waters.     

Flow injection determination of selenium by successive retention of Se(IV) and tetrahydroborate(III) on an anion-exchange resin and hydride generation electrothermal atomization atomic absorption spectrometry with in-atomizer trapping: Part 1. Method development and investigation of interferences

A sample solution was passed at 20 ml min⁻¹ through a column (150×4 mm²) of Amberlite IRA-410Stron anion-exchange resin for 60 s. After washing, a solution of 0.1% sodium borohydride was passed through the column for 60 s at 5.1 ml min⁻¹. Following a second wash, a solution of 8 mol l⁻¹ hydrochloric acid was passed at 5.1 ml min⁻¹ for 45 s. The hydrogen selenide was stripped from the eluent solution by the addition of an argon flow at 150 ml min⁻¹ and the bulk phases were separated by a glass gas–liquid separator containing glass beads. The gas stream was dried by passing through a Nafion® dryer and fed, via a quartz capillary tube, into the dosing hole of a transversely heated graphite cuvette containing an integrated L’vov platform which had been pretreated with 120 μg of iridium as trapping agent. The furnace was held at a temperature of 250°C during this trapping stage and then stepped to 2000°C for atomization. The calibration was performed with aqueous standards solution of selenium (selenite, SeO₃²⁻) with quantification by peak area. A number of experimental parameters, including reagent flow rates and composition., nature of the gas–liquid separator, nature of the anion-exchange resin, column dimensions, argon flow rate and sample pH, were optimized. The effects of a number of possible interferents, both anionic and cationic were studies for a solution of 500 ng 1⁻¹ of selenium. The most severe depressions were caused by iron (III) and mercury (II) for which concentrations of 20 and 10 mg  1⁻¹ caused a 5% depression on the selenium signal. For the other cations (cadmium, cobalt, copper, lead,. magnesium, and nickel) concentrations of 50–70 mg 1⁻¹ could be tolerated. Arsenate interfered at a concentration of 3 mg⁻¹, whereas concentrations of chloride, bromide, iodide, perchlorate, and sulfate of 500–900 mg l⁻¹ could be tolerated. A linear response was obtained between the detection limit of 4 ng 1⁻¹, with a characteristic mass of 130 pg. The RSDs for solutions containing 100 and 200 ng 1⁻¹ selenium were 2.3% and 1.5%, respectively.     

Determination of Rh, Pd and Pt in urine samples using a pre-concentration sequential injection analysis system coupled to a quadrupole-inductively coupled plasma-mass spectrometer

The proposed flow system was developed in order to minimize the drawbacks related to the PGEs determination by quadrupole-inductively coupled plasma-mass spectrometry (Q-ICP-MS). It was intended not only to lower the limits of detection (LODs) but also to eliminate the interferences originating from some atomic and molecular ions produced in the argon plasma. This was accomplished by means of an on-line sample clean-up/pre-concentration step, using a chelating resin (Metalfix™ Chelamine™) in which Rh, Pd and Pt were preferably retained when compared with the interfering species.

The results obtained by using the developed flow system in the analysis of urine samples are presented. With a sampling rate of 9 samples h⁻¹ (i.e., 27 determinations) and a sample consumption of ca. 10 mL, the developed flow system allowed linear calibration plots up to 100 ng L⁻¹ with detection limits of 1.2 ng L⁻¹ (Rh), 0.4 ng L⁻¹ (Pd) and 0.9 ng L⁻¹ (Pt). Repeatability studies showed good precision (R.S.D.%, n = 5): 3.7% (Rh); 2.6% (Pd) and 2.4% (Pt), for 10 ng L⁻¹; 2.4% (Rh); 1.4% (Pd) and 1.9% (Pt), for 50 ng L⁻¹; and 1.3% (Rh); 0.58% (Pd) and 0.62% (Pt), for 100 ng L⁻¹. By spiking human urine samples, recovery tests were performed, and the values obtained ranged between 89% and 105% (Rh); 90% and 104% (Pd); and 93% and 105% (Pt).     

Spectrophotometric simultaneous determination of nitrite, nitrate and ammonium in soils by flow injection analysis

A new rapid flow injection procedure for the simultaneous determination of nitrate, nitrite and ammonium in single flow injection analysis system is proposed. The procedure combines on-line reduction of nitrate to nitrite and oxidation of ammonium to nitrite with spectrophotometric detection of nitrite by using the Griess-llosvay reaction. The formed azo dye was measured at 543 nm. The influence of reagent concentration and manifold parameters were studied. Nitrite, nitrate and ammonium can be determined within the range of 0.02–1.60 μg mL⁻¹, 0.02–1.60 μg mL⁻¹ and 0.05–1.40 μg mL⁻¹, respectively. R.S.D. values (n = 10) were 2.66; 1.41 and 3.58 for nitrate, nitrite and ammonium, respectively. This procedure allows the determination and speciation of inorganic nitrogen species in soils with a single injection in a simple way, and high sampling rate (18 h⁻¹). Detection limits of 0.013, 0.046 and 0.047 μg mL⁻¹were achieved for nitrate, nitrite and ammonium, respectively. In comparison with others methods, the proposed one is more simple, it uses as single chromogenic reagent less injection volume (250 mL in stead of 350 mL) and it has a higher sampling rate.     

Micellar effects on photoinduced redox reactions of Fe(bpy)2(CN)2

Excitation of solutions of Fe(bipy)₂(CN)₂ by a 266-nm laser pulse produces a hydrated electron and the oxidized complex, Fe(bipy)₂ (CN)₂⁺, in the primary photochemical step, in homogeneous aqueous solution as well as in aqueous solutions containing cetyltrimethylammonium bromide (CTAB) or sodium dodecyl sulfate (SDS) micelles. In all cases nascent hydrated electrons react with ground state Fe(bipy)₂(CN)₂ to form Fe(bipy)₂(CN)₂⁻, and comparison of the decay constants in the three media (H₂O: k = 2.8 × 10¹⁰ M⁻¹ s⁻¹; CTAB: k = 2.9 × 10¹⁰ M⁻¹ s⁻¹; SDS: k = 5.5 × 10⁹ M⁻¹ s⁻¹), shows that the reaction is essentially unaffected by CTAB micelles but is much slower in SDS solution. Similar micellar effects were found for the back reaction between e_aq⁻ and Fe(bpy)₂(CN)₂⁺. Rate constants for the scavenging of the photogenerated hydrated electrons by methyl viologen (MV²⁺) cations and NO₃⁻ anions were measured in the three systems, and the results indicate that for scavenging by MV²⁺ the rate constants are decreased in the micelle systems (k in H₂O, 8.4 × 10¹⁰; CTAB, 3.5 × 10¹⁰ and SDS, 1.58 × 10¹⁰ M⁻¹ s⁻¹), whereas for NO₃⁻ the CTAB micelle decreases while the SDS micelle enhances the scavenging compared to water solution (k in H₂O, 8.3 × 10⁹; CTAB, 7 × 10⁸; and SDS, 2.05 × 10¹⁰ M⁻¹ s⁻¹). For the comproportionation reaction between Fe(bipy)₂(CN)₂⁺ and Fe(bipy)₂(CN)₂⁻ both micelles reduce the rate (k in H₂O, 3.3 × 10¹⁰; CTAB, 2.3 × 10¹⁰; and SDS, 1.05 × 10¹⁰ M⁻¹s⁻¹), but while the reaction of Fe(bipy)₂(CN)₂⁺ with MV⁺ is increased in CTAB compared to water, it is slowed in SDS (k in H₂O, 2.4 × 10¹⁰; CTAB, 8.9 × 10¹⁰; and SDS, 1.8 × 10¹⁰ M⁻¹s⁻¹). All effects observed in these microheterogeneous systems can be uniformly interpreted in terms of Coulombic interactions between the actual reactants and the charged surface of the micelles.     

Repetitive determination of ascorbic acid using iron(III)-1.10-phenanthroline-peroxodisulfate system in a circulatory flow injection method

Ascorbic acid (AA) could be determined in large quantities of a co-existing oxidant. The incorporation of an on-line reagent regeneration step based on redox reaction eliminates the baseline drift in the procedure. This makes it possible to adopt a circulatory flow injection method (cyclic FIA) and to determine AA repetitively. The method is based on the reduction of iron(III) to iron(II) by the analyte, the reaction of the produced iron(II) with 1,10-phenanthroline (phen) in a weak acidic medium to form a colored complex, and the subsequent oxidation reaction of iron(II) to iron(III) by the co-existing peroxodisulfate. A solution (50 ml) of 3.0×10⁻⁴ mol l⁻¹ ferric chloride, 9.0×10⁻⁴ mol l⁻¹ phen and 5.0×10⁻² mol l⁻¹ ammonium peroxodisulfate in acetate buffer (0.2 mol l⁻¹, pH 4.5) is continuously circulated at a constant flow rate of 1.0 ml min⁻¹. Into this stream, an aliquot (20 μl) of the sample solution containing AA is quickly injected by means of a six-way valve. The complex formed is monitored spectrophotometrically (at 510 nm) in the flow system. The stream then returns to the reservoir after passing through a time-delay coil (50 m). The iron(II)–(phen)₃ complex is oxidized to iron(III)–(phen)₃ complex by peroxodisulfate which exists excessively in the circulating reagent solution. The proposed method allows as many as 300 repetitive determinations of 15 mg l⁻¹ AA with only 50 ml reservoir solution. The contents of AA in commercial pharmaceutical products were analyzed to demonstrate the capability of the developed system.     

Ab initio GB study of prebiotic synthesis of purine precursors from aqueous hydrogen cyanide: dimerization reaction of HCN in aqueous solution

Ab initio MO GB theory which includes the continuum model of solvent effect using generalized Born formula has been applied to the dimerization reaction of HCN in aqueous solution which is the starting step in prebiotic synthesis of purine precursors from aqueous hydrogen cyanide. Three steps considered were: (i) the reaction of HCN and H₂O to produce the CN⁻ anion, (ii) the reaction of CN⁻ with HCN to give the NC–CH=N⁻ anion, and (iii) the addition of a proton to the anion to give iminoacetonitrile. The formation of CN⁻ ion from HCN in aqueous solution requires 15.1 kcal/mol (the experimental value estimated from the dissociation constant of HCN in water is 14.8 kcal/mol). The reaction of CN⁻ with HCN requires the activation energy of 32.2 kcal/mol (MP2/6-31++G^**//HF/6-31++G^**) to give the dimer. This barrier height is reduced to 26.1 kcal/mol when HCN is associated with H₃O⁺. In the presence of NH₃ in aqueous solution, CN⁻ is produced easily by the reaction of HCN and NH₃ with a low activation energy of 4.3 kcal/mol. It was shown that the formation of CN⁻ becomes easier in ammoniacal solution, and the dimerization occurs efficiently in aqueous solutions which contain NH₃.     

Sequential injection spectrophotometric determination of trace amounts of iodide by its catalytic effect on the 4,4'-methylenebis(N,N-dimethylaniline)-chloramine-T reaction

A simple and sensitive sequential injection spectrophotometric procedure is proposed for the determination of trace amounts of iodide in pharmaceutical preparations. The method is based on the catalytic effect of iodide on the (tetra base) 4,4′-methylenebis(N,N-dimethylaniline)-chloramine-T reaction in acidic solution. The method involves a sequential aspiration of 255 μl sample/standard followed by 170 μl tetra base and then 128 μl chloramine-T solutions into a carrier stream to be stacked inside a holding coil and flow reversed through a reaction coil towards a detector. The resulting colored compound is measured at 600 nm using an UV/Vis-spectrophotometer. All the parameters that affect the reaction were evaluated and the calibration curve is linear over a range of 0.1–6.0 μg l⁻¹ of iodide concentration with detection limit of 0.05 μg l⁻¹. A sample throughput of 80 samples per hour and relative standard deviation of less than 2.0% was achieved. The method is successfully applied for the determination of iodide in three different samples (tablets).     

On-line dynamic fractionation and automatic determination of inorganic phosphorus in environmental solid substrates exploiting sequential injection microcolumn extraction and flow injection analysis

Sequential injection microcolumn extraction (SI-MCE) based on the implementation of a soil-containing microcartridge as external reactor in a sequential injection network is, for the first time, proposed for dynamic fractionation of macronutrients in environmental solids, as exemplified by the partitioning of inorganic phosphorus in agricultural soils. The on-line fractionation method capitalises on the accurate metering and sequential exposure of the various extractants to the solid sample by application of programmable flow as precisely coordinated by a syringe pump.

Three different soil phase associations for phosphorus, that is, exchangeable, Al- and Fe-bound, and Ca-bound fractions, were elucidated by accommodation in the flow manifold of the three steps of the Hieltjes–Lijklema (HL) scheme involving the use of 1.0 M NH₄Cl, 0.1 M NaOH and 0.5 M HCl, respectively, as sequential leaching reagents. The precise timing and versatility of SI for tailoring various operational extraction modes were utilized for investigating the extractability and the extent of phosphorus re-distribution for variable partitioning times.

Automatic spectrophotometric determination of soluble reactive phosphorus in soil extracts was performed by a flow injection (FI) analyser based on the Molybdenum Blue (MB) chemistry. The 3σ detection limit was 0.02 mg P L⁻¹ while the linear dynamic range extended up to 20 mg P L⁻¹ regardless of the extracting media. Despite the variable chemical composition of the HL extracts, a single FI set-up was assembled with no need for either manifold re-configuration or modification of chemical composition of reagents.

The mobilization of trace elements, such as Cd, often present in grazed pastures as a result of the application of phosphate fertilizers, was also explored in the HL fractions by electrothermal atomic absorption spectrometry.     

Biosorption of heavy metals on Aspergillus fumigatus immobilized Diaion HP-2MG resin for their atomic absorption spectrometric determinations

A solid phase extraction procedure based on biosorption of copper(II), lead(II), zinc(II), iron(III), nickel(II) and cobalt(II) ions on Aspergillus fumigatus immobilized Diaion HP-2MG has been investigated. The analytical conditions including amounts of A. fumigatus, eluent type, flow rates of sample and eluent solutions were examined. Good recoveries were obtained to the spiked natural waters. The influences of the concomitant ions on the retentions of the analytes were also examined. The detection limits (3sigma, N = 11) were 0.30 μg l⁻¹ for copper, 0.32 μg l⁻¹ for iron, 0.41 μg l⁻¹ for zinc, 0.52 μg l⁻¹ for lead, 0.59 μg l⁻¹ for nickel and 0.72 μg l⁻¹ for cobalt. The relative standard deviations of the procedure were below 7%. The validation of the presented procedure is performed by the analysis of three standard reference materials (NRCC-SLRS 4 Riverine Water, SRM 1515 Apple leaves and GBW 07605 Tea). The procedure was successfully applied for the determination of analyte ions in natural waters microwave digested samples including street dust, tomato paste, black tea, etc.     

Sequential injection spectrophotometric determination of iron as Fe(II) in multi-vitamin preparations using 1,10-phenanthroline as complexing agent

A sequential injection analysis (SIA) system is proposed for the determination of iron (II). Fe(II) was determined by SIA based on the reaction between 1,10-phenanthroline and iron (II), yielding an orange–red colour complex with absorption maximum at 512 nm. The method involved aspiration of 187 μl sample/standard zone followed by a zone of a reagent solution containing 140 μl of 7.8 × 10⁻⁴ mol l⁻¹ 1,10-phenanthroline into a carrier stream to be stacked inside a holding coil and flow reversed through a reaction coil to a detector. The optimum condition was evaluated and the calibration curve is linear over a range of 0.25 to 5.0 mg l⁻¹ of Fe(II) with detection limit of 18 μg l⁻¹. A sample throughput of 40 h⁻¹ was established. This technique is found to be simple, accurate, reproducible and sensitive. The proposed method was successfully applied for the determination of total iron as Fe(II) in pharmaceutical products (multi-vitamin tablets) and is especially useful for the determination of iron (II) in tablets with lower iron (II) contents. The results were found to be in good agreement with the results obtained by manual UV/Vis spectrophotometry and flame atomic absorption spectrometry (FAAS) and with claimed values by the manufacturers.     

Kinetic method for the determination of traces of thyroxine by its catalytic effect on the Mn(III) metaphosphate-As(III) reaction

A new, highly sensitive and simple kinetic method for the determination of thyroxine was proposed. The method was based on the catalytic effect of thyroxine on the oxidation of As(III) by Mn(III) metaphosphate. The kinetics of the reaction was studied in the presence of orthophosphoric acid. The reaction rate was followed spectrophotometrically at 516 nm. It was established that orthophosphoric acid increased the reaction rate and that the extent of the non-catalytic reaction was extremely small. A kinetic equation was postulated and the apparent rate constant was calculated. The dependence of the reaction rate on temperature was investigated and the energy of activation and other kinetic parameters were determined.

Thyroxine was determined under the optimal experimental conditions in the range 7.0 × 10⁻⁹ to 3.0 × 10⁻⁸ mol L⁻¹ with a relative standard deviation up to 6.7% and a detection limit of 2.7 × 10⁻⁹ mol L⁻¹. In the presence of 0.08 mol L⁻¹ chloride, the detection limit decreased to 6.6 × 10⁻¹⁰ mol L⁻¹. The proposed method was applied for the determination of thyroxine in tablets. The accuracy of the method was evaluated by comparison with the HPLC method.