Docstoc

Electron probe microanalysis Electron microprobe analysis EPMA

Document Sample
Electron probe microanalysis Electron microprobe analysis EPMA Powered By Docstoc
					UW- Madison Geology 777


            Electron probe microanalysis -
            Scanning Electron Microscopy
                    EPMA - SEM

            Preface:
  What‟s EPMA and SEM all about?
   How does Geology 777 work?
               What can you learn?
                                        Updated 9/5/09
UW- Madison Geology 777




                              Why?
   You are taking this class because you need to be able to
   intelligently use one or both of the Department‟s electron
   beam instruments -- the Cameca SX51 electron microprobe
   in Weeks 306 or the Hitachi S3400 SEM in Weeks 308.
   Both instruments work on the same physical principles, but
   have differences.
   Traditionally this class has focused primarily upon the
   electron microprobe, but over the past few years more
   material has been added to address specific issues with
   increased usage of the SEM. It is a work in progress…
UW- Madison Geology 777




                          EPMA - what is it?
   EPMA is a tool to get precise
   and accurate quantitative
   chemical analyses of micron-
   size domains of our samples.
   A focused beam of high
   energy electrons interact with
   the atoms in the sample,
   yielding X-rays (and other
   signals), which we quantify
   and compare with counts from
   standards. It is nominally non-
   destructive.
UW- Madison Geology 777




                          SEM - what is it?
   SEM is a tool to produce
   images -- pictures -- of our
   samples. A rastered
   (scanned) beam of high
   energy electrons sweeps
   across the surface, interacting
   with the atoms in the sample,
   yielding backscattered
   electrons, secondary electrons,
   auger electrons, and in some
   cases photons in the visible
   light range (CL). It is
   nominally non-destructive.
UW- Madison Geology 777




                          EPMA - is it for me?
    This technique has its own characteristics, strengths, weaknesses. It pays
        to consider whether it is the best technique to get the information you
        need.
    1) It is a micro-technique, and for multiphase samples provides discrete
       compositions, not the bulk composition.
    2) It samples volumes (depths) on the order of ~1 um, limiting its
       usefulness for small inclusions or films.
    3) It provides major and minor element quantification, and has limited
       capacity for trace element analysis. (What do you mean by “trace”?)
    4) Despite being non-destructive, samples need to be mounted and
       polished; they can be reanalyzed many times.
    5) It is relatively inexpensive and accessible
UW- Madison Geology 777




                          SEM - is it for me?
    1) This technique is rather simple and one can learn the essentials in a
       short time.
    2) It provides images easily, though one needs to understand the various
       parameters (e.g. working distance, resolution, etc) to not make
       mistakes compromising image quality.
    3) Samples may be imaged with little or no preparation (coating,
       mounting+polishing), though this may complicate detailed
       examination.
    4) It is very easy to make mistakes using the easy EDS software,
       especially for attempts to get chemistry of small particles.
UW- Madison Geology 777




                          Goal of this course
     The goal is to provide useful background information to make
     SEM and EPMA less a „black box‟ for you and to help you
     make better decisions about how to analyze your samples,
     and to understand when data is good and when it is not.
     This class will provide the basic instructions for the use of our
     Hitachi SEM and Thermo-Fisher EDS. It will point out errors
     that can occur with EDS spectral interpretation.
     This course provides some directed exercises with our
     Cameca SX51. The electron probe is much more complicated
     than the SEM and experience has shown that individualized
     training is the best way to go. Which means this happens only
     when the student has his/her samples ready to analyze do we
     set up a 4-8 hour appointment.
UW- Madison Geology 777




              How this course is structured
         Weekly class meetings: ~1.5 hours, discussion of
         assigned materials (PP presentations, readings); students
         will be responsible for many of them
         Weekly quiz: at start of each class, on the assigned
         material
         Weekly labs: ~2 hours. Complete lab report and turn in
         following week
         Weekly assignments: Calculations and computer
         exercises.
         Each student will present an assigned published paper
         Projects
UW- Madison Geology 777




      Use for
   Reference--In
    Library on
     Reserve
         Goldstein et al,
         3rd Edition.
         2003 New:$75
UW- Madison Geology 777




         Also On Reserve in Geo Library




                     Reed (1996) 201 pages    Reed (1993)
                     Paper: New:$36           Paper: New:$55 Used:$35?
                     Hard:New: $95 Used $80   Hard:New ~$95
UW- Madison Geology 777




                          EPMA - “ideal” case
         Simple assumptions: We have stuck our sample in
         epoxy, cut and polished it (or made a thin section
         epoxied to a glass slide and polished it). There are
         standards, either user-supplied, or in the probe lab. We
         sign up on the schedule, get some “on the job” training,
         and analyze our samples. We return to our lab with the
         data we need.
UW- Madison Geology 777




                          The devil in the details
         Optimal case for “easy” EPMA: The samples are flat,
         well polished, conductive, non-porous, infinitely thick
         (to e- beam), homogeneous, clean. Standards exist and
         have the same 5 features. Materials are oriented at 90°
         to the electron beam (not tilted). Background positions
         well chosen with no peak or background interferences
         (in unknown and standard). Detector pulse distribution
         well centered. Constants for matrix correction (e.g.
         mass absorption coefficients) well known. Sample is
         not able to be altered by beam.
UW- Madison Geology 777




                  EPMA - worst case details
         Possible complications: The actual materials being
         probed are scratched or etched, insulators, porous,
         multiphase (eutectic) assemblages, with polishing oil in
         pores. 1” round has surface that is not normal to walls,
         resulting in tilted surface to electron beam. Background
         and peak positions have interferences. Detector pulse
         distribution on standard depressed (cut off) on low end.
         Mass absorption coefficients poorly known. Specimen
         is a particle -- or a thin film. Specimen is hydrous or
         sensitive to alteration or damage by the beam electrons.
         A main goal of this course is for you to understand
         when the optimal conditions are met -- and when they
         are not -- and if there is a way to make this thing work!
UW- Madison Geology 777




                          EPMA - so what to do?
         How to trust the results?: Evaluate “secondary”
         standards. “Should” get 100 wt% totals (98.5-101).
         Evaluate stoichometry if able to.
         How to get the best results: Get the sample preparation
         right. Have multiple standards for difficult samples.
         Take some time at the start: Don‟t be in a hurry with a
         new sample/suite of elements. Do wavescans first to
         consider peak and background interferences.