Consideration of the uncertainty in the CRM value with the analysis of solid materials using atomic spectrometry

 The principles of the ISO "Guide to the expression of uncertainty in measurement" are applied to the analysis of solid materials by graphite furnace atomic absorption spectrometry. Assessment of trueness is achieved by the analysis of a certified reference material (CRM) under fixed instrumental conditions. For the evaluation of the method's uncertainty (as part of the validation) a model equation is derived, considering all significant contributions, including the uncertainty in the CRM value and the uncertainty in the CRM analysis. A concrete example is presented and discussed (soya flour as an unknown sample and CRM BCR rye grass). For routine analysis a pooled estimate from the validation experiment can be used, leading to an acceptable small uncertainty range even in the case of "duplicate determination". The test for trueness (acceptance) of the result of the CRM analysis is applied and the correction for detected bias is discussed. Received: 28 June 1998 · Accepted: 13 July 1998     

Influence of two grinding methods on the uncertainty of determinations of heavy metals in atomic absorption spectrometry/electrothermal atomisation of plant samples

 Chemical analyses of trace elements are affected by relatively high analytical errors due to the different steps of the laboratory procedures: samples grinding, mineralisation and instrumental measurements. In the present communication, the influence of the grinding phase on the global uncertainty of Pb, Cd, Ni and Cr determinations in plant samples by the classical method of atomic absorption spectrometry/electrothermal atomisation (AAS-ETA) after dry ashing is quantified. Two grinding machines, a planetary mill with balls and jars of agate versus a stainless steel grinder were compared by analysing leaf samples of cucumber, strawberry, kiwivines, apple trees and grapevines from agricultural experimental plots under controlled conditions. Variance components due to the difference between grinding methods and experimental plots were estimated. Further, the simultaneous effects of the grinding methods on all considered metals have been evaluated by analysis of variance. With the stainless steel grinder, on average, higher levels of the considered heavy metals were obtained (up to 67% of the mean values). On average, the increments were similar for metals contained in steel (Ni and Cr) and those not contained (Pb and Cd). The true causes of these differences need further investigation to determine whether the higher metal detection is due to possible contamination, to a different grinding quality or to other reasons. Finally, the grinding methods did not seem to affect the combined uncertainty of the analyses. Received: 3 November 1997 · Accepted: 29 November 1997     

Analysis of small amounts of solid samples by spark-source mass spectrometry

Various elements were determined in solid samples weighing < 2 mg by spark-source mass spectrometry. The samples were fixed on the top of a cylindrical graphite electrode using a conductive silver paint. After baking, the samples were sparked against a tantalum or gold wire. The method was used for the determination of impurities in steel, iron, molybdenum and CdSe. For samples weighing ca. 1 mg, detection limits of the order of 1 μg g^?1 were obtained. A relationship between the relative sensitivity factor and physical properties is proposed.     

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.     

An uncertainty evaluation for multiple measurements by GUM

An approach for uncertainty evaluation is proposed to determine the overall uncertainty by combining the uncertainties of the individual results from multiple measurements. It is accomplished by the separate combinations of the individual random and systematic components of the uncertainties of the individual results. The approach is useful when the individual results are not statistically different. It is recognized that, owing to the correlation, the uncertainty resulting from systematic effects is not reduced by multiple measurements. On the contrary, the uncertainty resulting from random effects can be reduced. Received: 3 May 2002 Accepted: 16 July 2002     

Extraction as a method for preparation of vegetable samples for the determination of trace metals by atomic absorption spectrometry

An alternative, simple and rapid technique was developed for a quantitative isolation of the group of eight elements: Al, Ca, Cd, Cu, Fe, Mg, Pb and Zn from vegetable material samples in an open system with the use of acids: conc. HCl, dil. HNO₃ and hydrofluoric acids. Equivalence of both the extractants, HCl and HNO₃, was found suitable for a quantitative isolation of Ca, Cu, Mg, Zn, Pb and Cd. For lead and cadmium, however, dil. HNO₃ proved to be more suitable because these elements are determined by the graphite furnace atomic absorption spectrometry (GF-AAS) technique. A quantitative dissolution of Al and Fe requires hydrofluoric acid as an additional extractant. The proposed method allows to obviate the organic matrix destruction stage, shorten the analyte dissolution time, reduce cost, and minimize hazards of loss and contamination. Validity and versatility of the method developed was verified by the analysis of standard reference materials.     

Validation of a computer program for atomic absorption analysis

 The approach to validation of a computer program for an analytical instrument as a component of the analytical method (using this instrument with the program) is discussed. This approach was used for validating a new program for atomic absorption analysis. The validation plan derived from this approach was based on minimising the influence of all steps of the analytical procedure on the analytical results obtained by the method. In this way significant changes in the results may be caused only by replacement of the previous program by the new one. The positive validation conclusion was based on the comparison of the results of the analysis of suitable reference materials obtained with the new program and with its precursor in the same conditions, and also on comparison of their deviations from the accepted reference values for these materials, with the corresponding uncertainties. Received: 25 January 1997 Accepted: 14 March 1997     

Application of a new resin functionalised with 6-mercaptopurine for mercury and silver determination in environmental samples by atomic absorption spectrometry

Polystyrene–divinylbenzene (8%) has been functionalised by coupling it through an ---N=N--- group with 6-mercaptopurine. The resulting chelating resin has been characterised by using elemental analysis, thermogravimetric analysis and infrared spectra. The resin is highly selective for Hg(II) and Ag(I) and has been used for preconcentrating Hg(II) and Ag(I) prior to their determination by atomic absorption spectrometry. The maximum sorption capacity for Hg(II) and Ag(I) was found to be 1.74 and 0.52 mmol g⁻¹, respectively, over the pH range 5.5–6.0. The calibration range for Hg(II) was linear up to 10 ng ml⁻¹ with a 3σ detection limit of 0.02 ng ml⁻¹; the calibration range for Ag(I) was linear up to 5 μg ml⁻¹ with a detection limit of 29 ng ml⁻¹. The recoveries of the metals were found to be 99.7±3.8 and 101.3±4.1% at the 95% confidence level for both Hg(II) and Ag(I). In column operation, it has been observed that Hg(II) and Ag(I) in trace quantities can be selectively separated from geological, medicinal and environmental samples.     

Atomic fluorescence spectrometry combined with reduction aeration for the determination of mercury in biological samples

A sensitive atomic fluorescence system for the determination of mercury was optimized. The system consists of a continuous-flow mercury vapour generator coupled to a fluorescence spectrometer specific for mercury. A new gas-liquid separator was developed. A detection limit of 0.9 ng Hg 1^?1 was obtained. The system was combined with a microwave oven for dissolving samples in closed Teflon PFA vessels. Accurate results were obtained for certified reference materials, and biological samples such as urine and hair were analysed.     

Flow injection on-line displacement/solid phase extraction system coupled with flame atomic absorption spectrometry for selective trace silver determination in water samples

A novel flow injection (FI) on-line displacement solid phase extraction preconcentration and/or separation method coupled with FAAS in order to minimize interference from other metals was developed for trace silver determination. The proposed method involved the on-line formation and subsequently pre-sorption of lead diethyldithiocarbamate (Pb-DDTC) into a column packed with PTFE-turnings. The preconcentration and/or separation of the Ag(I) took place through a displacement reaction between Ag(I) and Pb(II) of the pre-sorbed Pb-DDTC. Finally, the retained analyte was eluted with isobutyl methyl ketone (IBMK) and delivered directly to nebulizer for measuring. Interference from co-existing ions with lower DDTC complex stability in comparison with Pb-DDTC, was eliminated without need for any masking reagent. With 120 s of preconcentration time at a sample flow rate of 7.6 mL min−1, an enhancement factor of 110 and a detection limit (3 s) of 0.2 μg L−1 were obtained. The precision (RSD, n = 10) was 3.1% at the 10 μg L−1 level. The developed method was successfully applied to trace silver determination in a variety of environmental water samples and certified reference material.     

Screening of antimony in PVC by solid sampling-graphite furnace atomic absorption spectrometry

The development of two-stage control systems is of great interest when a large number of samples are analysed in order to check that they fulfil certain requirements. If the first stage is carried out using an inexpensive method with a high throughput which makes it possible to filter out the majority of the samples that fulfil the requirements, the procedure is not only less time-consuming but also more economical. Direct determination of metals in solid samples by graphite furnace atomic absorption spectrometry (GFAAS) appears, in principle, to be a suitable analytical technique for screening purposes since it provides sufficiently reliable results in a reasonably short time. In this paper it is applied with satisfactory results to antimony content control in a PVC sample.     

Use of atomic absorption spectrometry for the determination of metals in sediments in south-west Louisiana

The relatively recent introduction of atomic absorption spectrometry has produced a rapid and relatively inexpensive method for the determination of metal concentrations in a wide variety of samples. One such application is in the determination of metal concentrations in soils and sediments. Soils and sediments represent concentrated reservoirs for these metals that serve as sinks for introduced trace metals or can become environmental sources. The coastal zone of Louisiana provides a ‘living laboratory’ to investigate the mechanisms of transport, deposition, and dissolution of trace metals into this fragile environment. Investigations done in the coastal zone have found trace metals tend to concentrate near pollution inputs and sources and have not migrated to or significantly impacted the coastal zone of Louisiana. Common trace metals determined and their range of concentrations in coastal soil and sediments are chromium (10–30 ppm), copper (10–25 ppm), iron (0.6–2.1%), manganese (200–600 ppm), nickel (6–20 ppm), lead (8–20 ppm), and zinc (30–55 ppm).     

Customers' needs in relation to uncertainty and uncertainty budgets

 The general requirement of Quality Management standards to include in test reports a statement of the uncertainty of the results reflects the fact that a test result is rather useless without a knowledge of its accuracy. After an outline of the basic concepts of uncertainty, the need for uncertainty statements is illustrated for different ranges of applications. Received: 23 December 1997 · Accepted: 3 March 1998     

Validation of mercury determination by solid sampling Zeeman atomic absorption spectrometry and a specially designed furnace

Certified reference materials (CRMs) of different origin were used to validate the direct determination of total mercury by solid sampling Zeeman atomic absorption spectrometry (SS-ZAAS) and a specially designed furnace. The temperature program provides only for one step. Atomisation of mercury and pyrolysis of the matrix is performed at a constant temperature in the range of 900–1000 °C. Calibration points achieved by CRMs and aqueous solutions are covered by one calibration line, indicating the absence of matrix effects. Relatively high amounts of chlorine, known for causing problems in mercury determination do not influence analytical results. The excellent accuracy of the method results in a very good agreement with the certified values. The precision of SS-ZAAS measurements in a range from 0.5 to 50 ng Hg does not exceed 3% R.S.D. A limit of quantification of 0.008 μg g⁻¹ Hg was achieved.     

On-line electrolytic dissolution of solid metal samples and determination of copper in aluminium alloys by flame atomic absorption spectrometry

A flow-injection system was developed in which alloy metal samples are electrolytically dissolved and the dissolved samples are analysed by flame atomic absorption spectrometry (FAAS). The effects of electrolyte composition and electrolysis parameters on the dissolution of the sample were studied. The method was used for the determination of copper in aluminium alloys. Electrolyte solutions consisting of 0.2–1.0 M nitric acid are better than other electrolytes tested with regard to both alloy sample dissolution and determination of copper by FAAS. The peak height increases linearly with the electrolysis time or current within a certain range. The detection limit depends on the sensitivity of the detector used, and can be improved by increasing the electrolysis time or current. Generally, aluminium alloys containing 0.5–10% copper can give suitable signals for FAAS determination. The reproducibility of electrolysis and determination is about 4% for the same sampling points and 5% for different sampling points on the alloy sample.     

Measurement uncertainty of selenium determination in the reference material Seronorm Trace Elements Serum by hydride generation atomic fluorescence spectrometry

 An analytical result is not considered to be complete without a statement of the measurement uncertainty associated with it. Evaluation of measurement uncertainty is therefore an essential part of every determination. In the present study the measurement uncertainty of the result of determination of selenium in the reference material Seronorm Trace Elements Serum was studied. The combination of a digestion procedure using sulfuric acid, V₂O₅ in H₂SO₄ and hydrogen peroxide and the continuous flow approach of hydride generation atomic fluorescence spectrometry (HG-AFS) detection was used for selenium determination. The total uncertainty budget was calculated with the help of the GUM Workbench program, in which computations follow the rules of the ’ISO guide to the expression of uncertainty in measurement’. The major sources of uncertainty were found to be due to the recovery of the procedure, measurement of peak heights and the purity of Na₂SeO₃. A selenium mass fraction of 77.1±4.8 ng/g (with a coverage factor of 2.1, 95% probability) was determined, which is comparable with the recommended value of 80 ng/g. Received: 13 September 2002 Accepted: 23 December 2002 Acknowledgements We would like to thank Tinkara Buˇcar, B.Sc. for useful discussions and the IAEA Vienna, for financial support of the project 11553/RO. Presented at CERMM-3, Central European Reference Materials and Measurements Conference: The function of reference materials in the measurement process, May 30–June 1, 2002, Rogaška Slatina, Slovenia Correspondence to V. Stibilj     

Evaluation of different permanent modifiers for the determination of arsenic in environmental samples by electrothermal atomic absorption spectrometry

Single noble metal permanent modifiers such as, Rh, Ir, and Ru, as well as mixed tungsten plus noble metal (W-Rh, W-Ru, W-Ir) permanent modifiers thermally deposited on the integrated platform of transversally heated graphite atomizer were employed for the determination of arsenic in sludges, soils, sediments, coals, ashes and waters by electrothermal atomic absorption spectrometry. Microwave digests of solid samples and water samples were employed for obtaining the analytical characteristics of the methods with different permanent modifiers. The performance of the modifiers for arsenic determination in the real samples depended strongly on the type of permanent modifier chosen. The single noble metal (Rh, Ir and Ru) permanent modifiers were suitable for the analyte determinations in simpler matrices such as waters (recoveries of certified values 95-105%), but the analyte recoveries of certified values in sludges, soils, sediments, coals, and ashes were always lower than 90%. On the other hand, for the determination of arsenic, using W-Rh, W-Ru, and W-Ir permanent modifiers presented recoveries of certified values within 95-105% for all the samples. Long-term stability curves obtained for the determination of arsenic in environmental samples with different permanent modifiers (Rh, Ir, Ru, W-Rh, W-Ir, W-Ru) showed that the improvement in the tube lifetime depends on the tungsten deposit onto the platform. The tungsten plus noble metal permanent modifier presents a tube lifetime of at least 35% longer when compared with single permanent modifier. The results for the determination of As employing different permanent modifiers in the samples were in agreement with the certified reference materials, since no statistical differences were found after applying the paired t-test at the 95% confidence level.     

Determination of heavy metals in soil, mushroom and plant samples by atomic absorption spectrometry

The concentrations of heavy metals in the soil, mushroom and plant samples collected from Tokat, Turkey have been determined by flame and graphite furnace atomic absorption spectrometry after dry ashing, wet ashing and microwave digestion. The study of sample preparation procedures showed that the microwave digestion method was the best. Good accuracy was assured by the analysis of standard reference materials. The relative standard deviations for all measured metal concentrations were lower than 10%. In all cases, quantitative analytical recoveries ranging from 95 to 103% were obtained. Metal accumulation factors were calculated for mushroom and plant samples. High ratio of plants to soil cadmium, zinc and copper concentrations indicate that these elements are accumulated by mushrooms. Results obtained are in agreement with data reported in the literature.     

On-line determination of manganese in solid seafood samples by flame atomic absorption spectrometry

Manganese is extracted on-line from solid seafood samples by a simple continuous ultrasound-assisted extraction system (CUES). This system is connected to an on-line manifold, which permits the flow-injection flame atomic absorption spectrometric determination of manganese. Optimisation of the continuous leaching procedure is performed by an experimental design. The proposed method allows the determination of manganese with a relative standard deviation of 0.9% for a sample containing 23.4 μg g⁻¹ manganese (dry mass). The detection limit is 0.4 μg g⁻¹ (dry mass) for 30 mg of sample and the sample throughput is ca. 60 samples per hour. Accurate results are obtained by measuring TORT-1 certified reference material. The procedure is finally applied to mussel, tuna, sardine and clams samples.     

Evaluation of polychlorotrifluoroethylene as sorbent material for on-line solid phase extraction systems: determination of copper and lead by flame atomic absorption spectrometry in water samples

Polychlorotrifluoroethylene (PCTFE) in the form of beads was applied, as packing material for flow injection on-line column preconcentration and separation systems coupled with flame atomic absorption spectrometry (FAAS). Its performance characteristics were evaluated for trace copper determination in environmental samples. The on-line formed complex of metal with diethyldithiophosphate (DDPA) was sorbed on the PCTFE surface. Isobutyl methyl ketone (IBMK) at a flow rate of 2.8 mL min⁻¹ was used to elute the analyte complex directly into the nebulizer-burner system of spectrophotometer. The proposed sorbent material reveal, excellent chemical and mechanical resistance, fast adsorption kinetics permitting the use of high sample flow rates up to 15 mL min⁻¹ without loss of retention efficiency. For copper determination, with 90 s preconcentration time the sample frequency was 30 h⁻¹, the enhancement factor was 250, which could be further improved by increasing the loading (preconcentration) time. The detection limit (3s) was c_L = 0.07 μg L⁻¹, and the precision (R.S.D.) was 1.8%, at the 2.0 μg L⁻¹ Cu(II) level. For lead determination, the detection limit was c_L = 2.7 μg L⁻¹, and the precision (R.S.D.) 2.2%, at the 40.0 μg L⁻¹ Pb(II) level. The accuracy of the developed method was evaluated by analyzing certified reference materials and by recovery measurements on spiked natural water samples.