Kenneth Pye Associates Ltd. - www.kpal.co.uk Kenneth Pye Associates Ltd.
 Scientific Research, Consultancy and Investigations

                     
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Scanning Electron Microscopy and Chemical Microanalysis
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Scanning Electron Microscopy

Secondary electron (SE) imaging is conventionally used to obtain high quality images of small objects with high resolution of fine surface detail.  For best results samples require coating with carbon or gold. However in most circumstances uncoated samples can be examined with a minimum of preparation using a variable-pressure SEM instrument, such as the Hitachi S-2600N.  SE images provide topographic information, while backscattered electron (BSE) images provide both topographic and compositional (atomic number) contrast. Applications include studies of quartz grain surface textures, industrial minerals, dust contaminants (including asbestos), glass fragments and gunshot residue particles.

Specifications for our Hitachi variable-pressure S-2600N scanning electron microscope are as follows:

  • Image resolution: 4.0 nm (SE mode) and 5.0 nm (VP mode)

  • Magnification range: 15x to 300,000x

  • Accelerating voltage: 0.5kV to 30kV

  • Variable pressure range: 1 to 270 Pa, selectable in 25 steps 

  • Maximum sample dimensions: 60 mm

  • Imaging detectors: Secondary Electron (high vacuum) and Backscattered Scanning Electron (4-quadrant solid-state)

  • Image capture: 1280 x 960 pixels

  • PGT energy-dispersive x-ray microanalysis system (see below)

    
Hitachi S-2600 Scanning Electron Microscope with                    Bluebell pollen grains (uncoated BSE VP-SEM
PGT energy-dispersive x-ray microanalysis system                      image)

Scanning electron micrograph of a sand grain    
Sand grain with extensive overgrowth development           Quartz grain with well-developed euhedral form
(uncoated BSE VP-SEM image)                                      (uncoated BSE VP-SEM image)

 

    
 Quartz sand grain with Na-Feldspar inclusions                  Polymineralic clay pellet (uncoated BSE VP-SEM
 (uncoated BSE VP-SEM image)                                      image)

 

    
Uncoated polished section of fine-grained concrete            Uncoated polished section of a sample of Chalk
(BSE VP-SEM image)                                                      rock (BSE VP-SEM image)

 


A montage of SEM images of a sandstone block (source: D.H. Krinsley)

Energy Dispersive X-Ray Microanalysis

The elemental composition of particles as small as 2 to 3 µm in diameter can be determined using an energy-dispersive x-ray microanalysis system attached to the SEM.  This method of analysis can also be used to obtain an indication of the bulk composition of powdered samples of soil or rock on the basis of elemental peak height ratios. Semi-quantitative results can be obtained if standards are also analysed, but fully-quantitative analysis is usually undertaken using a dedicated electron microprobe.

Specifications for our PGT Avalon 8000 X-ray microanalysis system are as follows:

  • Permanent thin window and light-element spectrometer

  • 10 mm square active area, 133 eV FWHM resolution

  • Detection down to boron

  • Computer selectable pulse processing modes

  • Count rates exceeding 40,000 counts per second

  • X-ray mapping capability

  • Spirit data processing software (includes x-ray mapping, line profile analysis and area fraction determination)

 


Energy-dispersive x-ray spectrum obtained from a fragment of terrestrial
gastropod shell (low magnesium calcite) 


 Energy-dispersive x-ray spectrum obtained from an ironstone ooid


Bivariate plots of (a) Si/O v Si/Al; (b) Si/Al v Si/Fe; and (c) Si/Al v
Si/Ca K-alpha peak height ratios for three area scan analyses of 
the ground <150 µm fractions of six different soil samples

Related Publications and Reports

Pye, K. and Croft, D.J. (2007). Forensic analysis of soil and sediment traces by scanning electron microscopy and energy-dispersive X-ray analysis: An experimental investigation. Forensic Science International 165, 52-63.
(click to view the abstract of this paper)

Pye, K. (2004). Forensic examination of rocks, sediments, soils and dusts using scanning electron microscopy and X-ray chemical microanalysis. In: Pye, K. and Croft, D. (Eds.) Forensic Geoscience - Principles, Techniques and Applications. Geological Society Special Publication No. 232, Geological Society Publishing House, Bath, pp. 103-121.
(click to view the abstract of this paper)

Krinsley, D.H., Pye, K., Boggs, S.and Tovey, K.N. (1998). Backscattered Scanning Electron Microscopy and Image Analysis of Sediments and Sedimentary Rocks. Cambridge University Press, Cambridge, 193 pp. Reprinted in paperback in 2005.


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Page last modified: November 04, 2008