Applications of Electron Microbeam Analysis in the Earth Sciences

 Naturally formed geological materials are not constrained in the number of elements which may be present and a wide range of concentrations can be expected. Oxides are a frequent component and water may be present. The particular problems posed for quantitative analysis in some representative applications are described and solutions suggested. Critical testing of the results for consistency with mineral formulae and for repeatability provides confidence in those results. Examples of applications of quantitative analysis of minerals are used to show the variety of problems encountered in electron microbeam analysis and how the analytical results can be used to determine the conditions (time, pressure, temperature) of the minerals or the rocks in which they occur.     

Application of Microbeam Techniques in the Steel Industry

 Steel is a very complex material, and nowadays all types of microbeam techniques are used to characterise it. This paper describes how and why microbeam techniques are used in the steel industry, with special emphasis on micro-analysis of precipitates and surface analysis.     

Application of Microbeam Techniques to Materials Problemsin a Service Laboratory

 Examples for providing analytical services with microbeam techniques are elucidated in the areas of failure analysis, chemical thin film analysis in glass coating, manufacture control of optical coatings, characterization of microelectronic devices and tribostressed surfaces in diesel motors of passenger cars. Besides the specific analytic or material problems and the techniques and methods used the characteristic aspects of surface and thin film analysis within the scope of analytical services are emphasized.     

Nanometer-Scale Chemical Surface Analysis by Scanning (Tunnelling) Atom Probes

 An overview is given of the developments in the area of chemical surface analysis on the nanometer scale by ion mass spectrometry. Two main types of instrumentation are described. In one (the ‘scanning atom probe’, SAP), an extraction electrode with a small aperture ( < 10 μm) is scanned at a short distance ( < 10 μm) from a rough surface. Field evaporation of atoms or small clusters from natural or artificial microtips at the surface and time-of-flight (ToF) analysis reveal the surface composition. In the other type (the ‘scanning tunnelling atom probe’, STAP), atoms or small clusters are transferred from a surface of a specimen to an STM tip. Subsequent field desorption from the tip and ToF analysis reveal the transferred particle’s identity. First applications of constructed S(T)AP instruments show the capabilities and limitations of this new technique. Various designs and applications are described and factors that steer maturation and further developments are discussed. These factors are either intrinsic to the technique (e.g., surface diffusion, field evaporation, material transfer, etc.) or related to general developments in the science of small objects or thin films. The final goal is chemical analysis of single atoms or molecules from specific atomic surface sites.     

Trace Analysis by Heavy Ion Induced X-Ray Emission

 Various K-, L- and M-shell X-ray production cross sections are measured for heavy ion impact on elements in the range Z ₂ = 13 to 83. The ion species range from Z ₁ = 10 to 36, and ion energies from 1 to 16 MeV are used. Enhanced cross sections are observed when the projectile K- or L- binding energy is similar to the energy of the target K-, L- or M-shell. This effect is used to improve the analysis sensitivity for selected elements. As an example trace analysis of Fe in glass with V, Mn, Co and Ni ions is investigated. Results are compared with proton induced X-ray emission analysis on the same samples. In these samples Fe-K_α X-ray production is similar for irradiation with 3 MeV protons and 14 MeV Ni ions. However the signal to background ratio is four times higher for the irradiation with Ni ions as compared to irradiation with protons. Advantages and drawbacks of heavy ion induced X-ray emission for quantitative analysis compared to proton induced X-ray emission analysis are discussed.     

Quantitative Trace Analysis by Wavelength-Dispersive EPMA

 “Trace” elements may be defined as elements whose concentrations are of a similar order to the detection limit. In WD analysis the detection limit is a function of the ‘figure of merit’ P²/B, where P is the pure-element peak intensity and B the background intensity. With normal analysis conditions detection limits of ∼100 ppm are typical, but substantial improvements can be achieved by using higher values of accelerating voltage and beam current. Long counting times are also advantageous, but should preferably be divided into relatively short alternating peak and background measurements to minimise the effect of instrumental drift. Using separate routines for trace and major element analysis is desirable owing to their different requirements. As the statistically defined detection limit is reduced, errors due to background nonlinearity and interferences (overlaps) from other elemental peaks become more probable. Spectrum simulation is useful for optimising background offsets and choice of crystal to minimise interferences, and estimating interference corrections when these are necessary. ‘Blank’ standards containing none of the trace elements of interest are also useful for quantifying background nonlinearity.     

Determination of Heavy Metals in Soils, Sediments and Geological Materials by ICP-AES and ICP-MS

 Sixteen soil and sedimentary geological reference materials were analysed for As and the heavy metals Cd, Co, Cr, Cu, Ni, Pb and Zn by inductively coupled plasma-atomic emission spectrometry (ICP-AES) and inductively coupled plasma-mass spectrometry (ICP-MS) in combination with total and partial dissolution of the samples. It can be demonstrated that none of the modern ICP methods is completely free from analytical problems. This applies in particular when the concentrations are close to the detection limits (e.g. in ICP-AES) and is mainly due to the wide variation in the bulk composition of soils resulting in complex matrix effects (e.g. in ICP-MS). In order to determine the extent of soil pollution by heavy metals, both partial and total dissolution have to be performed prior to analysis.     

EDXTOOLS — Computer Programmes for the Determination of Critical EDX Spectrometer Parameters

 Quality assurance according to ISO or EN norms entails a periodical check of critical instrumental parameters. Not yet all commercial software purchased with instruments supports the related measurements and their fast evaluation. EDXTOOLS consist of a programme library which complements the existing software for electron excited energy dispersive X-ray spectrometry (EDS) in this respect. EDXTOOLS can be used to check the detection efficiency by the evaluation of the L/K intensity ratio in a copper or nickel spectrum or by the calculation of the thickness of absorbing detector layers from experiments, which can be performed on any scanning electron microscope. Moreover, measured spectra can be modified by the transmission curve of absorbing media to estimate their influence on the result of quantitative analysis. EDXTOOLS allow the determination of the signal to background ratio from an Fe-55 spectrum and the fitting of measured FWHM’s of K-lines to find the resolution curve . They are completed by the possibility of calculating EDX spectra for the K-lines of light elements and to compare the resulting spectra for different formulae, physical data tables, and detector parameters chosen for the calculation. EDXTOOLS are written in MATLAB?, a wide spread interpreter language. This has the advantage that the programmes are readable text files. A user can check each computational step and modify it. The installation of MATLAB? and its optimization toolbox is necessary to work with EDXTOOLS.     

Solid State Analysis with the New Leipzig High-Energy Ion Nanoprobe

 The high-energy ion nanoprobe LIPSION at the University of Leipzig has been operational since October 1998. The ultrastable single ended 3.5 MV SINLETRON^TM accelerator supplies the H⁺ or He⁺ ion beam. A magnetic scanning system moves the focused beam across the sample. At present, a resolution of 150 nm in the low current mode and 300 nm at 5 pA could be achieved. The UHV grade experimental chamber is equipped with electron-, energy dispersive X-ray-, and particle detectors. They can be used simultaneously to analyse the sample by means of PIXE (particle induced X-ray emission), RBS (Rutherford backscattering) and in the case of thin samples STIM (scanning transmission ion microscopy). A goniometer allows the application of channeling measurements in single crystals in combination with these methods. The detection limits depend on the elements to be analysed and range from (1000⋯1) μg/g relative and (1⋯0.01) pg absolute. The analysis is nondestructive, but the sample has to be vacuum resistant. Applications of the nanoprobe in the field of semiconductor research, biomedicine, and archaeology will be described.     

INA-X: An Advanced Instrument for Electron-Gas SNMS

 The design and the specifications of a prototype instrument for the electron gas version of secondary neutral mass spectrometry SNMS which will optionally enable measurements with X-ray induced photoelectron spectroscopy XPS, too, are described. By operating the SNMS plasma inside an ultra high vacuum vessel interfering signals from residual gas species are reduced to the level of the mass independent background. The influence of varying angular distributions of sputter-ejected neutrals at low ion energies for sample bombardment can be widely reduced by an oblique take-off for the postionized particles. First examples of SNMS-studies with the new system reveal a detection power well below 1 ppm. Analytically difficult elements as C or O become quantitatively detectable down to a level of several 10 ppm.     

Basics, Possibilities and Limitations of the Microscopic X-Ray Fluorescence Analysis

 Microscopic X-ray fluorescence (MXRF) analysis was used to investigate different samples: meteorites, Jasper, coated glas and, reference materials. The element distribution within sections of two different meteorites have been determined – one metal rich and one oxide rich. The metal rich showed a matrix of Fe with Ni-, Ti-, and Si-enriched regions. The oxide rich also showed a Fe rich matrix and regions with different concentrations of other elements. A reference sample with a flat and polished but systematically tilted surface was used to assure, that roughness of the sections of the meteorites has only negligible influence. Nondestructive investigations in Jasper with included Stromatolithes, which were fossilized more than 2 billion years ago, showed the Stromatolithes to have Fe as main element instead of Si in the Jasper matrix. The thickness of Yb-layers on glas was determined from the intensity of the Yb fluorescence peak. Calibration was done by using a sample without coating and a reference sample whose thickness of the layer was determined by XRD reflectometry. Futhermore it has been shown that materials can be analysed even if mounted in glas capillaries or covered by plastic foils. By using Mark capillaries the elements from S to U may be detected instead of Na to U while working in vacuum mode.     

Nano-HPLC of Oligosaccharides – Method Development and Optimization

 The separation of homologous maltodextrins using methyl-, ethyl- and n-butyl esters of aminobenzoic acid and n-heptyloxyaniline by means of reversed phase nano-HPLC is presented. Fused silica capillaries packed with four different stationary phases were tested, the separation of the derivatized sugars was optimized and the separation properties of these columns were tested on the basis of the van Deemter plots. UV/VIS detection was used (285 nm for the aminobenzoic esters and 275 nm for the aniline derivatives) in combination with on-line ESI triple quadrupole mass spectrometry and off-line MALDI-TOF mass spectrometry. The choice of the stationary phase had a tremendous influence on the separation. Optimal results were obtained with an encapsulated ODS stationary phase and n-butyl aminebenzoate as label for the oligosaccharides. Malto-oligosaccharides with a degree of polymerization of up to of 25 could be resolved to baseline. On-line coupling with ESI-MS yielded additional information as a result of induced fragmentation and increased sensitivity by monitoring precursor ion. Off-line mass detection with MALDI-TOF was performed on the separated derivatized saccharides collected from nano-HPLC runs. The MALDI-TOF mass spectra confirm the ESI-MS data. Received January 25, 2001. Revision April 28, 2001.     

Application of ICP-MS for Multielement Analysis in Small Sample Amoun ts of Pathological Thyroid Tissue

 As a result of the Chernobyl nuclear power plant accident in 1986, thyroid pathologies occurred among children in some regions of Belarus. Besides the irradiation of children’s thyroids by radioactive iodine and caesium nuclides, toxic elements from fallout are a direct risk to health. Inductively coupled plasma quadrupole-based mass spectrometry was used for multielement determination in small amounts (1–10 mg) of thyroid tissue samples obtained during thyroid operations in Minsk hospitals. The accuracy of the applied analytical technique for small biological sample amounts was checked using NIST Standard Reference Material oyster tissue (SRM 1566b) and by comparison with independent analytical methods, such as neutron activation analysis. Almost all essential elements as well as a number of toxic elements such as Cd, Pb, Hg, U etc. were determined in a multitude of human and animal thyroid tissues (up to 1 mg sample material) by quadrupole-based ICP-MS using micronebulization. In general, the thyroid tissue affected by pathology is characterized by higher calcium content. Some other elements, among them Fe, Mn, V, As, Cr, Ni, Pb, U, Ba, Sb, were also accumulated in such tissue. The results obtained will be used as initial material for further specific studies of the role of particular elements in thyroid pathology development. Received November 15, 1999. Revision March 15, 2000.     

Relative Cross Sections for L- and M-Shell Ionization by Electron Impact

 Results from measurements and calculations of relative L- and M-shell ionization cross sections by electron impact are presented. Measurements were performed for elements Te, Au and Bi on an electron microprobe with specimens consisting of extremely thin films of the studied element deposited on thin, self-supporting, carbon layers. The relative variation of the ionization cross section was obtained by counting the number of characteristic X-rays from the considered element and shell, for varying incident electron energies, from the ionization energy up to 40 keV. Measured data were corrected to account for the energy-dependent spread of the electron beam within the active film and for the ionization due to the electrons backscattered from the carbon layer, using Monte Carlo simulation. Cross sections were evaluated in the Born approximation using an optical-data model with numerically evaluated dipole photoelectric cross sections. Calculated ionization cross section were converted to vacancy production cross sections, which can be directly compared with our experimental data.     

Particle Analysis by SEM/EDXS and Specimen Damage

 Quantitative analysis of particles, especially environmental particles in the submicron region, is hampered mainly by radiation damage. It can already be observed for probe currents smaller than 1 nA and analysis times of 10 seconds. The main reason for radiation damage is the storage of thermal energy in the particle, until the melting temperature is reached, and the subsequent loss of one or more of the elements of the respective material, e.g. SO_x in K₂SO₄. The lower the melting point, the more severe the specimen damage. The amount of specimen damage can vary substantially even for particles of the same size and composition. Specimen damage on inorganic materials can be observed for energies as low as 1 keV. Thus radiation damage is the greatest obstacle to accurate quantitative analysis for particles in the submicron region, even for analysis times as low as those used in automated particle analysis by CCSEM/EDXS.     

Indirect Measurement of Biologically Important Compounds by Means of X-Ray Microanalysis

 The main objective of our work was to investigate the possibility and usefulness of indirect methods in X-ray microanalysis for the quantification of biologically important compounds. Metallothionein-like proteins (MT-like proteins) from kidney and liver, rich in sulfur were chosen as an indicator of heavy metal presence in cells and their environment. Tissues from goldfish (Carassius auratus gibelio) were sampled after short and prolonged periods of exposure to Co⁺², CrO₄ ⁻², Pb⁺², Cu⁺² and control treatment and prepared for histochemical staining for peroxidated thiolate groups. Commonly used –S–S– bonds dye (Nitro Red) was replaced with iodine atoms and they were quantified at L line by means of X-ray microanalysis combined with SEM. After fish treatments with heavy metal solutions changes in MT-like proteins and in I atom contents were expected. There was statistically significant decrease in MT-like proteins level in kidney after lead treatment (Pb/C = 0.62). In liver a statistically significant increase in MT-like proteins concentration was observed after chromium, cobalt and lead ions treatment in comparison to control animals. The following ratios were noted: 3.04 for Cr/C, 2.18 for Co/C and 2.10 for Pb/C. Our finding indicates that the method of indirect measurement of MT-like proteins in fish and other animal tissues is possible. The concentration of iodine atoms is above their detection level by EDS and their changes are possible to identify. During histochemical procedures it is worth taking into account sample preparation methods which might disturb the quality and quantity of the analysed material.     

Energy Dispersive X-Ray Spectrometry by Microcalorimetry for the SEM

 Analytical X-ray spectrometry for electron beam instruments has advanced significantly with the development of the microcalorimeter energy dispersive X-ray spectrometer (μcal EDS). The μcal EDS operates by measuring the temperature rise when a single photon is absorbed in a metal target. A cryoelectronic circuit with electrothermal feedback and a superconducting transition edge sensor serves as the thermometer. Spectral resolution approaching 4.5 eV for high energy photons (6000 eV) and 2 eV for low energy photons below 2000 eV has been demonstrated in energy dispersive operation across a photon energy range from 250 eV to 8 keV. Spectra of a variety of materials demonstrate the power of the μcal EDS to solve practical problems while operating on a scanning electron microscope platform.     

Identification of Composite Cement Hydration Products by Means of X-Ray Diffraction

 In this paper the effect of limestone, fly ash, slag and natural pozzolana on the cement hydration products is studied. Four composite cements containing limestone, natural pozzolana from the Milos Island, slag and fly ash have been produced by intergrinding clinker (85%), the above main constituent (15%) and gypsum. The grinding process was designed in order to produce cements of the same 28d compressive strength. The hydrated products, formed after 1–28 days, were studied by means of X-ray diffraction. Unhydrated calcium silicate compounds of clinker and hydration products such as C*H, C*S*H and ettringite are clearly observed. Although there is not significant differentiation among samples hydrated for the same period of time, modifications of calcium aluminate hydrates as well as sulfoaluminate hydrates, are indicated by the XRD patterns. In samples of limestone cement, monocarboaluminate is formed in the first 24 hours and is still present after 28 days.     

Quantitative Mass Spectrometry of Decarburisation and Denitridation of Cemented Carbonitrides During Sintering

 Quantitative quadrupole mass spectrometry of (Ti, W)C-based cemented carbonitrides was carried out in order to monitor the evolution of the gas species as a function of time and temperature during vacuum sintering. Solid standards and gas mixtures as well as precise flow control were employed for calibration. Upon integration of the outgassing rates the carbon and nitrogen evolutions and mass losses during sintering of raw hardmetal powder mixtures (WC, TiC,…) and cemented carbonitrides could be quantified. Outgassing occurs in the range of temperatures 490–1480 °C. Nitrogen outgassing of (Ti, W)(C, N) was greater than that of Ti(C, N) due to the presence of W which increases the nitrogen activity in the solid causing a higher nitrogen equilibrium pressure. TiN showed a CO (0.2 wt%) and N₂ (0.5 wt%) weight loss only in the presence of the binder phase. For powder mixtures, it was observed that the amount of CO, CO₂ and N₂ liberated during vacuum sintering increased with the addition of cobalt in comparison with the same powders without binder phase. Nitrogen containing cermet alloys showed a greater loss of carbon than (Ti, W)C-based hardmetals. A careful control of carbon content by doping seems to be necessary in such alloys to avoid η-phase formation. Received July 7, 2000. Revision December 4, 2000.     

Rapid Procedure to Determine Wear Metals in Lubricating Oils and the Analysis of Variance in the Evaluation of Sample Preparation Procedures

 A new sample treatment is proposed based on a partial wet digestion in closed reactors assisted by microwaves for wear metal determinations by flame atomic absorption spectrometry (FAAS). It is rapid, precise and sensitive and allows the use of inorganic standards. Problems concerning solubility have been overcome with mixed solvents. The repeatability, stability, calibration curve and limit of detection were established. Six samples, with different degree of wear were analyzed for Fe, Cu, Pb and Cr by FAAS using the procedure proposed and other sample treatment procedures from the literature. Pb and Cr were not found in all samples. The one-way analysis of variance for Fe applied to each sample confirms the need of an acid attack and the importance of HCl for the digestion. The results obtained for Cu led us to use a two-way analysis of variance for all the samples considering the preparation procedures and the samples as variation sources. It showed no significant differences for the preparation procedures used. Therefore, treatments implying the use of acids are recommended when several wear metals are analyzed. On the other hand, the differences between the simple dilution and the procedures, implying the dissolution of metallic particles is an index to predict an imminent engine failure. Received March 1, 2001. Revision July 10, 2001.