P. Van Espen

A computer analysis of X-ray fluorescence spectra

Abstract A method is presented for the non-linear least-squares analysis of low-energy X-ray spectra obtained by tube-excited X-ray fluorescence analysis. The method is based on a description of the full energy peaks as Gaussian distributions and the background as a polynomial. System constraints are used to keep peak position and resolution consistent with predetermined calibration expressions. The least-squares routine is based on the Marquardt algorithm but parameters are confined within predetermined physically significant intervals. The method is implemented in the computer program AXIL written in Fortra IV for a PDP 11/45 system. Tests are described which prove that the program is able to provide reliable values for the parameters.

Biomass burning in Southern Africa: individual particle characterization of atmospheric aerosols and savanna fire samples

Ambient atmospheric aerosols and savanna fireparticulate emission samples from southern Africa werecharacterised in terms of particle classes and theirnumber abundance by electron probe X-ray microanalysis(EPXMA). About ten particle classes were identifiedfor each sample. The major classes werealuminosilicates and sea salts for ambient coarse(2–10 μm equivalent aerodynamic diameter (EAD))samples, and K-S and S-only particles for ambient fine(<2 μm EAD) samples. The K-S particles are oneof the major products of biomass burning. The EPXMAresults were found to be consistent with the resultsfrom bulk analyses on a sample by sample basis. Forsavanna fire fine samples, quantitative EPXMA revealedthat many particles had a composition of simple saltssuch as KCl. Some particles had a deviatingcomposition in the sense that more ionic species wereinvolved in sustaining the balance between cations andanions, and they were composite or mixed salts.Because of extensive processing during the atmospherictransport, the composition of the K-S particles in theambient samples was different from K2SO4,and such particles were enriched with S. The finepyrogenic KCl particles and the fine sea-saltparticles were much depleted in chlorine.

A method for the accurate description of the full-energy peaks in non-linear least-squares analysis of x-ray spectra☆

Abstract Accurate fitting of X-ray spectra obtained with Si(Li) detectors is accomplished by a model consisting of a Gauss function and numerical corrections. The procedure is incorporated into a non-linear least-squares fitting program used for the analysis of X-ray induced X-ray fluorescence spectra.

PIXE analysis of aerosol samples collected over the atlantic ocean from a sailboat

Abstract Size-fractionated aerosol samples, collected over the Atlantic Ocean, were analyzed for up to 20 elements by PIXE. Using a sailboat as sampling platform, duplicate samples were taken for two-day periods by means of battery operated 6-stage cascade impactors, positioned about 8 m above the sea surface. In the PIXE analysis of the fine particle stages (stages 3 to 5) a 5 times smaller beam size was used than for stages 1 and 2. This led to significant improvement in the detection limits for the former stages. The results from the duplicate impactor samples were normally in good agreement, indicating that the combined uncertainty of sampling and PIXE analysis was of the order of 20%. The precision of the PIXE analysis alone was investigated by rebombarding some samples six months after the first analysis. The trends with time of the fine particle sulfur and the coarse particle iron concentrations are discussed in some detail.

Particle-induced x-ray emission (PIXE) analysis of biological materials: Precision, accuracy and application to cancer tissues

Abstract An autopsy kidney, a human serum sample and NBS bovine liver were analyzed by both PIXE and instrumental neutron activation analysis (INAA). Several target preparation procedures were investigated. The reproducibility of the PIXE analysis, as determined by analyzing up to 20 targets from the same material, was of the order of 10% or better. For most elements a good agreement was obtained between PIXE and INAA, indicating that PIXE can yield data which are accurate to within 10%. The PIXE technique was also applied to cancerous and normal tissue sections of the same organ of patients, showing renal cell and other types of carcinoma. Substantial differences were often observed between the trace element concentration patterns of the cancerous and normal sections.

A program for the processing of analytical data (DPP)

Abstract Computer-assisted interactive data presentation and analysis facilities are needed to handle the vast amount of information produced by automated instruments. The data processing program, DPP, presented here in a FORTRAN-77 program designed to solve this problem. The program is equipped with a leading verb command language for input and job scheduling, thus providing an efficient and user-friendly operator/program interface, and with a data-base organization that accommodates a wide variety of data structures. Data presentation and analysis procedures include tabulation and plotting, regression analysis, non-linear least-squares fitting, polynomial fitting, principal component analysis, hierarchal clustering and non-hierarchal clustering. Data matrices with up to 10 000 data points, distributed over a maximum of 3000 variables and 3000 samples, can be examined. Because of the open-ended structure of the program, it is straightforward to incorporate additional data analysis procedures when they are needed. Recent applications are discussed.

Comparison of several background compensation methods useful for evaluation of energy-dispersive X-ray fluorescence spectra

Abstract A number of background estimation and modelling strategies suitable for evaluating energy-dispersive X-ray spectra by means of non-linear least squares fitting are evaluated and intercompared. As background modelling functions, exponential and linear combinations of mutually orthogonal polynomials are considered. These functions allow the shape of the background to be determined together with the photopeak intensities. As background estimation algorithms, an iterative stripping algorithm and a background channel selection procedure which is also based on the use of orthogonal polynomials are studied. The last two methods calculate the spectral background prior to the actual fitting process. For the intercomparison, the various methods were incorporated in the software package AXIL (Analysis of X-ray spectra by means of Iterative Least Squares). By using simulated spectra in which the intensity of all lines is a priori known, the accuracy and noise-sensitivity of the different background compensation strategies are evaluated. The method in which the background is modelled as a linear combination of orthogonal polynomials is identified as being the most robust and yielding the most accurate results.

Classification of Chinese Tea Samples According To Origin and Quality By Principal Component Techniques

Abstract Three categories of tea, black, green and oolong tea, with those varieties in each category were analyzed for their contents of cellulose, hemicellulose, lignin, polyphenols, caffeine and amino acids. The data were subjected to multivarige analysis. Principal component analysis and principal component classification provide discrimination between the different categories and varieties. The quality index, assigned to these tea standards by experts, could be predicted form the principal component scores.

Study of the Composition of Zirconium based Chromium free Conversion Layers on Aluminium

SummaryThe composition of chromium free conversion layers on aluminium, based on zirconium has been determined using complementary surface analytical techniques: SEM, AES, XPS, SIMS. The zirconium based chromium free conversion layer consists of a two layered structure, with total thickness less than 10 nm, in which the bottom layer contains only Al and O, while the top layer contains also a fluorinated zirconium compound and probably a polymer that is concentrated towards the outer surface. In a second part the influence of the conversion time on the thickness of the film was studied using AES, SE and EIS. It is concluded that the zirconium based conversion layers are formed within ten seconds and a successive increase in conversion time results only in a very slight thickening of the layer.

The 2000 IAEA intercomparison of PIXE spectrum analysis software

Seven programs for analysis of PIXE spectra were compared using the 2000 IAEA test spectra, i.e. Geopixe, Gupix, Pixan, Pixeklm, Sapix, Winaxil and Witshex. A systematic statistical study of the analysis results was performed based on z-scores. The results indicate that most of the programs perform reasonably well with respect to peak areas. Except for a very rare exception, the statistical analysis shows that the participants generally reported smaller uncertainties than would have been expected from the reference uncertainty values. The results show that all the participants reported a number of statistically significant “false hits” and “misses” in their reports. All this indicates that programs in general still leave room for further improvements.