A Small Sample Mounting Technique for Scanning Electron Microscopy and X-Ray Analysis by Ward (Forensic Science Communications, July 1999)
July 1999 - Volume 1 - Number 2
and X-Ray Analysis
Dennis C. Ward
Materials and Devices Unit
Federal Bureau of Investigation
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.
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.