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Analytical Instrumentation
article
Measurement of Tungsten in Molybdenum using High-Resolution Continuum Source
author
Atomic Absorption Spectrometry (HR-CS AAS) Heike Gleisner, Alf Liebmann
Thomas Furche
Analytik Jena AG Konrad-Zuse-Strasse 1 07745 Jena, Germany Tel.: +49/3641/7770 Fax: +49/3641/779279
[email protected]
H.C. Starck Hermsdorf GmbH Robert-Friese-Strasse 4 07629 Hermsdorf Germany Tel.: +49/36601/922101 Fax: +49/36601/922111
Looking back over the last few years at the development of AAS, there appear to have been no outstanding technical and analytical innovations in this field. Changes have primarily involved accessories and software and serve to enhance automation and improve device handling. Firstly, the optimisation of software improves user-friendliness, and secondly, the steady growth in quality control and data security standards are addressed. Nevertheless, by virtue of its simple operability, rapid readiness for measurement, low operating costs and high degree of interference immunity, AAS is still well established in inorganic analytics today. The commercial introduction of HR-CS AAS technology marks a developmental leap opening up a whole new generation of atomic spectroscopy devices to the user. Introduction In High-Resolution Continuum Source AAS (HR-CS AAS), the element-specific radiation source (hollow cathode lamp) is replaced with just one continuum source – a short arc xenon lamp [1] emitting continuously across the entire spectral range from near vacuum UV through to infrared with very high radiation intensity. This makes all analysis lines of interest available at all times without reservation and without the technical restrictions implicit with element-specific HCLs. Analytic selectivity is achieved with a high-resolution double monochromator based on a prism and echelle grating [1, 3]. This concept is realised in practice through the Becker-Roß team at ISAS in Berlin in the first commercially available flame HR-CS AAS instrument from Analytik Jena – the contrAA 300 (Fig. 1). The following application described demonstrates the advantages and the information lead offered by this new technology.
based on classical line source AAS (LS AAS) however has its limitations. Measurement with flame AAS based on deuterium background correction is limited by spectral interference caused by molybdenum absorption lines within the effective slit width of 0.2 nm. Until now, the measurement itself was preceded by laborious sample preparation to minimise this interference. Once it has been cracked with hydrofluoric and nitric acids, the molybdenum matrix has to be shaken out by time-consuming and chemical-intensive repeated extraction with acetic acid / butyl acetate. Sample extraction takes around 4 hours and with a capacity of 5 parallel measurements, the daily sample throughput is low. Alternatively, in larger control laboratories, ICP-OES or ICPMS can be deployed for analysis. Although these analytic techniques save the time-consuming sample preparation, the procurement and operating costs are often unjustifiably high. Besides the high level of daily utilisation of the equipment required, they often also present a problem for smaller laboratories in respect of the more demanding evaluation procedures. Here High Resolution Continuum Source AAS technology presents a real cost-effective alternative. Cracking and calibration Approx. 2 g Mo powder was added to 10 ml HNO3 (65 % m/v) and 10 ml HF (40 % m/v) for cracking. After cracking, 1 ml (0.1 % m/v) CsCl was added to the solution and then filled with water to 100 ml. The cracked sample was measured directly with the contrAA?300 in a fuel gas rich nitrous oxide / acetylene flame using the SFS 6 (Segmented Flow Star) [4]. Acid-matched standards in the concentration range 10-350 mg/l W were used. The W main resonance line 255.01235 nm was used as the analysis wavelength. To compare analysis results, identical calibration and sample measurement was performed with the novAA? 400, a flame AAS instrument with deuterium background correction. Calibration curves and detection limits for LS AAS and HR-CS AAS as well as the relevant characteristics (SD, c0) are included in Table 1.
Results and discussion With the HR-CS AAS instrument contrAA 300, it is possible to investigate the spectral vicinity of the analysis line during performance of the method. There are two W absorption lines, the resonance line at 255.135 nm with maximum sensitivity and a further W absorption line at 255.039 nm around a factor of 10 times less sensitive, to be seen in the spectrum of the 200 mg/l calibration standard, which covers a range of 0.43 nm (Fig. 2). As
Fig. 2: Absorption spectrum of the 200 mg/l W calibration standard, spectral observation width 0.43 nm (W main resonance line 255.135 nm, W secondary line 255,039 nm)
both lines lie in detection range of the smallest selectable slit of 0.2 nm in the LS AAS instrument and have different absorption coefficients, poorer linearity results for LS AAS. In HR-CS AAS,
Fig. 1: Atomic absorption spectrometer contrAA 300 Measurement of tungsten in molybdenum using HR-CS AAS The measurement of tungsten in ultra-pure molybdenum is a quality control task encountered in the metalworking industry and medical technology. For example, pure molybdenum is used in the manufacture of wear resistant and highly temperature stable electrodes for glass smelting processes. Contamination of the Mo electrodes would be transferred to the glass smelt and would also reduce the lifespan of the electrodes. Another interesting application is the analysis of pure molybdenum powder used for bearing components, axles and rotating anodes in x-ray instruments. The measurement of W in Mo
Table 1: Calibration curve for W, detection limit and characteristics for LS AAS and HR-CS AAS
Fig. 3: Energy scan of the W-HCL (GLE) in novAA 400, spectral observation width 1.0 nm
Analytik Jena Article (P?&?)
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Analytical Instrumentation only the main resonance line is taken for analytical evaluation in this example. The second absorption line can be used if the concentration is exceeded in order to expand the dynamic working range for calibration. As a comparison, an energy scan over 1.0 nm around the analysis wavelength of the W-HCL used was performed in the classical LS AAS (Fig. 3). The two W lines can hardly be distinguished given the significantly lower resolution of LS AAS. It was also apparent that not every HCL can be deployed, as not all lamp manufacturers use UV transparent quartz windows for their W-HCLs., in which case one has to turn to the longer wavelength and somewhat less sensitive W absorption line at 294.4 nm. In the Mo sample spectrum (Fig. 4) there are three additional W absorption lines in the spectral vicinity of the 255.135 nm analysis line, which could be assigned to the molybdenum matrix. The 255.086 nm Mo line lies directly in the spectral transmission band of the slit in the LS AAS instrument with deuterium background correction and consequently further attenuates the D2 broadband source. This attenuation distorts the background measurement on the analysis line leading to over-correction of the background and hence to a lower analysis result (Table 2). For this reason, the Mo
Table 2: Results of W measurement with HR-CS AAS and LS AAS
Fig. 4: Absorption spectrum of the W sample Mo-HZ, spectral observation width 0.43 nm (W main resonance line 255.135 nm, W secondary line 255,039 nm, Mo lines: 255.017 nm; 255.086 nm; 255.287 nm)
Summary The W content values measured for molybdenum samples lie in the anticipated range and show good correspondence with the internal reference values. The performance of the method is simplified, because, as a result of the possibility of measuring the spectral vicinity of the analysis line simultaneously, significantly more information is available on the sample under investigation. Fluctuations in the radiant intensity of the lamp, in the detector sensitivity and in the permeability of the flame – and therefore all continuous background absorption – are measured simultaneously and are automatically corrected at selected reference pixels. Through the use of a high-resolution double monochromator, discontinuous spectral disturbances, e.g. through absorption lines from the Mo matrix, are not identified by the pixels used for analysis and are thereby eliminated. There is therefore no necessity for background correction as exists for LS AAS. The detection limit in HR-CS AAS is fundamentally improved, as no second lamp noise source is present in the optical system. The use of an extremely low noise CCD array detector in the contrAA 300 is also superior to the photomultipliers common in LS AAS and the use of a high-energy Xe short arc lamp with very high radiation intensity further improves the signal-to-noise ratio significantly. The detection limit for tungsten was consequently improved by a factor of 5. Finally, it can be seen that the measurement of tungsten in molybdenum using the contrAA 300 with HR-CS AAS can be performed easily, correctly and without laborious sample preparation. The minimisation of investment and operating costs produces in a significant increase in laboratory effectiveness and flexibility together with a noticeably higher sample throughput.
matrix must be separated using special sample preparation prior to measurement of W with LS AAS. The high-resolution double monochromator in the contrAA?300 separates the Mo absorption lines significantly from the W absorption line. As HR-CS AAS always identifies the background and the analysis line simultaneously and measures selected correction pixels, the measurement of tungsten using this method is not affected by the molybdenum absorption line.
Bibliography: [1] B. Welz, H. Becker-Ross, S. Florek, U. Heitmann, High-Resolution Continuum Source AAS – The better Way to do Atomic Absorption Spectrometry, ISBN 3-527-30736-2, Wiley-VCH, Weinheim, 2005 [2] H. Gleisner, K. Eichardt, G. Schlemmer, U. Heitmann: Die AAS wird neu definiert, LABO 4/ 2004, S.64-67 [3] U. Heitmann, H. Becker-Roß: AtomabsorptionsSpektrometrie mit einem Kontinuumstrahler (CS AAS), GIT 7/2001, S.728-731 [4] H. Gleisner: Applikationsberichte Analytik Jena, CSAA_FL_01_04_d | 11 / 2004
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