The reduction and elimination of matrix interferences in graphite furnace atomic absorption spectrometry

The approaches to reduction or elimination of matrix interferences encountered in graphite furnance atomic absorption spectrometry is reviewed. These techniques include matrix modification, application of active gas, and coating tubes with metallic compounds. The research work carried out in the author's laboratory is emphasized. A more universal matrix modifier, palladium, is proposed for the determination of mercury, lead, tellurium, bismuth, arsenic, thallium and indium in environmental samples.     

Organophosphorus acid interferences in flame atomic absorption spectrometry

The interference of the organophosphorus acids, 1-hydroxyethane-1,1-bisphosphonic acid, aminotris(methylenephosphonic acid), ethylenediaminetetrakis(methylenephosphonic acid) and hexamethylenediaminetetrakis(methylenephosphonic acid) on the determination of eighteen metal ions by flame atomic absorption spectrometry is reported. Comparisons with the effect of orthophosphoric acid reveal similarities and distinct differences in their interfering effects. In the air/acetylen flame, depressive interferences are attributed to the formation of phosphates, M₃(PO₄)₂, or hydroxynpatite-like compounds, M₅(OH)(PO₄)₃, in the flame aerosol particles for Mg, Ca, Sr, Ba, Mn, Co and Ni. Iron(III) and chromium(III) appear to form stable oxide phosphates, M₂O₃?MPO₄ or M₃O₄?MPO₄. Evidence for the formation of stable molybdenum carbides, MoC and MoC₂, is also presented. In the nitrous oxide/acetylene flame, serious interferences perssisted only for molybdenum but were eliminated by the addition of sodium sulphate.     

Feasibility of eliminating interferences in graphite furnace atomic absorption spectrometry using analyte transfer to the permanently modified graphite tube surface

A procedure is proposed to avoid spectral and/or non-spectral interferences in graphite furnace atomic absorption spectrometry (GF AAS) by transferring the analyte during the pyrolysis stage from a solid sampling platform to the graphite tube wall that has been coated with a permanent modifier, e.g. by electrodeposition of a platinum-group metal. The direct determination of mercury in solid coal samples was chosen as a model to investigate the feasibility of this idea. The graphite tube surface was coated with palladium and the analyte was transferred from the solid sampling platform to the tube wall at a temperature of 500±50 °C. A characteristic mass of m₀=64 pg Hg was obtained for an atomization temperature of 1300 °C, proposing a quantitative transfer of the analyte to the tube wall. Calibration against aqueous mercury standards was not feasible as this element was lost in part already during the drying stage and could not be trapped quantitatively on the modified graphite tube surface. However, the results for all except one of the coal reference materials were within the 95% confidence interval of the certificate when the slope of a correlation curve between the integrated absorbance, normalized for 1 mg of sample, and the certified value for mercury was used for calibration. A detection limit of 0.025–0.05 μg g⁻¹ Hg in coal, calculated from three times the standard deviation of the investigated coal samples, could be obtained with the proposed method. The spectral interference due to excessive background absorption in the direct determination of mercury in coal could be eliminated completely. It is expected that this analyte transfer can be used in a similar way to eliminate other spectral and/or non-spectral interferences in the GF AAS determination of other volatile analytes.     

Time resolution of interferences in electrothermal atomic absorption spectrometry

A fast detector-amplifier-readout system is used for studying interferences in electrothermal graphite atomizers. The effects of different matrix components (K, B, Ca, Mg, and Cl), and graphite tube surfaces significantly alter the atomization processes of lead.     

Abuse of the analyte addition technique in atomic absorption spectrometry

Summary The analyte addition technique is frequently used in atomic absorption spectrometry, and is often applied routinely to avoid errors in graphite furnace and other techniques for trace element determination. In fact, however, the analyte addition technique can be applied reasonably only if certain prerequirements are fulfilled. The two most important basic rules are that the analyte addition technique can only correct for multiplicative but not for additive systematic errors, and that the added analyte element and the element present in the sample must behave in an analytically identical manner. Careful isoformation is typically required in the graphite furnace technique to fulfil this second requirement. Doing so, however, completely eliminates numerous interferences if at the same time proper atomization conditions and evaluation of integrated absorbance signals are used. This means that the analyte addition technique has become redundant in this case, and that the analytical curve technique can be used for evaluation.

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Mi\brauch des Additionsverfahrens in der Atomabsorptionsspektrometrie

Zusammenfassung Das Additionsverfahren wird in der Atomabsorptionsspektrometrie sehr häufig verwendet und in der Graphitrohrofentechnik und anderen Techniken der Spurenelementbestimmung oft routinemäßig zur Vermeidung von Fehlmessungen eingesetzt. In Wirklichkeit ist das Additionsverfahren aber nur dann sinnvoll einsetzbar, wenn gewisse Vorbedingungen erfüllt sind. Die zwei wichtigsten Grundregeln sind, daß das Additionsverfahren nur multiplikative, nicht aber additive, systematische Fehler beseitigen kann sowie daß sich der zugesetzte Analyt und der in der Probe vorhandene analytisch identisch verhalten müssen. Um diese zweite Bedingung zu erfüllen, muß bei der Graphitrohrofentechnik üblicherweise eine sorgfältige Isoformierung erfolgen. Dadurch werden aber, bei gleichzeitigem Einhalten günstiger Atomisierungsbedingungen und Auswertung der integrierten Extinktion, zahlreiche Störungen vollständig beseitigt. Das bedeutet, daß das Additionsverfahren in diesem Fall überflüssig geworden ist und die Auswertung nach dem Standard-Kalibrierverfahren erfolgen kann.

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Dedicated to Prof. Dr. H. Hartkamp on the occasion of his 60th birthday     

Halide interferences in an electrothermal graphite furnace atomic absorption spectrometry with group IIIB elements as studied by atomic and molecular absorption signal profiles

Molecular absorptions of monohalides (MX; X = F, Cl and Br) of alkali metals, alkaline-earth metals and Group IIIB elements produced in a conventional electrothermal graphite furnace atomizer have been observed by a rapid measurement system. Their characteristic data are summarized as appearance and peak temperatures and the analytical sensitivities. Furthermore, molecular absorptions ofAlF and InF are utilized to study the interference of fluoride with aluminum and indium atomic absorptions. Although the formations of AlF and InF suppress the atomic absorptions ofAl and In, respectively, the time-overlap in signal appearance of atomic absorptions of these two elements and molecular absorptions of their monofluorides is not observed. This is contradictory to pure “vapor phase mechanism”, and indicates that processes occurring on the graphite surface are also important. Usefulness of strontium nitrate as a matrix modifier for the determination of indium was also demonstrated.     

Condensation of analyte vapor species in graphite furnace atomic absorption spectrometry

A shadow spectral digital imaging technique (SSDI) with charge coupled device (CCD) camera detection was used to investigate the spatial and temporal distribution of condensation clouds of analyte species generated in a graphite furnace during the atomization of 5–40-μg masses of metals. Complex, non-uniform structures of aerosol species located away from both the graphite tube axis and walls are prevalent. Species giving rise to observed signals are likely clusters of metals in the case of Ag, Au and Pd, and oxide aerosols for elements such as Al, Ca and Mg. Source scatter often arises during the atomization of relatively large masses of analytes (or during the atomization of real samples containing high concentrations of concomitant matrix species) in the graphite furnace due to such aerosol formation. The SSDI technique is an extremely useful aid to the elucidation of this phenomenon and imaging at several wavelengths using both line and continuum sources in combination with (thermal, incandescent) emission from these structures permits a more complete picture to be developed.     

Non-spectral interferences in flameless atomic absorption spectrometry using graphite mini-tube furnaces

This work concerns non-spectral interferences occurring in flameless atomic absorption spectrometry using a Varian Techtron CRA model 63 instrument. Special attention has been paid to the influence exerted by concomitants on the vaporization and atomization behaviour of the test elements Be, Mn and Zn. Tracer analysis permitted the direct determination of the vaporization patterns of the test elements during the atomization phase. The variation of the transient absorption signals with time was measured using an electronic recording system sufficiently fast to obtain instrumentally undistorted pulses. The concomitants were selected on the basis of literature dealing with thermochemical processes in are emission spectrometry. Results are discussed on the basis of (i) the decrease of the content of the test element in the furnace, (ii) the characteristics of the absorption signal, and (iii) the surface temperature of the furnace. It turned out that the class “atomization efficiency interferences” might constitute one of the most serious sources of systematic error in frameless atomic absorption spectrometry.     

A calibration model to overcome non-spectral interferences in flame atomic absorption spectrometry

A calibration model has been developed in order to overcome matrix effects in atomic absorption measurements. The model uses two independent variables for analyte quantification (the amount of the sample and the amount of analyte added). The dependent variable is the absorbance measured. The method also allows matrix interferences to be controlled without prior knowledge of matrix composition. The method is applied to iron determination by FAAS in the presence of large amounts of copper. Direct calibration and standard addition are also performed in order to compare them with the new empirical model. Results show that the error in iron determination could be –42% when direct calibration is applied and +10% when the standard addition method is used, whereas the proposed model decreases the error to –20%.     

Electrochemical hydride generation as a sample-introduction technique in atomic spectrometry: fundamentals,interferences, and applications

Electrochemical hydride generation (EC-HG) has been proposed as a valid alternative to chemical generation as a sample-introduction technique in atomic spectrometry. In this review fundamental aspects of the technique are revised, including designs of electrolytic cells, mechanisms of the generation process, and interferences caused by the presence of different species. Special attention is paid to the role of the configuration of the cathodes and their materials on the efficiency of hydride generation and on interferences from concomitant species. An overview of the application of EC-HG to the analysis of real samples is also given.     

Nebulization effects in analytical atomic spectrometry

The effects of solvent vaporization and of element redistribution between the various fractions of an aerosol during sample nebulization in atomic spectrometry are examined. These effects govern the fluctuations of element concentration in the analytical zone of the flame. Sample composition modification can increase random errors to 5–10% in the absence of matrix matching.     

Effects of analyte reduction by flame gases through heterogenous reactions in flame spectrometry

A simplified model describing vaporisation with simultaneous reduction to an involatile species and assuming that the rate of reduction is limited by the diffusion of reducing species from the flame gases to the particle surface is presented. Under these conditions the fraction vaporised may be independent of the particle size and linear calibration curves may be found even with incompletely vaporised aerosol particles.     

Reduction of interferences in the determination of germanium by hydride generation and atomic emission spectrometry

Effects of interferences and methods for the reduction of interferences in the determination of germanium were studied. Simultaneous signal enhancement and reduction of interference were achieved using l-cysteine, l-cystine, penicillamine and thioglycerol. Amino acids such as glycine and alanine showed some capacity to reduce interferences, but this was both different in mechanism from, and inferior to, the reagents mentioned above. Histidine, on the other hand, proved to be superior to l-cysteine in its ability to reduce interferences from high levels of nickel. Sodium citrate, sodium oxalate, thiourea and thiosemicarbazide also reduced interferences to some extent. Mechanisms are proposed to explain reduction of interferences and enhancement of signals by the thiol-containing compounds and to explain the behaviour of amino acids as interference-reducing reagents.     

Systematic investigation of aluminium interferences with the alkaline earths in flame atomic absorption spectrometry

Summary Aluminium interferes with the absorption of Mg and Ca in the air-acetylene flame to such an extent that the corresponding absorbances may fall even to zero. This well-known chemical interference can be overcome with the nitrous oxide-acetylene flame, completely in the case of Mg, however only to a restricted extent in the case of Ca. Mg and Ca with concentrations of the AAS-working range in aqueous solutions and Cl^– or NO ₃ ^– as anions (in an aqueous HCl or HNO₃ matrix, respectively), were determined in the air-acetylene flame with continuously rising Al portions and with (or without) 0.25% Cs as radiation buffer, in order to quantify the degree of these interferences. The same was done to evaluate the extent of the suppression of those interference when using a releaser or protector reagent in both the air-acetylene and the nitrous oxide-acetylene flame. After the decrease of absorption in the air-acetylene flame by forming thermally stable Mg or Ca aluminates, a rapid increase (positive interference) occurs unexpectedly in the presence of Cl^–±Cs and with further rising Al contents. This effect still appears for Ca also in the hotter nitrous oxideacetylene flame, however, only in a restricted extent. In the air-acetylene flame the undisturbed absorptions for Mg and Ca (i.e. the starting data without Al) are nearly reached again within the range of the positive interference. This supports the assumption that in consequence of a continuous equilibrium change in the flame because of the rising Al content and in the presence of Cl^– and ±Cs the formation of only pure Al oxides now generates the release of Mg and Ca (instead of the thermally stable aluminates in the beginning). In the air-acetylene flame interferences of 1000 mg/l Al are completely removed by an addition of 1% releaser-La, when measuring up to 0.2 mg/l Mg and up to 4 mg/l Ca. The extent of releasing Mg and Ca is effective only up to that Al concentration range which leads to the absorption maximum of Mg and Ca. In the nitrous oxideacetylene flame 5000 mg/l Al are compensated when determining up to 1 mg/l Mg. In the case of Ca, which is detected up to 4 mg/l, interferences of 1000 mg/l Al are only avoided by using the nitrous oxide acetylene flame together with 1% releaser-La. The excellent sensitivity of Ca in this flame (in contrast to the air-acetylene flame) permits a strong dilution, lowering thereby the interfering Al concentration, too. For Mg the same option is valid because of its high sensitivity in the air-acetylene flame.     

Reduction of interferences in graphite furnace atomic absorption spectrometry by multiple linear regression modelling

The multivariate effects of Na, K, Mg and Ca as nitrates on the electrothermal atomisation of manganese, cadmium and iron were studied by multiple linear regression modelling. Since the models proved to efficiently predict the effects of the considered matrix elements in a wide range of concentrations, they were applied to correct the interferences occurring in the determination of trace elements in seawater after pre-concentration of the analytes. In order to obtain a statistically significant number of samples, a large volume of the certified seawater reference materials CASS-3 and NASS-3 was treated with Chelex-100 resin; then, the chelating resin was separated from the solution, divided into several sub-samples, each of them was eluted with nitric acid and analysed by electrothermal atomic absorption spectrometry (for trace element determinations) and inductively coupled plasma optical emission spectrometry (for matrix element determinations). To minimise any other systematic error besides that due to matrix effects, accuracy of the pre-concentration step and contamination levels of the procedure were checked by inductively coupled plasma mass spectrometric measurements. Analytical results obtained by applying the multiple linear regression models were compared with those obtained with other calibration methods, such as external calibration using acid-based standards, external calibration using matrix-matched standards and the analyte addition technique. Empirical models proved to efficiently reduce interferences occurring in the analysis of real samples, allowing an improvement of accuracy better than for other calibration methods.     

Inelastic collisional deactivation in plasma-related non-spectroscopic matrix interferences in inductively coupled plasma atomic emission spectrometry

Inelastic collisional deactivation of the analyte excited state is demonstrated as a dominant cause for non-spectroscopic matrix interference in inductively coupled plasma atomic emission spectrometry (ICP-AES) for commonly used plasma operating conditions in routine analysis. A mathematical simulation of the inelastic collisional model was examined. Comparison between the theoretical model and experimental results using atomic and ionic lines of the analytes Zn, Ba, Mg, Mn and Sr validates the inelastic collisional deactivation model as a dominant cause for non-spectroscopic matrix effect. Matrices evaluated were NH₄Cl, NH₄SCN, (NH₄)₂SO₄, and H₂SO₄ to represent difficult-to-ionize matrices (DIE) and NaCl and CaCl₂ to represent easy-to-ionize element matrices (EIE).     

Catalytic effects as the origin of some interferences in flame spectrometry

A new explanation for a range of hitherto quite unexplained interference phenomena is put forward and subjected to experimental verification. It is suggested that the interfering metal may be acting through a catalysis of the recombination of excess free radical concentrations in the flame, thereby leading to the possibility of significant disturbance of the proportion of analyte metal bound up in compound form. It is suggested, for example, that the metals Mg, Ca, Sr, Ba, Sn, Cr, U, and Mn may, in described circumstances, interfere with certain of the (either) atomic or molecular emissions from each other or with certain of the (either) atomic or molecular emissions from Li, K, Rb, Cs, Al, Ga, In and Cu.     

Flow-injection techniques for the removal of stable-compound interferences in flame atomic absorption spectrometry of calcium

The effects of phosphate and aluminum on calcium in an air/acetylene flame were studied in a flow-injection system. The depressive effect of phosphate was eliminated by operating the nebulizer under starvation conditions with air compensation. Pulse dampers were incorporated into the flow line and the instrument operating conditions were optimized for minimum interference. The depressive effect of aluminum could not be removed in this way. To achieve this, a two-line manifold was used in which the calcium was precipitated as oxalate, retained on a membrane filter and redissolved in hydrochloric acid. Up ot 200 mg l^?1 aluminum could be tolerated with a precision of 2% for 10 mg l^t?1 calcium.     

Determination of selenium in sediments by hydride generation atomic absorption spectrometry. Elimination of interferences

Summary The hydride generation/atomic absorption spectrometry (AAS) with an automated flow system is useful for the routine analysis of selenium in environmental samples. This method is, however, subject to interferences from transition metal ions and other hydride forming ions. The conditions to minimize the interferences were established: the concentration of hydrochloric acid 6 mol/l; the concentration of tetrahydroborate 0.5%. Iron(III) chloride released the depression of selenium signals by metal ions such as copper(II) and bismuth(III). Selenium in several standard reference materials including sediment samples was determined by the present method and by fluorimetry with 2,3-diaminonaphthalene. The results obtained by the two methods agreed with an acceptable precision. This means that hydride generation/AAS offers good precision and accuracy in the determination of selenium in sediment samples as well as DAN fluorimetry. However, the former is much simpler in operation. The method was applied to the determination of selenium in estuarine sediments collected in Nagoya harbor and Ise Bay. The results can be used to assess the pollution state of these places.

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Selenbestimmung in Sedimenten durch AAS mit Hydriderzeugung. Eliminierung von Störungen

     

Study of interferences by transition metal ions in hydride generation atomic absorption spectrometry of antimony

The mechanism of the interference of metal ions in the hydride generation atomic absorption spectrometry (AAS) of antimony was studied. Experiments on the decomposition rate of sodium tetrahydroborate in acidic media were done in the presence of interfering metals, with and without the addition of masking agents. The results were compared with experiments on the sensitivity of AAS in the presence of the same interfering metals with and without the addition of the same masking agents. AAS experiments are described in which the product of the reaction between sodium tetrahydroborate and one of the interfering metals was present during hydride formation. This reaction product was certainly one of the causes of interference, but the contribution of the catalytic decomposition of sodium tetrahydroborate to interference is not clear.