The electron microprobe is the perfect tool for chemical analyses of rock-forming minerals, and in recent decades the availability of microprobe data has led to major advances in many aspects of mineralogy and petrology. The ease of sample preparation, speed of analysis, and potential to evaluate the spatial relationship of the analysed mineral and its neighbours have led to it being the analytical method of choice in many situations. Indeed, if it were not for the capabilities of EPMA and the anticipated knowledge gained, the Apollo astronauts may not have been asked to return with lunar samples! (... but, while you're there, and have no golf balls remaining, ...)
To simply qualify is to perceive without measurement, and the microprobe extends our ability detect many different properties of a specimen. A researcher may simply want to know if iron is present, or if relatively abundant, without a need to measure it. Or, a researcher may want to know if there is more iron in one area relative to another. Both questions ask only to qualify, and indeed, a very important piece of information is to first determine presence and relative amount, without any need to measure exactly.
- Imaging is the best example of a qualitative tool available to the researcher, and the microprobe not only extends our ability to see many different types of information, but at a very small scale too.
To quantify is to measure and have confidence in the measurement. It is this capability of EPMA, especially with regard to its spatial resolution and ease of sample preparation, which makes it a unique and valuable tool. EPMA is the logical extension of optical petrography; that is, the study of a rock begins with the optical microscope. However, to visually identify a mineral as plagioclase feldspar is very important, but questions about genesis beg the question of exact sodium and calcium content. Furthermore, EPMA can be very accurate and precise (to within 2%), and detection for many elements can extend to below 100ppm.
Wait, there is more ... EPMA can provide you with distinct quantitative analyses at specific points, which are separated only by 3 thousandths of a millimeter!
As alluded to before, the technique needs to measure x-rays emitted from the sample, and it is therefore necessary that the x-ray energy and intensity overcome absorption on the way out of the sample. Energy is related to the element and the wavelength of the x-ray, whereas intensity is related to composition (... i.e., lots of atoms). So it is conceivable the x-ray has enough energy to escape absorption, ... or so many x-rays are generated some will escape. Relative to elements heavier than magnesium (At#=12), which can be detected to below 100ppm, the EPMA's sensitivity begins to suffer increasingly for elements lighter than sodium (At#=11). All other elements can be measured with good to excellent precision, but each element's sensitivity needs to be understood relative to it's x-ray's wavelength.
Specific X-ray wavelengths can only be accommodated by specific wavelength discriminating crystals. For example, the lithium fluoride (LiF) crystal can only accommodate wavelengths between 0.30 and 0.10 nanometer, which for K-shell ionizations, is elements Sc (21) through Br (35). Synthetic crystals are required for longer wavelengths ... e.g., TAP for fluorine (9) through silicon (14), and PET for silicon through Ti (22). For even longer wavelengths, manufactured x-ray reflectors are required ... e.g., PC0 for nitrogen (7) through fluorine (9), PC2 for boron (5) through oxygen (8), and PC3 for beryllium (4) through boron. Our Cameca is configured with all these possibilities.
Precision is one aspect of good analytical technique. Accuracy, on the other hand, is only as good as the reference standard the measurement is relative to. This Electron Microprobe Facility's standards' library is adequate enough measure all elements which occur in nature at levels capable of EPMA. Our library is also diverse enough for accommodating lighter elements in a very similar chemical matrix ... e.g., aluminum in feldspar versus garnet. Researchers interested in our facility should contact us regarding elements of interest and anticipated levels.
More on the EPMA technique can be found here. Also, Dr. James Wittke, Northern Arizona University, has made available excellent and enlightening EPMA graduate course notes. Thanks to Northern Arizona University's Electron Microprobe Laboratory.
- Dr. Aphrodite Indares: Selected major & trace elements in metamorphic minerals, such as garnet, pyroxene, feldspar, and biotite; compositional maps of garnet
- The concepts of modal analysis with digital images
- Other MUN ESD research cases (coming soon)