TXRF technique detection limits

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). 

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]. 

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. 

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. 

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... 

Total reflection X-ray fluorescence (TXRF) spectrometry is a trace elemental microanalysis technique based on conventional energy dispersive X-ray fluorescence. It has become increasingly popular in the last decade and is applied in almost every field of trace elemental analysis where low detection limits and multielement capabilities are required. Like all X-ray techniques, TXRF is nondestructive making it extremely useful and important in areas where samples are precious and/or need to be used for further characterization. New tabletop instruments make this technique affordable and more versatile as it can be used also for field research. In the semiconductor industry, TXRF is now routinely applied to scan wafers for impurities on the surface and in near-surface layers. The following article introduces the basic principle of TXRF and its instrumental features and discusses various applications of this 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. 

TXRF is characterised by multi-element determination, matrix independent calibration and single-element internal standardization, and low detection limits. In addition, the technique requires very low sample masses. Another significant feature of TXRF is its inherent surface sensitivity which makes it useful for surface analysis, a topic not discussed in this book. 

Jg is the background intensity, t the measuring time and S the sensitivity (cps ppm or cps ng i). Either increasing the sensitivity or reducing the background leads to a reduction of detection limits. Therefore, special techniques of XRF, mainly EDXRF techniques, are applied, like TXRF, MXRF or SRXRF, as methods for trace element analysis. Detection limits range from ng to fg or pg g i to pg g... 

It is obvious that the sample preparation technique used influences the detection limits. Table 1 shows this influence on various samples from different fields of application. Table 2 gives an overview of applications of TXRF already analysed. Figure 3 shows a spectrum of a water standard reference sample (NIST 1643c) obtained with a TXRF vacuum chamber, constructed at Atominstitut, Vienna. Generally, an excellent field of application of TXRF in trace element analysis can be seen in liquid samples. All kinds of liquids, ranging from different kinds of water to acids and oils, as well as body fluids, can be analysed. Environmental samples, like airborne particles, plant material or medical and biological samples such as tissue can be analysed directly on a reflector. 

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