Fred C. Adams

A general Monte Carlo simulation of energy-dispersive X-ray fluorescence spectrometers. I: Unpolarized radiation, homogeneous samples

Abstract A general Monte Carlo program for the simulation of X-ray fluorescence (XRF) spectrometers is presented. The global layout of the program is discussed and the way in which variance reduction techniques have been employed to improve the efficiency of the code is described. For the case of polychromatic excitation in a direct excitation energy-dispersive (ED) XRF instrument, experimentally collected ED-XRF spectra are compared with simulated spectral distributions. Applications of the software in the field of quantitative analysis and thickness estimation of samples of intermediate thickness illustrate the potential of the method.

A general Monte Carlo simulation of ED-XRF spectrometers. II: Polarized monochromatic radiation, homogeneous samples

Abstract A general Monte Carlo code for the simulation of X-ray fluorescence spectrometers, described in a previous paper is extended to predict the spectral response of instruments employing polarized exciting radiation. Details of the calculation method specific for the correct simulation of photon-matter scatter interactions in case of polarized X-ray beams are presented. Comparisons are made with experimentally collected spectral data obtained from a monochromatic X-ray fluorescence setup installed at a synchrotron radiation source. The use of the simulation code for quantitative analysis of intermediate and massive samples is also demonstrated.

A general Monte Carlo simulation of energy-dispersive X-ray fluorescence spectrometers. Part 3. Polarized polychromatic radiation, homogeneous samples

Abstract A general Monte Carlo (MC) code, capable of simulating photon-matter interactions in the energy range of 1–80 keV, has been adapted to model the behavior of synchrotron radiation X-ray fluorescence (SR-XRF) spectrometers using polarized, polychromatic radiation as means of excitation. A brief description of the simulation method specific for polychromatic excitation is presented for homogeneous, multi-element samples. The code has been verified by comparing calculated spectral distributions of samples of various nature with experimentally collected data, taken at the bending magnet source based X26A SR-XRF beamline installed at the National Synchrotron Light Source (NSLS). Simulated results have also been compared with previously developed analytical calculations. Applications of the program in the field of quantitative analysis of biological and glass materials are presented and the use of the software to study the influence of the changes in the excitation-detection geometry on the predicted spectral distributions is demonstrated. Depending on the thickness and matrix type of the samples, the average deviations of the predicted fluorescent line and scatter background intensities relative to the experimentally obtained values are in the 3–12% range for homogeneous samples.

Laser microprobe mass spectrometry of environmental soot particles

Soot from an experimental oil shale retort has been examined by laser microprobe mass spectrometry to discern the composition of this material and to evaluate the capability of laser mass spectrometry to analyse carbonaceous environmental particles. Organic compounds indicated by laser mass spectrometry were principally polycyclic aromatic hydrocarbons (PAH), this analysis being confirmed by HPLC, GC/MS and EI mass spectrometry. The influence of the laser intensity on the mass spectra of this soot sample was examined and found to be consistent with the influence of laser intensity on the mass spectra of pure compounds. The qualitative elemental analysis of this soot by laser microprobe mass spectrometry was confirmed by X-ray fluorescence analysis. The laser microprobe mass spectrometer was shown to be capable of the detection of a wide molecular weight range of PAH simultaneously with multielemental inorganic analysis on individual particles 1 to 10 microns in diameter.

Ion kinetic energy measurements on laser-induced plasmas in laser microprobe mass analysis (LAMMA). Part I. Methodology

Abstract A method for the measurement of kinetic energy distributions of ions produced in a laser microprobe mass spectrometer employing the energy cut-off property of an ion reflector has been developed. The method is insensitive to time of ion formation and to miscalibration of the flight-time equation, but requires accumulation of many spectra from uniform samples. When applied to elemental carbon foil, measurement of ion kinetic energy reveals that ionic species occur with different kinetic energy distributions. A substantial fraction of the ions having less kinetic energy than the full acceleration energy was observed.

Speciation of ionic alkyllead in grass and tree leaves

Abstract A preliminary study aimed at outlining the potential of the extraction/derivatization/gas chromatography-atomic absorption spectrometry methodology for the species-specific determination of ionic organolead compounds in grass and tree leaves is described. A pretreatment procedure based on leaching the species from the matrix using tetramethylammonium hydroxide appeared to be efficient. In grass, concentrations up to 100 ng Pb g-1 were detected; a reliable differentiation between the amount taken up and the amount superficially adsorbed could not yet be achieved.

Arsenic speciation in water by hydride cold trapping-quartz furnace atomic absorption spectrometry: an evaluation

Arsenate, arsenite, monomethylarsonate, monoethylarsonate and dimethylarsinate are speciated in environmental waters. After pH-dependent hydride generation, the arsines are collected on a cold trap and volatilised sequentially into a quartz furnace atomic absorption spectrometer.The various parameters of the methodology are optimised, and their influence critically discussed and compared with other methods. Aspects such as interferences in organoarsenic speciation, the effect of atomisation support gases in collection mode techniques and unambiguous identification of dimethylarsine are emphasised in particular. An evaluation of the analytical characteristics and performance of the method is presented, using representative environmental samples. This paper attempts to illustrate the advantages and drawbacks of the technique and includes a review of recent progress in arsenic hydride methodology.

Laser microprobe mass analysis (LAMMA) as a tool for particle characterization: a study of coal fly ash.

Laser microprobe mass analysis (LAMMA) is used for the characterization of a fly ash derived from a conventional coal-fired electrical generating plant. The mass spectra provide data in the chemical composition including speciation data on the matrix and minor constituents and organic constituents present at the surface. It appears that LAMMA constitutes a fast and probably cost-effective tool for qualitative screening and for establishment of some physical and chemical characteristics of fly ash. However, at present the quality of the information is insufficiently high to warrant its use as a screening tool for particle characterization.

Ion kinetic energy measurements on laser-induced plasmas in laser microprobe mass analysis (LAMMA). Part 2. Titanium dioxide

Abstract Kinetic energy distributions of various ionic species from a TiO 2 film have been obtained using the energy cut-off property of an ion reflector in a laser microprobe mass spectrometer. The energy distributions of elemental ions are more positive and broader than those of polyatomic ions. They all have a substantial contribution in the “negative” energy range. At higher laser energy densities, the ion kinetic energy distributions become broader and shift towards “lower” energy values. Chemically different ion species have different energy distributions. The energy distributions are attributed predominantly to the locus of ion formation in the accelerating electrostatic field.

Occurrence of monoalkyllead species during the speciation of organolead

Abstract When tri-levels are determined in environmental samples by a method based on Grignard derivatization and subsequent gas chromatography/atomic absorption spectrometry, monoalkyllead species are sometimes detected. A rearrangement process during or following Grignard derivatization explains this occurrence at least partly. The accuracy of the determination does not suffer significantly from this limited-scale reaction but the detection of a monoalkyltributyllead species does not necessarily mean that the sample contains monoalkyllead compounds.