M. Maetz

Properties of Interplanetary Dust: Information from Collected Samples

The properties of hundreds of interplanetary particles have been determined by direct laboratory analysis of recovered samples. The particles that span the 1 μm to 1 mm size range have been collected from the stratosphere, from polar ice, and from deep sea sediments. Typically, these particles are black, somewhat porous and have chondritic elemental compositions. They are rather complex mineral assemblages in that they are mixtures of very large numbers of sub-micrometer-sized components. While the data are not totally representative of small interplanetary meteoroids at 1 AU they provide significant insight into the common physical properties of meteoroids. These properties can be used as guidelines for analysis of spacecraft and astronomical observations and for modeling solar system dust as well as some circumstellar dust in systems around other stars.

Element distribution in mycorrhizal and nonmycorrhizal roots of the halophyte Aster tripolium determined by proton induced X-ray emission

Summary.The salt aster (Aster tripolium L.) colonized by the arbuscular mycorrhizal fungus Glomus intraradices Sy167 and noncolonized control plants were grown in a greenhouse for nine months with regular fertilization by Hoagland nutrient solution supplemented with 2% NaCl. Mycorrhizal roots showed a high degree of mycorrhizal colonization of 60–70% and formed approximately 25% more dry weight and much less aerenchyma than the nonmycorrhizal controls. Cryosectioning essentially preserved the root cell structures and apparently did not cause significant ion movements within the roots during cuttings. The experimental conditions, however, did not allow to discriminate between fungal and plant structures within the roots. Quantification of proton-induced X-ray emission (PIXE) data revealed that in control roots, Na+ was mainly concentrated in the outer epidermal and exodermal cells, whereas the Cl− concentration was about the same in all cells of the roots. Cross sections of roots colonized by the mycorrhizal fungus did not show this Na1 gradient in the concentration from outside to inside but contained a much higher percentage of NaCl among the elements determined than the controls. PIXE images are also presented for the four other elements K, P, S, and Ca. Both in colonized and control roots, the concentration of potassium was high, probably for maintaining homoeostasis under salt stress. This is seemingly the first attempt to localize both Na+ and Cl− in a plant tissue by a biophysical method and also demonstrates the usefulness of PIXE analysis for such kind of investigation.

Micro-PIXE studies of elemental distribution in Cd-accumulating Brassica juncea L.

Abstract Brassica juncea L. is a high biomass producing crop plant, being able to accumulate Cd and other heavy metals in their roots and shoots. It is a good candidate for efficient phytoextraction of heavy metals – such as Cd – from polluted soils. PIXE and STIM analyses were applied to investigate Cd-uptake in roots and the resulting effects on the elemental distribution of Cd stressed plants. The axial distribution of trace elements as a function of distance from the root tip as well as the radial distribution within cross-sections were analysed. The results are compared with the elemental distribution in control plants.

The new Heidelberg proton microprobe: The success of a minimal concept

Abstract The new proton microprobe is characterized by modular mounting resulting in easy maintenance, low cost personal computer (PC) system and several new features. The most important of these being the design of the lens, its location inside the vacuum, and an elaborate system for beam monitoring. The PC system allows for all kinds of standard data acquisition, beam and target positioning and an automated and quick routine for beam profile analysis. The ionoptical properties fulfil the expectations, which is not true for the brightness delivered from a Penning ion source. Focusing to a submicron beam spot results therefore in a beam current of ≈1 pA only. Tuning to 100 pA enlarges the spot to 3 × 3 μm2.

Low-dose nuclear microscopy as a necessity for accurate quantitative microanalysis of biological samples

Abstract Some aspects of quantitative analysis of biological samples – including a review of the relevant literature – are discussed. Special emphasis is put on the accuracy of trace elemental results despite severe sample damage which occurs during analysis. The occuring mass loss can be corrected by combining quantitative STIM and PIXE. It is also possible to characterize the volatility of most of the minor and trace elements of the sample. The comparison between the presented model and matrix determination with RBS, a widely used method, definitely demonstrates that the concentrations obtained using RBS correction only may be too high by a factor of two. We recommend at least one low-dose analysis to do correct quantification.

SUBCELLULAR TRACE ELEMENT DISTRIBUTION IN GEOSIPHON PYRIFORME

Geosiphon pyriforme is a unique endosymbiotic consortium consisting of a soil dwelling fungus and the cyanobacterium Nostoc punctiforme. At present this symbiosis becomes very interesting because of its phylogenetic relationship to the arbuscular mycorrhizal (AM) fungi. Geosiphon pyriforme could be an important model system for these obligate symbiotic fungi, which supply 80–90% of all land plant species with nutrients, in particular phosphorous and trace elements. Combined PIXE and STIM analyses of the various compartments of Geosiphon give hints for the matter exchange between the symbiotic partners and their environment and the kind of nutrient storage and acquisition, in particular related to nitrogen fixation and metabolism. To determine the quality of our PIXE results we analysed several geological and biological standards over a time period of three years. This led to an overall precision of about 6% and an accuracy of 5–10% for nearly all detectable elements. In combination with the correction model for the occurring mass loss during the analyses this holds true even for biological targets.

High resolution PIXE analyses of interplanetary dust particles with the new Heidelberg proton microprobe

Abstract Besides meteorites and lunar material, interplanetary dust particles (IDPs) collected in the stratosphere represent another form of extraterrestrial matter available for studies in the laboratory. One major goal of IDP research activities is the search for an IDP classification scheme based on their chemical composition that is obtained by modern microanalytical methods, to ascertain the source and perhaps also the specific history of individual IDPs. Excitation of characteristic X-rays in a sample by 2–4 MeV protons (Proton-Induced X-ray Emission, PIXE) enables the non-destructive detection of elements with Z > 11 with high sensitivity. If a focused proton beam is provided as with a proton microprobe, measurements with a spatial resolution in the micrometer range are possible. This method turns out to be useful to study the chemical composition of individual IDPs with high spatial resolution and with sensitivities down to ppm levels for elements between Mg and Br. Between 1990 and 1992 in Heidelberg a new proton microprobe was developed and constructed. Here we will report some of its properties, especially those which are necessary to perform PIXE analyses of small particles. We will then present the first bulk and high resolution PIXE results on IDPs using the new machine.

Heavy metal uptake of Geosiphon pyriforme

Abstract Geosiphon pyriforme represents the only known endosymbiosis between a fungus, belonging to the arbuscular mycorrhizal (AM) fungi, and cyanobacteria (blue-green algae). Therefore we use Geosiphon as a model system for the widespread AM symbiosis and try to answer some basic questions regarding heavy metal uptake or resistance of AM fungi. We present quantitative micro-PIXE measurements of a set of heavy metals (Cu, Cd, Tl, Pb) taken up by Geosiphon-cells. The uptake is studied as a function of the metal concentration in the nutrient solution and of the time Geosiphon spent in the heavy metal enriched medium. The measured heavy metal concentrations range from several ppm to some hundred ppm. Also the influence of the heavy metal uptake on the nutrition transfer of other elements will be discussed.

Investigation of hydrolysed TaCl5-graphite intercalation compounds with PIXE

Graphite intercalation compounds combine low density with high electrical conductivity. These physical properties makes these compounds interesting as components of carbon epoxy composites, but the high reactivity of graphite intercalation compounds to moisture is an obstacle for practical applications.Earlier investigations showed that tantalum chloride forms with graphite flakes an intercalation compound, which is stable against humidity for a few days. We studied the stability against moisture of a stage-2 tantalum chloride intercalation compound after one year. A fresh tantalum chloride intercalation compound was exposed for 2 h in water and afterwards stored at room humidity for ca. 1 year. Some of the flakes were embedded in epoxy resin after a few months. After 1 year all flakes were highly altered. The non-embedded flakes showed coatings of tantalum oxide on the surface. The epoxy-resin-mounted flakes showed an irregular surface with many cracks. The edges of the flakes were sealed by epoxy resin, whereas the basal planes of the flakes are not coated. With particle-induced X-ray emission (PIXE) we performed Ta and Cl analyses across the flakes. The Cl∶Ta ratios varied over a wide range and, especially near the cracks, were very low. In other regions of the flake we found that tantalum oxychloride or a mixture of tantalum chloride and tantalum oxide had been formed. In a few regions we found evidence that tantalum chloride was encapsulated in the interior of the flakes.

Accurate quantification resulting from precise beam monitoring and calibration

Abstract PIXE investigations in Heidelberg utilize very different target types for which analogous standards are very rarely available. Standardless quantitative measurements are therefore the preferred method. This asks for precise beam monitoring and accurate calibration of the setup for the conversion from X-ray counts to concentrations. It will be shown that our beam monitor enables us to measure beam currents down to 1 pA with a precision of ≈ 0.5%. The computer code GUPIX which we use to quantify our results offers the possibility to use a calibration function H(E) rather than one constant only. H(E) has been measured, leading to an overall accuracy for quantification of 5% even for trace elements.