A direct solid sampling electrothermal atomic absorption spectrometric method for determination of trace elements in zirconium dioxide

A direct solid sampling electrothermal atomic absorption spectrometric method for determination of Cd, Co, Cr, Cu, Fe, Li, Mn, Ni and Zn in zirconium dioxide has been developed. Using optimised experimental conditions, very effective in-situ analyte/matrix separation was achieved without any chemical modification. After the measurement, the matrix residue could easily be tipped out from the platform. In the determination of Cr, before sampling, the platform bottom was covered with carbon powder. Quantification was performed using calibration curves measured with aqueous standard solutions pipetted onto the matrix residue from a previous sample run. Sample amounts between 0.5 and 40 mg were applied for each analysis cycle. The accuracy was determined by comparison of the results with those obtained by radiochemical neutron activation analysis and by electrothermal atomic absorption spectrometry using liquid sampling of digests and slurry sampling. For the nine elements assayed, the limits of detection achievable by this method are between 0.06 ng g^–1 (Cd) and 2.3 ng g^–1 (Fe).Dedicated to the memory of Wilhelm Fresenius     

Chromium determination in pharmaceutical grade barium sulfate by solid sampling electrothermal atomic absorption spectrometry with Zeeman-effect background correction

A procedure for chromium (Cr) determination in pharmaceutical grade barium sulfate by direct solid sampling electrothermal atomic absorption spectrometry (DSS-ET AAS) with Zeeman-effect background correction was developed. Operational conditions for the proposed procedure and the use of citric acid, ammonium phosphate, palladium and magnesium nitrate as chemical modifiers were evaluated. Pyrolysis and atomization temperatures were set at 1500 and 2400 °C, respectively and the use of matrix modifiers did not improve these conditions. Graphite platform presented high degradation rate, but minima changes were observed in the sensitivity or signal profile. Samples (0.3-1 mg) were weighted and introduced into the furnace using a manual solid sampling system. The linear concentration range of the calibration curve was from 100 to 1800 pg (R² > 0.995). The characteristic mass was 7.7 pg and the limit of detection was 2.4 pg. Chromium concentration in commercial samples ranged from 0.45 to 1.06 μg g⁻¹ and these results were confirmed by standard addition method. The mean reproducibility was 12% (n = 20 in a 3-day period) and repeatability was less than 9%. Results obtained using inductively coupled plasma optical emission spectrometry and conventional electrothermal atomic absorption spectrometry after extraction with HNO₃ were around 20% lower than those obtained by the proposed procedure. It was assumed that the low results were due to incomplete extraction even using hard conditions related to temperature and pressure. The proposed procedure by DSS-ET AAS provided some advantages related to recommended pharmacopoeias methodology, as lower risks of contamination and analyte losses, higher specificity, accuracy and sensitivity, no toxic or unstable reagents are required, and calibration with aqueous standards was feasible.     

Direct determination of manganese in vitamin-mineral tablets using solid sampling electrothermal atomic absorption spectrometry

Manganese in vitamin-minerals tablets was determined by solid sampling electrothermal atomic absorption spectrometry (SS-ETAAS) using three different calibration methods, namely calibration against aqueous standards, standard addition with aqueous standards on solid samples and calibration against solid certified standards. Samples were only finely ground and introduced directly into the furnace by means of solid autosampler system without any dissolving process. Effects of different calibration techniques, temperatures and heating rates of atomization and pyrolysis steps on the accuracy and precision of the analyte elements were investigated. After optimization of the experimental parameters, there is good agreement (at 95% confidence level) between the results obtained by solid sampling and those obtained by acid digestion of samples.     

Determination of trace impurities in titanium dioxide by direct solid sampling electrothermal atomic absorption spectrometry

A true direct solid sampling electrothermal atomic absorption spectrometry method with Zeeman-effect background correction (Analytik Jena ZEEnit 60 AAS) was developed for the determination of As, Cd, Hg, Pb, Sb and Zn in powdered titanium dioxide of pharmaceutical, food and cosmetics grade. The interaction of the titanium matrix and graphite surface of the sample carrier boat in a transversely heated graphite tube atomizer was investigated. Conversion of titanium dioxide to interfering TiO₂–TiC-liquid phase, running out the sampling boat, was observed at temperatures above 2000 °C. The temperature program was optimized accordingly for these volatile analytes in atomization and cleaning steps in order to prevent this interference and to prolong significantly the analytical lifetime of the boat to more than one thousand runs. For all elements, calibration by aqueous standard addition method, by wet-chemically analyzed samples with different content of analytes and/or by dosing one sample in different amounts, were proved as adequate quantification procedures. Linear dynamic calibration working ranges can be considerably expanded up to two orders of magnitude within one measurement run by applying three-field dynamic mode of the Zeeman background correction system. The results obtained by true direct solid sampling technique are compared with those of other independent, mostly wet-chemical methods. Very low limits of detection (3σ criterion) of true solid sampling technique of 21, 0.27, 24, 3.9, 6.3 and 0.9 ng g^− 1 were achieved for As, Cd, Hg, Pb, Sb and Zn, respectively.     

Cadmium determination in biological samples by direct solid sampling flame atomic absorption spectrometry

A direct solid sampling flame atomic absorption spectrometric procedure for trace determination of cadmium in biological samples has been developed. Test samples (0.05–2.00 mg) were ground and weighed into small polyethylene vials, which were connected to the device for solid sample introduction into a conventional air/acetylene flame. Test samples were carried as a dry aerosol to a quartz cell, placed between the burner and the optical path, which had a perpendicular entrance and a slit in the upper part. The atomic vapor generated in the flame produced a transient signal that was totally integrated within 1 s. The effect of operating conditions and the extent of grinding on the analytical signal were evaluated. Background signals were always low and a characteristic mass of 0.29 ng Cd was obtained. Calibration was performed using different masses of solid certified reference materials. Results obtained for certified and in-house reference materials were typically within the 95% confidence interval of the certified and/or reference value, and the precision, expressed as relative standard deviation, was between 3.8 and 6.7%. The proposed system is simple and it might be adapted to conventional atomic absorption spectrometers allowing the determination of Cd in more than 80 test samples per hour, excluding weighing.     

Method development for the determination of manganese, cobalt and copper in green coffee comparing direct solid sampling electrothermal atomic absorption spectrometry and inductively coupled plasma optical emission spectrometry

A method has been developed for the determination of cobalt, copper and manganese in green coffee using direct solid sampling electrothermal atomic absorption spectrometry (SS-ET AAS). The motivation for the study was that only a few elements might be suitable to determine the origin of green coffee so that the multi-element techniques usually applied for this purpose might not be necessary. The three elements have been chosen as test elements as they were found to be significant in previous investigations. A number of botanical certified reference materials (CRM) and pre-analyzed samples of green coffee have been used for method validation, and inductively coupled plasma optical emission spectrometry (ICP OES) after microwave-assisted acid digestion of the samples as reference method. Calibration against aqueous standards could be used for the determination of Mn and Co by SS-ET AAS, but calibration against solid CRM was necessary for the determination of Cu. No significant difference was found between the results obtained with the proposed method and certified or independently determined values. The limits of detection for Mn, Cu and Co were 0.012, 0.006 and 0.004 μg g⁻¹ using SS-ET AAS and 0.015, 0.13 and 0.10 μg g⁻¹ using ICP OES. Seven samples of Brazilian green coffee have been analyzed, and there was no significant difference between the values obtained with SS-ET AAS and ICP OES for Mn and Cu. ICP OES could not be used as a reference method for Co, as essentially all values were below the limit of quantification of this technique.     

Progress in direct solid sampling analysis using line source and high-resolution continuum source electrothermal atomic absorption spectrometry

The literature about direct solid sample analysis of the past 10–15 years using electrothermal atomic absorption spectrometry has been reviewed. It was found that in the vast majority of publications aqueous standards were reported as having been used for calibration after careful program optimization. This means the frequently expressed claim that certified reference materials with a matrix composition and analyte content close to that of the sample have to be used for calibration in solid sample analysis is not confirmed in the more recent literature. There are obviously limitations, and there are examples in the literature where even calibration with certified reference materials did not lead to accurate results. In these cases the problem is typically associated with spectral interferences that cannot be corrected properly by the systems available for conventional line source atomic absorption spectrometry, including Zeeman-effect background correction. Using high-resolution continuum source atomic absorption spectrometry, spectral interferences become visible owing to the display of the spectral environment at both sides of the analytical line at high resolution, which makes program optimization straightforward. Any spectrally continuous background absorption is eliminated automatically, and even rapidly changing background absorption does not cause any artifacts, as measurement and correction of background absorption are truly simultaneous. Any kind of fine-structured background can be eliminated by “subtracting” reference spectra using a least-squares algorithm. Aqueous standards are used for calibration in all published applications of high-resolution continuum source atomic absorption spectrometry to direct solid sample analysis. This contribution is based on a presentation given at the Colloquium for Analytical Atomic Spectroscopy (CANAS ‘07) held March 18–21, 2007 in Constance, Germany.     

Indium determination in different environmental materials by electrothermal atomic absorption spectrometry with Amberlite XAD-2 coated with 1-(2-pyridylazo)-2-naphthol

Methods were developed for indium (In) determination in complex ores by electrothermal atomization atomic absorption spectrometry using matrix modification after its separation with Amberlite XAD-2 coated with 1-(2-pyridylazo)-2-naphthol (PAN). Palladium-magnesium, nickel, and zinc nitrates were used as matrix modifiers and were compared in terms of maximum pyrolysis temperature, sensitivity and background signal. They have enhanced the absorption signals for indium, respectively eliminating the matrix interferences. The standard additions method was applied. The relative standard deviations for six replicate determinations were in the range 0.3-4.0% for indium in different ores samples for indium concentrations 7.6-209 μg g⁻¹. The recommended method was applied to the indium determination in real samples. The data obtained by this method were in good agreement with those obtained by ICP-AES.     

Determination of trace impurities in boron nitride by graphite furnace atomic absorption spectrometry and electrothermal vaporization inductively coupled plasma optical emission spectrometry using solid sampling

Two digestion-free methods for trace analysis of boron nitride based on graphite furnace atomic absorption spectrometry (GFAAS) and electrothermal vaporization inductively coupled plasma spectrometry optical emission (ETV-ICP-OES) using direct solid sampling have been developed and applied to the determination of Al, Ca, Cr, Cu, Fe, Mg, Mn, Si, Ti and Zr in four boron nitride materials in concentration intervals of 1–23, 54–735, 0.05–21, 0.005–1.3, 1.6–112, 4.5–20, 0.03–1.8, 6–46, 38–170 and 0.4–2.3 μg g^− 1, respectively. At optimized experimental conditions, with both methods, effective in-situ analyte/matrix separation was achieved and calibration could be performed using calibration curves measured with aqueous standard solutions. In solid sampling GFAAS, before sampling, the platform was covered with graphite powder and, for determination of Si, also the Pd/Mg(NO₃)₂ modifier was used. In the determination of all analyte elements by solid sampling ETV-ICP-OES, Freon R12 was added to argon carrier gas. For solid sampling GFAAS and ETV-ICP-OES, the achievable limits of detection were within 5 (Cu)–130 (Si) ng g^− 1 and 8 (Cu)–200 (Si) ng g^− 1, respectively. The results obtained by these two methods for four boron nitride materials of different purity grades are compared each with the other and with those obtained in analysis of digests by ICP-OES. The performance of the two solid sampling methods is compared and discussed.     

Chemometric investigation of systematic error in the analysis of biological materials by flame and electrothermal atomic absorption spectrometry

Systematic errors observed when using flame atomic absorption spectrometry (FAAS) and electrothermal atomic spectrometry (ETAAS) for the analysis of biological solid materials (seafood products) were evaluated. The effect of the sample pre-treatment method (microwave-assisted acid digestion, ultrasound-assisted acid leaching and slurry sampling) as well as the number of times that a certain pre-treatment process is repeated, were two factors evaluated. They give information about the effect of the sample pre-treatment on the uncertainty in the analysis. In addition, the number of measurements (i.e., number of times that an acid digest, an acid leachate or aqueous slurry are analysed) and the calibration technique used (aqueous calibration method or standard addition technique) were other two variables taken into account. This last factor gives information about the effect of the calibration on the results, while the replicate measurements showed the repeatability. A fifth variable named as sample matrix tests the influence of the matrix sample on the systematic error through the use of different reference materials. This variable allows the study of the effect of the trace element concentrations on the uncertainty because the trace elements contents are different in each reference material. Experimental design and principal component analysis approaches were used as chemometric tools. It has been found that the use of the slurry sampling technique in ETAAS and FAAS and the determination of high element concentrations by ETAAS have led to poor precision.     

An ion-exchange method for speciation of antimony by flow injection electrothermal atomic absorption spectrometry

In this work, a simple preconcentration system, achieved by replacing the sample tip of the autosampler arm by a micro-column packed with Amberlite IRA-910 or silica gel chelating resin functionalised with 1,5-bis(di-2-pyridyl)methylene tbiocarbohydrazide (DPTH-gel), is developed for the determination of Sb(V) and total antimony, respectively. Different factors including pH of sample solution, ionic strength, concentration and volume of eluent, sample flow rate, sample loading time and matrix effects for preconcentration were investigated. The method has been applied to the determination of antimony species in different samples.     

Determination of vanadium content in soils by slurry sampling electrothermal atomic absorption spectrometry using KO300G as the stabilizing agent

A direct and sensitive method for the determination of vanadium concentrations in soil is developed using ultrasonic slurry sampling electrothermal atomic absorption spectrometry (USSSETAAS). The surfactant, KO300G, is used as the stabilizing agent. The precision and accuracy of the method are investigated. The detection limits are 0.6 and 0.7 μg 1⁻¹ for SRM Montana Soil 2711 and SRM Soil — S, respectively. The method is applied to determine the vanadium content in 10 soil samples from the Wielkopolska region.     

Feasibility of employing permanent chemical modifiers for the determination of cadmium in coal using slurry sampling electrothermal atomic absorption spectrometry

Iridium and ruthenium, alone and in combination with tungsten, thermally deposited on the platform of a transversely heated graphite tube, were investigated for their suitability as permanent chemical modifiers for the determination of cadmium in coal slurries by electrothermal atomic absorption spectrometry (ET AAS). The conventional mixed palladium and magnesium nitrates (Pd–Mg) modifiers, added in solution, were also investigated for comparison. The latter one showed the best performance for aqueous solutions, and the mixed W–Ir and W–Ru permanent modifiers had the lowest stabilizing power. All of the investigated modifiers lost some of their stabilizing power when coal slurries were investigated. The Pd–Mg modifier, pure Ir and Ru, and a mixture of 300 μg W + 200 μg Ir could stabilize Cd at least to a pyrolysis temperature of 600 °C, whereas all the other combinations already failed at temperatures above 500–550 °C. Additional investigations of the supernatant liquid of the slurries supported the assumption that the high acid concentration of the slurries and/or a concomitant leaching out of the coal might be responsible for the reduced stabilizing power of the modifiers. The maximum applicable pyrolysis temperature of 600 °C was not sufficient to reduce the background absorption to a manageable level in the majority of the coal samples. High-resolution continuum source ET AAS revealed that the continuous background absorption was exceeding values of A = 2, and was overlapping with the analyte signal. Although the latter technique could correct for this background absorption, some analyte was apparently lost with the rapidly vaporizing matrix so that the method could not be considered to be rugged. A characteristic mass of 1.0 pg and a detection limit of 0.6 ng g^− 1 could be obtained under these conditions.     

Selenium determination by electrothermal atomic absorption spectrometry in petroleum refinery aqueous streams containing volatile organic compounds

An electrothermal atomic absorption spectrometry (ETAAS) with polarized Zeeman background correction was used for determining selenium in petroleum refinery aqueous streams containing large amounts of volatile unknown organic compounds. Some parameters that might affect the measurement were investigated such as the amount of matrix modifier added, the temperature program and the calibration mode employed. Obtained results indicate that, in this kind of sample, selenium must be determined by standard addition procedure with a careful control of the dry step temperature and ramp pattern. Also, the results show that 2.5 μg of Pd must be added as matrix modifier to stabilize the analyte in the range of 2-20 ng Se. In order to evaluate the accuracy of the procedure, selenium was determined in 18 samples by ETAAS and hydride generation atomic absorption spectrometry (HGAAS) (as reference methodology). In both techniques the results agreed well.     

Utilization of electrodeposition for electrothermal atomic absorption spectrometry determination of gold

Gold was determined by electrothermal atomic absorption spectrometry after electrochemical preconcentration on the graphite ridge probe used as a working electrode and sample support. The probe surface was electrochemically modified with Pd, Re and the mixture of both. The electrolysis of gold was performed under galvanostatic control at 0.5 mA. Maximum pyrolysis temperature for the probe surface modified with Pd was 1200 °C, with Re 1300 °C. The relative standard deviation for the determination of 2 μg l^− 1 Au was not higher than 5.6% (n = 8) for 2 min electrodeposition. The sensitivity of gold determination was reproducible for 300 electrodeposition and atomization cycles. When the probe surface was modified with a mixture of Pd and Re the detection limit was 31 ng l^− 1 for 2 min electrodeposition, 3.7 ng l^− 1 for 30 min, 1.5 ng l^− 1 for 1 h and 0.4 ng l^− 1 for 4 h electrodeposition, respectively. The procedure was applied to the determination of gold in river water samples. The relative standard deviation for the determination of 2.5 ng l^− 1 Au at 4 h electrodeposition time at 0.5 mA was 7.5%.     

Determination of trace impurities in aluminum nitride by direct solid sampling graphite furnace atomic absorption spectrometry

The trace impurities Cr, Cu, Fe, K, Mn, Sb and Zn were determined in powdered aluminum nitride by direct solid sampling graphite furnace atomic absorption spectrometry using a ZEEnit 60 atomic absorption spectrometer. This spectrometer features inverse Zeeman-effect background correction and a variable magnetic field enabling measurements in two sensitivity modes over a concentration range of three orders of magnitude. The measurement sensitivity can be adjusted to the analyte concentration in the sample. The use of chemical modifiers was not necessary. Calibration was carried out by means of calibration curves obtained with aqueous standard solutions. Accuracy was checked mainly by comparison of the results with those obtained by instrumental and radiochemical neutron activation analysis whereby, excluding the results for potassium, no significant differences were found by carrying out the t-test at the significance level 0.05. The limits of detection were between 0.05 ng g⁻¹ (Zn) and 80 ng g⁻¹ (Fe) and the relative standard deviations below 11 %. With the proposed method, up to ten measurement cycles can be carried out in one hour.     

Lead ultra-trace on-line preconcentration and determination using selective solid phase extraction and electrothermal atomic absorption spectrometry: applications in seawaters and biological samples

In this work, a new chelating resin [1,5-bis (2-pyridyl)-3-sulphophenyl methylene] thiocarbonohydrazide immobilised on aminopropyl-controlled pore glass (550 A; PSTH-cpg) was synthesised and packed in a microcolumn which replaced the sample tip of the autosampler arm. The system was applied to the preconcentration of lead. When microliters of 10% HNO3, which acts as elution agent, pass through the microcolumn, the preconcentrated Pb(II) is eluted and directly deposited in a tungsten-rhodium coated graphite tube. With the use of the separation and preconcentration step and the permanent modifiers, the analytical characteristics of the technique were improved. The proposed method has a linear calibration range from 0.012 to 10 ng ml(-1) of lead. At a sample frequency of 36 h(-1) with a 90 s preconcentration time, the enrichment factor was 20.5, the detection and determination limits were 0.012 and 0.14 ng ml(-1), respectively and the precision, expressed as relative standard deviation, was 3.2% (at 1 ng ml(-1)). Results from the determination of Pb in biological certified reference materials were in agreement with the certified values. Seawaters and other biological samples were analysed too.     

Determination of aluminium and manganese in human scalp hair by electrothermal atomic absorption spectrometry using slurry sampling

Methods for the determination of aluminium and manganese in human scalp hair samples by electrothermal atomic absorption spectrometry using the slurry sampling technique were developed. Palladium and magnesium nitrate were used as chemical modifiers. Hair samples were pulverized using a zirconia vibrational mill ball, and were prepared as aqueous slurries. Determinations can be performed in the linear ranges of 1.9–150 μg l⁻¹ Al³⁺ and 0.03–10.0 μg l⁻¹ Mn²⁺. Limits of detection of 0.9 mg kg⁻¹ and 27.6 μg kg⁻¹ were obtained for aluminium and manganese, respectively. The analytical recoveries were between 99.6 and 101.8% for aluminium and in the 98.3–101.3% range for manganese. The repeatability of the methods (n=11), slurry preparation procedure and ETAAS measurement, was 16.0 and 7.9% for aluminium and manganese, respectively. The methods were finally applied to the aluminium and manganese determination in 25 scalp hair samples from healthy adults. The levels for aluminium were between 8.21 and 74.08 mg kg⁻¹, while concentrations between 0.03 and 1.20 mg kg⁻¹ were found for manganese.     

Solid sampling-graphite furnace atomic absorption spectrometry for the direct determination of boron in plant tissues

In this work, the potential of graphite furnace atomic absorption spectrometry for the direct determination of B in plant tissues has been investigated. Three certified reference materials (NIST SRM 1570a spinach leaves, NIST SRM 1573a tomato leaves and BCR CRM 679 white cabbage) were selected for this study, the goal always being to develop a fast procedure that could be robust enough to provide a satisfactory performance for all of them, without any modifications in the conditions applied.The use of a suitable chemical modifier was found to be essential for obtaining a reproducible and sufficiently sensitive signal for boron solutions. In this regard, the performance of the combination of citric acid plus W (added as a permanent modifier) was noteworthy, resulting in well-defined signal profiles, a remarkable analyte stabilization during the pyrolysis step (up to 2100 °C) and minimal memory effects. This mixture of modifiers provided a good performance for the direct analysis of solid samples as well, but only if a suitable temperature program, favoring the interaction between the analyte and the modifiers, was used. Thus, such a temperature program, with two pyrolysis steps and the addition of NH₄NO₃ in order to carry out the in situ sample microdigestion, was optimized. Under these conditions, the peak areas obtained for both solid samples and aqueous standards were comparable.Finally, the analysis of the samples was carried out. In all cases, a good agreement with the certified values was obtained, while R.S.D. values ranged between 6 and 10%. It can be concluded that the method proposed shows significant advantages for the determination of this complicated element in solid samples such as the use of aqueous standards for calibration, a high sample throughput (20 min per sample), a suitable limit of detection (0.3 μg g⁻¹) and reduced risk of analyte losses and contamination.     

Comparative determination of Ba, Cu, Fe, Pb and Zn in tea leaves by slurry sampling electrothermal atomic absorption and liquid sampling inductively coupled plasma atomic emission spectrometry

The comparative determination of barium, copper, iron, lead and zinc in tea leaf samples by two atomic spectrometric techniques is reported. At first, slurry sampling electrothermal atomization atomic absorption spectrometry (ETAAS) was applied. The results of Ba and Pb determination were calculated using the method of standard additions, and results of Cu, Fe and Zn from the calibration graphs based on aqueous standards. These results were compared with the results obtained after microwave-assisted wet (nitric+hydrochloric+hydrofluoric acids) digestion in closed vessels followed by inductively coupled plasma-atomic emission spectrometric (ICP-AES) determination with the calibration by means of aqueous standards. The exception was lead determined after a wet digestion procedure by ETAAS. The accuracy of the studied methods was checked by the use of the certified reference material Tea GBW-07605. The recoveries of the analytes varied in the range from 91 to 99% for slurry sampling ETAAS, and from 92.5 to 102% for liquid sampling ICP-AES. The advantages of slurry sampling ETAAS method are simplicity of sample preparation and very good sensitivity. Slurry sampling ETAAS method is relatively fast but if several elements must be determined in one sample, the time of the whole microwave-assisted digestion procedure and ICP-AES determination will be shorter. However, worse detection limits of ICP-AES must also be taken into the consideration in a case of some analytes.