Calibration model of simultaneous multielement atomic-emission analysis using analytical line groups of each determined element

A calibration model of multielement methods for simultaneous determination of micro- and macro-concentrations of elements by computing the arc atomic-emission spectra has been developed. A calibration procedure for the analytical line group of the elements to be determined is offered. It allows the lower and upper limits of the concentration range for each line of the determined element to be calculated by means of the least-square method (LSM) and the Weibull distribution law is used to extend the concentration region. The calibration model was successfully tested for different arc optical emission spectroscopy (OES) methods. Received: 17 June 1997 / Revised: 3 November 1997 / Accepted: 7 November 1997     

Determination of uranium, thorium, and 20 other elements in high-purity tungsten by radiochemical neutron activation analysis

A radiochemical neutron activation analysis method for the determination of 22 elements in high-purity tungsten has been developed. For the assay of indicator radionuclides with long half-lives, the radiochemical separation was performed from HF/H₂O₂, HF/NH₄F and HCl/H₂SO₄ media by a combination of cation and anion exchange on a Dowex 50 W × 8 and Dowex 1 × 8 column. An effective removal of the matrix-produced radionuclides of W and Re was achieved. U was determined via ¹⁴⁰La, the daughter nuclide of the fission product ¹⁴⁰Ba. Limits of detection of 2 ng/g for U and 0.02 ng/g for Th can be achieved. For the other elements, the limits of detection are between 0.004 ng/g (Sc) and 200 ng/g (Sr). The elements Hf, Ta and Sb could be determined by instrumental neutron activation analysis. This method was applied to the analysis of two tungsten powder samples of different purity grade. The results and limits of detection are compared with those of other methods. Received: 28 July 1997 / Revised: 29 October 1997 / Accepted: 1 November 1997     

Determination of metal-cofactors in respiratory chain complexes by total-reflection X-ray fluorescence spectrometry (TXRF)

Total Reflection X-Ray Fluorescence Spectrometry (TXRF) offers many advantages for the detection of trace elements in enzymes as compared to other well known analytical techniques like flame-AAS or ICP-AES because of the significantly smaller amounts of sample required. Without any decomposition, elements like Fe, Ni, Cu, Zn, Mn and Mo could be determined with high accuracy, in spite of the large bio-organic matrix. Besides the metals also sulfur can be determined in protein samples. The two terminal oxidases, cytochrome c oxidase and quinol oxidase, isolated from the soil bacterium Paracoccus denitrificans, were transferred from their usual salt buffer into a solution of 100 mmol/L tris(hydroxymethyl)aminomethane (TRIS) acetate containing an appropriate detergent. By this procedure an improved signal/noise ratio is attained. The data for cytochrome c oxidase are in good agreement with values obtained by ICP-AES. Further results of quinol oxidase, which has different element ratios, also fit the expected values. The investigations lead to the conclusion that the method is well suited for the quantitative determination of metals in enzymes, and in particular their molar ratios, and requires only small amounts of the biological sample without any extensive pretreatment. Received: 17 June 1997 / Revised: 21 November 1997 / Accepted: 27 November 1997     

Investigations on the applicability of two ELISA types for the determination of chlorpyrifos in soil samples compared with a gas chromatographic method

Two commercially available ELISA kits for water analysis, a microtiter plate-ELISA based on polyclonal antibodies (p-ELISA) and a magnetic particle tube-ELISA based on monoclonal antibodies (t-ELISA), were used to determine chlorpyrifos residues in soils. Comparison with a gas chromatographic method frequently applied was carried out by fortification experiments and by analyses of real soil samples. At concentration levels of 1.0, 0.1, 0.05 and 0.01 mg/kg, chlorpyrifos was reliably determined by the GC method. Application of the p-ELISA did not permit a reliable quantitation, while the t-ELISA was applicable in a concentration range of 0.05–1.0 mg chlorpyrifos/kg dry soil. Received: 23 July 1997 / Revised: 10 November 1997 / Accepted: 14 November 1997     

Limits of detection, identification and determination: a statistical approach for practitioners

 For the estimation of the limits of detection, identification and determination, considerations from analytical practice were applied to the statistics of the calibration line and its prediction interval. The detection limit was the concentration calculated from the maximum height of the prediction interval at zero spiking concentration. The identification limit was twice the detection limit and was the lowest concentration that could safely be detected. The determination limit was the lowest concentration fulfilling three criteria: 1. None of the signals resulting from determination limit concentration should interfere with any signal from detection limit concentration, thus providing an unambiguous separation between the two limits. 2. Recovery should be between 70% and 120%. 3. Lowest and highest predictable signal at determination limit concentration should not deviate more than ± 30% from the average. Practical analytical guidance and the necessary mathematical formulae are presented. Received: 12 November 1997 · Accepted: 7 December 1997     

Multielement trace determination in SiC powders: assessment of interlaboratory comparisons aimed at the validation and standardization of analytical procedures with direct solid sampling based on ETV ICP OES and DC arc OES

The members of the committee NMP 264 “Chemical analysis of non-oxidic raw and basic materials” of the German Standards Institute (DIN) have organized two interlaboratory comparisons for multielement determination of trace elements in silicon carbide (SiC) powders via direct solid sampling methods. One of the interlaboratory comparisons was based on the application of inductively coupled plasma optical emission spectrometry with electrothermal vaporization (ETV ICP OES), and the other on the application of optical emission spectrometry with direct current arc (DC arc OES). The interlaboratory comparisons were organized and performed in the framework of the development of two standards related to “the determination of mass fractions of metallic impurities in powders and grain sizes of ceramic raw and basic materials” by both methods. SiC powders were used as typical examples of this category of material. The aim of the interlaboratory comparisons was to determine the repeatability and reproducibility of both analytical methods to be standardized. This was an important contribution to the practical applicability of both draft standards. Eight laboratories participated in the interlaboratory comparison with ETV ICP OES and nine in the interlaboratory comparison with DC arc OES. Ten analytes were investigated by ETV ICP OES and eleven by DC arc OES. Six different SiC powders were used for the calibration. The mass fractions of their relevant trace elements were determined after wet chemical digestion. All participants followed the analytical requirements described in the draft standards. In the calculation process, three of the calibration materials were used successively as analytical samples. This was managed in the following manner: the material that had just been used as the analytical sample was excluded from the calibration, so the five other materials were used to establish the calibration plot. The results from the interlaboratory comparisons were summarized and used to determine the repeatability and the reproducibility (expressed as standard deviations) of both methods. The calculation was carried out according to the related standard. The results are specified and discussed in this paper, as are the optimized analytical conditions determined and used by the authors of this paper. For both methods, the repeatability relative standard deviations were <25%, usually ~10%, and the reproducibility relative standard deviations were <35%, usually ~15%. These results were regarded as satifactory for both methods intended for rapid analysis of materials for which decomposition is difficult and time-consuming. Also described are some results from an interlaboratory comparison used to certify one of the materials that had been previously used for validation in both interlaboratory comparisons. Thirty laboratories (from eight countries) participated in this interlaboratory comparison for certification. As examples, accepted results are shown from laboratories that used ETV ICP OES or DC arc OES and had performed calibrations by using solutions or oxides, respectively. The certified mass fractions of the certified reference materials were also compared with the mass fractions determined in the interlaboratory comparisons performed within the framework of method standardization. Good agreement was found for most of the analytes.     

TUNA FISH (T-30) – A new proficiency testing material for the determination of As and Hg in seafood

The quality of the aquatic and marine environment can be monitored by the determination of pollutants in organisms living in this environment. Certified reference materials and well-organised proficiency tests are powerful means of ensuring a constant level of quality and verifying the correct application of standardised methods. The preparation of a tuna fish proficiency testing material for the evaluation of quality of As and Hg monitoring in seafood is described. Preparation and characterisation of the material as well as studies on its homogeneity and stability are described. Concentrations of 3.4 ± 0.2 mg/kg total arsenic and 2.91 ± 0.09 mg/kg total mercury have been determined as target values. Moreover indicative values for some trace elements (Cd, Cu, Ni, Pb, Sr) and some major constituents (Al, Br, C, Ca, Cl, Fe, H, K, Mg, N, Na, P, S, Si, Zn) have also been measured. Received: 29 November 1996 / Revised: 17 February 1997 / Accepted: 22 February 1997     

Direct-reading d.c. arc spectrometry for rapid geochemical surveys

A technique developed for the rapid quantitative analyses of metals in large suites of silicate materials is described. A direct-reading emission spectrometer interfaced to a dedicated minicomputer is used with d.c. arc excitation. Sample, buffer and arc parameters were chosen to promote reproducible selective volatilization. By partial integration of the line emission for the volatile metals, the spectrum background contribution is reduced, improving line to background ratios and permitting both “volatile” and “nonvolatile” metals to be determined in one burn.Preformed electrode cups are used in a specially designed arc chamber which incorporates a gas jet for controlled atmosphere and arc stabilization, and permits automatic electrode alignment as well as rapid and efficient cleaning between samples. No adjustment of the electrodes is made during the burn.One standard only is required for calibration since the working curves are linear over the concentration ranges of interest and a method of background correction using this same standard is described.A high rate of analysis as well as good accuracy and precision is achieved for 21 metals.     

Determination of copper, lead, cadmium, zinc, and iron in calcium fluoride and other fluoride-containing samples by means of direct solid sampling GF-AAS

Direct solid sampling graphite furnace AAS (SS-GF-AAS) provides an advantageous alternative to the conventional AAS which requires wet digestion of the samples. This method is suitable for trace element determination in calcium fluoride and other fluoride-containing samples. Matrix effects were studied by using calibration standards, certified reference materials with different matrices and by means of three-dimensional calibration. 3D calibration is suitable for selection of calibration samples for more reliable analyses. Cu, Pb, Cd, Zn, and Fe were determined by means of calibration with variation of sample weight using certified reference materials and suitable fluoride-containing calibration samples. Received: 25 July 1997 / Revised: 15 December 1997 / Accepted: 20 December 1997     

Determination of copper, lead, cadmium, zinc, and iron in calcium fluoride and other fluoride-containing samples by means of direct solid sampling GF-AAS

Direct solid sampling graphite furnace AAS (SS-GF-AAS) provides an advantageous alternative to the conventional AAS which requires wet digestion of the samples. This method is suitable for trace element determination in calcium fluoride and other fluoride-containing samples. Matrix effects were studied by using calibration standards, certified reference materials with different matrices and by means of three-dimensional calibration. 3D calibration is suitable for selection of calibration samples for more reliable analyses. Cu, Pb, Cd, Zn, and Fe were determined by means of calibration with variation of sample weight using certified reference materials and suitable fluoride-containing calibration samples. Received: 25 July 1997 / Revised: 15 December 1997 / Accepted: 20 December 1997     

Experimental studies for the characterization of analytical performance in axially-observed inductively coupled plasma atomic emission spectrometry

Investigations were carried out on the optimization of excitation and projection conditions of the axially-observed inductively coupled plasma (ICP) concerning simultaneous measurements. For minimized background equivalent concentration (BEC) it can be shown that the optimal excitation conditions of the atomic and ionic lines also vary in the case of axial plasma observation. A relationship was confirmed between the high frequency power and the excitation energy of the analytical lines. The main contribution of this work is the displacement of the axial viewing inductively coupled plasma by means of an x,y,z sliding carriage.The displacement showed that under the selected experimental conditions the point of observation is spatially identical for all analytical lines. Received: 30 May 1997 / Revised: 20 November 1997 / Accepted: 25 November 1997     

Experimental studies for the characterization of analytical performance in axially-observed inductively coupled plasma atomic emission spectrometry

Investigations were carried out on the optimization of excitation and projection conditions of the axially-observed inductively coupled plasma (ICP) concerning simultaneous measurements. For minimized background equivalent concentration (BEC) it can be shown that the optimal excitation conditions of the atomic and ionic lines also vary in the case of axial plasma observation. A relationship was confirmed between the high frequency power and the excitation energy of the analytical lines. The main contribution of this work is the displacement of the axial viewing inductively coupled plasma by means of an x,y,z sliding carriage.The displacement showed that under the selected experimental conditions the point of observation is spatially identical for all analytical lines. Received: 30 May 1997 / Revised: 20 November 1997 / Accepted: 25 November 1997     

A simple method of target preparation for the bulk analysis of powder samples by laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS)

A simple and rapid procedure using a glue technique has been developed for the preparation of stable targets from powder samples for bulk analysis by LA– ICP–MS. The procedure was evaluated for the analysis of trace elements in SiC, of rare-earth elements in different types of silicate (rocks, sediments, and soils), and of Au and platinum-group elements in geological silicates. The test analysis was conducted using an IR laser in combination with a quadrupole mass spectrometer. The recommended preparation procedure offers the possibility of different types of calibration, for example application of certified reference samples in combination with prepared spiked samples on a base of a natural or synthetic matrix, or addition calibration. The resulting calibration functions are linear over a range of several decades. The trueness of the results was evaluated by use of certified reference samples. Analytical concentration ranges, detection limits, and the relative standard deviations are reported. Received: 20 November 2000 / Revised: 13 March 2001 / Accepted: 17 March 2001     

Capabilities of double focussing magnetic sector-ICP-MS for the determination of trace elements in body fluids (blood, blood serum, urine) at the example of control materials

The capability of double focussing magnetic sector inductively coupled plasma mass spectrometry (HR-ICP-MS) for the determination of trace elements in different body fluids was investigated in commercially available control materials. The elements included in this study were: Al, As, Cd, Cr, Co, Cu, Hg, Mn, Ni, Pb, Sb, Se, Tl and Zn. Sample preparation, instrument stability and the results, which are in very good agreement to the given values, are presented and discussed. Received: 27 August 1997 / Revised: 19 November 1997 / Accepted: 23 November 1997     

Capabilities of double focussing magnetic sector-ICP-MS for the determination of trace elements in body fluids (blood, blood serum, urine) at the example of control materials

The capability of double focussing magnetic sector inductively coupled plasma mass spectrometry (HR-ICP-MS) for the determination of trace elements in different body fluids was investigated in commercially available control materials. The elements included in this study were: Al, As, Cd, Cr, Co, Cu, Hg, Mn, Ni, Pb, Sb, Se, Tl and Zn. Sample preparation, instrument stability and the results, which are in very good agreement to the given values, are presented and discussed. Received: 27 August 1997 / Revised: 19 November 1997 / Accepted: 23 November 1997     

A comparative study of laser induced breakdown spectroscopy analysis for element concentrations in aluminum alloy using artificial neural networks and calibration methods

A comparative study of analysis methods (traditional calibration method and artificial neural networks (ANN) prediction method) for laser induced breakdown spectroscopy (LIBS) data of different Al alloy samples was performed. In the calibration method, the intensity of the analyte lines obtained from different samples are plotted against their concentration to form calibration curves for different elements from which the concentrations of unknown elements were deduced by comparing its LIBS signal with the calibration curves. Using ANN, an artificial neural network model is trained with a set of input data of known composition samples. The trained neural network is then used to predict the elemental concentration from the test spectra. The present results reveal that artificial neural networks are capable of predicting values better than traditional method in most cases.     

Calibration of titration methods

Summary A calibration method is proposed which makes it possible to use titration techniques in the presence of systematic errors, even if these errors depend on the concentrations of the analytes. The approach uses an empirical calibration model which approximates the relationship between apparent (found) and true concentrations of the analytes. Also a calibration of the physical model of the titration process is proposed, as well as a method of determination of the model parameters, which are useful when analytes are determined by fitting the model to experimental data of titration. Both approaches, empirical and based on the physical model, may be applied jointly. The example presented reveals high efficiency of the proposed approach in cases when a deficient physicochemical model of the titration process is used in the determination of an analyte concentration (simulated titration data applied). The calibration proposed may be considered as a generalization of the titrant standardization used in the conventional volumetric analysis. It may be applied to all titration techniques and for all methods of end-point detection and determination of the concentration of analytes. It opens new possibilities for the development of titration methods. Permanent address: Department of Analytical Chemistry, Jagiellonian University, Karasia 3, PL-30-060 Krakow, Poland     

Efficiency of decomposition procedures for the determination of some elements in soils by atomic spectroscopic methods

Nine decomposition procedures for soil samples, such as via acid mixtures and fusion in open systems and microwave and autoclave dissolution (in closed systems under elevated temperature and pressure) were assessed using certified soil reference materials. The determination of various elements in solutions was performed by FAAS and ICP-OES and the results were compared with certified values and with direct current arc OES analysis. Received: 3 March 1997 / Revised: 30 May 1997 / Accepted: 3 June 1997     

Classical and modern algorithms for the evaluation of data from sensor-arrays

Sensors have found wide application in process control, environmental analysis, and other analytical problems in recent years. Optical sensor arrays can be used to monitor organic solvent vapour mixtures by use of reflectometric interference spectroscopy. Lack in selectivity of the sensitive polymer films requires multivariate algorithms for evaluation. Two major aspects are of interest: the random error of calibration and the interpretation of the influence of a single sensor in an array with redundant information. Due to the partial selectivity of the different sensitive layers, non-linearities, cross-sensitivities, and differences in sensitivity, the selection of the most suitable sensitive polymer layers is not trivial. Model based algorithms allow the interpretation of variables whereas the model free algorithms provide better results concerning the random error of calibration. We choose the pruning algorithm to optimize a neural network topology in order to obtain the qualitative information on the sensor elements from the remaining links between the input layer and the hidden layer. We compare these results to the ones obtained for linear and non-linear PLS1 by partial least squares (PLS1) and calculate the errors for the calibration. Received: 3 December 1996 / Revised: 27 February 1997 / Accepted: 4 March 1997     

A calibration algorithm for atomic absorption

The equation

is shown to be applicable in fitting the analytically useful concentration range of atomic absorption calibration curves. In this equation C is the concentration, A the observed absorbance, and the K's are coefficients which must be determined during calibration. Typical calibration data for four diverse elements (Ag, Bi, Co and Cu) determined in an air-acetylene flame are shown and rules for choosing the standards to be used are discussed.