Between Chemistry and Other Disciplines

  1. Physics. In addition to color and density, refractive index is a useful way for identifying glass. Refractive index is the degree to which a beam of light bends as it passes from air into a solid or liquid.

  2. Geology. Soil is a common form of physical evidence. Soil samples are analyzed to determine if they have a common origin. The methods include a density profile and settling rate curve. For example, a soil sample is dried and sieved. (Sieves containing 200 wires per in 2 = 200 mesh; 50 wires per in 2 = 50 mesh, etc. The higher the mesh size, the finer the particles.) A small amount of a desired mesh size (usually 30-45 g) is placed on the top of a column containing layers of immiscible liquids of different densities. Heavy particles will settle to their level in a few minutes. Light particles may take a few hours to stop moving. The soil profile at the scene is then compared to the soil profile from the suspect. (The same technique can be used to determine the density of small glass fragments.)

  3. Biological Science . Blood contains characteristic factors based on blood groups. A person's blood contains proteins called antibodies. Human blood can be grouped into types A, B, O, and AB, depending upon the antibodies present in the serum. If blood from people of other blood types are mixed, agglutination (clumping together) will occur. All four types can be identified by this method. Blood typing can be useful (but not conclusive) in establishing possible parents of children.

    Blood proteins are controlled by genetics and are rather specific for an individual. The best tool for distinguishing blood proteins is electrophoresis. Proteins are electrically charged at a given pH. In the presence of an electric current and at the appropriate pH, the proteins migrate at different rates toward the positive electrode separating into different spots along the way. The spots are stained with ninhydrin, making them visible. Their relative positions are used to identify individual proteins (see Enzymes module).

  4. Biological Science . DNA Profiling is a new technique that helps identify suspects from bodily traces (blood, semen, hair) often left at a crime scene. DNA consists of four bases attached to each other in pairs and connected like a zipper into a double helix pattern. The exact sequence of bases is unique to each person (except identical twins), somewhat like a fingerprint. To link a suspect to the crime, DNA is extracted from physical evidence found at the scene of a crime. Scientists split the DNA pairs into fragments and the fragments are separated into bands by electrophoresis. The fragments are mixed with radioactive bases that bind to the DNA as pairs as in the original sequence. When exposed to X-ray film, the DNA pattern appears in bands resembling a bar code on supermarket products. The DNA pattern from the evidence is compared to the DNA pattern in the suspect's blood sample.