X-Ray Spectrometry - Survival-training.info

10 CONSIDERING THE ROLE OF X-RAY SPECTROMETRY IN CHEMICAL ANALYSIS AND OUTLINING THE VOLUMEhas been overcome in many synchrotron facilitiesnowadays.Chapter 6 reviews some advances in computerisationconcerning XRS. The first subchapter, writtenby L. Vincze, K. Janssens, B. Vekemans andF. Adams (Department of Chemistry, Universityof Antwerp, Belgium) deals with modern MonteCarlo (MC) simulation as an aid for EDXRF.The use of MC simulation models is becomingmore and more viable due to the rapid increaseof inexpensive computing power and the availabilityof accurate atomic data for photon-matterinteractions. An MC simulation of the completeresponse of an EDXRF spectrometer is interestingfrom various points of view. A significantadvantage of the MC simulation based quantificationscheme compared to other methods, such asFP algorithms, is that the simulated spectrum canbe compared directly to the experimental data inits entirety, taking into account not only the fluorescenceline intensities, but also the scatteredbackground of the XRF spectra. This is linkedwith the fact that MC simulations are not limitedto first- or second-order approximations andto ideal geometries. Moreover, by considering thethree most important interaction types in the 1–100keV energy range (photoelectric effect followedby fluorescence emission, Compton and Rayleighscattering), such models can be used in a generalfashion to predict the achievable analytical characteristicsof e.g. future (SR)XRF spectrometersand to aid the optimisation/calibration of existinginstruments. The code illustrated in this subchapterhas experimentally been verified by comparisonsof simulated and experimental spectral distributionsof various samples. With respect to the simulationof heterogeneous samples, an example isgiven for the modeling of XRF tomography experiments.The simulation of such lengthy XRF imagingexperiments is important for performing feasibilitystudies and optimisation before the actualmeasurement is performed. Subchapter 6.2 by P.Lemberghe (Department of Chemistry, Universityof Antwerp, Belgium) describes progress in spectrumevaluation for EDXRF, where it remainsa crucial step, as important as sample preparationand quantification. Because of the increasedcount rate and hence better precision due to newdetectors, more details became apparent in thespectra; fortunately, the availability of inexpensiveand powerful PCs now enables the implementationof mature spectrum evaluation packages. In thissubchapter, the discussed mathematical techniquesgo from simple net peak area determinations, to themore robust least-squares fitting using referencespectra and to least-squares fitting using analyticalfunctions. The use of linear, exponential or orthogonalpolynomials for the continuum fitting, and of amodified Gaussian and Voigtian for the peak fittingis discussed. Most attention is paid to partial leastsquaresregression, and some illustrative analyticalexamples are presented.The final chapter, Chapter 7, deals with fivegrowing application fields of XRS. J. Börjesson(Lund University, Malmö and the Department ofDiagnostic Radiology, County Hospital, Halmstad,Sweden) and S. Mattsson (Department of DiagnosticRadiology, County Hospital, Halmstad Sweden)focus on applications in the medical sciences since1995, i.e. on recent advances in in vivo XRF methodsand their applications, and on examples of invitro use of the technique. The latter deals mostlywith the determination of heavy metals in tissues,in well-established ways. But there have beensignificant developments lately in in vivo analysiswith respect to sources, geometry, use of polarisedexciting radiation, MC simulations and calibration,and the analytical characteristics have beenimproved. Examples of novel in vivo determinationsof Pb, Cd, Hg, Fe, I, Pt, Au and U arediscussed. The next subchapter deals with novelapplications for semiconductors, thin films and surfaces,and is authored by Y. Mori (Wacker-NSCECorp., Hikari, Japan). Progress in the industrialapplication of TXRF in this field is first discussed.The use of TXRF for semiconductor analysis cameinto popular use in the 1990s; today, more than 300TXRF spectrometers are installed in this industryworldwide, meaning that almost all leading-edgesemiconductor factories have introduced TXRF.Since the main purpose of TXRF is trace contaminationanalysis, improvements in the elementalrange (including light elements), detection ability(e.g. by preconcentration) and standardisation