Elemental mapping of biological samples using a scanning proton microprobe

Elemental mapping using a scanning proton microprobe (SPM) can be a powerful technique for probing trace elements in biology, allowing complex interfaces to be studied in detail, identifying contamination and artefacts present in the specimen, and in certain circumstances obtaining indirect chemical information.Examples used to illustrate the advantages of the technique include the elemental mapping of growing pollen tubes, honey bee brain section, a mouse macrophage cell, human liver section exhibiting primary biliary cirrhosis, and the attack by a mildew fungus on a pea leaf.     

Recent advances with the scanning proton microprobe

The scanning proton microprobe has established itself as an essential tool for quantitative and trace elemental microscopical analysis and for nondestructive depth profiling, This paper discusses the major strengths and limitations of the instrument, the advances that have been made and the directions in which instrumental research is progressing to enable further improvements in probe performance. Recent applications in medical and in semi-conductor research are presented in order to illustrate the significance of these improvements.     

Development of a new sample chamber for proton microprobe analysis of mineral samples

We describe a newly developed proton microprobe sample chamber designed for analysis of heterogeneous mineral samples. The instrument features a computer-driven stage and is equipped with novel beam and sample optics, external microscope and filter exchange system. Under the employed optical geometry, axes of beam and sample optics are collinear and normal to the sample surface, an essential aspect allowing high spatial resolution of analyses and accurate micrometer-scale sample and beam positioning. Use of a long working distance in conjunction with integral reflectance mirror are the primary design constraints satisfied for successful development. Based on measurements, the presented proton microprobe is expected to enhance studies requiring effective and easy-to-perform nondestructive microanalytical analysis of minerals.     

Optical dot gain on newsprint determined with the Lund nuclear microprobe

A technique for measuring optical dot gain, i.e. the relative difference between the actual screen dot and the optically perceived one, is presented. By combining measurements from the non-optical nuclear microprobe with data from image analyzing technique the optical dot gain can be determined. The procedure to reach pixel by pixel correlation on a micrometer scale is discussed. In the newsprint sample studied in this investigation a typical optical dot gain between 15 and 20% was deduced. The variation in the optical dot gain was correlated with other characteristic parameters of the print and newsprint and especially a positive correlation to the mass density of the newsprint was observed.     

A scanning and data acquisition system for the Lund proton microprobe

A computer system for scanning control and the evaluation of data obtained using a scanning proton microprobe is described. The system consists of a μVax-II host computer and a VME bus data acquisition and beam scanning system. The VME bus computer controls the front-end electronics, such as ADCs, and sends data in blocks to the μVax-II which does the on-line sorting of data and elemental mapping. To facilitate fast and effective analyses, a software package has been developed which gives the user the possibility of manipulating data acquisition, beam scanning etc. during sample analysis.     

High speed acquisition and handling of scanning proton microprobe data

The collection of high count rate data on a scanning proton microprobe from a number of detectors simultaneously requires much attention to both hardware and software detail. With fast data acquisition and rapid data handling coupled with high resolution graphics, a microprobe provides a powerful analytical instrument.Several considerations favour the use of event by event mode for data collection. Rapid data handling requires the provision of individual, fast ADCs for each detector. A controlling host computer must record each energy event together with the positional coordinates of the event as it was generated at the specimen. The task of the resident host software ranges from dumping the data on to magnetic tape or disk to a full online, realtime SORT to enable the mapping of elemental distributions on high resolution colour monitors as data is being collected.The evolution of total quantative scanning analysis will be described from its inception on a small mini-computer to the present time. The principles of high speed data capture and processing as associated with the more powerful, present day computers integrating high resolution graphics will be discussed, including the development of fast data acquisition hardware and multidimensional display programs.     

Some aspects of statistical distribution of trace element concentrations in biomedical samples

Concentrations of trace elements in biomedical samples were studied using X-ray fluorescence (XRF), total reflection X-ray fluorescence (TRXRF) and particle-induced X-ray emission (PIXE) methods. Used analytical methods were compared in terms of their detection limits and applicability for studying the trace elements in large populations of biomedical samples. In a result, the XRF and TRXRF methods were selected to be used for the trace element concentration measurements in the urine and woman full-term placenta samples. The measured trace element concentration distributions were found to be strongly asymmetric and described by the logarithmic–normal distribution. Such a distribution is expected for the random sequential process, which realistically models a level of trace elements in studied biomedical samples. The importance and consequences of this finding are discussed, especially in the context of comparison of the concentration measurements in different populations of biomedical samples.     

扫描质子微探针的安装调试及象差控制

讨论了扫描质子微探针(SPM)安装调试的方法,通过象差调整,提高了SPM系统的空间分辨率。该系统可提供空间分辨率为2-3μm、束流强度为100pA的3MeV H~+束和2MeV He~+束。     

用扫描质子微探针研究包裹金和微细粒金的赋存状态

用质子微探针设备研究了500μm×500μm区域内的金驹山金银矿和金牙微细粒金矿样品。结果表明,前者Au与Ag具有正相关关系,Au与Fe在微区范围内具有反相关关系;后者矿中Au与As、S、Fe,特别是与As具有密切的相关关系。     

A proton microprobe study of the hippocampus from rats subjected to cerebral ischaemia

The versatility of the proton microprobe in the analysis of biological material is illustrated in a study of the hippocampus in rat brain with the objective of investigating the accumulation of calcium and loss of potassium in specific cell layers following transient cerebral ischaemia.     

The design of a versatile scanning proton microprobe of high resolution and efficiency

The designer of a scanning proton (or nuclear) microprobe must make many decisions, some of which may be compromises. There is a wide range of lens types and configurations. Microprobe performance will depend on performance of the accelerator and its ion source, on stability and control of the lens current supply, on the nature of the microprobe supports, on the vacuum system, on magnetic shielding and connection to the accelerator.There are many possible modes of observation and analysis to be considered when the specimen chamber is designed and a versatile chamber should make provision for most of them. They include optical microscopy of front and back surfaces of the specimen, secondary electron imaging, X-Ray imaging, channelling contrast microscopy, Rutherford backscattering and forward scattering, nuclear reaction analysis and scanning transmission ion microscopy in brightfield and darkfield modes.Microprobe performance will also inevitably depend on the ease of operation and the extent to which the operator has been considered in the overall design and layout of the microprobe. The equally important considerations involved in data collection and analysis are discussed in a second paper.     

High resolution imaging and elemental analysis of PAGE electrophoretograms by scanning proton microprobe

Metal content of metalloproteins can be detected and even quantified by the PIXE–PAGE method. In this technique the proteins are separated by thin layer electrophoresis (by polyacrylamide gel electrophoresis (PAGE) in most cases) and the properly dried gel sections are analyzed by PIXE using “band-shaped” proton milli-beam. This PIXE–PAGE method was adapted for our scanning proton microprobe. The microPIXE–PAGE version provides two-dimensional elemental mapping of the protein bands. In addition, the fast continuous scanning reduces the risk of the thermal deterioration of the sample and the X-ray contribution from dust-impurities can be filtered out in the data evaluation process.     

Analysis of lichen thin sections by PIXE and STIM using a proton microprobe

In order to better understand the distribution pattern of mineral elements in lichen tissues, thin sections (15 μm) of the foliose, vagrant soil lichen Xanthoparmelia chlorochroa were examined using proton microprobe Particle induced X-ray emission (PIXE). This technique was used to make two-dimensional scans, with 5 μm resolution, across tissue cross sections of the test species. Element maps for Si, P, S, Cl, K, Ca, Ti, Mn, Fe, Cu, Zn, and As have been prepared. Several elements are strongly localized in the element maps. PIXE data are complimented with STIM, light micrographs, and SEM images. Preliminary data suggest that nuclear microprobe techniques may be useful in elucidating element absorption and transport mechanisms in lichens.     

Mercury and gold concentrations of highly polluted environmental samples determined using prompt gamma-ray analysis and instrument neutron activation analysis

The authors have established a method of determining mercury and gold in severely polluted environmental samples using prompt gamma-ray analysis (PGA) and instrumental neutron activation analysis (INAA). Since large amounts of mercury are constantly being released into the environment by small-scale gold mining in many developing countries, the mercury concentration in tailings and water has to be determined to mitigate environmental pollution. Cold-vapor atomic absorption analysis, the most pervasive method of mercury analysis, is not suitable because tailings and water around mining facilities have extremely high mercury concentrations. On the other hand, PGA can determine high mercury concentrations in polluted samples as it has an appropriate level of sensitivity. Moreover, gold concentrations can be determined sequentially by using INAA after PGA. In conclusion, the analytical procedure established in this work using PGA and INAA is the best way to evaluate the degree of pollution and the tailing resource value. This method will significantly contribute to mitigating problems in the global environment.     

On the accuracy of measuring proton fluence by beam integration,for the determination of stopping power

The energy dependence of the stopping power can be determined from the height of backscattering spectra, when for different energies the fluence of the projectiles is measured. We list the possible errors when the number of protons hitting the target is deduced from the integrated beam current. It is shown, e.g., that a proper correction for dead time in the multichannel analyzer requires a stable beam current. Taking account of all effects discussed, we find that for proton energies larger than 60 keV the fluence can be determined with a precision better than 1%. This is confirmed by comparing the calculated and the measured backscattering yields from a Au- and a Ta-target. To facilitate the determination of the backscattering cross section of the shielded target nuclei we use a fit formula applicable to all atoms heavier than Al, which was obtained from calculations using the scattering integral.     

On the accuracy of protein determination in large biological samples by prompt gamma neutron activation analysis

A prompt gamma neutron activation analysis (PGNAA) facility has been developed for the determination of nitrogen and thus total protein in large volume biological samples or the whole body of small animals. In the present work, the accuracy of nitrogen determination by PGNAA in phantoms of known composition as well as in four raw ground meat samples of about 1 kg mass was examined. Dumas combustion and Kjeldahl techniques were also used for the assessment of nitrogen concentration in the meat samples. No statistically significant differences were found between the concentrations assessed by the three techniques. The results of this work demonstrate the applicability of PGNAA for the assessment of total protein in biological samples of 0.25-1.5 kg mass, such as a meat sample or the body of small animal even in vivo with an equivalent radiation dose of about 40 mSv.     

A study of barium accumulation in desmids using the Oxford scanning proton microprobe (SPM)

Within a group of unicellular freshwater algae called desmids there is a genus, Closterium, the cells of which are known to store BaSO₄ microcrystals in vacuoles situated at each end of the cell.In studies involving Closterium moniliferum cells grown in cultures containing different concentrations of BaCl₂, it was observed that the barium uptake, as measured by the Oxford SPM, was not lowest at the lowest concentrations of BaCl₂ to which the cells were exposed (0.001 and 0.01 mg/l but unexpectedly at a median concentration of 1.0 mg/l.In a second study on Closterium moniliferum cells grown in a culture containing 0.010 mg Ba/l, the rate of uptake of barium was observed to level off over a 24 h period.SPM elemental maps of Closterium cells are presented, one sample of which was grown in a normal medium and shows BaSO₄ crystals present in the terminal vacuoles, and the other sample grown in a Ba rich medium and showing BaSO₄ crystals distributed throughout the cell.     

On the analysis of neonatal hamster tooth germs with the photon microprobe at Daresbury, UK

Complementary to the micro-PIXE experiments performed on hamster tooth germs to elucidate the role of fluoride during the growth, the photon microprobe at Daresbury was used to obtain information on the distribution of Zn. The germs of fluoride-administered hamsters, together with a control group, were analyzed with the micro-synchrotron radiation fluorescence method (micro-SXRF).