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

1ppb 1ppt 1ppq 10nm 100nm 1 m m 10 m m 100 m m 1mm

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1ppb 1ppt 1ppq 10nm 100nm 1 m m 10 m m 100 m m 1mm 10mm Lateral resolution ICP - MS ICP - MS AAS ICP - OES ICP - OES S - XRF ED - XRF WD - XRF S - XRF OES OES EPMA EPMA m- XRF m- XRF XRF SIMS SIMS PIXE PIXE LA - ICP - MS LA - ICP - MS AAS Concentration Fig. 7.1. Analytical methods and their spatial resolution and concentration range it can be seen that micro-XRF already covers a relatively large concentration range together with an acceptable range of spatial resolution. Hence, it can be expected that this method will find a wide range of applications in the near future. With the availability of compact X-ray optics, laboratory instruments were made possible for micro-XRF [6–10]. All these instruments have a very similar configuration–an X-ray tube as source irradiates the sample and an energy dispersive detector system performs the spectrum acquisition. X-ray optical components can be found either between source and sample or between sample and detector. In the first case, a large solid angle of the X-ray tube radiation is captured and concentrated by the X-ray optic onto a small area of the sample. If the optic is placed between sample and detector a large area of the sample is irradiated but the fluorescence radiation is collected by the detector from only a small area of the sample. In both cases, the use of an X-ray optic defines a small sample volume analysed. In the case of using an X-ray optic between “source—sample” and “sample—detector” a confocal set-up is established which enables depth-sensitive investigations. This will be described in the last part of this chapter. The concentration of the source radiation onto a small sample area can be achieved very effectively by using a low power micro-focus X-ray tube. On the other hand when the X-ray optic is placed between sample and detector a high power X-ray tube is necessary in order to get sufficient fluorescence intensity from a small sample area [11]. In general, for the concentration of the source radiation onto the sample capillary optics (i.e. mono-capillaries or polycapillary lenses) are used. Also
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436 B. Kanngießer and M. Haschke for collecting the fluorescence radiation from a selected part of the sample, capillary optics are the best choice. However, in special cases it could be advantageous to use mirrors for collecting the fluorescence radiation. The peak-to-background ratio might be improved which enhances the sensitivity significantly. It should be noted that all the X-ray optics have an energy dependent transmission function and will, therefore, influence the spectrum finally mea- sured. For BRAGG-optics it is obvious that only the BRAGG-reflected radia- tion can be used. Placing a BRAGG-optic between source and sample a quasi- monochromatic excitation is produced which might be of interest for special applications. An example is the micro-XRF laboratory unit of XOS/Horiba which is especially designed for the analysis of sulphur in oil. It excites the S fluorescence radiation with a high efficiency but produces a low background yielding very low limits of detection [12].
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