Practical X-Ray Spectrometry

X-ray fluorescenct" spectrometry is now widely accepted as a highly versatile and potentially accurate method of instrumental elemental analysis and so it is somewhat surprising that although the volume of published work dealing with the technique is high the number of textbooks dealing exclusively with its application is relatively few. Without wishing to detract from the excellence of the textbooks which are already available we have both felt for some time, that a great need exists for a book dealing with the more practical aspects of the subject. For a number of years we have been associated with the provision and arrangement of X-ray schools for the training of new X-ray spectroscopists as well as in the organisation of conferences and sym­ posia whose aims have been to keep the more experienced workers abreast with the latest developments in instrumentation and techniques. In all of these ventures we have found a considerable dearth of reference work dealing with the reasons why an X-ray method has not succeeded as opposed to the multitude of success stories which regularly saturate the scientific press. In this book, which is based on lecture notes from well established courses in X-ray fluorescence spectrometry, we have tried to cover all of the more usual practical difficulties experienced in the application of the method and we have endeavoured to keep the amount of purely theoretical data at a minimum.

1: Physics of X-Rays.- 1.1 Origin of X-rays.- 1.1.1 General.- 1.1.2 Continuous radiation.- 1.1.3 Characteristic radiation.- 1.1.4 Non-diagram lines.- 1.1.5 Auger effect.- 1.1.6 Fluorescent yield.- 1.2 Production of X-rays.- 1.2.1 General.- 1.2.2 X-ray tubes.- 1.2.3 Light element excitation.- 1.3 Properties of X-rays.- 1.3.1 Absorption.- 1.3.2 Scattering of X-rays.- 1.3.3 Absorption by composite materials.- 1.3.4 Diffraction of X-rays.- 1.3.5 Conditions for diffraction.- 1.4 Excitation of fluorescence radiation in the sample.- 1.4.1 General.- 1.4.2 Excitation by monochromatic radiation.- 1.4.3 Excitation by continuous spectra.- 2: Dispersion.- 2.1 General.- 2.2 Geometric arrangement of the spectrometer.- 2.3 Effective range of the spectrometer.- 2.4 Dispersion efficiency.- 2.5 Broadening of line profile.- 2.6 Collimation requirements of the spectrometer.- 2.7 Reflection efficiency.- 2.8 Filters to increase resolving power.- 2.8.1 Reducing secondary radiation.- 2.8.2 Reducing primary radiation.- 2.9 Problems experienced in the application of crystal dispersion.- 2.9.1 General condition of the crystal.- 2.9.2 Temperature effects.- 2.9.3 Crystal fluorescence.- 2.9.4 Abnormal reflections.- 2.10 Dispersion of soft X-rays.- 2.10.1 Organo-metallic compounds.- 2.10.2 Pseudo crystals.- 2.10.3 Diffraction gratings.- 2.11 Comparison of the methods for long wavelength dispersion.- 3: Detection.- 3.1 General.- 3.2 Gas filled detectors.- 3.2.1 Dead time.- 3.2.2 The Geiger-Müller counter.- 3.2.3 The proportional counter.- 3.2.4 The gas flow proportional counter.- 3.3 The scintillation counter.- 3.3.1 The phosphor.- 3.3.2 The photomultiplier.- 3.3.3 Characteristics of the scintillation counter.- 3.4 Comparison of detectors.- 4: Pulse Height Selection.- 4.1 Principle of pulse height selection.- 4.2 Automatic pulse height selection.- 4.2.1 Variation of pulse amplitude.- 4.2.2 Variation of base line and channel settings.- 4.3 Applications of pulse height selection.- 4.4 Theoretical application of pulse height selection.- 4.4.1 Flow counter.- 4.4.2 Scintillation counter.- 4.5 Practical problems arising in pulse height selection.- 4.6 Pulse amplitude shifts.- 4.6.1 Effect of counter voltage.- 4.6.2 Count rate effect.- 4.6.3 Gas density effect.- 4.6.4 Effect of ionisable gas atoms to quench gas ratio.- 4.7 Pulse amplitude distortions.- 4.7.1 Additional peaks arising from the same wavelength giving main peak.- 4.7.2 Additional peaks not arising from the measured wavelength.- 5: Counting Statistics.- 5.1 introduction.- 5.2 Definition of statistical terms.- 5.3 Random distribution of X-rays.- 5.4 Choice of fixed time or fixed count.- 5.5 Limit of counting error.- 5.6 Counting error in the net intensity.- 5.7 Selection of optimum counting times.- 5.8 Selection of best conditions for analysis.- 5.9 Selection of best conditions for low concentrations.- 5.10 Errors in using the ratio method.- 5.11 Selection of ratio or absolute counting method.- 5.12 Counting error versus stability.- 5.13 Counting error as a function of total numbers of counts.- 6: Matrix Effects.- 6.1 Errors in X-ray analysis..- 6.2 Elemental interactions.- 6.2.1 Absorption.- 6.2.2 Enhancement effects.- 6.3 Physical effects ..- 6.3.1 Particle size and surface effects.- 6.3.2 Effects due to chemical state.- 7: Quantitative Analysis.- 7.1 General.- 7.2 Use of standards.- 7.2.1 External standards.- 7.2.2 Internal standard (different element).- 7.2.3 Internal standard (same element).- 7.2.4 Use of scattered tube lines.- 7.3 Dilution techniques.- 7.4 Thin film techniques.- 7.5 Mathematical corrections.- 7.5.1 Principle of the influence factor method.- 7.5.2 Absorption correction methods.- 8: Sample Preparation.- 8.1 General.- 8.2 Samples requiring only a simple treatment.- 8.2.1 Bulk solids (a) metals.- 8.2.2 Bulk solids (b) non-metals.- 8.2.3 Powders.- 8.2.4 Liquids.- 8.3 Samples requiring significant pre-trcatment.- 8.3.1 Bulk solids.- 8.3.2 Powders.- 8.4 Samples requiring special handling treatment.- 8.4.1 Very small samples.- 8.4.2 Very dilute samples.- 8.4.3 Radioactive samples.- 9: Trace Analysis.- 9.1 General.- 9.2 Analysis of low concentrations.- 9.3 Theoretical considerations.- 9.4 Statistical definition.- 9.5 Figure of merit (or quality function).- 9.6 Generator stability.- 9.7 Effect of long term drift.- 9.8 Variation of detection limit with atomic number.- 9.9 Choice of excitation conditions.- 9.9.1 Choice of X-ray tube.- 9.9.2 Choice of tube current and potential.- 9.10 Effect of background.- 9.11 Removal of background by polarization.- 9.12 Use of filters.- 9.13 Effect of the matrix.- 9.14 Analysis of limited quantities of material.- 9.15 Theoretical considerations I.- 9.16 Ultimate requirements in sample size.- 9.17 Handling of small samples.- Appendix 1(a).- Appendix 2(a).- Appendix 2(b).- Appendix 3(a).- Appendix 3(b).- Appendix 4.