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July 1999 Volume 1 Number
2
A
Small Sample Mounting Technique for Scanning Electron Microscopy
and X-Ray Analysis
Dennis C. Ward
Chemist
Materials and Devices Unit
Federal Bureau of Investigation
Washington, DC
Frequently, applications
of scanning electron microscopy (SEM) and X-ray analysis in forensic
science include the analysis of small (less than 100 µm)
individually handled samples. A mounting method for analyzing
small samples has been implemented that has several advantages
over commonly used double-sided specimen mounting tapes and liquid
adhesives (Basu and Ferris 1980; Wrobel et al. 1998). This method
uses pyrolytic carbon planchets (available from electron microscopy
suppliers) as specimen mounts. These disks are nonporous, glass-like,
flat, and reusable.
Duro-Tak® 80-1061, a
pressure-sensitive adhesive, is available from National Starch
and Chemical (10 Finderne Avenue, Bridgewater, New Jersey 08807-3300;
phone: 800-797-4992; web site: http://www.nationalstarch.com).
Duro-Tak® 80-1061 can be applied to a mount as a spot, line,
or thin film. A spot or line is applied with a capillary tube
or microdropper. A thin film is applied by drawing a drop of
adhesive across the planchet with the leading edge of a coverslip.
The dilution of the adhesive is adjusted with acetone to give
the desired thickness. Once applied, the adhesive remains tacky
indefinitely. Samples to be analyzed are then transferred to
the adhesive layer.
This method of sample mounting
for SEM and X-ray analysis has several advantages over conventional
methods:
- The elemental contribution
of the substrate is only carbon.
- The adhesive layer is stable
under conditions of high-beam current and high vacuum.
- The adhesive layer is very
thin. Therefore, a particle can easily be removed from the mount
after analysis, and charging of an uncoated sample during observation
is minimized.
- The background is structurally
featureless, providing optimal photographic presentation of the
sample.
- The noninvasive, nondestructive
characteristic of the adhesive permits additional analytical
procedures to be performed.
This method is not recommended
for handling populations of very small particles such as gunshot
primer residue.
References
Basu, S. and Ferris, S. A.
Refined collection technique for the rapid search of gunshot
residue particles in the SEM, Scanning Electron Microscopy
(1980) 1:375-384.
Wrobel, H. A., Millar, J.
J., and Kijek, M. Comparison of properties of adhesive tapes,
tabs, and liquids used for the collection of gunshot residue
and other trace materials for SEM analysis, Journal of Forensic
Sciences (1998) 43:178-181.
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FORENSIC SCIENCE COMMUNICATIONS JULY 1999 VOLUME
1 NUMBER 2 |