Comparative methods for determination of rare earths in geological and mineral samples

Particle induced X-ray emission (PIXE) has been used for the quantitative analysis of rare earth elements (REE) in thick targets prepared from geological and mineral samples. Measurements were made with 1 and 3 MeV proton beams. For comparison, determination of the same elements was made by X-ray fluorescence (XRF) using a²⁴¹Am annular source. Minimal detectable limits (MDL) have been estimated for the two situations. Neutron activation analysis (NAA) has also been used for the determination of REE at the ppm level.     

Comparison of different excitation methods for X-ray spectral analysis: the case of synchrotron radiation

Different excitation means, protons, photons and electrons (PIXE, XRFA, and EPMA, respectively) have previously been compared with regard to the figures of merit, i.e. detection power, precision and accuracy. The aim in this article is to compare synchrotron radiation SR as another excitation method with the methods mentioned above. From this point of view the evaluation of (SR) was missing as an again independently optimized excitation method and was based as previously on practical problems of trace analysis, in this case on the determinations of traces in lead. The experiment has been performed with thick homogeneous samples of lead, the same samples already used in the former work, so that a direct comparison is possible. The calculation of the figures of merit is based on the measurements of the blank values and their relative standard deviation for the detection limit and on the random errors for the precision. Regarding the thick homogeneous target of lead XRFA still turns out to be the best method, whereas Synchrotron X-Ray Fluorescence SY-XRF compares favorably, at least for larger atomic numbers Z. Even PIXE is inferior to (SY-XRF) in the case of larger Z; when information on the lateral distribution of the elements is of interest, PIXE is indispensable. Received: 13 June 2000 / Revised: 2 August 2000 / Accepted: 5 August 2000     

Two decades with PIXE spectrometry at the University of Liège

The different important stages of the development of proton induced X-ray emission (PIXE) spectrometry utilised with success since 1976 at Liège University are outlined. The methodological efforts (irradiation, target preparation, calibration, matrix effect corrections) to improve quantitative trace element analysis are emphasized. Examples of applications in geological, biomedical and environmental sciences are selected in order to illustrate the potential of the technique in these fields.     

XSQR: The technique of pixe-induced XRF

The XSQR technique uses nearly monoenergetic X-rays from the proton bombardment of a primary target to excite X-ray fluorescence for analytical purposes. With the proper choice of primary target, interferences from high concentration components in a matrix may be eliminated. An overview is given of the technique, the equipment it uses and its methodology. The application of the technique is described for the determination of Cr in steels using an Fe primary, Cu and Zn in gold objects using a Ge primary, multielemental analysis of geological and biological materials using primary targets such as Mo, Rh and Pd and Si in oils using a Ti primary target. The method serves as a useful complementary method to PIXE for the determination of the medium mass elements.     

Determination of aluminium and silicon in zeolites by PIXE

Using 1.0 MeV protons on thick targets of zeolite pellets, aluminium and silicon were determined by PIXE. Results were compared with XRF data. The relative root mean square error was 3.7% and the results of the two methods agreed with a correlation factor of 0.991. PIXE analysis can be carried out on very small samples, while sample preparation involves minimal handling.     

Comparison of different excitation methods for X-ray spectral analysis: the case of synchrotron radiation

Different excitation means, protons, photons and electrons (PIXE, XRFA, and EPMA, respectively) have previously been compared with regard to the figures of merit, i.e. detection power, precision and accuracy. The aim in this article is to compare synchrotron radiation SR as another excitation method with the methods mentioned above. From this point of view the evaluation of (SR) was missing as an again independently optimized excitation method and was based as previously on practical problems of trace analysis, in this case on the determinations of traces in lead. The experiment has been performed with thick homogeneous samples of lead, the same samples already used in the former work, so that a direct comparison is possible. The calculation of the figures of merit is based on the measurements of the blank values and their relative standard deviation for the detection limit and on the random errors for the precision. Regarding the thick homogeneous target of lead XRFA still turns out to be the best method, whereas Synchrotron X-Ray Fluorescence SY-XRF compares favorably, at least for larger atomic numbers Z. Even PIXE is inferior to (SY-XRF) in the case of larger Z; when information on the lateral distribution of the elements is of interest, PIXE is indispensable.     

Standardless PIXE analysis of thick biomineral structures

The particle-induced X-ray emission (PIXE) of thick biomineral targets provides pertinent surface analysis, but if good reference materials are missing then complementary approaches are required to handle the matrix effects. This is illustrated by our results from qualitative and semiquantitative analysis of biomaterials and calcified tissues in which PIXE usually detected up to 20 elements with Z > 14 per sample, many at trace levels. Relative concentrations allow the classification of dental composites according to the mean Z and by multivariate statistics. In femur bones from streptozotocin-induced diabetic rats, trace element changes showed high individual variability but correlated to each other, and multivariate statistics improved discrimination of abnormal pathology. Changes on the in vitro demineralization of dental enamel suggested that a dissolution of Ca compounds in the outermost layer results in the uncovering of deeper layers containing higher trace element levels. Thus, in spite of significant limitations, standardless PIXE analysis of thick biomineral samples together with proper additional procedures can provide relevant information in biomedical research.Abbreviations AAS Atomic absorption spectrometry - ERDA Elastic recoil detection analysis - ESR Electron spin resonance - FDA Factorial discriminant analysis - FTIR spectroscopy Fourier transform infra-red spectroscopy - HP Ge detector Hyperpure Ge detector - ICP-AES Inductively coupled plasma atomic emission spectrometry - NAA Neutron activation analysis - NRA Nuclear reaction analysis - PCA Principal component analysis - PIXE Particle-induced X-ray emission - PIGE Particle-induced -ray emission - RBS Rutherford backscattering spectroscopy - SRIXE Synchrotron radiation-induced X-ray emission     

XRF induced by PIXE: Comparison with PIXE

Two different modes of sample excitation have been used for elemental analysis of thin standard reference materials and of chemical standards with known composition. PIXE /2.5 MeV protons/ and XRF induced by PIXE, will be called XRF-PIXE /2.5 MeV protons on Mo primary target/. The same samples were alternatively exposed to protons and X-ray beams. The sensitivities under standard running conditions are determined for both an XRF-PIXE and a PIXE analysis system. It is shown that the sensitivity of the PIXE spectrometer depends strongly on the sample matrix, whereas the XRF-PIXE sensitivity is rather constant with respect to different kinds of samples. In addition, the advantages of one mode of excitation on the other are discussed. It is shown that XRF-PIXE can be a useful complement to PIXE analysis.     

Applicability of PIXE to the analysis of biological reference materials

Summary Optimized PIXE procedures were applied to the analysis of up to 18 elements in certified biological reference materials. The exercise indicated that the total random error of the results from one single target analysis is in the range of 2–5% for elements heavier than K and for levels from 2 g/g on. The accuracy of the PIXE procedures is shown to be better than 5%.     

Gamma-ray spectroscopic and PIXE analysis of selected samples from the phosphorite deposits of Northwestern Saudi Arabia

Phosphorite deposits from northwestern Saudi Arabia were analyzed by natural radioactivity measurements to detect the presence of radioactive elements. PIXE was used to obtain an average elemental composition of these deposits. From the analysis of radiations from ²³⁸U, ²³²Th and ⁴⁰K isotopes, the samples were found to contain U, Th and K in concentrations up to 130 ppm, 30 ppm and 2.5 wt%, respectively. PIXE showed the presence of a number of trace elements such as Ca, V, Cr, Fe, Ni, Cu, Zn, As, Sr, Y and U in concentrations ranging from a few ppm to several hundred ppm. The occurrence of these radioactive and non-radioactive elements are discussed for their geological significance in the phosphorite deposits.     

Simultaneous PIXE and PIGME Analysis of a Nigerian Tar Sand Sample from a Deep Borehole

Tar sands from a deep borehole in southwestern Nigeria were subjected to elemental analysis by simultaneous proton induced X-ray emission (PIXE) and proton induced gamma-ray emission (PIGME) techniques. The concentration of 22 major, minor and trace elements, namely Al, Ca, Cr, Cu, F, Fe, Ga, Ge, K, Li, Mg, Mn, Na, P, Pb, Rb, S, Si, Sr, Ti, Zn and Zr were determined. The accuracy and precision of the techniques for chemical analysis were assured by analysing the NBS 278 geological standard.     

Preparation and characterization of a set of IAEA reference air filters for quality control in air-pollution studies

Several sets of reference air filters were prepared as part of an IAEA evaluation of the performance of laboratories involved in air-pollution studies. Each set comprised three polycarbonate membrane filters, two of which were loaded with urban air particulate matter (APM) obtained in Vienna or Prague, and one unloaded filter. The filters were loaded by filtration of a suspension of the APM materials in water. The homogeneity both of bulk APM materials and of the loaded filters was evaluated and found suitable by determining several elements by instrumental neutron-activation analysis (INAA), proton-induced X-ray emission (PIXE), and micro-X-ray energy-dispersive fluorescence analysis (micro-EDXRF). After evaluation of the homogeneity, INAA, PIXE, EDXRF, atomic absorption spectrometry (AAS), inductively coupled plasma optical emission spectrometry (ICP-OES), and ICP mass spectrometry (ICP-MS) were used to characterize the filter materials and establish "target values" and their associated standard deviations for 15 elements. Problems encountered during the preparation of these unique, simulated air filters and the criteria for setting both the target values and standard deviations are presented.     

Speciation of the inorganic components in brown coal

Brown coal samples from different deposits have been analyzed for the bonding forms of their inorganic components. Besides the analysis of the dried coals, ashing techniques (high and low temperature ashing) and extraction procedures with different solvents (acids, bases, complexation agents, organic solvents with different polarity) have also been investigated. ESCA, PIXE, INAA, ICP-Atomic Emission Spectroscopy, NMR and Ionchromatography have been applied to the analysis of coals, ashes, wet ashing and extraction products. The bonding behavior of more than 40 elements could be characterized. Conclusions about geological and geochemical processes during and after coalification could be drawn.     

Measurement of “enhanced” L and M X-ray yields from1H2 + and2H2 + molecule-ion beams and alpha particles,in the energy range of 500–700 keV

Charge-induced X-ray emission (CHIX) has been studied using infinitely thick, non-conducting targets of some fluorides, bombarded with hydrogen and deuterium molecule-ion beams, and -particles, in the energy range of 500–700 keV, where the PIXE contribution to the X-ray yield was relatively small. Protons and deuterons with effective energies of 250–350 keV were obtained in this way. Continuous bombardment on the same spot in the targets caused significant decreases in the X-ray yields in some cases, as a result of target deterioration.     

Preparation and characterization of a set of IAEA reference air filters for quality control in air-pollution studies

Several sets of reference air filters were prepared as part of an IAEA evaluation of the performance of laboratories involved in air-pollution studies. Each set comprised three polycarbonate membrane filters, two of which were loaded with urban air particulate matter (APM) obtained in Vienna or Prague, and one unloaded filter. The filters were loaded by filtration of a suspension of the APM materials in water. The homogeneity both of bulk APM materials and of the loaded filters was evaluated and found suitable by determining several elements by instrumental neutron-activation analysis (INAA), proton-induced X-ray emission (PIXE), and micro-X-ray energy-dispersive fluorescence analysis (μ-EDXRF). After evaluation of the homogeneity, INAA, PIXE, EDXRF, atomic absorption spectrometry (AAS), inductively coupled plasma optical emission spectrometry (ICP–OES), and ICP mass spectrometry (ICP–MS) were used to characterize the filter materials and establish “target values” and their associated standard deviations for 15 elements. Problems encountered during the preparation of these unique, simulated air filters and the criteria for setting both the target values and standard deviations are presented.     

Precision and accuracy in standards analysis by PIXE

Four different approaches to PIXE data obtained in repeated measurements on thick standards have been evaluated in terms of precision and accuracy. Both were found to be the best when determinations relative to an external standard were normalized to a composition assumed to be 100% oxides.     

Proton induced X-ray emission (PIXE) analysis on thick samples exemplified on the rare earth elements

Summary A semiempirical procedure is developed for the calculation of the signal intensity for thick sample PIXE experiments. The calculated X-ray intensity emerged out of a thick sample is factorized into the overall produced intensity within a thick sample and an effective transmission factor for the X-ray self-absorption.The accuracy of this procedure is limited by the reliability of the experimental input parameters to about ±25%.We find as an important result of this study: The signal intensity is calculated as a function of the various experimental parameters, i.e. proton energy and sample composition including interelement- and matrix effects. This function is presented as a closed equation, which can be applied very easily, i.e. without large scale computers, to calculate the sample elemental concentration. The mass absorption coefficient of the sample most strongly influences the effective transmission term.Enhancement corrections are calculated for additional interfering rare earth elements j. No significant interelement-effects appear at concentrations c _j 2 weight%, where analysis by PIXE may be interesting. The calculated results agree well with experimental data for the rare earth elements in different matrices.Dedicated to Karl Gleu on the occasion of his 80th birthday     

PIXE analysis of muscovite samples from tourmaline bearing granite pegmatites of Southwestern Nigeria

Summary Muscovites from pegmatite bodies from southwestern Nigeria have been characterized by PIXE. The chemical analysis reveals that in these muscovites, the interlayer sites are occupied mainly by K, Fe, Rb Na, and traces of Mg and Ca in the octahedral positions whilst Si, Al, Ti Mn occur in the tetrahedral ones. The low Na/(Na+ K) ratios, low Ti content and enrichment in Rb in the muscovite suggest postmagmatic genesis of the host rocks. The variability in trace element composition may reflect differences in geological environment of emplacement of the pegmatite bodies.     

PIXE analysis of prehistoric and protohistoric Caborn-Welborn phase copper artifacts from the lower Ohio River Valley

Particle-induced X-ray emission (PIXE) spectrometry is being used to nondestructively determine the elemental composition of copper-based artifacts excavated from prehistoric/protohistoric sites in the Ohio River Valley. Copper objects from Caborn-Welborn (C-W) and contemporary Fort Ancient sites are being studied so as to differentiate between native American and European copper. The trace element analysis of metal artifacts enables archaeologists to more accurately assess the material culture and chronological development of C-W society (A.D. 1400–1700) with particular reference to geological sources of copper and brass.     

Problems associated with routine PIXE analysis in quantifying elemental concentrations of leukocytes from Coronary Artery Bypass Grafting (CABG) surgery patients

Whole blood from patients undergoing Coronary Artery Bypass Grafting (CABG) operations was separated into leukocyte subfractions of polymorphonuclearcytes (PMN) and peripheral blood mononuclear cells (PBMC). Blood samples were collected and analyzed at various timepoints to determine the elemental composition to provide a better understanding of recovery mechanisms and to indicate complications that may occur post-operatively. Proton induced X-ray emission (PIXE) analysis and Rutherford backscattering spectrometry (RBS) using the University of Surrey microprobe was employed to determine the concentrations of a range of elements. Accurate two-dimensional PIXE analysis however, requires knowledge of the sample matrix composition. These samples, on the other hand, showed varying thickness, lacked matrix homogeneity and displayed non-uniform trace element distribution. This paper discusses the results and problems associated with routine PIXE analysis and demonstrates the potential ability of ion beam analysis (IBA) depth profiling software, previously unused in PIXE analysis, to model a RBS spectrum of inhomogeneous, multi-layered samples prior to performing PIXE analysis.