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Ge 116 Module 2: Electron Probe Part 1: instrument basics, WDS X- ray analysis and standardization

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Ge 116 Module 2: Electron Probe Part 1: instrument basics, WDS X- ray analysis and standardization Slide 2 Electron Probe: Parts (5 of these) Slide 3 Electron Microprobe Instrumentation What Makes a Microprobe? High Stability Electron Source Focussing WDS X-ray Optics High Precision Stage Reflected Light Optics Beam Current (Faraday Cup) Slide 4 Electron Probe: WDS spectro- meters Slide 5 WDS Spectrometers This is a Cameca spectrometer, but JEOL design is pretty similar Crystals (2 pairs) Proportional Counting Tube (note tubing for gas) PreAmp Slide 6 Electron Probe: WDS spectrometers Rowland Circle: crystal moves in straight line away from sample focus point, detector must move along curve to remain on circle With vertical Rowland circle, can pack lots of spectrometers around machine but X-ray counts very sensitive to Z focus Slide 7 Electron Probe: diffracting crystals Crystal2dE (eV) K elements LiF [lithium fluoride, (200)] 4.027 3333-10841Ca to Ge PET [Penta- erythritol (001)] 8.742 1535 - 4994Si to Ti TAP [Thallium acid phthalate (001)] 25.76 521 - 1695O To Al LDE1 (W-Si) 61.1 220 - 716C to F LDE2 98.0 137 - 446B, C, N LDEB 145.0 93 - 302Be, B, C Slide 8 Electron Probe: diffracting crystals Slide 9 Pseudocrystals/LSMs Goldstein et al, p. 280 Slide 10 Crystals in our probe SP1: TAP, PET, LDE1, LDEB SP2: PET, LIF SP3: PET-L SP4: TAP, LDE2 SP5: PET-H, LIF-H Example protocol: SP1 TAP: Na, Mg SP2 PET: Ca, Ti SP3 PET-L: K, P SP4 TAP: Al, Si SP5 LIF-H: Mn, Fe Slide 11 Electron Probe: gas proportional counters With increasing bias, gas counters go from ionization chamber to proportional counting to Geiger cascade region (in proportional region, can use energy of X-ray to suppress counting of wavelength multiples) Slide 12 Electron Probe: gas proportional counters Trade-off between window thickness and gas pressure Gas flow P10 vs. sealed Xe Slide 13 Spectral Resolution WDS provides roughly an order of magnitude higher spectral resolution (sharper peaks) compared with EDS. Plotted here are resolutions of the 3 commonly used crystals, with the x-axis being the characteristic energy of detectable elements. Note that for elements that are detectable by two spectrometers (e.g., Y L by TAP and PET, V K by PET and LIF), one of the two crystals will have superior resolution. When there is an interfering peak and you want to try to minimize it, this knowledge comes in very handy. Reed, 1995, Fig 13.11, in Williams, Goldstein and Newbury (Fiori volume) Slide 14 Spectral Resolution Slide 15 Electron Probe: wavelength scans and peaking WDS has much higher spectral resolution than EDS, which is a big advantage, but peak shifts can be significant, so you have to check peak positions and match your standards well BaSO 4 PbS Al Ka Peaks Slide 16 Electron Probe: background correction Can use wavelength scans to find featureless regions near peak for background fitting. For standard elements, this has already been done; for exotics you have to do it yourself Low bkg High bkg Bkg under peak Slide 17 Quantitative Analysis Background-corrected on-peak counting rate on standard (composition known), normalized to Faraday cup current: I i std (counts/sec/nA) Background-corrected on-peak counting rate on unknown, normalized to Faraday cup current: I i unk (counts/sec/nA) Ratio I i unk / I i std is called the k-ratio. To first order it equals the ratio of element concentration in unknown to that in standard. Slide 18 Raw data needs correction This plot of Fe Ka X- ray intensity data demonstrates why we must correct for matrix effects. Here 3 Fe alloys show distinct variations. Slide 19 Absorption and Fluorescence The Fe-Ni alloys plot above the 1:1 line (have apparently higher Fe), because Ni atoms present produce 7.278 keV X- rays, above Fe K edge of 7.111 keV.Thus, additional Fe K are produced by this secondary fluorescence. The Fe-Cr alloys plot below the 1:1 line (have apparently lower Fe), because Fe atoms produce X-rays of 6.404 keV, greater than the Cr K edge of 5.989 keV. Thus, Cr K is increased while Fe K are used up. Slide 20 Z A F In addition to absorption (A) and fluorescence (F), there are two other matrix corrections based upon the atomic number (Z) of the material: one dealing with electron backscattering, the other with electron penetration (or stopping). These deal with corrections to the generation of X-rays. C is composition as wt% element (or elemental fraction). Slide 21 The matrix corrections assume that all elements present (and interacting with the X-rays) will be included. There are situations, however, where either an element cannot be measured, or not easily, and thus the analyst must make explicit in the quantitative setup the presence of unanalyzed element/s -- and how they are to be input into the correction. Unanalyzed elements Typically oxygen (in silicates) is calculated by stoichometry (which requires valence of cations). Elements can also be defined in set amounts, or relative proportions, or by difference although this later method is somewhat dangerous as it assumes that there are no other elements present. Slide 22 Some remarks on standards EPMAs claim to fame as a microanalytical tool rests upon (1) faith in a correct matrix correction and (2) use of good, correct, true standards. How do you know whether to trust a standard? Slide 23 Standards In practice, we hope we can start out using the best standard we have.* There have been 2 schools of thought as to what is the best standard is: a pure element, or oxide, or simple compound, that is pure and whose composition is well defined. Examples would be Si or MgO or ThF 4. The emphasis is upon accuracy of the reference composition. a material that is very close in composition to the unknown specimen being analyzed, e.g. silicate mineral or glass; it should be homogenous and characterized chemically, by some suitable chemical technique (could be by epma using other trusted standards). The emphasis here is upon having a matrix that is similar to the unknown, so that (1) any potential problem with the matrix correction will be minimized, and (2) any specimen specific issues (i.e. element diffusion, volatilization, sub-surface charging) will be similar in both standard and unknown, and largely cancel out. * This is based upon experience, be it from prior probe usage, from a more experienced user, from a book or article, or trial and error (experience comes from making mistakes!) It is commonly a multiple iteration, hopefully not more than 2-3 efforts. Slide 24 Standards - Optimally Ideally the standard would be stable under the beam and not be altered (e.g., oxidizable or hygroscopic) by exposure to the atmosphere. It should be large enough to be easily mounted, and able to be easily polished. If it is to be distributed widely, there must be a sufficient quantity and it must be homogeneous to some acceptable level. However, in the real world, these conditions dont always hold. Round Robins On occasion, probe labs will cooperate in round robin exchanges of probe standards, where one physical block of materials will be examined by several labs independently, using their own standards (usually there will be some common set of operating conditions specified). The goal is to see if there is agreement as to the compositions of the materials. Slide 25 Sources for standards : Purchased as ready-to-go mounts from microscopy supply houses as well as some probe labs ($1200-2000) Alternately, most probe labs develop their own suite of standards based upon their needs, acquiring standards from: Minerals and glasses from Smithsonian (free) Alloys and glasses from NIST (~$100 ea) Metals and compounds from chemical supply houses (~$20-60 ea) Specialized materials from researchers (synthesized for experiments, or starting material for experiments) both at home institution as well as globally (some $, most free) Swap with other probe labs Materials from your Departments collections, local researchers/ experimentalists, local mineral shop or national suppliers (e.g., Wards) Slide 26 Thoughts on beam current Is more beam current always better? No. Detector saturationtoo much deadtime Sample heating (example: Mica at 50 nA, 1 m spot -> 514 C of heating!) Element migration (Na diffuses away from beam, especially in hydrous glasses) Loss of spatial resolution (higher current = bigger beam) T = max temp rise E = accelerating potential in keV I = beam current in A k = thermal conductivity in W/mK d = beam diameter in m Slide 27 Reducing beam damage For beam-sensitive samples (like hydrous, high-alkali glass) Reduce beam current (trade-off with counts) Reduce counting time (trade-off with counts) Defocus or raster beam over area (need homogeneous, clear spots); need to run standards in same geometry Run sensitive elements in first pass Examine counts vs. time experiments to see rate of damage; extrapolate to zero time? Slide 28 Thoughts on temperature stability Electron microprobes only operate well in very constant temperature rooms Changing T causes thermal expansion of the diffracting crystals -> peak shifts Changing T changes pressure in gas- flow proportional counters -> changes counting efficiency Others? Slide 29 Some tricks Normalization With replicate data, can test which is more reproducible - raw or normalized data Secondary standards No physics here, just empirical adjustment for machine performance on a given day Slide 30 X-ray mapping Usually done in qualitative mode, that is not background corrected, normalized, or referenced to a standard Can introduce empirical two-point calibration to semi-quantify maps Map quality is a function of counting time, mostly Slide 31 X-ray mapping Slide 32 Statistics Measurement is always a statistical process, and mature understanding of the statistics is essential to proper interpretation of data There are several ways to look at the precision and sensitivity of e

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