Ray Fluorescence

X-ray fluorescence A method of analysis used to identify and measure heavy elements in the presence of each other in any matrix. The sample is irradiated with a beam of primary X-rays of greater energy than the characteristic X-radiation of the elements in the sample. This results in the excitation of the heavy elements present and the emission of characteristic X-ray energies, which can be separated into individual wavelengths and measured. The technique is not suitable for use with elements of lower atomic number than calcium. 

The methods most generally used are combustion and X-ray fluorescence. 

We will begin by a brief review of the concept of the X-ray fluorescence analytical method widely used in the petroleum industry for studying the whole range of products and for analyzing catalysts as well. 

The choice between X-ray fluorescence and the two other methods will be guided by the concentration levels and by the duration of the analytical procedure X-ray fluorescence is usually less sensitive than atomic absorption, but, at least for petroleum products, it requires less preparation after obtaining the calibration curve. Table 2.4 shows the detectable limits and accuracies of the three methods given above for the most commonly analyzed metals in petroleum products. For atomic absorption and plasma, the figures are given for analysis in an organic medium without mineralization. 

Elemental Analysis Atomic absorption spectrometry X-Ray fluorescence spectrometry Plasma emission spectrometry... 

Elemental and chemical-state resolution affords the possibility of detecting only a monolayer or even a fraction of a monolayer. This approach is prevalent in PD and in metiiods based on x-ray fluorescence. 

Figure Bl.25.1. Photoemission and Auger decay an atom absorbs an incident x-ray photon with energy hv and emits a photoelectron with kinetic energy E = hv - Ej. The excited ion decays either by the indicated Auger process or by x-ray fluorescence.

Acronyms abound in phofoelecfron and relafed specfroscopies buf we shall use only XPS, UPS and, in Sections 8.2 and 8.3, AES (Auger elecfron specfroscopy), XRF (X-ray fluorescence) and EXAFS (exfended X-ray absorption fine sfmcfure). In addition, ESCA is worth mentioning, briefly. If sfands for elecfron specfroscopy for chemical analysis in which elecfron specfroscopy refers fo fhe various branches of specfroscopy which involve fhe ejection of an elecfron from an atom or molecule. Flowever, because ESCA was an acronym infroduced by workers in fhe field of XPS if is mosf often used to refer to XPS rather than to electron spectroscopy in general. 

Figure 8.21 shows schematically a set of lx, 2s, 2p and 3s core orbitals of an atom lower down the periodic table. The absorption of an X-ray photon produces a vacancy in, say, the lx orbital to give A and a resulting photoelectron which is of no further interest. The figure then shows that subsequent relaxation of A may be by either of two processes. X-ray fluorescence (XRF) involves an elecfron dropping down from, say, fhe 2p orbifal fo fill fhe lx... 

Figure 8.21 The competitive processes of X-ray fluorescence (XRF) and Auger electron emission...

Figure 8.28 shows how the X-rays fall on the solid or liquid sample which then emits X-ray fluorescence in the region 0.2-20 A. The fluorescence is dispersed by a flat crystal, often of lithium fluoride, which acts as a diffraction grating (rather like the quartz crystal in the X-ray monochromator in Figure 8.3). The fluorescence may be detected by a scintillation counter, a semiconductor detector or a gas flow proportional detector in which the X-rays ionize a gas such as argon and the resulting ions are counted.

In scanning the wavelength range of the fluorescence the crystal must be smoothly rotated to vary the angle 6, and the detector must also be rotated, but at twice the angular speed since it is at an angle of 26 to the direction of the X-ray fluorescence beam. 

An alternative type of spectrometer is the energy dispersive spectrometer which dispenses with a crystal dispersion element. Instead, a type of detector is used which receives the undispersed X-ray fluorescence and outputs a series of pulses of different voltages that correspond to the different wavelengths (energies) that it has received. These energies are then separated with a multichannel analyser. 

Figure 8.29 X-ray fluorescence transitions forming (a) a K emission spectrum and (b) an L emission spectrum. The energy levels are not drawn to scale...

The example of a X emission X-ray fluorescence spectrum of a solid sample of tin, shown in Figure 8.30, shows four prominent transitions. The method of labelling the transitions is, unfortunately, non-systematic but those in the lower-energy group are labelled a and the... 

In the X-ray fluorescence spectmm of tin, as in those of other elements, transitions such as 3d- s and Ad- s, which are forbidden by the selection mles, may be observed very weakly due to perturbations by neighbouring atoms. 

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