Disposable Nafion-modified micro-fabricated bismuth-film sensors for voltammetric stripping analysis of trace metals in the presence of surfactants

This work is a study of the analytical utility of Nafion-modified microfabricated bismuth film electrodes (BiFEs) for the determination of Pb(II) and Cd(II) by anodic stripping voltammetry (ASV) in the presence of surfactants. Micro-fabricated BiFEs were prepared by depositing a thin film of bismuth on the surface of a silicon substrate by sputtering while the two-dimensional geometry of the final sensors was defined by photolithography. The BiFEs were further drop-coated with a Nafion film. These devices were applied to the determination of Pb(II) and Cd(II) by square wave ASV (SWASV) in the presence of Triton X-100 (a non-ionic surfactant), cetyltrimethylammonium bromide (CTAB) (a cationic surfactant) and sodium dodecyl sulphate (SDS) (an anionic surfactant). It was found that the presence of Nafion afforded an increase in sensitivity and the tolerance against surfactants but these properties were severely influenced by both the thickness of the Nafion film and the nature of the interfering surfactant. Using a Nafion of 0.4 μm thickness and 120 s of preconcentration, the repeatability (expressed as the % relative standard deviation on the same sensor (n = 8)) at the 20 μg l⁻¹ level was 3.8% for Pb(II) and 3.1% for Cd(II) and the limits of detection were 0.5 μg l⁻¹ for Cd(II) and Pb(II). The sensors were applied to Cd(II) and Pb(II) determination in a certified lake-water sample.     

Anodic-stripping voltammetry of heavy metals in the presence of organic surfactants

The efficacy of fumed silica for removal of sorption interferences by organic surfactants in the anodic-stripping voltammetry of heavy metals is demonstrated. Appropriate addition of silica to the sample solution rapidly "purifies" it from interfering surfactants during the nitrogen purge step. Up to at least 6 ppm of gelatin, Triton X-100, albumin or Liqui-Nox then does not affect the stripping response of cadmium, lead and zinc at the hanging mercury drop electrode. A relative standard deviation of 5.5% is obtained for 20 successive measurements of 1 x 10(-7)M lead in the presence of 2 ppm Triton X-100. Analogous improvements are observed at the mercury film electrode (in the presence of up to 60 ppm of these surfactants). The use of silica thus possesses the advantages of speed, efficiency, simplicity and low cost compared to other schemes for dealing with surfactant interferences in anodic-stripping voltammetry.     

Analytical procedures for the simultaneous voltammetric determination of heavy metals in meals

An analytical procedure regarding the determination of copper(II), lead(II), cadmium(II), zinc(II) and antimony(III) in matrices involved in foods and food chain as wholemeal, wheat and maize meal is proposed. The digestion of each matrix was carried out using concentrated HCl suprapure at 130 °C for 3 h. Differential pulse anodic stripping voltammetry (DPASV) was employed for simultaneously determining all the elements, using a conventional three-electrode cell and 0.5 M HCl as supporting electrolyte. The analytical procedure has been verified on the reference standard materials Wholemeal BCR-CRM 189, Wheat Flour NIST-SRM 1567a and Rice Flour NIST-SRM 1568a. For all the elements in the certified matrix, the precision as repeatability, expressed as relative standard deviation (s_r), and the accuracy, expressed as relative error (e), were of the order of 3 to 6%. The limits of detection were in the range 0.009–0.096 μg/g.     

Effect of ethylenediamine on the anodic stripping voltammetric determination of heavy metals in the presence of humic acid

Ligand competition coupled with differential pulse anodic stripping voltammetry has been investigated for the determination of copper, lead and cadmium. Ethylenediamine displaces humic acid in its metal complexes forming kinetically labile compounds. It also eliminates interferences associated with the oxidation of copper. This enhances the sensitivity of the determination of the metals over a wide range of humic acid concentration (up to 30 mg/l). The procedure has been applied to the determination of heavy metals in a real water sample.     

Determination of some heavy metals in sea water by atomic absorption spectrophotometry

Summary Direct determination of trace metals in sea water by flame atomic absorption technique suffers from serious interferences caused by the high salt content. Separation of the metals from the matrix has been effected by chelation and extraction with ammonium pyrrolidine dithiocarbamate and methyl-isobutyl-ketone. Besides a separation preconcentration and increased sensitivity is achieved with this extraction. Organic substances and metal-organic complexes are destroyed with UV-radiation before extraction is carried out. Flameless atomic absorption has been tried using the Heated Graphite Furnace combined with a) a selective volatilization procedure, b) removing of sodium from the samples using hydrated antimon pentoxide (HAP). Both methods were used to remove the interfering substances (mainly sodium salt). All the techniques have been compared by analysing the same sea water samples, some of which were also analysed by other laboratories.

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Bestimmung einiger Schwermetalle in Meereswasser durch Atomabsorptions-Spektralphotometrie

Zusammenfassung Die direkte Bestimmung der Spurenmetalle in Meereswasser durch Flammen-Atomabsorption wird durch den hohen Salzgehalt behindert. Die Abtrennung der Metalle von der Matrix wurde durch Komplexierung und Extraktion mit Ammoniumpyrrolidindithiocarbamat und Methylisobutylketon erreicht, wodurch außerdem eine Anreicherung und Steigerung der Empfindlichkeit erzielt wird. Organische Substanzen und metallorganische Komplexe können durch UV-Bestrahlung vor der Extraktion zerstört werden. Die flammenlose Technik mit Hilfe des erhitzten Graphitrohres wurde in Kombination mit a) einer selektiven Verdampfungsmethode und b) der Entfernung des Natriums mit hydratisiertem Antimonpentoxid eingesetzt. Alle Verfahren wurden an Hand der gleichen Wasserproben verglichen. Tests wurden auch von anderen Laboratorien durchgeführt.

This work was made possible by substantial grants from International Atomic Energy Agencies (IAEA) and from the Royal Norwegian Council for Scientific and Industrial Research.     

A reactive electrode (reactrode) for the voltammetric determination of heavy metals in laboratories and for use as a passive monitor in remote analysis

The use of a reactive electrode (reactrode) consisting of graphite, a solid ion exchanger (HYPHAN) and paraffin for the batch analysis of Cd²⁺, Cu²⁺, Pb²⁺ and Hg²⁺ in aqueous samples and as a passive monitor for these metal ions is described. The metal ions are accumulated on the reactrode surface at an open-circuit potential in an ion-exchange reaction. After the accumulation, the ion exchanger-bonded metal ions are reduced to the metals which remain on the electrode surface. In a following step, the metals are anodically dissolved which is recorded by differential-pulse voltammetry. The 3s detection limits for the analysis of drinking water are: 1.1×10^-7 mol/l for Pb²⁺, 5×10^-8 mol/l for Hg²⁺ and 2.4×10^-7 mol/l for Cu²⁺.The reactrode developed can be used for the passive monitoring of heavy metals in aqueous streams if the reactrode is mounted in a wall-jet cell which is part of a flow-through system. Using this arrangement, it has been possible to determine Hg²⁺, Cu²⁺ and Pb²⁺ in drinking water after 20 hours of accumulation.     

Flotation of traces of heavy metals coprecipitated with aluminum hydroxide from water and sea water

Microgram quantities of heavy metals in 1-l samples of water and sea water are quantitatively coprecipitated with aluminum hydroxide at pH 9.5. The precipitate is floated with the aid of sodium oleate and small nitrogen bubbles, separated and dissolved in 2 M nitric acid, and the heavy metals are determined by atomic-absorption spectrometry. The method is rapid and applicable to 9 heavy metals at the low p.p.b. level.     

Differential pulse voltammetric determination of tin in the presence of noble metals

A voltammetric method for the determination of tin is proposed to minimise interferences from noble metals that are commonly encountered with other analytical techniques. Strong distortions of voltammetric peaks are observed in the presence of platinum. On the basis of a full investigation, the formation of an intermediate Sn(II)–Pt mixed chloro-complex at the electrode surface is identified as being responsible for the platinum interference, as it competes with the normal Sn(IV)→Sn(0)_Hg reduction. The use of a higher scan rate prevents the relatively low reaction kinetics and thus gets rid of this interference. No problems are encountered with other noble metals such as Pd, Ir, Re, Rh and Ru when using the modified method, although a baseline subtraction is necessary for the latter one. The proposed method is validated with real Pt–Sn catalysts.     

Determination of heavy metals in sea water and in marine organisms by graphite furnace AAS

Summary This paper is a plea for a reference material containing extremely low concentrations of Pb and Cd. A reasoning is given by a historical overview on the main topics of pollution research in relation to those metals. Emphasis has changed from toxicological tests (with high concentrations) over hot spots in the environment (certain organisms and organs, such as kidney and liver, showing significantly elevated concentrations) to mechanisms of transport and modelling. That development has been accompanied and promoted by methodological improvement. Those concentrations, which have to be determined with a sufficiently high precision are now in the lower background range. A proper standard material for such purpose could be fish muscle which, over and above, could be spiked, if necessary, for measurements in a higher concentration range.

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Bestimmung von Schwermetallen in Meerwasser und marinen Organismen mit Hilfe der Graphitofen-AASXXII. Fischmuskel an Stelle von Leber und Algen als Referenzmaterial für Blei- und Cadmiumbestimmungen

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Part XXI. Cadmium determination in coastal water samples from the German Bight (1985) Vom Wasser 64:53–68     

Determination of heavy metals in environmental bio-indicators by voltammetric and spectroscopic techniques

The determination of copper, lead, cadmium and zinc in matrices involved in the food chain as algae, species Ulva rigida, and clams, species Tapes philippinarum by differential pulse anodic stripping voltammetry (DPASV) was carried out. For the mercury determination in these matrices, a new accurate and precise method was developed employing a mixture of concentrated acids H₂SO₄-K₂Cr₂O₇ for digestion and subsequent cold vapor atomic absorption spectrometry (CV-AAS) by reduction with SnCl₂. The analytical procedures were verified for four reference standard materials: Ulva lactuca BCR-CRM 279, Lagarosiphon major BCR-CRM 060, Oyster tissue NBS-SRM 1566, Mussel tissue BCR-CRM 278. For all the elements the precision, expressed as relative standard deviation (s_r), and the accuracy, expressed as relative error (e), were in the order of 3 to 5%, while the detection limits were in the range 0.010–0.100 μg/g. The standard addition technique improved the resolution of the voltammetric method even in the case of very high element concentration ratios. The analytical procedure was used for real matrices sampled in the Adriatic Sea south to Po river mouth, in the zone “Goro bay”, and at open sea north to the Ravenna shore.     

Determination of heavy metals in environmental bio-indicators by voltammetric and spectroscopic techniques

The determination of copper, lead, cadmium and zinc in matrices involved in the food chain as algae, species Ulva rigida, and clams, species Tapes philippinarum by differential pulse anodic stripping voltammetry (DPASV) was carried out. For the mercury determination in these matrices, a new accurate and precise method was developed employing a mixture of concentrated acids H₂SO₄-K₂Cr₂O₇ for digestion and subsequent cold vapor atomic absorption spectrometry (CV-AAS) by reduction with SnCl₂. The analytical procedures were verified for four reference standard materials: Ulva lactuca BCR-CRM 279, Lagarosiphon major BCR-CRM 060, Oyster tissue NBS-SRM 1566, Mussel tissue BCR-CRM 278. For all the elements the precision, expressed as relative standard deviation (s_r), and the accuracy, expressed as relative error (e), were in the order of 3 to 5%, while the detection limits were in the range 0.010–0.100 μg/g. The standard addition technique improved the resolution of the voltammetric method even in the case of very high element concentration ratios. The analytical procedure was used for real matrices sampled in the Adriatic Sea south to Po river mouth, in the zone “Goro bay”, and at open sea north to the Ravenna shore. Received: 7 June 1998 / Revised: 2 November 1998 / Accepted: 5 November 1998     

Optimal peak area determination in the presence of noise

A method is presented for determining optimal peak integration intervals on the basis of known peak shapes and noise characteristics. General theoretical considerations lead to conditions yielding optimal integration intervals. Examples of frequently occurring peak shapes and noise types are given, such as gaussian or skewed peak shapes, with band-limited first-order noise or flicker noise superimposed. The optimal integration intervals are approximately independent of the signal-to-noise ratio, and they are considerably smaller than the integration intervals normally used. The resulting expected peak area estimation errors are compared with the estimation error resulting from peak maximum amplitude measurement. On the basis of this comparison, rules of thumb are proposed to determine whether piak maximum measurement or peak integration yields the best results. Simulation of different peak shapes with noise superimposed confirms the results obtained. A flexible method is presented for the optimal measurement of the area of peaks with an unknown shape. This on-line method is simple, and could be used as a simple peak-finding procedure. The method requires almost no computer memory, and can be implemented on a microcomputer. A simulated example of this procedure is given.     

Determination of heavy metals in sea water and in marine organisms by flameless atomic absorption spectrophotometry

Summary This paper presents results of an intercalibration of cadmium determination by means of flameless AAS in biological materials from toxicological experiments. Furthermore, the philosophy of this kind of investigation is discussed in some detail. The goal and the result of this investigation was a correspondence of the measurement results within an acceptable range. Contamination during further handling has been identified as the main reason for severe deviations. The authors made measurements in the labs of several colleagues. The equipment itself did not produce individual variations. Repeated intercomparison after elimination of identified systematic errors showed sufficient correspondence of the results.Teil XII: Fresenius Z. Anal. Chem.301, 294–299 (1980)     

Determination of heavy metals in sea water and in marine organisms by flameless atomic absorption spectrophotometry

Summary In natural waters, heavy metals such as Cd may be present in bound form. It is, therefore, important to consider chemical speciation, especially at low concentrations of the metal where a significant part can be bound by other trace constituents so tightly that it escapes several forms of its determination. It has been demonstrated that even the very strong complexant APDC is unable to leach out all of the heavy metal present in several coastal water samples from the German Bight.This paper describes a new micro method for the determination of total Cd in natural waters. First a small sample is dried in a polypropylene vial. Then the salt residue is heated to 90° C with a mixture of sulphuric and nitric acid for digestion of organic trace constituents (and oxidation of sulphides if present). After dilution subsamples are neutralized using an excess of NaHCO₃ buffer solution. Extraction results are then independent from the possible slight variations of pH. The extractant, a solution of APDC in CCl₄ is stable for weeks, and even the extract is stable for at least 16h. Precision is in the range of 7% at a concentration of 0.066 g Cd l^–1; the detection limit is below 0.002 g l^–1 and can be lowered, if necessary, by enlargement of the original sample. According to the sensitivity of the method the typical blank of 0.022 g Cd l^–1 is high, but it is rather constant (mean variation from the mean: 0.0024 g l^–1).Teil XI: Fresenius Z. Anal. Chem. 299, 206–207 (1979)     

Determination of heavy metals in sea water and in marine organisms by flameless atomic absorption spectrophotometry

Summary This publication is a part of a series in which the author describes a set of methods for the determination of cadmium in environmental samples by means of flameless AAS. Common properties of these methods are their miniaturization and standardization. These have been attained by the utmost reduction of the number of steps and by their simplification. The method described in this publication consists of an acid digestion procedure followed by neutralization, extraction, and flameless AAS measurement. The main characteristics are: digestion takes place in small 1.5 ml tubes of quartz, polyvinyldifluoride or polypropylene. Neutralization is not performed by titration but only by an excess of saturated NaHCO₃ solution. Extraction results are independent of pH in the region to be expected. Extraction is carried out by a (stable) solution of APDC in CCl₄. The organic extract is also stable for at least 16h. Precision of the method lies in the range of 8.4%, at a concentration of 18.9 ng Cd g^–1; sensitivity is at least in the range of 1.2 ng Cd g^–1. The latter can be increased, if necessary, by a several-step extraction procedure.Teil VIII: Z. Lebensm.-Unters. Forsch. 168, 193-194 (1979)     

Determination of heavy metals in sea water and in marine organisms by flameless atomic absorption spectrophotometry

Summary An ultraquick light-beam oscillograph has been used to track the growing and destruction of the atomic cloud in a graphite tube atomizer. In view of the results obtained it was stated that the analytical signal is mainly determined by the stepwise growing of the atomic cloud and its simultaneous destruction by thermal expansion of the inert gas from the tube. Thermodiffusion and recondensation seem to be much slower processes. The dependence of the analytical signal on the settings of the atomizer or the presence of matrix components and on thermal ballast like the L'vov platform is discussed in view of the above thesis. Finally, some proposals are made to overcome the matrix effects by separating the atomization process from the heating and thermal expansion of the surrounding inert gas. For this purpose atomizers with two separate electric current circuits are recommended.

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Bestimmung von Schwermetallen im Meerwasser und in marinen Organismen durch flammenlose AtomabsorptionsspektralphotometrieXV. Matrixeffekte in Graphitrohr-Atomisatoren und Wege zu ihrer Überwindung

Zusammenfassung Ein schneller Lichtpunktschreiber wurde eingesetzt, um das Entstehen und den Abbau der Atomwolke in einem Graphitrohr-Atomisator zu verfolgen. Aufgrund der erhaltenen Ergebnisse wird geschlossen, daß das analytische Signal im wesentlichen durch das allmähliche Wachsen der Atomwolke und deren gleichzeitigen Abbau infolge thermischer Ausdehnung des Inertgases im Rohr bestimmt wird. Thermodiffusion und Rekondensation verlaufen offenbar langsamer. Die Abhängigkeit des analytischen Signals von den Geräteeinstellungen, von der Gegenwart bestimmter Matrixkomponenten und von thermischem Ballast wie der L'vov-Plattform wird im Lichte der o.a. These diskutiert. Schließlich werden einige Wege zur Behebung der Matrixabhängigkeit vorgeschlagen. Sie zielen auf eine möglichst weitgehende Trennung der eigentlichen Atomisierung vom Erhitzen und Sichausdehnen des umgebenen Inertgases. Hierzu werden Atomisatoren mit zwei getrennten elektrischen Heizkreisen vorgeschlagen.

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XIV. Mitt. Fresenius Z Anal Chem (1982) 310:254–256

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Presented at the Colloquium Spurenanalytik, Konstanz, 7.–9.4. 1981