Simultaneous separation and preconcentration of trace elements in water samples by coprecipitation on manganese dioxide using D-glucose as reductant for KMnO(4)

A field oriented and economical method of coprecipitation of trace elements like Al, Au, Bi, Cd, Co, Cu, Fe, Mo, Ni, Pb, Pd, Ti, V, W, Zn and REE has been developed. A novel reductant D-glucose, reduces KMnO₄ in solution to form a precipitate of MnO_2. Two liters of clear natural water sample is adjusted to pH 3.5–4.0, and is treated with 10 ml of 1% KMnO₄ and 20 ml of 0.1% D-glucose. The sample is heated at a temperature of 75–80 °C, MnO₂ is formed which coprecipitates the above trace elements. The precipitate is separated by filtration, dissolved in 2 ml of 50% HCl and 2 ml of 30% H₂O₂ and diluted to 25 ml for analysis using AAS and ICP-AES. The recoveries were found to be 96–105%. The preconcentration factor is 80. Limits of determination by the proposed method in natural waters are 1 μg l⁻¹ for Al, Cd, Mo, V, W, Ti and Zn, 5 μg l⁻¹ for Au, Bi, Co, Cu, Fe, Ni, Pb and Pd and 8 μg l⁻¹ for REE. The RSD of the present procedure (n=5) is 8% at 5 μg l⁻¹ level. Twenty water samples can be analyzed by an analyst in an 8-h day.     

Flow injection for the determination of Se(IV) and Se(VI) by hydride generation atomic absorption spectrometry with microwave oven on-line prereduction of Se(VI) to Se(IV)

An on-line flow injection system has been developed for the selective determination of Se(IV) and Se(VI) in citric fruit juices and geothermal waters by hydride generation atomic absorption spectrometry with microwave-aided heating prereduction of Se(VI) to Se(IV). The samples and the prereductant solutions (4 mol l⁻¹ HCl for Se(IV) and 12 mol l⁻¹ HCl for Se(VI)) which circulated in a closed-flow circuit were injected by means of a time-based injector. This mixture was displaced by a carrier solution of 1% v/v of hydrochloric acid through a PTFE coil located inside the focused microwave oven and mixed downstream with a borohydride solution to generate the hydride. The linear ranges were 0–120 and 0–100 μg l⁻¹ of Se(IV) and Se(VI), respectively. The detection limits were 1.0 μg l⁻¹ for Se(IV) and 1.5 μg l⁻¹ for Se(VI). The precision (about 2.0–2.5% RSD) and recoveries (96–98% for Se(IV) and 94–98% for Se(VI)) were good. Total selenium values were also obtained by electrothermal atomic absorption spectrometry which agreed with the content of both selenium species. The sample throughput was about 50 measurements per hour. The main advantage of the method is that the selective determination of Se(IV) and Se(VI) in citric fruit juices and geothermal waters is performed in a closed system with a minimum sample manipulation, exposure to the environment, minimum sample waste and operator attention.     

Reverse flow injection spectrophotometric determination of iron(III) using norfloxacin

A reversed flow injection colorimetric procedure for determining iron(III) at the μg level was proposed. It is based on the reaction between iron(III) with norfloxacin (NRF) in 0.07 mol l⁻¹ ammonium sulfate solution, resulting in an intense yellow complex with a suitable absorption at 435 nm. Optimum conditions for determining iron(III) were investigated by univariate method. The method involved injection of a 150 μl of 0.04% w/v colorimetric reagent solution into a merged streams of sample and/or standard solution containing iron(III) and 0.07 mol l⁻¹ ammonium sulfate in sulfuric acid (pH 3.5) solution which was then passed through a single bead string reactor. Subsequently the absorbance as peak height was monitored at 435 nm. Beer's law obeyed over the range of 0.2–1.4 μg ml⁻¹ iron(III). The method has been applied to the determination of total iron in water samples digested with HNO₃–H₂O₂ (1:9 v/v). Detection limit (3σ) was 0.01 μg ml⁻¹ the sample through of 86 h⁻¹ and the coefficient of variation of 1.77% (n=12) for 1 μg ml⁻¹ Fe(III) were achieved with the recovery of the spiked Fe(III) of 92.6–99.8%.     

Determination of As(III) and total inorganic arsenic by flow injection hydride generation atomic absorption spectrometry

A simple procedure was developed for the direct determination of As(III) and As(V) in water samples by flow injection hydride generation atomic absorption spectrometry (FI–HG–AAS), without pre-reduction of As(V). The flow injection system was operated in the merging zones configuration, where sample and NaBH₄ are simultaneously injected into two carrier streams, HCl and H₂O, respectively. Sample and reagent injected volumes were of 250 μl and flow rate of 3.6 ml min⁻¹ for hydrochloric acid and de-ionised water. The NaBH₄ concentration was maintained at 0.1% (w/v), it would be possible to perform arsine selective generation from As(III) and on-line arsine generation with 3.0% (w/v) NaBH₄ to obtain total arsenic concentration. As(V) was calculated as the difference between total As and As(III). Both procedures were tolerant to potential interference. So, interference such as Fe(III), Cu(II), Ni(II), Sb(III), Sn(II) and Se(IV) could, at an As(III) level of 0.1 mg l⁻¹, be tolerated at a weight excess of 5000, 5000, 500, 100, 10 and 5 times, respectively. With the proposed procedure, detection limits of 0.3 ng ml⁻¹ for As(III) and 0.5 ng ml⁻¹ for As(V) were achieved. The relative standard deviations were of 2.3% for 0.1 mg l⁻¹ As(III) and 2.0% for 0.1 mg l⁻¹ As(V). A sampling rate of about 120 determinations per hour was achieved, requiring 30 ml of NaBH₄ and waste generation in order of 450 ml. The method was shown to be satisfactory for determination of traces arsenic in water samples. The assay of a certified drinking water sample was 81.7±1.7 μg l⁻¹ (certified value 80.0±0.5 μg l⁻¹).     

Simultaneous determination of selenium and lead in whole blood samples by differential pulse polarography

The polarographic reduction of lead in the presence of selenite gives rise to an additional peak corresponding to the reduction of lead (Pb) on adsorbed selenium (Se) on mercury at −0.33 V. The selenium and lead content can be determined using this peak by the addition of a known amount of one of these ions first and then the second ion. The linear domain range of lead is 5.0×10⁻⁷–2.0×10⁻⁵ M and for selenium 5.0×10⁻⁷–1.0×10⁻⁵ M. Using this method 4.90×10⁻⁷ M Se(IV) and 1.47×10⁻⁶ M Pb(II) in a synthetic sample could be determined with a relative error of +2.0% and 1.8%, respectively (n=4). A recovery test after acid digestion for a synthetic sample was 97% for selenium and 96.5% for lead. The method was applied to 1 ml of digested blood, and 328±23 μg l⁻¹ Se(IV) and 850±62 μg l⁻¹ Pb(II) could be determined with a 90% (n=5) confidence interval.     

Determination of nanomolar concentrations of phosphate in freshwaters using flow injection with luminol chemiluminescence detection

A simple and rapid flow injection (FI) method is reported for the determination of phosphate (as molybdate reactive P) in freshwaters based on luminol chemiluminescence (CL) detection. The molybdophosphoric heteropoly acid formed by phosphate and ammonium molybdate in acidic conditions generated chemiluminescence emission via the oxidation of luminol. The detection limit (3× standard deviation of blank) was 0.03 μg P l⁻¹ (1.0 nM), with a sample throughput of 180 h⁻¹. The calibration graph was linear over the range 0.032–3.26 μg P l⁻¹ (r²=0.9880) with relative standard deviations (n=4) in the range 1.2–4.7%. Interfering cations (Ca(II), Mg(II), Ni(II), Zn(II), Cu(II), Co(II), Fe(II) and Fe(III)) were removed by passing the sample through an in-line iminodiacetate chelating column. Silicate interference (at 5 mg Si l⁻¹) was effectively masked by the addition of tartaric acid and other common anions (Cl⁻, SO₄²⁻, HCO₃⁻, NO₃⁻ and NO₂⁻) did not interfere at their maximum admissible concentrations in freshwaters. The method was applied to freshwater samples and the results (26.1±1.1–62.0±0.4 μg P l⁻¹) were not significantly different (P=0.05) from results obtained using a segmented flow analyser method with spectrophotometric detection (24.4±4.45–84.0±16.0 μg P l⁻¹).     

Speciation of chromium in mineral waters and salinas by solid-phase extraction and graphite furnace atomic absorption spectrometry

A simple GF-AAS method for speciation analysis of chromium in mineral waters and salinas was developed. Cr(VI) species were separated from Cr(III) by solid-phase extraction with APDC (ammonium pyrrolidinedithiocarbamate). The APDC complexes were formed in the sample solution under proper conditions, adsorbed on Diaion HP-2MG resin and the resin was separated from the sample. After elution with concentrated nitric acid Cr(VI) was determined by GF-AAS. Total chromium was determined by GF-AAS directly in the sample and Cr(III) concentration was calculated as the difference between those results.

The detection limit of the method defined as 3 s of background variation was 0.03 μg l⁻¹ for Cr(VI) and 0.3 μg l⁻¹ for total chromium. RSD for Cr(VI) determination at the concentration of 0.14 μg l⁻¹ was 9%, and for total chromium at the concentration of 5.6 μg l⁻¹ was 5%. The recovery of Cr(VI) was in the range of 94–100%, dependently on type of the sample.

The investigation of recovery of the spiked Cr(VI) showed that at concentration levels near 1 μg l⁻¹ and lower recovery may be reduced significantly even by pure reagents that seem to be free from any reductants.     

The determination of molybdenum in water and biological samples by graphite furnace atomic spectrometry after polyurethane foam column separation and preconcentration

A system for molybdenum separation and enrichment aiming its determination in water and biological samples by graphite furnace atomic absorption spectrometry (GFAAS) is proposed. The procedure is based on the sorption of the molybdenum (VI) thiocyanate complex onto a mini-column packed with polyurethane foam (PUF). The elution is accomplished by a 3.0 mol l⁻¹ nitric acid solution. Flow variables were optimized and an enrichment factor of 10 as well as a limit of detection (LOD) (3 s) of 0.08 μg l⁻¹ in the sample solution were achieved. The coefficient of variation showed values of 3 and 2% for molybdenum solutions of 2.0 and 10.0 μg l⁻¹, respectively. The accuracy of the method was confirmed by the good concordance between found and certified values in the analysis of certified reference materials (CRMs) (CASS-3 Nearshore Seawater, NIST 1547 Peach Leaves, NIST 1515 Apple Leaves and NIST 1572 Citrus Leaves). The procedure was also applied for the molybdenum determination in mineral waters as well as in produced water samples. The results obtained for the mineral water samples compared well with those obtained by ICP-MS. Concerning the produced water samples, in spite of their large salinity, recoveries of 90 to 120% at the 1 μg l⁻¹ were observed.     

Evaluation of ammonia as diluent for serum sample preparation and determination of selenium by graphite furnace atomic absorption spectrometry

A method for the determination of total selenium in serum samples by graphite furnace atomic absorption spectrometry was evaluated. The method involved direct introduction of 1:5 diluted serum samples (1% v/v NH₄OH+0.05% w/v Triton X-100^®) into transversely heated graphite tubes, and the use of 10 μg Pd+3 μg Mg(NO₃)₂ as chemical modifier. Optimization of the modifier mass and the atomization temperature was conducted by simultaneously varying such parameters and evaluating both the integrated absorbance and the peak height/peak area ratio. The latter allowed the selection of compromise conditions rendering good sensitivity and adequate analyte peak profiles. A characteristic mass of 49 pg and a detection limit (3s) of 6 μg 1⁻¹ Se, corresponding to 30 μg l⁻¹ Se in the serum sample, were obtained. The analyte addition technique was used for calibration. The accuracy was assessed by the determination of total selenium in Seronorm™ Trace Elements Serum Batch 116 (Nycomed Pharma AS). The method was applied for the determination of total selenium in ten serum samples taken from individuals with no known physical affection. The selenium concentration ranged between 79 and 147 μg l⁻¹, with a mean value of 114±22 μg l⁻¹.     

Analysis of acrylamide in coffee and dietary exposure to acrylamide from coffee

An analytical method for analysing acrylamide in coffee was validated. The analysis of prepared coffee includes a comprehensive clean-up using multimode solid-phase extraction (SPE) by automatic SPE equipment and detection by liquid chromatography tandem mass spectrometry using electrospray in the positive mode. The recoveries of acrylamide in ready-to-drink coffee spiked with 5 and 10 μg l⁻¹ were 96±14% and 100±8%, respectively. Within laboratory reproducibility for the same spiking levels were 14% and 9%, respectively. Coffee samples (n = 25) prepared twice by coffee machines and twice by a French Press Cafetière coffee maker contained 8±3 μg l⁻¹ and 9±3 μg l⁻¹ acrylamide. Five ready-to-drink instant coffee prepared twice contained 8±2 μg l⁻¹. Hence, the results do not show significant differences in the acrylamide contents in ready-to-drink coffee prepared by coffee machine, French Press or from instant coffee. Medium roasted coffee contained more acrylamide (10 μg l⁻¹) than dark roasted coffee (5 μg l⁻¹). Males aged 35–45 years, drinking on average 1.1 l coffee per day are exposed to the highest doses of acrylamide from coffee. The dietary intake of acrylamide from coffee comprises, on an average, 10 μg day⁻¹ for males and 9 μg day⁻¹ for females aged 35–45 years. Probabilistic modelling of the exposure of Danish consumers (all adults) to acrylamide from coffee shows a mean exposure of 6.5 μg day⁻¹ and a 95 percentile of 18 μg day⁻¹.     

Sequential determination of Se(IV) and Se(VI) by flow injection-hydride generation-atomic absorption spectrometry with HCl/HBr microwave aided pre-reduction of Se(VI) to Se(IV)

In this study a flow injection (FI) system used in conjunction with hydride generation (HG), atomic absorption spectrometry (AAS) and microwave (MW) aided pre-reduction of selenite (Se(IV)) to selenate (Se(IV)) with HCl:HBr has been developed in order to differentiate both inorganic selenium species. As full control of the MW reduction step is possible, the experimental approach allows the use of milder acidic conditions (10% v/v of HCl and HBr) than those conventionally accomplished with hydrochloric acid alone (≥50% v/v). Experimental parameters were optimized by the univariate optimization method. In either case, the linear range was from 1.0 to 30 μg l⁻¹. The detection limits based on 3σ of the blank signal were 0.25 μg l⁻¹ for Se(IV) and 0.30 μg l⁻¹ for Se(VI). The reproducibility, about 3% RSD and recoveries of different amounts of Se(VI) and Se(IV) added to water and orange juice samples (97–103%) were good. The main advantage of the proposed method is that the sequential determination of Se(IV) and Se(VI) is performed at a high sampling frequency (ca. 50 samples per h) in a closed system without Se losses, and with a minimum sample waste, operator attention, and sample manipulation.     

Flow-through fluorescence immunosensor for atrazine determination

A new flow-through fluoroimmunosensor for atrazine determination based on the use of protein A immobilized on controlled pore glass as immunoreactor is reported. The support, placed in the optical path of the flow cell, allows the ‘in situ’ quantification of atrazine by on-line antigen–antibody binding upon successive injections of both substances. The immunosensor has a detection limit of 2.1 μg l⁻¹, a sample speed of about 10 samples per hour, and provides high reproducibility both within-day (3.2% for 5 μg l⁻¹ and 2.2% for 30 μg l⁻¹) and between days. The optimum working concentration range was 2.1–50 μg l⁻¹. Possible interferences of other triazines like simazine, desethylatrazine (DEA) and desisopropylatrazine (DIA) were evaluated. Simazine and DIA were not cross-reactive; however, the cross-reactivity for DEA was CR=7.7%. The proposed immunosensor was successfully applied to the determination of atrazine in drinking water and citrus fruits.     

Determination of chlorite in drinking water by on-line coupling of capillary isotachophoresis and capillary zone electrophoresis

An isotachophoresis (ITP)–capillary zone electrophoresis (CZE) combination was used for the determination of chlorite in drinking waters. No sample preparation is needed and no interfering by other anions in tap water was observed. The reached limits of detection with conductivity detector were 0.012–0.017 mg l⁻¹. By four-fold sample loading with a 30 μl valve, 0.005 mg l⁻¹ of chlorite was determined with R.S.D.=3.3%. The concentrations of 0.05 and 0.20 mg l⁻¹ were measured with R.S.D. of 2.2 and 2.7%, respectively. The recoveries of chlorite from drinking water were 96–106% in the range of 0.02–0.20 mg l⁻¹. The R.S.D. values of migration times (inter-day) were up to 1.3%. The time for analysis is about 15 min.     

HPLC determination of Vitamin D(3) and its metabolite in human plasma with on-line sample cleanup

A HPLC method with automated column switching and UV-diode array detection is described for the simultaneous determination of Vitamin D₃ and 25-hydroxyvitamin D₃ (25-OH-D₃) in a sample of human plasma. The system uses a BioTrap precolumn for the on-line sample cleanup. A sample of 1 ml of human plasma was treated with 2 ml of a mixture of ethanol–acetonitrile (2:1 (v/v)). Following centrifugation, the supernatant was evaporated to dryness under a stream of dry and pure nitrogen. The residue was reconstituted in 250 μL of a solution of methanol 5 mmol l⁻¹ phosphate buffer, pH 6.5 (4:1 (v/v)), and a 200 μl aliquot of this solution was injected onto the BioTrap precolumn. After washing during 5 min with a mobile phase constituted by a solution of 6% acetonitrile in 5 mmol l⁻¹ phosphate buffer, pH 6.5 (extraction mobile phase), the retained analytes were then transferred to the analytical column in the backflush mode. The analytical separation was then performed by reverse-phase chromatography in the gradient elution mode with the solvents A and B (Solvent A: acetonitrile–phosphate buffer 5 mmol l⁻¹, pH 6.5; 20:80 (v/v); solvent B: methanol–acetonitrile–tetrahydrofuran, 65:20:15 (v/v)). The compounds of interest were detected at 265 nm. The method was linear in the range 3.0–32.0 ng ml⁻¹ with a limit of quantification of 3.0 ng ml⁻¹. Quantitative recoveries from spiked plasma samples were between 91.0 and 98.0%. In all cases, the coefficient of variation (CV) of the intra-day and inter-day-assay precision was ≤2.80%. The proposed method permitted the simultaneous determination of Vitamin D₃ and 25-OH-D₃ in 16 min, with an adequate precision and sensitivity. However, the overlap of the sample cleanup step with the analysis increases the sampling frequency to five samples h⁻¹. The method was successfully applied for the determination of Vitamin D₃ and 25-OH-D₃ in plasma from 46 female volunteers, ranging from 50 to 94 years old. Vitamin D₃ and 25-OH-D₃ concentrations in plasma were found from 4.30–40.70 ng ml⁻¹ (19.74 ± 9.48 ng ml⁻¹) and 3.1–36.52 ng ml⁻¹ (7.13 ± 7.80 ng ml⁻¹), respectively. These results were in good agreement with data published by other authors.     

Electrochemiluminescence sensor using tris(2,2′-bipyridyl)ruthenium(II) immobilized in Eastman-AQ55D–silica composite thin-films

An electrochemiluminescence (ECL) sensor with good long-term stability and fast response time has been developed. The sensor was based on the immobilization of tris(2,2′-bipyridyl)ruthenium(II) (Ru(bpy)₃²⁺) into the Eastman-AQ55D–silica composite thin films on a glassy carbon electrode. The ECL and electrochemistry of Ru(bpy)₃²⁺ immobilized in the composite thin films have been investigated, and the modified electrode was used for the ECL detection of oxalate, tripropylamine (TPA) and chlorpromazine (CPZ) in a flow injection analysis system and showed high sensitivity. Because of the strong electrostatic interaction and low hydrophobicity of Eastman-AQ55D, the sensor showed no loss of response over 2 months of dry storage. In use, the electrode showed only a 5% decrease in response over 100 potential cycles. The detection limit was 1 μmol l⁻¹ for oxalate and 0.1 μmol l⁻¹ for both TPA and CPZ (S/N=3), respectively. The linear range extended from 50 μmol l⁻¹ to 5 mmol l⁻¹ for oxalate, from 20 μmol l⁻¹ to 1 mmol l⁻¹ for TPA, and from 1 μmol l⁻¹ to 200 μmol l⁻¹ for CPZ.     

Investigation of on-line coupling electrothermal atomic absorption spectrometry with flow injection sorption preconcentration using a knotted reactor for totally automatic determination of lead in water samples

A flow injection on-line sorption preconcentration electrothermal atomic absorption spectrometric system for fully automatic determination of lead in water was investigated. The discrete non-flow-through nature of ETAAS, the limited capacity of the graphite tube and the relatively large volume of the knotted reactor (KR) are obstacles to overcome for the on-line coupling of the KR sorption preconcentration system with ETAAS. A new FI manifold has been developed with the aim of reducing the eluate volume and minimizing dispersion. The lead diethyldithiocarbamate complex was adsorbed on the inner walls of a knotted reactor made of PTFE tubing (100 cm long, 0.5 mm i.d.). After that, an air flow was introduced to remove the residual solution from the KR and the eluate delivery tube, then the adsorbed analyte chelate was quantitatively eluted into a delivery tube with 50 μl of ethanol. An air flow was used to propel the eluent from the eluent loop through the reactor and to introduce all the ethanolic eluate onto the platform of the transversely heated graphite tube atomizer, which was preheated to 80°C. With the use of the new FI manifold, the consumption of eluent was greatly reduced and dispersion was minimized. The adsorption efficiency was 58%, and the enhancement factor was 142 in the concentration range 0.01–0.05 μg l⁻¹ Pb at a sample loading rate of 6.8 ml min⁻¹ with 60 s preconcentration time. For the range 0.1–2.0 μg l⁻¹ of Pb a loading rate of 3.0 ml min⁻¹ and 30 s preconcentration time were chosen, resulting in an adsorption efficiency of 42% and an enhancement factor of 21, respectively. A detection limit (3σ) of 2.2 ng l⁻¹ of lead was obtained using a sample loading rate of 6.8 ml min⁻¹ and 60 s preconcentration. The relative standard deviation of the entire procedure was 4.9% at the 0.01 μg l⁻¹ Pb level with a loading rate of 6.8 ml min⁻¹ and 60 s preconcentration, and 2.9% at the 0.5 μg l⁻¹ Pb level with a 3.0 ml min⁻¹ loading rate and 30 s preconcentration. Efficient washing of the matrix from the reactor was critical, requiring the use of the standard addition method for seawater samples. The analytical results obtained for seawater and river water standard reference materials were in good agreement with the certified values.     

Determination of arsenic(III) and arsenic(V) by electrothermal atomic absorption spectrometry after complexation and sorption on a C-18 bonded silica column

A flow injection procedure for the separation and pre-concentration of inorganic arsenic based on the complexation with ammonium diethyl dithiophosphate (DDTP) and sorption on a C-18 bonded silica gel minicolumn is proposed. During the sample injection by a time-based fashion, the As³⁺-DDTP complex is stripped from the solution and retained in the column. Arsenic(V) and other ions that do not form complexes are discarded. After reduction to the trivalent state by using potassium iodide plus ascorbic acid, total arsenic is determined by electrothermal atomic absorption spectrometry (ETAAS). Arsenic(V) concentration can be calculated by difference. After processing 6 ml sample volume, the As³⁺-DDTP complexes were eluted directly into the autosampler cup (120 μl). Ethanol was used for column rinsing. Influence of pH, reagent concentration, pre-concentration and elution time and column size were investigated. When 30 μl of eluate plus 10 μl of 0.1% (w/v) Pd(NO₃)₂ were dispensed into the graphite tube, analytical curve in the 0.3–3 μg As l⁻¹ range was obtained (r=0.9991). The accuracy was checked for arsenic determination in a certified water, spiked tap water and synthetic mixtures of arsenite and arsenate. Good recoveries (97–108%) of spiked samples were found. Results are precise (RSD 7.5 and 6% for 0.5 and 2.5 μg l⁻¹, n=10) and in agreement with the certified value of reference material at 95% confidence level.     

Continuous flow system for lead determination by faas in spirituous beverages with solid phase extraction and on-line copper removal

A continuous flow system for the determination of lead in home made spirituous beverages was developed. The determination was based on the formation of a neutral chelate of the element with ammonium pyrrolidine dithiocarbamate, its adsorption onto a minicolumn packed with sodium faujasite type Y synthetic zeolite, followed by elution with methyl isobutyl ketone and determination by flame atomic absorption spectrometry. Ethanol and copper interfere strongly in the determination and therefore, must be separated prior to the analysis. Copper is removed by precipitation with rubeanic acid, while ethanol is eliminated by rotaevaporation. Sample solutions containing Pb²⁺ in the concentration range from 5 to 120 μg l⁻¹ at pH 2.5 could be analyzed, by using a preconcentration time of 3 min. Preconcentration factors from 80 to 140 were achieved for a sample volume of 6 ml and the detection limit varied from 1.4 to 3.5 μg l⁻¹, depending on the matrix composition. The relative standard deviations for 60 μg l⁻¹ Pb was 3.2% (n = 10) and the recovery of spikes (20, 40, 60 and 80 μg l⁻¹) added to the samples was estimated within 92–105% range, suggesting that lead can be quantitatively determined in such samples. Determining lead in several samples by an alternative technique further checked the accuracy. Finally, the concentrations of Pb²⁺ determined in 28 samples of Venezuelan spirituous beverages were in 12.6–370.0 μg l⁻¹ range, depending on the fermenting material based on different mixtures of agave, raw sugar cane and white sugar.     

Simultaneous flow injection determination of calcium and fluoride in natural and borehole water with conventional ion-selective electrodes in series

An on-line automated system for the simultaneous flow injection determination of calcium and fluoride in natural and borehole water with conventional calcium-selective and fluoride-selective membrane electrodes as sensors in series is described. Samples (30 μl) are injected into a TISAB II (pH=5.50) carrier solution as an ionic strength adjustment buffer. The sample-buffer zone formed is first channeled to a fluoride-selective membrane electrode and then via the calcium-selective membrane electrode to the reference electrodes. The system is suitable for the simultaneous on-site monitoring of calcium (linear range 10⁻⁵–10⁻² mol l⁻¹ detection limit 1.94×10⁻⁶ mol l⁻¹ recovery 99.22%, RSD<0.5%) and fluoride (linear range 10⁻⁵–10⁻² mol l⁻¹ detection limit 4.83×10⁻⁶ mol l⁻¹ recovery 98.63%, RSD=0.3%) at a sampling rate of 60 samples h⁻¹.     

A new method for the voltammetric determination of molybdenum(VI) using carbon paste electrodes modified in situ with cetyltrimethylammonium bromide

Molybdenum(VI) is determined by anodic stripping voltammetry using a carbon paste electrode modified in situ with cetyltrimethylammonium bromide (CTAB). The preconcentration of molybdenum is performed by adsorption and reduction of ion-pairs of cetyltrimethylammonium and molybdenum(VI) oxalate at a potential of −0.4 V vs. the saturated calomel electrode (SCE). The supporting electrolyte contains 0.01 M oxalic acid and 0.075 mM CTAB. Differential pulse anodic stripping voltammetry exploiting the reoxidation signal is used for the determination of trace levels of molybdenum(VI). Linearity between current and concentration exists for a range of 0.5–500 μg 1⁻¹ Mo with proper preconcentration times; the limit of detection (calculated as 3σ) is 0.04 μg 1⁻¹ with an accumulation period of 10 min.