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

Info icon This preview shows pages 456–458. Sign up to view the full content.

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
Image of page 456

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

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].
Image of page 457
Image of page 458
This is the end of the preview. Sign up to access the rest of the document.
  • Spring '14
  • MichaelDudley

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern