Full-Field X-ray Fluorescence Microscopy Using a Color X-ray Camera
Ursula Elisabeth Adriane Fittschen , 1 * Oliver Scharf , 2 and Magnus Menzel 3 1 Washington State University , Chemistry Department , Pullman WA 99164-4630 2 IfG-Institute for Scientifi c Instruments GmbH , Rudower Chaussee 29/31 , 12489 Berlin , Germany 3 University of Hamburg , Institute for Applied and Inorganic Chemistry , Martin-Luther-King Plz. 6 , 20163 Hamburg , Germany
*
ursula.fi ttschen@wsu.edu Introduction
Elemental imaging of several elements simultaneously and with detection limits in the ppb range is achieved by synchrotron-based X-ray fluorescence microscopy, also often referred to as micro-X-ray fluorescence (MXRF). This has been shown, for example, by imaging Cu and U distribution in contaminated sediments [ 1 ] and P, Ca, and Zn distribution imaging of single cells and mitochondria [ 2 ]. A review on environmental application can be found in reference [ 3 ]. XRF micro-probes are available at synchro- trons all around the world and allow for 2D imaging with spatial resolution from several micrometers down to the nanometer range (30–100 nm); the latter mainly at third- generation synchrotrons. X-ray fluorescence (XRF) . Interaction of X-rays with matter is in general dominated by the absorption of photons to generate photoelectrons. Because of the relatively high energy of X-ray photons, core shell electrons are often targeted by this process. Relaxation of the core hole occurs by a transition of an outer shell electron and emission of the transition energy as either an Auger electron or a photon, usually in the X-ray energy range. The emitted fluorescent X-ray photon is characteristic of the excited element. This process is the basis for qualitative and quantitative determi- nation of the elements present in the specimen, as well as XRF microscopy. As in other types of microscopy, MXRF can be performed in scanning mode or in full-field mode (Figure 1). Full-fi eld MXRF . In the full-fi eld MXRF mode, the full sample is illumi- nated by the X rays from the source, and the fl uorescence is guided by an optic to the fl uorescence array detector. T is is illustrated in Figure 1a . Horizontal and vertical slit systems can be used to shape the beam. However, most MXRF setups operate in scanning mode, which means the sample is moved through a focused primary X-ray beam that excites the fl uorescent X rays. A single element fl uorescence detector can be used. T is is illustrated in Figure 1b . T e scanning mode comes with disadvan- tages regarding in situ applications where the sample must remain fairly static or where the sample is brittle or in other ways sensitive to movements.
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Here full-fi eld MXRF is advantageous. An example is the imaging of elemental distributions in droplets (10–20 µL containing Mn, Ni, Cu, and Sc) while drying. T is is shown in Figure 2 . T e droplets were allowed to dry undisturbed while the elemental information was recorded. Full-fi eld MXRF allows for fast imaging of large areas (for example, 12×12 mm 2 at 1,000 frames per second and 264×264 pixels) and therefore simultaneous detection of elemental changes over the entire fi eld of view, which can be important for certain in situ applications. However, the detectability of each element will depend on the fl uorescence yield of the element and the total counts acquired. T us, the recording frequency will be limited by the need to acquire enough counts for detecting specifi c elements. Full-fi eld MXRF also allows fast 3D elemental imaging by taking images at diff erent depths of the sample using a sheet beam.
Materials and Methods Energy-sensitive camera/detector . Full-fi eld MXRF in the
past has suff ered from low spectral (energy resolution) and low sensitivity, which were in part caused by the event processing and the low quantum effi ciency of the array detectors used, as well as by optics with low transmission for fl uorescent X-ray photons. Recently full-fi eld MXRF has become signifi cantly more powerful by the use of a two-dimensional energy-sensing camera/detector, increasing the sensitive thickness of the
Figure 1 : (a) Schematic of a typical full-fi eld MXRF setup. The entire sample is illuminated at once, and the spatially resolved elemental information is obtained by an array detector combined with a suitable optic. (b) Schematic of a typical scanning MXRF setup. The sample is scanned through a focused beam. The primary beam is usually at 45° to the sample surface.
doi: 10.1017/S155192951500022X
www.microscopy-today.com • 2015 May
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