TXRF technique

TXRF is a descendant of conventional energy-dispersive XRF, however, with detection limits improved by 4-5 orders of magnitude, to date below KT g. It is important to point out that the TXRF technique differs fundamentally frcnn classical X-ray fluorescence (EDXRF), in sample preparation, calibration, data analysis and detection performance, as well as in the objects under investigation. 

Three techniques involving the use of X-ray emission to obtain quantitative elemental analysis of materials are described in this chapter. They are X-Ray Fluorescence, XRF, Total Reflection X-Ray Fluorescence, TXRF, and Particle-Induced X-Ray Emission, PIXE. XRF and TXRF use laboratory X-ray tubes to excite the emission. PIXE uses high-energy ions from a particle accelerator. 

The term direct TXRF refers to surface impurity analysis with no surface preparation, as described above, achieving detection Umits of 10 °—10 cm for heavy-metal atoms on the silicon surface. The increasit complexity of integrated circuits fabricated from silicon wafers will demand even greater surfrce purity in the future, with accordingly better detection limits in analytical techniques. Detection limits of less than 10 cm can be achieved, for example, for Fe, using a preconcentration technique known as Vapor Phase Decomposition (VPD). 

The sensitivity enhancement achieved by VPD is determined by the ratio of the substrate area to the area of the detector aperture (analyzed area), provided there is fiill collection of the impurities. This has been demonstrated for Fe and Zn. For Cu and Au, however, only a small percentage can be collected using this technique, due to electrochemical plating. An example comparing direct TXRF with VPD-TXRF on the same substrate is shown in F%ure 4. 

Atomic absorption spectroscopy of VPD solutions (VPD-AAS) and instrumental neutron activation analysis (INAA) offer similar detection limits for metallic impurities with silicon substrates. The main advantage of TXRF, compared to VPD-AAS, is its multielement capability AAS is a sequential technique that requires a specific lamp to detect each element. Furthermore, the problem of blank values is of little importance with TXRF because no handling of the analytical solution is involved. On the other hand, adequately sensitive detection of sodium is possible only by using VPD-AAS. INAA is basically a bulk analysis technique, while TXRF is sensitive only to the surface. In addition, TXRF is fast, with an typical analysis time of 1000 s turn-around times for INAA are on the order of weeks. Gallium arsenide surfaces can be analyzed neither by AAS nor by INAA. 

VPD-TXRF is also a facile technique for interface analysis [4.78, 4.79]. Automated VPD equipment (Fig. 4.16) improves both the detection limit (upper range 10 atoms cm ) and the reliability (by > 50%) of the VPD-TXRF measurement [4.14]. Current research focuses on sample holders [4.80, 4.81] and light-element detection capability [4.82-4.84]. 

Different analytical procedures have been developed for direct atomic spectrometry of solids applicable to inorganic and organic materials in the form of powders, granulate, fibres, foils or sheets. For sample introduction without prior dissolution, a sample can also be suspended in a suitable solvent. Slurry techniques have not been used in relation to polymer/additive analysis. The required amount of sample taken for analysis typically ranges from 0.1 to 10 mg for analyte concentrations in the ppm and ppb range. In direct solid sampling method development, the mass of sample to be used is determined by the sensitivity of the available analytical lines. Physical methods are direct and relative instrumental methods, subjected to matrix-dependent physical and nonspectral interferences. Standard reference samples may be used to compensate for systematic errors. The minimum difficulties cause INAA, SNMS, XRF (for thin samples), TXRF and PIXE. 

Due to the different working principles of WDXRF and EDXRF, the applications differ strongly (Table 8.43). Simultaneous WDXRF with ten channels (elements) and increased sensitivity for the low atomic number elements (e.g. a few ppm of phosphorous in a low atomic number matrix) has been used for QC of polymer granules [252], To detect elements at trace levels (ppm-ppt), generally the special XRF modes, mainly EDXRF techniques, are applied like TXRF, SR-XRF or pXRF. Detection limits with SR-XRF are now at the attogram level. 

Activation analysis is based on a principle different from that of other analytical techniques, and is subject to other types of systematic error. Although other analytical techniques can compete with NAA in terms of sensitivity, selectivity, and multi-element capability, its potential for blank-free, matrix-independent multielement determination makes it an excellent reference technique. NAA has been used for validation of XRF and TXRF. 

In the field of RM certification, NAA represents a major analytical technique. It possesses unique quality assurance and self-verification aspects. Not surprisingly, therefore, NAA has been used to certify NIST standard reference materials [470]. By analogy, NAA has also been instrumental in analysing the EC polymer reference materials within the framework of the PERM project [1]. NAA was also used to validate a TXRF procedure for the determination of additives containing Ti, Zn, Br, Cd, Sn, Sb and Pb [56],... 

In the simplest case, experimental calibration can be carried out by direct reference measurements where the sensitivity factor b is given by the relation of measured value to concentration of a reference material (RM), b = yRvi/xRv,. Direct reference calibration is frequently used in NAA and X-ray analytical techniques (XRF, EPMA, TXRF). 

TXRF was used to characterize high-viscosity polymer dispersions [83], with special attention being paid to the different drying techniques and their effect on the uniformity of the deposited films. TXRF was also used as a means to classify different polymers on the basis of their incoherently scattered peaks [84], Dispersive XRF has been used to assess the level of aluminum in antacid tablets [85]. 

In addition to the electrochemical techniques, many surface analytical techniques are constantly in use, such as ellipsome-try for the surface thin oxide thickness, multiple reflection infrared spectroscopy (MIR), and X-ray photoelectron spectroscopy (XPS) for surface layer composition, total reflection X-ray fluorescence spectroscopy (TXRFS) for the metal surface contaminants, and naturally atomic force microscopy (AFM) for the surface roughness profile. 

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. 

EAAS and ICP-AES (Ihnat 1984). Klock-enkamper (1997) has a plot of absolute TXRF DLs for residues of aqueous solutions versus element atomic number, under various experimental conditions, and an instructive graphical presentation of actual TXRF detection limits for real samples after specific preparations. Tblgyessy and Klehr (1987) compare detection limits of some analytical methods, including classical methods, AAS, LAS, polarography, mass spectrometry, NAA, and isotope dilution analysis, and include more detailed DL information for NAA techniques. A nice comparison of DLs (in pg) for NAA, ETV-ICP, GF-AAS and ETV-ICP-MS is tabulated by Dybczynski (2001). Naturally, a wealth of information is available in handbooks, an example of which is the one by Robinson (1974) containing detailed listings for various spectroscopic methods. 

In TXRF, involving irradiation of an optically flat sample with a parallel X-ray beam below the angle of total reflection, the depth penetration of the primary X-rays is confined to a few tens of nanometers below the surface. The technique of a-XRF, based on the confinement of the analytical region of the sample, involves the localized excitation and analysis of a microscopically small area of the surface, providing information on the lateral distribution... 

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,... 

Reus U, Markeit B, Hoffmeister C, Spott D, Guhr H (1993) Determination of trace metals in river water and suspended solids by TXRF spectroscopy A methodical study on analytical performance and sample homgeneity. Fresenius J Anal Chem 347 430-435 Sanchez HJ. (2001) Detection limit calculations for the total reflection techniques of X-ray fluorescence analysis. Spectrochimica Acta 56 2027-2036... 

---