Fast spectrophotometric determination of fluoride in ground waters by flow injection using partial least-squares calibration

The presence of sulphate constitutes a serious interference in the usual zirconium lake-based spectrophotometric method for the determination of fluoride in water. In this report, full spectral data have been recorded for the zirconium lake of 2-(parasulfophenylazo)-1,8-dihydroxy-3,6-naphthalene-disulfonate (SPADNS) in the simultaneous presence of fluoride and sulphate, as obtained with a flow injection system with a diode-array detector. The information has been processed with partial least-squares (PLS) multivariate calibration. Adequate modeling using a sixteen-sample calibration set allows fluoride to be determined in ground waters by the automated flow injection method, even in the presence of sulphate in concentrations up to 1000 mg l⁻¹. In the calibration range 0-1.50 mg l⁻¹ for fluoride, the limit of detection is 0.1 mg l⁻¹. The fluoride contents in real samples, as determined with the present method, were satisfactorily compared with those provided by ion selective potentiometry.     

Flow injection-chemiluminescence determination of propranolol in pharmaceutical preparations

A rapid and precise continuous-flow method is described for the determination of propranolol based on the chemiluminescence (CL) produced by its reaction with potassium permanganate in a sulphuric acid medium. The optimum chemical conditions for the chemiluminescence emission were investigated. Two manifolds were tested and their characteristics such as the length of the reactor, injection volume and flow rate were compared. When using the selected manifold, propranolol gives a linear calibration graph over the concentration range 1.0-17.5 mg l⁻¹. The detection limit calculated as proposed by IUPAC was 70 ng ml⁻¹ and the detection limit calculated as proposed by Clayton was 0.87 mg l⁻¹. For analysis of 10 solutions of 10.0 mg l⁻¹ propranolol, if error propagation theory is assumed, the relative error was 0.1%. The standard deviation (S.D.) for 10 replicate samples was 0.07 mg l⁻¹. The method has been validated versus a published fluorimetric method.The present chemiluminescence procedure was applied to the determination of propranolol in simple British and Spanish pharmaceutical formulations, with excellent recoveries, as the determination is free from interference from common excipients. However, some drugs, such as hydralazine and bendroflumethizide which may also be present in the formulation, increase the emission intensity.     

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.     

Analytical strategy photodegradation/chemiluminescence/continuous-flow multicommutation methodology for the determination of the herbicide Propanil

The present paper is dealing with an analytical strategy based on coupling photodegradation, chemiluminescence and multicommutation continuous-flow methodology for the determination of the pesticide Propanil, a common herbicide. The pesticide solution is inserted as small segments sequentially alternated with segments of the solution for adjusting the suitable medium for the photodegradation. Both flow-rates (sample and medium) are adjusted to required time for photodegradation, 2.0 min; and then, the resulting solution is also sequentially inserted as segments alternated with segments of the oxidizing solutions system, 1.00 × 10⁻⁴ mol l⁻¹ potassium permanganate in 2.00 mol l⁻¹ sulphuric acid medium. The calibration range, from 10 μg l⁻¹ to 25 mg l⁻¹, resulted in a linear behaviour over the range 10 μg l⁻¹-5 mg l⁻¹ and fitting the linear equation: I = 780.30C + 95.28; correlation coefficient 0.9999. The limit of detection was 8 μg l⁻¹ and the sample throughput 20 h⁻¹. After testing the influence of a large series of potential interferents the method is applied to water samples obtained from different places and to one formulation. The method is valid for the determination of other pesticides from the same chemical family, namely: alachlor, flumetsulam, furalaxyl and ofurace. Calibration graphs, limits of detection, repeatability and determination in water samples are obtained for each reported pesticide.     

Selective stopped-flow sequential injection method for the spectrophotometric determination of titanium in dental implant and natural Moroccan phosphate rock

The present work reports the first sequential injection (SI) method for the spectrophotometric determination of Ti(IV). The method is based upon the reaction of Ti(IV) with chromotropic acid (CA) in acidic medium to form a water-soluble complex (λ_max = 420 nm). The chemical and instrumental variables of the system that affected the reaction were studied. Selectivity was greatly enhanced using ascorbic acid. A linear calibration graph was obtained in the range 0.2-10.0 mg l⁻¹ Ti(IV) at a sampling frequency of 24 h⁻¹. The precision was satisfactory (s_r = 1.5% at 5.0 mg l⁻¹ Ti(IV), n = 12) and the 3σ limit of detection, c_L, was 0.7 mg l⁻¹ (n = 10). The developed method proved to be adequately selective and was applied successfully to the analysis of real samples (dental implant and natural Moroccan phosphate rock) giving accurate results based on recovery studies (98-105%).     

Study on photocatalytic oxidation for determination of the low chemical oxygen demand using a nano-TiO2-Ce(SO4)2 coexisted system

A new photocatalytic system, nano-TiO₂-Ce(SO₄)₂ coexisted system, which can be used to determine the low chemical oxygen demand (COD) is described. Nano-TiO₂ powders is used as photocatalyst in this system. The measuring method is based on direct determination of the concentration change of Ce(IV) resulting from photocatalytic oxidation of organic compounds. The mechanism of the photocatalytic oxidation for COD determination was discussed and the optimum experimental conditions were investigated. Under the optimum conditions, a good calibration graph for COD values between 1.0 and 12 mg l⁻¹ was obtained and the LOD value was achieved as low as 0.4 mg l⁻¹. When determining the real samples, the results were in good agreement with those from the conventional methods.     

Chemiluminometric photo-induced determination of Strychnine in a Multicommutation flow assembly

This paper deals on the determination of Strychnine, a potent and dangerous pesticide and the analytical procedure is based on the photo-induced chemiluminescence of the pesticide by means of the Multicommutation continuous-flow methodology. Small segments of the pesticide solution were sequentially alternated with segments of the solution for adjusting the suitable medium for the photodegradation. The required time of UV irradiation was obtained by stopped-flow during 150 s; then, the resulting solution formed alternated segments with the oxidizing solution containing 5 × 10⁻³ mol l⁻¹ Ce(IV) in 0.6 mol l⁻¹ nitric acid. The calibration range, from 2 μg l⁻¹ to 50 mg l⁻¹, resulted in a linear behaviour over the range 25 μg l⁻¹ to 20 mg l⁻¹ and fitting the equation: I = 4706x + 624 with a correlation coefficient of 0.9955. The limit of detection was 2 μg l⁻¹ and the sample throughput 15 h⁻¹. After testing the influence of a large series of potential interferents, the method was applied to different kinds of samples.     

Method for monitoring urea and ammonia in wine and must by flow injection-pervaporation

An easy to automate flow injection-pervaporation method for monitoring urea and ammonia in must and wine was developed. The method is based on separation of the ammonia from the sample matrix by pervaporation followed by its reaction with salicylate, hypochlorite and nitroprusside to form a diazonium salt with maximum absorption at 647 nm. Conversion of urea into ammonia catalysed by urease was mandatory before pervaporation. After optimisation by either the univariate or multivariate approaches as required, the linear range was established (between 0 and 25 mg l⁻¹) for both analytes. Then, the assessment of the proposed method versus a reference one for urea and ammonia was studied in terms of repeatability (0.52 and 0.43 mg l⁻¹, respectively), reproducibility (1.34 and 1.21 mg l⁻¹, respectively), detection and quantification limits (LOD=0.9 and 0.6 mg l⁻¹, LQ=1.02 and 0.67 mg l⁻¹, respectively) and traceability. The sample throughput was 16 samples h⁻¹. The method can be applied to the monitoring of the target analytes in must and young wine in order to control their contents, preventing formation of ethyl carbamate.     

Spectrofluorimetric determination of tannins based on their activative effect on the Cu(II) catalytic oxidation of rhodamine 6G by hydrogen peroxide

A simple and highly sensitive kinetic fluorimetric method is proposed for the determination of trace tannins, based on the activation of tannins on the oxidation of rhodamine 6G (Rh 6G) by hydrogen peroxide catalyzed by Cu(II) ion. The calibration graph was rectilinear in the range 0.08-1.28 mg l⁻¹ for tannin, the 3σ detection limit for tannin is 0.0455 mg l⁻¹. The relative standard deviation for 11 determinations of 0.4 mg l⁻¹ tannin is 0.96%. The proposed method has been successfully used to determine tannins in tea and Chinese gall. The results obtained were compared with those provided by the Folin-Ciocalteu method. This is the first procedure to be reported for the determination of tannins based on fluorimetric measurements.     

A multisyringe sequential injection method for monitoring water in the energy cogeneration system of a municipal waste incinerator

Leading-edge urban solid waste ashing plants use burning heat energy to obtain electrical power. Water fed to their boilers for conversion into steam should be highly pure in order to minimize corrosion, scaling and similar phenomena, which can lead to malfunctioning and a reduced useful life but can be avoided by proper management and control of the water supply. In this work, we developed a multiparameter monitor based on multisyringe sequential injection for the sequential determination of up to eight important parameters, namely: pH, specific and acid conductivity, hydrazine, ammonium, phosphate, silicate and total iron.Acid conductivity was determined by passing the sample through a cation-exchange resin in order to retain ammonium ion and release protons. This parameter was deemed the most accurate indicator of dissolved solids in boiler water. Chemical parameters were determined spectrophotometrically: hydrazine by reaction with p-dimethylaminobenzaldehyde, ammonium by the modified Berthelot reaction, iron with o-phenanthroline, and phosphate and silica by formation of a molybdoheteropoly blue dye in the presence of ascorbic acid as reductant. Use of the optimum chemical and physical operating conditions provided 3s_blank detection limits of 0.01 mg l⁻¹ N₂H₄, 0.13 mg l⁻¹ NH₄⁺, 0.04 mg l⁻¹ Fe, 0.03 mg l⁻¹ SiO₂ and 0.05 mg l⁻¹ PO₄³⁻, and relative standard deviations not greater than 2.5%. The methods integrated in the proposed monitor were successfully applied to real samples from the water-steam cycle at the Son Reus ashing plant in Palma de Mallorca (Spain).     

Study of the voltammetric behaviour of maleic hydrazide and its determination at a hanging mercury drop electrode

Voltammetric behaviour of maleic hydrazide pesticide dissolved in a Britton-Robertson buffer was investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). It was found that the process at the Hg electrode was diffusion controlled; the reaction was irreversible and involved a change of one proton and a transfer of one electron. A quantitative differential pulse voltammetric method for determination of maleic hydrazide was developed on the basis of these studies involving the reduction of the compound at a hanging mercury drop electrode. A linear calibration was obtained in the range of 0.5-5.5 mg l⁻¹, and the developed DPV methodology was then applied for the determination of maleic hydrazide in spiked vegetable samples by the standard addition method. Satisfactory percentage R.S.D. (∼2%), percentage recovery values (∼85%) and LOD (0.215 mg l⁻¹) were obtained. These compared well with the results from the alternative spectrophotometric method.     

Development and validation of ultra-high performance supercritical fluid chromatography method for determination of illegal dyes and comparison to ultra-high performance liquid chromatography method

A novel simple, fast and efficient ultra-high performance supercritical fluid chromatography (UHPSFC) method was developed and validated for the separation and quantitative determination of eleven illegal dyes in chili-containing spices. The method involved a simple ultrasound-assisted liquid extraction of illegal compounds with tetrahydrofuran. The separation was performed using a supercritical fluid chromatography system and CSH Fluoro-Phenyl stationary phase at 70 °C. The mobile phase was carbon dioxide and the mixture of methanol:acetonitrile (1:1, v/v) with 2.5% formic acid as an additive at the flow rate 2.0 mL min⁻¹. The UV–vis detection was accomplished at 500 nm for seven compounds and at 420 nm for Sudan Orange G, Butter Yellow, Fast Garnet GBC and Methyl Red due to their maximum of absorbance. All eleven compounds were separated in less than 5 min. The method was successfully validated and applied using three commercial samples of chili-containing spices – Chili sauce (Indonesia), Feferony sauce (Slovakia) and Mojo sauce (Spain). The linearity range of proposed method was 0.50–9.09 mg kg⁻¹ (r ≥ 0.995). The detection limits were determined as signal to noise ratio of 3 and were ranged from 0.15 mg kg⁻¹ to 0.60 mg kg⁻¹ (1.80 mg kg⁻¹ for Fast Garnet) for standard solution and from 0.25 mg kg⁻¹ to 1.00 mg kg⁻¹ (2.50 mg kg⁻¹ for Fast Garnet, 1.50 mg kg⁻¹ for Sudan Red 7B) for chili-containing samples. The recovery values were in the range of 73.5–107.2% and relative standard deviation ranging from 0.1% to 8.2% for within-day precision and from 0.5% to 8.8% for between-day precision. The method showed potential for being used to monitor forbidden dyes in food constituents. The developed UHPSFC method was compared to the UHPLC-UV method. The orthogonality of Sudan dyes separation by these two methods was demonstrated. Benefits and drawbacks were discussed showing the reliability of both methods for monitoring of studied illegal dyes in real food constituents.     

Microwave-assisted digestion of organoarsenic compounds for the determination of total arsenic in aqueous, biological, and sediment samples using flow injection hydride generation electrothermal atomic absorption spectrometry

A microwave assisted wet digestion method for organoarsenic compounds and subsequent determination of total arsenic in aqueous, biological and sediment samples by means of flow injection hydride generation electrothermal atomic absorption spectrometry (FI-HG-ETAAS) is described. Sodium persulfate, sodium fluoride and nitric acid serve as digestion reagents, which allow a quantitative transformation of organoarsenic compounds to hydride forming species in a commercial microwave sample preparation system. The maximum operating pressures of the applied tetrafluorometoxil (TFM) liners are 75 bar (high pressure vessels) and 30 bar (medium pressure vessels), corresponding to maximum solution temperatures of 300 and 260 °C. For the investigated samples, digestion temperatures of 210-230 °C (medium pressure vessels) and 240-280 °C (high pressure vessels) were obtained.In medium pressure vessels, arsenic recovery from aqueous testing solutions of dimethylarsinic acid (DMA), phenylarsonic acid (PAA) and tetraphenylarsonium chloride (TPA) at initial concentrations of 100 and 10 μg l⁻¹ is complete, even in the presence of an excess of organic carbon (potassium hydrogen phthalate, 2000 mg l⁻¹) or fatty acids (linolenic acid 70%; linoleic acid ≈20-25%; Oleic acid ≈3%, 900-4500 mg l⁻¹).Arsenic recovery from aqueous arsenobetaine (ASB) solutions with the same initial concentrations is also complete if high pressure vessels and a higher concentration of fluoride ions are used, whereas the addition of organic carbon (potassium hydrogen phthalate, 2000 mg l⁻¹, fatty acids, 900-4500 mg l⁻¹) leads to a decrease in arsenic recovery of about 2-5%. In all cases, residual carbon contents are close to the limit of detection for the applied analytical method (15 mg l⁻¹).Results of arsenic analysis in reference standard materials revealed a significant dependence on the material’s nature (sediment samples, plant materials and seafood samples). Sediment samples and plant materials show recoveries for arsenic around 100% after a single-step digestion in medium pressure TFM liners. Seafood (fish/lobster/mussel samples) usually require either the use of high pressure vessels or a second digestion step, if medium pressure vessels are used.     

Chemiluminometric determination of the pesticide 3-indolyl acetic acid by a flow injection analysis assembly

A new method is proposed for the chemiluminescent determination of the pesticide 3-indolyl acetic acid by means of an flow injection analysis system. The chemiluminescence emission is obtained by oxidation of the analyte with Ce (IV) in nitric acid and presence of β-cyclodextrine.The continuous-flow method allows the determination of 159 samples h⁻¹ of 3-indolyl acetic acid in an interval of concentrations over the range 0.5-15.0 mg l⁻¹. The limit of detection was 0.1 μg l⁻¹ and the R.S.D. (n, 17) at 2.0 mg l⁻¹ of the pesticide level was 2.7%. The method was applied to water samples.     

Sequential injection spectrophotometric determination of phenylephrine hydrochloride in pharmaceutical preparations

A fully automated sequential injection spectrophotometric method for the determination of phenylephrine hydrochloride in pharmaceutical preparations is reported. The method is based on the condensation reaction of the analyte with 4-aminoantipyrine in the presence of potassium ferricyanide. The absorbance of the condensation product was monitored at 503 nm. A linear relationship between the relative peak height and concentration was obtained in the range 0.5-17.5 mg l⁻¹. The detection limit (as 3σ value) was 0.09 mg l⁻¹ and repeatability was 0.8 and 0.6% at 2.5 and 5 mg l⁻¹, respectively. Results obtained by this method agreed very well with those obtained by the AOAC official method.     

Sequential injection lab-on-valve simultaneous spectrophotometric determination of trace amounts of copper and iron

A sequential injection (SI) method in a lab-on-valve (LOV) format for simultaneous spectrophotometric determination of copper and iron has been devised. The detection chemistry is based on the complex formation of 2-(5-bromo-2-pyridylazo)-5-[N-n-propyl-N-(3-sulfopropyl)amino]aniline (5-Br-PSAA) with copper(II) and/or iron(II) at pH 4.6. Copper(II) reacts with 5-Br-PSAA to form the complex which has an absorption maximum at 580 nm but iron(III) does not react. In the presence of a reducing agent only iron(II)-5-Br-PSAA complex is formed and detected at 558 nm. Under the optimum experimental conditions, the determinable ranges are 0.1-2 mg l⁻¹ for copper and 0.1-5 mg l⁻¹ for iron, respectively, with a sampling rate of 18 h⁻¹. The limits of detection are 50 μg l⁻¹ for copper and 25 μg l⁻¹ for iron. The relative standard deviations (n = 15) are 2% for 0.5 mg l⁻¹ copper and 1.8% for 0.5 mg l⁻¹ iron when determined in standard solutions. The recoveries range between 96 and 105% when determining 0.25-2 mg l⁻¹ of copper and 0.2-5 mg l⁻¹ of iron in artificial mixtures at copper/iron ratios of 1:10 to 5:1. The proposed SI-LOV method is successfully applied to the simultaneous determination of copper and iron in multi-element standard solution and in industrial wastewater samples.     

Sequential injection method for the direct spectrophotometric determination of bismuth in pharmaceutical products

A spectrophotometric method is reported for the determination of bismuth in pharmaceutical products using sequential injection analysis. Methylthymol blue (MTB) was used as a color forming reagent and the absorbance of the Bi(III)-MTB complex was monitored at 548 nm. The various chemical and physical variables that affected the reaction were studied. A linear calibration graph was obtained in the range 0.0-75.0 mg l⁻¹ Bi(III) at a sampling frequency of 72 h⁻¹. The reagent consumption was considerably reduced compared to conventional flow injection systems, as only 150 μl of MTB were consumed per run. The precision was very satisfactory (s_r=0.5%, at 50.0 mg l⁻¹ Bi(III), n=12) and the limit of detection, c_L, was 0.250 mg l⁻¹. The developed method was applied successfully to the analysis of various pharmaceutical products containing Bi(III). The relative errors e_r, were <1.5% in all cases and were evaluated by comparison of the obtained results with those found using atomic absorption spectrometry as the reference method.     

A multicommuted flow system for sequential spectrophotometric determination of hydrosoluble vitamins in pharmaceutical preparations

A multicommuted flow system is proposed for spectrophotometric determination of hydrosoluble vitamins (ascorbic acid, thiamine, riboflavine and pyridoxine) in pharmaceutical preparations. The flow manifold was designed with computer-controlled three-way solenoid valves for independent handling of sample and reagent solutions and a multi-channel spectrophotometer was employed for signal measurements. Periodic re-calibration as well as the standard addition method was implemented by using a single reference solution. Linear responses (r=0.999) were obtained for 0.500-10.0 mg l⁻¹ ascorbic acid, 2.00-50.0 mg l⁻¹ thiamine, 5.00-50.0 mg l⁻¹ riboflavine and 0.500-8.00 mg l⁻¹ pyridoxine. Detection limits were estimated as 0.08 mg l⁻¹ (0.5 μmol l⁻¹) ascorbic acid, 0.8 mg l⁻¹ (2 μmol l⁻¹) thiamine, 0.2 mg l⁻¹ (0.5 μmol l⁻¹) riboflavine and 0.1 mg l⁻¹ (0.9 μmol l⁻¹) pyridoxine at 99.7% confidence level. A mean sampling rate of 60 determinations per hour was achieved and coefficients of variation of 1% (n=20) were estimated for all species. The mean reagent consumption was 25-fold lower in relation to flow-based procedures with continuous reagent addition. Average recoveries between 95.6 and 100% were obtained for commercial pharmaceutical preparations. Results agreed with those obtained by reference methods at 95% confidence level. The flow system is suitable for application in quality control processes and in dissolution studies of vitamin tablets.     

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.