The Importance of Trace Evidence

Trace evidence can prove to be invaluable to the forensic analyst.  Trace evidence comes in many varieties and is easily overlooked by those who are not familiar with where to look for it.  In the given scenario, we have several items, which we can classify as trace evidence.  The first item we will look at is the glass fragments found near the victim's head, which came from the driver side window. 

            The composition of most window glass is made from what we call float glass.  This type of glass gets its name from the process that is normally used to cool the glass on a layer of molten tin.  In comparing glass fragments the pieces need to fit together perfectly to make a positive comparison.  Fortunately, for us, the suspects left the vehicle behind and this greatly facilitates our investigation.  In this case, we do not have to look for suspect glass for comparison to known glass; we have both in one convenient location.  However, we still have to make comparisons for the benefit of legalities in a court of law.  Next, we will discuss the comparison methods. (Safferstein, Criminalistics: An Introduction to Forensic Science, 2011, p. 101).

            As stated earlier, we should have no problem in demonstrating a perfect physical fit of the glass fragments.  Two or more pieces of glass fitting together perfectly exclude the possibility of the fragments originating from different sources.  Next, we will make a density comparison of the glass fragments.  (Safferstein, Criminalistics: An Introduction to Forensic Science, 2011, p. 101).

            In our density comparison, we will use a method called flotation.  In this method, we will add a piece of reference glass, which we collected from near the head of the victim, into a solution of bromoform.  The density of the solution will be greater than that of the glass fragment causing the fragment to float.  Next, we carefully and slowly add small amounts of a less dense liquid, bromobenzene, to the solution.   We now add bromobenzene to the solution until the glass is suspended in the mixture.  At this point, the densities of the glass fragment and the solution are the same.  Next, we will add fragments of the suspect glass to the solution.  The fragments should be comparable in size.  If the solution also suspends these fragments, then the densities of the fragments are the same, indicating that the two or more fragments that we compared, originated from the same source.  (Safferstein, Lab Manual for Criminalistics: An Introduction to Forensic Science (Tenth ed), 2011, p. 9).   There are varying degrees of densities contained in any given sheet of glass.  We will need to adjust our solution to allow for a specified variable of densities to match the imperfections in density of the glass fragments. (Safferstein, Criminalistics: An Introduction to Forensic Science, 2011, p. 103).

            The more proof we have linking the suspects to the evidence and the crime, the better chance we have of obtaining a conviction.  Therefore, we will now perform another comparison test on the glass fragments.  This time we will compare the refraction indices of the fragments.  In this comparison, we are seeking the disappearance of the Becke line.  The Becke line is a bright halo that appears around the edges of a glass fragment when immersed in a liquid with a different refractive index than that of the fragment.  When the refractive indices of the immersion liquid and the particle of glass are the same, which is known as the match point, the halo disappears, thus the disappearance of the Becke line, this method is known as the immersion method.  We can change the refractive index of the immersion liquid in a couple different ways, the first, is by submerging them in a specific liquid medium, which is usually a mixture of silicone oil.  The first method in which the refractive index of the immersion liquid is changed is by changing the temperature of the liquid.  This we do by slowly heating the liquid at about 0.2 degrees C per minute until the solution reaches the match point.  Heating the liquid has no noticeable effect on the refractive index of the glass particle.  We need to carefully control the temperature of the liquid and accurately record it during this procedure.  When the solution reaches the match point it is time to submerge the particles to be compared to check for matching refractive indices of the fragments.  (Safferstein, Criminalistics: An Introduction to Forensic Science, 2011, p. 103).

            Another method of changing the refractive index of the immersion liquid is by much the same way that we changed the density of the liquid in the first analysis.  This time however we will use a mixture of bromoform and olive oil, because we are changing the refractive index instead of changing the density.  The refractive index of most glass ranges from 1.47 for headlight glass to about 1.53 for ophthalmic lenses.  The refractive indices for olive oil and bromoform are 1.45 and 1.59 respectively.  By mixing these two liquids until the refractive index of the glass particle is reached, we will notice the disappearance of the Becke line in much the same way as when we heated the liquid to change its refractive index.  Now we perform the comparisons in the same way, we immerse the remaining fragments that need comparison, to determine if the indices are the same, which would be an indication of like origins.  (Safferstein, Lab Manual for Criminalistics: An Introduction to Forensic Science (Tenth ed), 2011, pp. 32-33).

            Now we will turn our attention to the second piece of trace evidence found at the crime scene, which would be the dark red paint found across the victim’s torso.  Paint is one of the most popular types of physical evidence, where the volume of comparison requests passing through the crime labs are concerned.  When it comes to examining paint, the layer construction is the most important characteristic to observe in ascertaining the paint’s origin.  Binder composition is another important factor in determining the origin of a paint specimen.  Due to the wide variety that manufacturers produce, it makes the task of determining paint’s origin easier.  (Safferstein, Criminalistics: An Introduction to Forensic Science, 2011, pp. 342-343)

            When it comes to analyzing paint, pyrolysis gas chromatography is extremely useful.  This technique is probably the most reliable when characterizing a paint sample.  This technique involves heating the sample until it acquires a gaseous state.  When these gases are sent through a chromatograph, we analyze the exiting gases.  The resulting pyrogram contains sufficient details to distinguish between polymers thus ascertaining the paint’s origin.  (Safferstein, Criminalistics: An Introduction to Forensic Science, 2011, pp. 344-345)

            Infrared spectrophotometry is another reliable technique for obtaining the same results.  This technique involves the infrared absorption characteristic of a paint sample.  (Safferstein, Criminalistics: An Introduction to Forensic Science, 2011, p. 345)

            When collecting paint chips at the crime scene, it is imperative to keep the paint chip intact.  The sample can be placed in a piece of paper, which is then folded around the sample or placed in a glass or plastic vial or test tube.  If the sample is imbedded into another object then the entire object, if possible, should be sent to the lab for analysis.  (Safferstein, Criminalistics: An Introduction to Forensic Science, 2011, pp. 347-348)

            Now we can start the analysis of the hair samples found at the crime scene.  This would be a black medium hair found in the driver side headrest and a fine short blonde hair found on the lower portion of the passenger seat.  Distinguishing human hair from animal hair is easily accomplished; however, arriving at a conclusion of the origin of a human hair when compared with another human hair is not as easily done.  This difficulty is due to the variation of morphological characteristics of hair samples taken from the same individual.  Color, length, and diameter of the hair are major concerns when comparing hairs, it is also necessary to collect an abundance of samples from a suspected individual to make adequate comparisons.  As with bullet comparisons, hair comparisons are best achieved by the use of a comparison microscope.  (Safferstein, Criminalistics: An Introduction to Forensic Science, 2011, pp. 324-327)

            The final piece of trace evidence we will examine is the pink fiber found on the bumper of the vehicle, which appears to match the pink shorts worn by the victim at the time of the crime.  Examination and linking of a piece of fiber to its origin depends largely on the experience and expertise of the forensic analyst.  Due to the mass production of fibers in today’s world, individual characteristics of a single fiber is hard to determine.  This phenomenon is also largely attributable to the production of so many manufactured fibers.  Fortunately, in this particular circumstance, we have the shorts the victim was wearing and the piece of fiber recovered from the bumper.  This should permit us to make a positive match just by piecing together the materials.  If this were not the case, it would make it extremely difficult, if not impossible, to determine the origin of the evidence fiber and match it with the reference fiber.  (Safferstein, Criminalistics: An Introduction to Forensic Science, 2011, pp. 330-335)

            We should now have enough corroborative evidence to charge and subsequently prosecute the suspects for this crime.

Definitions of Terms

1.    Chromatograph>A method of finding out which components a gaseous or liquid mixture contains that involves passing it through or over something that absorbs the different components at different rates.

2.    Float Glass>Flat polished transparent glass made by solidifying molten glass as it floats on liquid of higher density such as tin.

3.    Flotation>A process for separating materials such as a mixture of minerals in an ore according to their different abilities to float in a given liquid.

4.    Infrared Spectrophotometry>The portion of the invisible electromagnetic spectrum consisting of radiation with wavelengths in the range of 750 nm to 1 mm, between light and radio waves.

5.    Pyrolysis Gas Chromatography>The use of heat to breakdown complex chemical substances into simpler substances.

6.    Refraction Index>The change in direction that occurs when a wave of energy such as light passes from one medium to another of a different density.

References

Safferstein, R. (2011). Criminalistics: An Introduction to Forensic Science (Tenth ed.). Upper Saddle River,

 NJ: Pearson Education.

Safferstein, R. (2011). Lab Manual for Criminalistics: An Introduction to Forensic Science (Tenth ed).

 Upper Sadle River, NJ: Pearson Education.