Hugo M. Ortner

Classical analysis including trace-matrix separation versus solid state mass spectrometry: A comparative study for the analysis of high purity Mo, W and Cr

The applicability of GDMS, SIMS, SSMS, NAA and TMS with AAS, ICP-OES and ICP-MS end determination for routine bulk ultratrace analysis of high purity refractory metals was investigated. Due to the heterogeneous distribution of trace elements in the sub-ppm range, sample consumption and analysis time have a tremendous influence on quantification with procedures of low sample consumption. As an example, GDMS, which is commonly used for ultrapure material certification by most of the manufacturers in Europe and the USA, exhibits discrepancies by more than one order of magnitude for repetitive analyses of a series of trace components in the same sample. Furthermore, results of different laboratories using the same instrument are frequently not comparable. Due to easy standardization and large sample consumption TMS procedures combined with FAAS, GFAAS, ICP-AES and ICP-MS as methods of end determination exhibit better precision and accuracy than GDMS and SIMS. Detection limits are comparably low or even better in case of ICP-MS end determination. TMS procedures are less expensive and less time consuming than highly sophisticated analytical techniques like GDMS, SIMS or NAA. Additionally, they can be easily applied by experienced personnel in a well equipped industrial analytical laboratory.

Mechanisms of the hydrogen reduction of molybdenum oxides

Abstract The two stages of the hydrogen reduction of MoO 3 to Mo were investigated in a thermal balance under well defined reaction conditions. Starting with different grain and agglomerate sizes for both stages, the influence of a set of parameters (temperature, local partial pressure of H 2 O, gas flow, etc.) on the reaction progress and the final result were studied in detail. Depending on the set of parameters used, different reaction mechanisms like pseudomorphic transformation or chemical vapour transport (CVT) were observed. Taking into account that grains and agglomerates deviate from a spherical shape and a definite grain size, the extent of reaction is well described by standard theoretical gas–solid reaction models such as the shrinking core model or the crackling core model (CCM). Thermo-gravimetric analysis, X-ray diffraction, scanning electron microscopy, surface area measurements (BET method) and laser diffraction were used for these studies. Under all conditions, the first stage shows a reaction path MoO 3 →Mo 4 O 11 →MoO 2 via CVT. The reaction extent follows the CCM. Depending on the local partial pressure of H 2 O during reduction, the formed Mo 4 O 11 and MoO 2 exhibit different size distributions and shapes of the grains. The extent of reaction of the second stage develops according to the shrinking core model. Depending on the local dew point, two different reaction paths can occur: Pseudomorphic transformation at low dew points and transformation via CVT at high dew points. This paper is an extract from the Ph.D. thesis of W.V. Schulmeyer “Mechanismen der Wasserstoffreduktion von Molybdanoxiden”, 1998, Darmstadt University of Technology, Institute of Material Science, Department of Chemical Analytics, FRG. It focuses on a phenomenological description of the most important results.

Modifiers and coatings in graphite furnace atomic absorption spectrometry—mechanisms of action (A tutorial review)

Abstract A multitude of different and often contradictory mechanisms for the effects of modifiers and coatings have been proposed. Many of these proposals lack sufficient experimental evidence. Therefore, a series of statements based on our own investigations is given as ‘facts’. Another series of statements is made as ‘fictions’ related to erroneous proposals on the functioning of modifiers and coatings in the pertinent literature. Two basic concepts are developed for the sequence of processes leading to analyte stabilization for the two most important groups of modifiers: refractory carbide forming elements of the IVa–VIa groups of the periodic system on the one hand and Pt-group metals on the other hand. These concepts are based on the main reactions of graphite with elements and compounds: carbide formation and intercalation. Most important experimental results leading to this understanding are described: Penetration measurements for modifiers and analytes indicated the subsurface zone down to approximately 10 μm as the essential place for graphite–analyte–modifier interactions. The reason for this phenomenon is an open porosity of the pyrocarbon coating of 5–10% (v/v) into which liquids penetrate upon sample application. This also indicates that modifiers are best applied by impregnation or electrolysis whereas dense coatings are not advantageous. It is also shown that graphite tube assemblies are dynamic systems with a limited lifetime and carbon losses are an essential feature of tube corrosion. Most frequently found erroneous statements are discussed: (a) Particles on the tube surface are responsible for analyte stabilization and retention during pyrolysis. (b) Analyte stabilization is taking place by formation of intermetallic compounds or thermally stable alloys. (c) Experiments are performed with unrealistic concentrations of analytes and/or modifiers. (d) Dense coatings are advantageous. Finally, a functional schedule is given for the three steps of graphite furnace atomic absorption spectrometry (GFAAS): sample application and drying; pyrolysis; atomization. Contrary to the vast amount of literature on this topic it tried to provide the analyst working with GFAAS and in an increasing number working with Solid Sampling-GFAAS with a set of most important statements. This might spare the experimentalist a lot of useless optimization procedures but should lead him to a basic understanding of the complex phenomena taking place in his instrument and during his analytical work.

Speciation of iron in atmospheric aerosol samples

Abstract The influence of iron in the atmosphere is manifold and a function of its concentration, chemical environment, and solubility. Several analytical methods were applied for the characterization of iron in aerosol samples: for the analysis of solid aerosol samples, instrumental neutron activation analysis, energy- and wavelength-dispersive X-ray fluorescence, Mossbauer spectrometry and electron probe microanalysis were used. For the analysis of the digestion or leaching solutions, total-reflection X-ray fluorescence, atomic absorption spectrometry with flame or graphite furnace atomization and ion chromatography were applied. The bulk iron content of some urban aerosol sample was determined to be about 7% w/w, predominantly occurring as oxides with goethite as the major phase. The major fraction of the investigated aerosol originates from anthropogenic sources. Only 2% of the total iron content is soluble in an aqueous phase.

Investigation of the wear mechanism of cubic boron nitride tools used for the machining of compacted graphite iron and grey cast iron

Various experiments were performed to investigate the wear mechanism of cubic boron nitride (cBN) tools used for the machining of compacted graphite iron (CGI). Comparative studies for tools used to machine grey cast iron (CI) were also performed in order to find out why in this case the tool lifetime is significantly higher. Two main effects were found that are responsible for tool wear, namely: (1) oxidation of the tool, and (2) interdiffusion of constituting elements between tool and CGI. These wear mechanisms are more or less the same for the machining of CGI and grey CI. The difference in tool lifetime can be explained by the formation of a MnS layer on the tool surface in the case of grey CI. This layer is missing in the case of CGI. The MnS layer acts as a lubricant and as a diffusion barrier and is the reason for the reduced wear in the case of grey CI.

Intercalation compounds of graphite in atomic absorption spectrometry

The significance of the formation of intercalation compounds of graphite in the atomization process for atomic absorption spectrometric measurements was investigated. The inspection of the graphite surface and subsurface regions was performed by electron probe microanalysis with energy dispersive X-ray spectroscopic (EDX) detection. It was found that chloride forms thermally stable graphite intercalation compounds (GIC) with the graphite of tubes, which was either non-modified or modified with noble metals.

Corrosion of transversely heated graphite tubes by mineral acids

Abstract Corrosive changes of transversely heated graphite atomizer (THGA)-tube and platform surfaces were studied by scanning electron microscopy in combination with tube lifetime measurements under recommended conditions for vanadium determination. This was done for the four mineral acid matrices HNO3, HF, HCl and HClO4. Rising corrosion and reduced tube lifetime are observed for these matrices in the sequence HNO3

Scanning electron microscopy studies on surfaces from electrothermal atomic absorption spectrometry. III: the lanthanum modifier and the determination of phosphorus

Abstract Morphological studies on graphite surfaces by scanning electron microscopy are presented for platforms made from pyrolytic graphite, and for polycrystalline electrographite tubes with pyrolytic graphite coating in which phosphorus was determined without and with the addition of higher concentrations of lanthanum as the modifier. Lanthanum causes severe pitting and corrosion of the graphite surface already after relatively few determinations, and definite indication was found for the formation of intercalation compounds between lanthanum and graphite. No sign was found, however, for the formation of a dense coating of lanthanum carbide as proposed by several authors. The mechanism for the increase of phosphorus sensitivity is most probably the formation of a thermally stable compound involving lanthanum and phosphorus which leads to vaporization of phosphorus at high enough temperatures to obtain sufficient atomization and useful analytical signals. This is supported by the morphological changes of the graphite surface observed after application of higher lanthanum concentrations, and the resulting increased number of active carbon sites. Phosphorus alone also causes substantial corrosion of graphite, but with a completely different pattern. A very pronounced secondary coating of tube and platform wall is observed in the absence of lanthanum which is most probably supported by the formation and decomposition of compounds between phosphorus and graphite.

Characterization of individual aerosol particles in workroom air of aluminium smelter potrooms

Aerosol particles with aerodynamic diameters between 0.18 and 10 microm were collected in the workroom air of two aluminium smelter potrooms with different production processes (Soderberg and Prebake processes). Size, morphology and chemical composition of more than 2000 individual particles were determined by high resolution scanning electron microscopy and energy-dispersive X-ray microanalysis. Based on chemical composition and morphology, particles were classified into different groups. Particle groups with a relative abundance above 1%(by number) include aluminium oxides, cryolite, aluminium oxides-cryolite mixtures, soot, silicates and sea salt. In both production halls, mixtures of aluminium oxides and cryolite are the dominant particle group. Many particles have fluoride-containing surface coatings or show agglomerations of nanometer-sized fluoride-containing particles on their surface. The phase composition of approximately 100 particles was studied by transmission electron microscopy. According to selected area electron diffraction, sodium beta-alumina (NaAl(11)O(17)) is the dominant aluminium oxide and cryolite (Na(3)AlF(6)) the only sodium aluminium fluoride present. Implications of our findings for assessment of adverse health effects are discussed.

LIMITS OF DETECTION IN MULTIVARIATE CALIBRATION

SummaryMultivariate calibration and prediction is applied to signal data measured for selected multicomponent systems using inductively coupled plasma—optical emission spectrometry (ICP-OES). A multivariate definition of the limit of detection is derived from the error propagation theory. No limiting assumptions of constant signal errors are made for derivation. The definition is successfully applied to estimate realistic limits of detection for the investigated systems. Contributions corresponding to the calibration error were included for calculation. For efficient application realistic error models are introduced to estimate the required errors of the signals without replicate measurements.