Download X-Ray Diffraction and Electron Microprobe Analysis: Techniques and Applications and more Study notes Hydrology in PDF only on Docsity!
X-Ray Diffraction
and Electron Microprobe
Analytical Instruments
21 September 2004
Agenda
- Collect homework
- Lecture: Survey of electron microprobe and
x-ray diffraction
- Tour of EMP and XRD labs; nine will start
in the EMP, the other nine in XRD. Switch
after 30-35 minutes.
What is X-ray Diffraction?
- A tool for determining the structure of
materials
- Single crystals or powdered samples made
up of a single phase can be studied.
- Powdered samples of multiple phases (e.g.,
rocks) can be quantitatively measured.
X-Ray Diffraction
(Info from UNM XRD lab)
- X-rays discovered by Roentgen in 1895.
- Three major uses of primary x-rays:
1) medical imaging
2) X-ray fluorescence (primary X-ray
produces x-rays in target sample allowing
chemical analysis
3) X-Ray crystallography to examine the
structure of materials
X-Ray Powder Diffraction
- Identification of single-phase materials such as minerals or synthetic phases (huge data base).
- Identification of a mixture of phases such as minerals in a rock or a combination of products and reactants.
- Crystal structure study.
- Clay mineral identification (requires special sample preparation).
- Identification of amorphous material in otherwise crystalline materials.
Advanced Powder XRD
- Structural analysis including unit-cell determinations.
- Quantitative analysis of modes of different phases in a mixture (by peak-ratio calculations).
- Crystallite size analysis (by broadening of x-ray peaks).
- Crystallite shape determination (by peak symmetry).
- Thermal expansion studies (if in-situ heating equipment is available).
Powder Diffractometer Schematic
Powder Diffractometer Example
From UNM web site
From Williams et al. (2001) Boron isotope geochemistry during diagenesis, Part I. Geochimica Cosmochimica Acta, 65 , 1769-1782.
Example: using XRD
to track structural
changes in clay held at
high P & T (350 ˚C)
Catches!
- To get believable results (especially for clay
minerals, determining crystal size
distribution, modal analysis), sample
preparation is very important.
- Difficult to detect the presence of one phase
if representing <5% of the sample.
- Best to have detailed discussions with lab
manager to ensure reliable results.
Single Crystal XRD
Single Crystal XRD
- Different instrumentation than the powder XRD.
- Put a crystal in the beam, observe reflections
and intensities at different orientations.
- Can learn everything about the structure.
- Inverting powder patterns to determine structure
is less reliable.
- Of course, you may not have a single crystal, or
realize that your sample has two phases, or be
able to align the crystal.
Problems With X-Ray Generation
- X-rays are isotropic, so only a small % from a traditional generator hits the sample. Increasing the electron beam hitting the X-ray target will increase the X-ray intensity (but the target melts)
- Cool the target (anode) with water. But it still melts with increasing electron beam current.
- Rotate the metal anode to distribute the heat over the target. This gives ~20X more X-ray intensity (but now it is complicated and may break more often).
Synchrotron X-ray Source
- A synchrotron is a way of keeping high-energy charged particles traveling in a circle.
- Acceleration of particles causes them to radiate.
- Radiation is intense, tunable, can be focused, pulsed, and polarized.
- Can study v. small samples (100 μm crystals) and do spectroscopy.
- Great for high-pressure experiments (small samples).
- Appropriate for materials returned to earth.
- Synchrotrons are uncommon.
Summary of XRD
- Both single crystal and powder diffraction
work can be performed at ASU
- The “advanced” techniques of XRD include
the study of crystal growth (rates and
mechanisms) and represent an important
research area in volcanology, metamorphic
petrology, and low-temperature diagenesis.
What is an Electron Probe?
- An instrument for quantitative analysis of
materials with a lateral resolution of a few
microns.
- Analyses are obtained through study of X-rays
generated by electron beam impact.
- Rapid qualitative analysis may be possible
because instruments often have additional
detectors for secondary electrons, back-
scattered electrons, and electron-induced light.
Secondary electron image
Relatively low energy electrons, derived only from top ~10Å of the sample surface. Diatom fossils.
Back-scattered Electrons (BSE)
Dark is low mean atomic number (Na 3 KAl 4 Si 4 O 16 -rich). Light is high mean atomic number (KAlSiO 4 -rich).
Another BSE Example
Quartz crystals (darkest) cemented by carbonate. Lithic fragments and detrital KAlSi 3 O 8 present. Cement shows some Mg and Ca variation.
Cathodoluminescence
<--CaCO 3 in transmitted light. Light emitted under electron bombardment. Variations related to chemistry and defects. <--- Same calcite. Pan- chromatic light detector. Close up of an area.
Counting X-rays
(Solid-State Detector)
- X-ray impact creates electron-hole pairs. Charges migrate to electrodes for detection. Number of charges is proportional to the energy of the X-ray (Energy Dispersive Spectrometry)
- EDS display is similar to the PIXE lab.
Which detector to use?
- Wavelength dispersive spectrometers have
higher resolution, and sensitivity (WDS
analysis). Can measure down to 20 ppm (in
favorable cases).
- Solid state detectors provide rapid analysis,
but EDS is less sensitive, and some argue
not as easily quantitative. Detection levels
between 0.4 and 0.8 wt.%)
Important Factors in
Quantitative Analysis
- Selecting suitable standards.
- Selecting the proper spectrometers (different analyzing crystals) and the order of analysis.
- Choice of appropriate conditions (electron current, energy, counting times on the different elements).
- Background correction approach.
- Matrix corrections, e.g., Z (mean atomic number), A (X-ray absorption), F (fluorescence).
Analysis is “Easy”
- CA(sp) = [IA(sp)/IA(st)]CA(st) Where:
- CA(sp)= concentration in specimen
- CA(st) = concentration in standard
- IA(sp) = X-ray intensity in specimen
- IA(st) = X-ray intensity in standard
- IA(sp)/IA(st) is known as the “K ratio” in electron probe labs.
- ZAF corrections can change CA(sp) by up to 20% or so.
Next Week
- Discuss homework
- Hand out new homework.
- No tour scheduled.
- Lecture on isotope ratios using SIMS, more
on electron probe (if time).
- Following week is IR/Raman lab
tour/lecture.
