Total reflection XRF

All these advantages lead to lower limits of detection (LD) compared to the standard EDXRF mode. 

The basic measuring strategy of microscopic X-ray fluorescence analysis (g-XRF) is illustrated in Fig. 11.23. This microanalytical variant of bulk EDXRF is based on the localized excitation and analysis of a microscopically small area on the surface of a larger sample, providing information on the lateral distribution of major, minor and trace elements in the material under study. Essentially, a beam of primary X-rays with (microscopically) small cross-section irradiates the sample and in- 

Several variants of EDXRF with different performance and character are developed by changing the beam size of primary X-rays or the geometrical setup of the XRF instrument. Two of them, micro-XRF and total reflection XRF, will be introduced for their wide use or attractive prospects in metallo-mics and metalloproteomics fields. 

When a beam of primary X-rays with (microscopically) small cross-section irradiates the sample, it induces the emission of fluorescent X-rays from a micro-spot, which carries information on the local composition of the sample. If the sample is moved either manually or under computer control in the X-ray beam path, spot or line analysis will be possible. 

When a monochromatic X-ray beam impinges upon an (optically) flat material under a very small angle (typically a few mrad), which is below the critical angle, ffcru, for the substrate, total external reflection occurs. The critical angle is given by  

In the TXRF condition, X-ray photons will only interact with the top few nm of material and then be reflected. Part of material that is present on top of the reflecting surface will be irradiated in the normal manner, and will interact with both the primary and the reflected X-rays. As a result, the double excitation of sample by both the primary and the reflected beam occurs. Hence, the fluorescent signal is practically twice as intense as in the standard EDXRF excitation mode. 

in particular, is applied for multielement determinations in water samples of various nature and for the routine analysis of Si-wafer surfaces employed in the micro-electronics industry. The samples with a few nanometers thickness are prepared by placing a small drop of the sample (5-100 pL of the substance dissolved in an appropriate solvent) on a silica carrier, then evaporating the solvent. In life sciences, the method is used for measuring amounts of trace elements in various tissues and body fluid.  

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Hernandez-Caraballo et al. [91,92] evaluated several classical chemometric methods and ANNs as screening tools for cancer research. They measured the concentrations of Zn, Cu, Fe and Se in blood serum specimens by total reflection XRF spectrometry. The classical chemometric approaches used were PCA and logistic regression. On the other hand, two neural networks were employed for the same task, viz., back-propagation and probabilistic neural networks. 

Historically, analysis for selenium has been difficult, partly because environmental concentrations are naturally low. Indeed, selenium analysis still remains problematic for many laboratories at concentrations below 0.01 mg a relatively high concentration in many environments (Steinhoff et al., 1999). Hence, selenium has often been omitted from multi-element geochemical surveys despite its importance (Darnley et al., 1995). Analytical methods with limits of detection of <0.01 mgL include colorimetry, total reflectance-XRF, HG-AFS, gas chromatography... 

Multi-collector ICP-MS is a powerful technique for precise determination of isotope ratios, as demonstrated for isotopic analysis of Ir after fusion by Ulfbeck et al. (2003). Total reflection XRF (TXRF) can also be used to determine PGM, the advantages being the need for only a small sample volume and also multi-element capability. To lower the LOD, Messerschmidt et al. 

In their reviews in the Journal of Analytical Atomic Spectrometry series. Atomic Spectrometry Update and Atomic Mass Spectrometry and X-Ray Fluorescence Spectrometry, Bacon etal. (1991, 1993) include various variants of X-ray fluorescence spectrometric techniques such as synchrotron radiation XRF microprobe (SRXRF-microprobe), X-ray microfluorescence (XRMF), total reflection XRF (TXRF), and synchrotron radiation XRF (SRXRF). This tradition continues in reviews in this journal dedicated to X-ray fluorescence spectrometry (Potts et al. 2001,... 

Examples of applications of X-ray spectrometric analytical techniques to elemental determinations in a variety of materials are presented in Table 2.12. Some recent applications papers may be mentioned. Total reflection XRF has been applied by Xie et al. (1998) to the multielement analysis of Chinese tea (Camellia sinensis), and by Pet-tersson and Olsson (1998) to the trace element analysis of milligram amounts of plankton and periphyton. The review by Morita etal. (1998) on the determination of mercury species in environmental and biological samples includes XRF methods. Alvarez et al. (2000) determined heavy metals in rainwaters by APDC precipitation and energy dispersive X-ray fluorescence. Other papers report on the trace element content of colostrum milk in Brazil by XRF (da Costa etal. 2002) and on the micro-heterogeneity study of trace elements in uses, MPI-DING and NIST glass reference materials by means of synchrotron micro-XRF (Kempenaers etal. 2003). 

Sanchez HJ (1999) Theoretical calculations of detection limits in total reflection XRF analysis. X-ray Spectrom 28 51-58... 

Total reflection XRF (TXRF) is already a competitive method for environmental trace analysis. The principle is that the sample is irradiated in grazing incidence by a narrow collimator X-ray beam. The sample has to he a very thin film on a polished quartz or Plexiglass carrier. The beam is totally reflected by the sample carrier and the sample is passed and excited. The advantages of this method are (Sansoni, 1987 ... 

Relative detection limits are useful figures-of-merit for bulk XRF equipment, where it is usually relevant to know the lowest concentration level at which the spectrometer can be used for qualitative or quantitative determinations. In instalments where very small sample masses are being irradiated (e. g., in the pg range for microscopic XRF (p-XRF) and total-reflection XRF (TXRF)), the absolute detection limit is another useful figure-ofmerit since that provides information on the minimal sample mass than can be analysed in a given set-up. 

Wavelength-dispersive XRF instramentation is almost exclusively used for (highly reliable and routine) bulk-analysis of materials, e. g., in industrial quality control laboratories. In the field of energy-dispersive XRF instrumentation, next to the equipment suitable for bulk analysis, several important variants have evolved in the last 20 years. Both total reflection XRF (TXRF) and micro-XRF are based on the spatial confinement of the primary X-ray beam so that only a Hmited part of the sample (+ support) is irradiated. This is realized in practice by the use of dedicated X-ray sources. X-ray optics, and irradiation geometries. 

X-ray diffraction and X-ray fluorescence (XRF) spectrometry have been utilized in the differentiation of man-made fibers, but the sample amounts required are large, so that some modifications for single fiber analysis are required. Total reflection XRF has many advantages in this respect. 

Total reflection XRF instruments (see Figure 5) are based on the principle that when an X-ray beam strikes an optically flat surface at grazing incidence (i.e., at below the critical angle), the beam will suffer total reflection from the surface, in theory, without absorption or scatter. In these circumstances, a thin sample deposited on the surface will be selectively excited with considerable suppression of the background component, which would otherwise result from scatter events in the substrate. This specialized XRF configuration incorporates ED... 

Figure 5 Total reflection XRF instrumentation showing the arrangement of X-ray tube, reflectors, and ED detector. The sample is normally prepared by evaporating a sample solution onto the appropriate quartz reflector plate.

All EDXRF systems are able to modify the primary signal to control the excitation of the sample. This is achieved by different means, which result in three groups of systems direct excitation, secondary or 3D excitation, and total reflectance XRF (TXRF). 

Two impoilant variants of the method have appeared during the last 2 decades. Both variants are based on confining the volume in which the primary X-ray beam interacts with the analyzed material. In total reflection XRF (TXRF), by irradiating a flat sample with a neargrazing X-ray beam below the angle of total reflection, the in-depth penetration of the primary X-rays can be confined to a few tens of a nanometer below the surface allowing very sensitive surface analysis. Alternatively, the method can be exploited for bulk elemental trace analysis of liquids when these are brought on a clean inert surface (Wobrauschek 2007). 

XRF is therefore a suitable technique to provide qualitative and quantitative elemental information. Furthermore, synchrotron XRF and also other set ups like total reflection XRF (TXRF) are sensitive to low concentrations and can detect elements on the 10 ppb concentration level. 

X-ray intensity due to a particular element is proportional to the concentration of that element in the sample. There are two types of instrument in production those in which the emitted radiation is separated by wavelength using crystals as gratings, i.e., total reflection XRF (wavelength dispersive XRF WDXRF or total reflection XRF TRXRF) and those in which the radiation is not separated but identified by energy dispersive electronic techniques using solid-state detectors and multi-channel analysers, i.e., energy dispersive XRF (EDXRF). 

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