This article incorporates, in modified form, material from the not-yet-published Illustrated Guide to Forensics Investigations: Uncover Evidence in Your Home, Lab, or Basement.
Forensics laboratories routinely use microscopic analysis to characterize soil specimens. Microscopic analysis is often used to identify and quantify the major mineral components of soil specimens, but the mix of non-mineral components present in a specimen is often at least as important for establishing a “fingerprint” of that specimen. For example, if a questioned soil specimen contains pollen or seeds, a forensic botanist may be able to identify unambiguously the plant or plants that produced the pollen or seeds. Known soil specimens taken from a vicinity where those plants are present can be compared against the questioned specimen to establish with high certainty that the questioned specimen originated in the same vicinity, particularly if the plant or mix of plants is relatively uncommon.
The initial microscopic examination is usually done with a stereo microscope at low magnification, typically 10X to 40X. This examination allows the forensic geologist to gain an overall impression of the soil specimen, note the presence and abundance of various natural and artificial non-mineral components such as seeds, bits of glass, plastic, or metal, and so on. The stereo microscope is used to view particles from as small as 10 micrometers (µm = 0.01mm) to as large as the physical capacity of the microscope stage. The eyepieces used for such examination often include a reticle that allows measurement of individual particles in the specimen, or a grid that simplifies counting individual particles in the specimen to provide the data necessary to determine particle size distribution.
Soil specimens to be examined are usually placed in a metal tray, white for dark specimens and black for light specimens, and examined by incident (reflected) light. The tray may also contain a comparison grid, and is often treated with a sticky substance to hold the particles in fixed positions while they are being examined and counted.
Subsequent microscopic examination is usually done with a petrographic microscope, which is essentially a modified version of a standard compound biological microscope. A petrographic microscope makes provision for examining specimens by transmitted polarized light, and may also provide such features as an electrically-heated stage, which is used in determining the density and index of refraction of individual soil particles.
Although it is not the ideal tool for the purpose, in a home lab a standard compound microscope is a reasonable substitute for the stereo and petrographic microscopes used in professional forensics labs. To examine our soil specimens, we used our standard compound microscope with a gooseneck lamp to provide incident light, as shown in Figure 5-15.
Figure 5-15. Standard compound microscope set up to examine soil specimens
Under magnification, soil specimens that appear very similar or identical to the naked eye take on distinct individual characteristics. For example, Figure 5-16 shows two soil specimen fractions taken within 50 cm of each other, near a child’s sandbox, and sieved with a kitchen flour sifter. Although it may not be obvious in the photograph, the specimen on the left appears very slightly redder and the specimen on the right very slightly browner, but otherwise it is difficult to discriminate these specimens by naked eye.
Figure 5-16. Two soil specimen fractions that appear nearly identical to the naked eye
The soil specimen fraction shown under low magnification in Figure 5-17 is from near the edge of the sandbox. Individual sand grains are readily visible as glass-like, colorless quartz crystals. The specimen gathered half a meter away from the sandbox appears similar, but has many fewer sand grains visible.
Figure 5-17. Many sand grains are visible at 40X magnification
Required Equipment and Supplies
- goggles, gloves, and protective clothing
- stereo microscope (optional; see Substitutions and Modifications)
- compound microscope
- reticle or grid eyepiece (if available)
- well slides (see Substitutions and Modifications)
- small scoop or spatula
- dry soil specimens, known and questioned
All of the specialty lab equipment and chemicals needed for this and other
lab sessions are available individually from Maker Shed or other laboratory
supplies vendors. Maker Shed also offers customized laboratory kits at special
prices, including the Basic Laboratory Equipment Kit, the Laboratory Hardware Kit, the Volumetric Glassware Kit, the Core Chemicals Kit, and the
Supplemental Chemicals Kit.
None of the activities in this lab session present any real hazard, but as a matter of good practice you should always wear splash goggles, gloves, and protective clothing when working in the lab.
Substitutions and Modifications
- If you do not have a stereo microscope, you may substitute your compound microscope set for its lowest magnification and using a gooseneck lamp to provide incident light. Alternatively, you can use a loop or magnifier.
- If you do not have well slides, you can substitute a bottle cap or similar small container. You can even use a small square of paper to hold the specimen.
- If you have not already done so, put on your splash goggles, gloves, and protective clothing. (In this lab session, the purpose of these safety items is less to protect you from the specimens than to protect the specimens from you. Forensics technicians and scientists always wear protective gear to avoid contaminating specimens.)
- Label six well slides Q1 and K1 through K5, and transfer small amounts of the corresponding soil specimens to each slide. You needn’t fill the well completely. Ideally, you want just enough soil in each well to provide a single layer of particles.
Begin at the lowest magnification available, and record detailed observations for each soil specimen you examine. For bulk soil specimens in a well slide, the physical depth of the specimen means you won’t be able to bring the entire specimen into focus at one time. Use the coarse and fine focus adjustments on your microscope to view all parts of the specimen. When you finish examining a specimen at low magnification, switch to medium and then high magnification, which may reveal additional details. Here are some questions to keep in mind as you observe your specimens.
Particle size and distribution
Are the particles of relatively uniform size, or do they differ significantly? If the sizes differ, are particle sizes distributed relatively evenly from smallest to largest, or are particles of only a few distinct sizes visible? Do the particles appear to be discrete, or aggregates made up of smaller particles? If you have a reticle or grid eyepiece, use the known spacings of the reticle or grid and the known magnification to estimate the particle sizes. If possible, do a count of particles in different size ranges and use that count to estimate particle size distribution.
Are the particles smooth or rough? Spherical or elongated? Crystalline or amorphous? Is there any correlation between particle size and appearance, for example, that the smallest particles appear rough and jagged while larger particles appear smooth?
What color are the particles? Are they transparent, translucent, or opaque? Is there any correlation between particle size or structure and particle color? Are individual particles of uniform color, or does the color vary within the particle?
Does the specimen contain non-mineral matter, natural or artificial? If so, observe it carefully, using high magnification if necessary, and describe it as completely as possible. For example, one of our specimens contained several tiny, transparent particles that were almost perfect spheres. In a professional forensics lab, there would be comparison specimens available to help identify those objects. We have no such specimens, but we speculate that they are tiny glass spheres that were originally part of a reflective warning sign or movie projection screen. Another specimen contained objects that appeared to be seeds, probably from the grass that covers our yard. We’re not botanists, so the most we can say is that those seeds appeared very similar to those in a bag of grass seed in the basement.
If you have the necessary equipment, photograph each soil specimen at low, medium, and high magnification and append copies of those images to your report.
Dispose of the soil specimens with household waste and wash the slides.
Q1: Based on your microscopic observations, how well can you discriminate between the soil specimens? With what level of certainty can you determine if the questioned specimen is consistent with one of the known specimens?