A Dissertation Methodology
The electro-microprobe technology
Modern geochemists (Donovan et al, 2003) generally agree that modern electron microprobe analysis (EMPA) is bringing the most precise and most accurate chemical analyses for solid specimens today. Moreover, these highest-quality results can be brought to routine achievements under carefully crafted analytical protocols. The modern EMPA technology already incorporates qualitative and quantitative analytical methods in a non-destructive mechanism even from micron-sized samples of surface materials at a sensitivity of many parts per million (ppm) (Cameca, 2012). In fact, a number of EMPA equipment today can incorporate one or two others of the four major surface and interface analytical techniques, such as SIMS, LEXES and APT, in addition to the usual microscopy tools available in older generation equipment. The latest Cameca EPMA machines (e.g. SX5, SX5FE, and Shielded 5X), for instance, can also perform SEM techniques. In effect, the electron microprobe and its improving technology makes the work of the geochemist more accurate, more precise, less time-consuming, and. thus, essentially more convenient.
Other concerns may as well be towards the development of mass cleaning techniques to speed up results turnaround time without the risks of contamination and sample alteration.
Electron microprobe analytical mechanism: Also referred to as the nuclear microprobe analysis (NMPA), the modern EMPA is a quantitative particle-beam analytical technique that conducts ratio analysis of the X-ray counts obtained from samples and those from standards in order to identify specific chemicals of interest and their respective concentration levels (Lane & Dalton, 1994; University of Minnesota [UM], 2016). Commonly used general standards, for instance, include pyrite, rhodonite, stibnite, synthetic troilite, and metals (Zakrzewski & Nugteren, 1984). It, however, makes the assumption that the material composition found in the sample is similar to that found in the standard. In case of significant difference between their material compositions, a matrix correction factor called ZAF is integrated into the computational equation. The ZAF consists of three corrections: the atomic number correction (Z); the absorption correction (A); and the characteristic fluorescence correction (F).
The functional mechanism of EMPA is well understood. Using a series of electromagnetic (EM) lenses, the electron microprobe, such as the CSIRO Nuclear Microprobe, sends a focused beam of accelerated electrons (typically, 5-30 kV) to the specimen surface. These high-energy electrons interacts with the target chemical particles, producing X-ray particles (quantified as “counts”) even within a small specimen volume, which is typically 1 to 9 cubic microns (μ3) (UM, 2016; Cameca, 2012). Birbilis et al (2015) called this mechanism as the micro-particle induced X-ray emission (Micro-PIXE) mechanism.
Lead (Pb) and zinc (Zn) have unique and specific X-ray characteristics, which are detected at specific wavelengths through wavelength dispersive spectroscopy (WDS) using an energy dispersive X-ray (EDX) spectrometer (e.g. Link Systems AN1000), and were pre-calibrated, or simultaneously compared, with their respective pure or high-content standards. The WDS technology operates on the basis of the Brigg’s law. Subsequently, the X-ray spectra of Pb and Zn are collected with a specific accelerator potential (e.g. 15 kV), a current (e.g. ca. 5 nA), and a counting time (e.g. of 100s) (Poitrasson, Chenery, & Shepherd, 2000; Cameca, 2012).
Analytical qualities: The EMP analytical technique has high sensitivity and spatial resolution with individual analyses usually as short as a minute or two in most cases (UM, 2016). The precision of EMPA can be about 1% relative standard deviation (RSD) for most major geological elements (except Thorium [Th] and light rare earths) and estimated to be 2-4% RSD (Poitrasson, Chenery, & Shepherd, 2000). Developments in the last 30 years, however, have contributed in narrowing the error distribution from the earlier ±5% RSD with the use of element standards to ±2% RSD when using alloy standards (Pouchou & Pichoir, 1991).
Donovan et al (2003) insisted that consistently highest-quality chemical analyses of solid materials can be routinely achieved using modern EMPA technology only under carefully designed procedures. These procedures include high-quality standards, stable and reliable equipment, meticulous sample preparation, and precise automated analytical execution, and proper data treatment for analysis.
Sample preparation
Geller and Engle (2002) prescribed a more sample-oriented approach to the preparation techniques as the large variety of samples make it unlikely to yield to mass production approaches. It means no automatic sample polishing should ever be used in EMPA sample preparation. Instead, manual techniques designed for each sample should be established as standard protocol for sample preparation.
Collection: Samples do not reach the mineralogical laboratory ready for EMPA. Often they came from enumerable sources and in various forms: corrosion scales on host surfaces; powders; coated substrates; or even bulk specimens (e.g. ceramics, metals, glasses, plastic, or organic substances). From collection to transportation to the final transfer in the analytical laboratory, contamination and sample alteration are highly possible. Geller and Engle (2002) warned that handling opens up to the entry of extraneous matters. Even preparatory procedures for transportation can introduce contamination as the in-field sawing or polishing tools may embed themselves into the samples. In a sense, even a pure contaminant does not represent the original sample with which analytical interest belongs. The crucial point at collection is to ensure that no other material is added to the sample, which may be mistaken as part of the analyte (Geller & Engle, 2002).
Mounting: Collected specimens must be embedded for grinding and polishing into acceptable pre-analysis characteristics. The Pb-Zn deposit must be held down in a stable state so that appropriate geometries can be easily defined for the eventual contact between the electron beams during the EPMA process (Geller & Engle, 2002). Slow curing epoxies are considered the best mounting medium because it cure harder; thus, providing better edge retention for the grinding process. Media with poor edge retention (e.g. acrylic) decompose easily under electron beam bombardment. Hot mount materials, which are hardened under high temperature and pressure, can cause particle redistribution. Drill point holes should be created where particles of interest can be placed. Large samples (e.g. can held manually or using a mechanical device during the grinding) may not need mounting.
Grinding: The hardened mount should then be grounded successively into finer grit abrasives while monitoring the location and depth of the markers below the mount surface (Geller & Engle, 2002). The process must continue until the sample particles are getting exposed or at least nearly exposed. The purpose of grinding is the creation of geometrical areas or surfaces wherein beam bombardment can occur during EMP procedure. However, it is inevitable that grinding can lead into surface relief formations. These irregularities lowers surface conductivity as much as thicker coats do, consequently decreasing the count rates. Consequently, the excellent samples have reduced surface relief; thus, are cleaner, smoother, and flatter. That feat requires excellent sampling preparation as well as polishing techniques. Birbilis et al (2015) suggested the use of ethanol in cleaning sample surfaces after each grinding step, and then drying them with compressed air.
Moreover, the samples must be reduced in size (as far down as possible) to fit into the analytical position in the EPMA chamber. However, highly aggressive methodology can permanently damage the mineral chemistry or morphology or both. Sizing may be achieved through cleaving, scribing fractioning scissors, sawing, cutting, torching, and wire electro discharge machining (EDM) (Geller & Engle, 2002). Sample deformation continues to be the primary concern in this preparatory stage.
Final polishing: Jercinovic and Williams (2005) suggested the use of colloidal silica as a final polish to substantially improve surface characteristics when applied during sample preparation. However, preparation does not end with the final polish.
The EMPA is a trace-element technique that requires precise and accurate analysis of Pb and Zn, both may be present at levels below 1000 ppm. Thus, an exceptionally high beam current (typically, hundreds of nA) for long count times (≥10 min) is needed to optimize counting statistics while maintaining maximal spatial resolution through a small beam size (Jercinovic & Williams, 2005). Thus, a high beam power density (100-1000 μW/cm2) may be necessary; but, increasing the risk of sample damage and charge effects. To prevent this, a carbon coat (250 A thick) or thin gold coat should be applied on the specimen surface. These coats also can significantly improve electrical performance. Moreover, gold coats possess substantially lower electrical resistivity than carbon coats.
Oftentimes, the presence of surface relief creates polishing-hardness that can result to loss of surface continuity of the coat and, thus, to charging effects (Jercinovic & Williams, 2005). This surface issue again must be managed to ensure that surface continuity between the coat and the final polish. Surface coatings, particularly metallic coats (e.g. gold), increase the absorption characteristics of the samples and the standards (Jercinovic & Williams, 2005). One major issue in the use of conductive coats involves potential interferences in X-ray characteristics, especially when using evaporated metals as coats. For instance, a heavy gold coat can create measurable interference between AuMγ and PbMβ counts.
Pre-analysis sample characteristics: Donovan et al (2013) insisted that samples or unknowns should also be clean, flat, and smooth on their surfaces, even including their carbon or gold coat. This ensures that the geometry between the sample and the EMPA electron beam (the takeoff angle) had to be known accurately as angular errors, even small ones, can bring significant errors into the final measurements. Geller and Engle (2002) required that any known limiting factor in the analysis must be addressed beforehand to ensure accurate results.
Sample analysis
Before any Pb-Zn sample analysis can begin, the electron microprobe must go through a standard start-up procedure, filament saturation, and then calibration. That is a long procedure, especially when using EMP equipment designed to process and to analyze large samples, starting from filling the cold trap with liquid nitrogen to the calibration of the element table or the individual elements of interest (Hagan, 1989). Within the quantitative analytical procedure itself, a calibration run should be conducted first.
Calibration of the primary standards: The choice of calibration standards can be as important as the sample preparation process itself in reducing analytical problems under the electron microscope (Geller & Engle, 2002). If the standard varies significantly from the samples, a ZAF adjustment must be performed. If the standard chosen is of the same composition as the unknown Pb-Zn samples, no such adjustment will be necessary. Thus, Donovan et al (2003) insisted that standards should be stable, homogeneous, of well-defined characteristics, containing a high concentration of the element of interest, and with a matrix similar to the unknowns; that is, chemically similar (Hagan, 1989), including the thickness of their carbon or gold coating. This homogeneity standard ensures minimal interference during analysis and, thus, also the need for a correction matrix (Jercinovic & Williams, 2005).
Thus, each element of interest from the Pb-Zn mine samples (e.g. Pb or Zn) is required to have its own respective primary and secondary standards as the case may demand. Primary Pb standards, for instance, may include cerussite (PbCO3) and alamosite (PbSiO3) (Donovan, et al., 2003). Cerussite has a molecular weight of 267.21 grams, 77.54% of which is Pb content (molecular weight: 207.19 g) (Webmineral, 2016a). Meanwhile, alamosite has a molecular weight of 283.28 grams and 73.14% Pb (Webmineral, 2016b). Other Pb containing minerals, such as boulangerite or bournonite (Zakrzewski & Nugteren, 1984), may also be used as known standards. Nonetheless, mineral standards of high characterization can be found in museums (e.g. Smithsonian Institution in Washington, DC), outside of which they are normally difficult to obtain; often available only on loan or trade. However, suppliers of certified pure elements may have the needed elements readily available. Alternatively, simple compounds may also be used.
Electron microprobe analysis: The Pb-Zn mine samples and the electrochemically dissolved pure or high-content standards of known Pb and Zn concentration are loaded into the EMPA chamber and bombarded with a focused proton beam with a specific energy level (e.g. 3.0 MeV) coming from a Pelletron accelerator (Birbilis et al., 2015). The resultant micro-particle induced X-ray emissions are then measured over an emission chamber of an appropriate size (e.g. 200 x 200 μm area), using a beam spot (e.g. 2 μm) with an appropriate current range (e.g. 0.3 to 0.5 nA). An appropriate filter may be used on the X-ray detector to limit the expected high count rate coming from the standards. A built-in program (e.g. MicrodaQ) collects the corrected dead-time image data for automated analysis.
References
Birbilis, N., Cain, T., Laird, J.S., Xia, X., Scully, J.R., & Hughes, A.E. (2015). Nuclear
microprobe analysis for determination of element enrichment following Magnesium dissolution. ECS Electrochemistry Letters, 4(10): C34-C35.
Cameca. (2012). Qualitative and quantitative elemental micro-analysis. Cameca.com. Retrieved
Donovan, J.J., Hanchar, J.M., Picolli, P.M., Schrier, M.D., Boatner, L.A., & Jarosewich, E.
(2003). A re-examination of the rare-earth-element orthophosphate standards in use for electron-microprobe analysis. The Canadian Mineralogist, 41(1): 221-2320-.
Geller, J.D. & Engle, P.D. (2002, November-December). Sample preparation for electron probe
microanalysis – Pushing the limits. Journal of Research of the National Institute of Standards and Technology, 107(6): 627-638.
Hagan, R. (1989, May 31). Microprobe operating procedure. Washington, DC: U.S. Nuclear
Regulatory Commission.
Jercinovic, M.J. & Williams, M.L. (2005). Analytical perils (and progress) in electron
microprobe trace element analysis applied to geochronology: Background acquisition, interferences, and beam irradiation effects. American Mineralogist, 90(1): 526-546.
Lane, S.J. & Dalton, J.A. (1994). Electron microprobe analysis of geological carbonates.
American Mineralogist, 79(1): 745-749.
Poitrasson, F., Chenery, S., & Shepherd, T.J. (2000). Electron microprobe and LA-ICP-MS study
of monazite hydrothermal alteration: Implications for U-Th-Pb geochronology and nuclear ceramics. Geochimica et Cosmochimica Acta, 64(19): 3283-3297.
Pouchou, J.L. & Pichoir, F. (1991). (Chapter title not available). In. Heinrich, K.F.J. & Newbury,
D.E. (eds.). Electron probe quantitation. New York: Plenum Press.
Retrieved from: http://probelab.geo.umn.edu/electron_microprobe.html <15 Jan. 2016>
Webmineral. (2016a). Cerussite mineral data. Webmineral.com. Retrieved from:
http://webmineral.com/data/Cerussite.shtml#.Vpjpvk-52-s <15. Jan. 2016>
Webmineral. (2016b). Alamosite mineral data. Webmineral.com. Retrieved from:
http://webmineral.com/data/Alamosite.shtml#.Vpjr7k-52-s <15 Jan. 2016>
Zakrzewski, M.A. & Nugteren, H.W. (1984). Mineralogy and origin of the distal
vocanosedimentary deposit at the Hallefors silver mine, Bergslagen, Central Sweden. Canadian Mineralogist, 22(1): 583-593.
