[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

The sample preparation technique can be improved by

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The sample preparation technique can be improved by reducing the size of the sample drop. Hence, the drying time is reduced significantly [29]. The drop size reduction is possible with a nanoliter injector similar in operation to that of an ink-jet printer that generates drops down to 2 nl. The drying time is reduced down to a few seconds because of the relatively larger surface area of the drops. Smaller dried spots can be generated by smaller drops, as well. Then the dried sample can be excited completely with the micro-beam of the spectrometer. In addition, the loss of volatile elements is minimised by the short evaporation time. The recovery rates should be better than for the larger droplets. Extended examinations of this nanodroplet technique [29] show an- other advantage—the sensitivity is enhanced, especially for the determination of absolute concentrations. Some typical limits of detection are displayed in Fig. 7.15. Limit of detection for Nano-droplets 2.5 1.5 0.5 2 1 0 LOD / pg Al Cl Ca Co Cu Zn Zr Fe Si Ni S P Pb Sr Y Fig. 7.15. Limits of detection obtained with micro-XRF using the “nano-droplet” technique [from 29]
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462 B. Kanngießer and M. Haschke This application shows that by using a special sample preparation tech- nique micro-XRF becomes a very sensitive analytical method that is applica- ble for ultra trace elemental analysis. The limits of detection are in the low and sub-pg range. 7.1.4 3D Micro X-Ray Fluorescence Spectroscopy The previous sub-chapters described the use of the microscopic excitation spot formed with the help of X-ray optics to perform very local analysis by lateral two-dimensional (2D) elemental mapping or line-scanning along the sample surface. The information retrieved has the disadvantage that it is not expli- citly depth sensitive, rather a convoluted product of the incident X-rays, pene- tration capability and the self-absorption correction depending on both the energy of the exciting radiation and the energy of the fluorescence radiation. Not long ago, Kanngießer et al. [30] and other groups expanded the capa- bilities of the micro-XRF spectroscopy to the depth, thus creating a true three- dimensional (3D) micro X-ray fluorescence spectroscopy (3D micro-XRF). The 3D micro-XRF is realised by a confocal arrangement, which consists of X-ray optics in the excitation as well as in the detection channel. A microvolume is defined by the overlap of the foci of both X-ray optics. If the sample is moved through this microvolume, its chemical composition can be non-destructively investigated not only laterally along the surface but also within the sample at the depth of interest, see Fig. 7.16 Furthermore, if synchrotron radiation is used as exciting radiation, the excitation energy is tunable, which facili- tates micro-XAFS investigation for chemical speciation at the same spot in the sample. Thus, not only depth information concerning the elemental dis- tribution but also chemical speciation and phase information are obtainable at the same point of interest in the sample.
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