The Use of Electrophoresis in Forensic Science

Fig. 1 - Gel Electrophoresis of DNA

Electrophoresis has proved to be an invaluable tool in the analysis of crime scene evidence, especially in the area of DNA fingerprinting.

Electrophoresis involves the separation of chemicals along a solid medium in the presence of an applied potential difference. In electrophoresis, chemicals such as blood proteins, DNA or inorganic ions can be separated according to differences in their mass and/or charge. The solid medium used in electrophoresis is usually an agarose or polyacrylamide gel

Electrophoretic separation has uses in forensic science because it can be used to isolate and compare DNA, blood proteins and inorganic substances such as gunshot residues from crime scenes with suspects, victims or standard reference material.

Blood Protein Analysis Using Gel Electrophoresis

This process involves the separation of ‘marker’ proteins that are found on the surface of red blood cells. Many of these are antigens that determine particular blood groupings such as A, B, AB and O, and they can therefore be used to exclude suspects from being present at a crime scene.

Blood protein analysis has now been largely replaced by DNA fingerprinting of blood, because this latter method is much more specific. It can still serve a role as collaborative evidence, however, and has also had great relevance in past criminal cases.

DNA Analysis Using Gel and Capillary Electrophoresis

Fig. 2 - Capillary Electrophoresis

Electrophoresis is most frequently used in forensic science to produce DNA fingerprints, as illustrated in figure 1. DNA evidence from a crime scene can be compared to DNA samples from different suspects, for instance, and suspects can either be included or excluded from suspicion using the results of such tests

In gel electrophoresis, DNA strands from crime scenes, victims or suspects are applied to an agarose gel that is subjected to an electric potential. The more traditional RFLP (restriction fragment length polymorphism) profiling procedure is now being replaced by the PCR (polymerase chain reaction) method, which often involves the use of shorter DNA segments known as STRs (single tandem repeats). This method is faster and requires less DNA.

Capillary electrophoresis (figure 2), in which a fused silica capillary is used instead of a gel slab, is now being used more frequently in DNA electrophoresis. Although applying the same principles of separation as the more traditional gel slab electrophoresis, it is more rapid and has a higher resolution.

Inorganic Ion Analysis Using Capillary Electrophoresis

Capillary electrophoresis can also be used to separate and analyse anions found in explosives and poisons so that the substances used in crimes can be identified and even linked to suspects.

Anions capable of being isolated from explosive residues include azides, chlorates, chlorides, nitrates, nitrites, perchlorates, sulfates and thiocyanates, while the anions of interest in toxic chemicals are azides, cyanide, arsenates, arsenites, chromates, thiosulfates, oxalates, bromides and iodides.

Capillary electrophoresis is often also used in conjunction with ion chromatography to achieve more effective separation of ions.

Criminal Cases Where Electrophoresis Has Been Employed

Blood protein analysis was used as recently as the O.J. Simpson trial in 1995, when electrophoretic comparisons between O.J’s blood and blood from the crime scene showed they both had the factors A, ESD1 and PGM2+2-. This evidence was highly incriminating, as the probability of two samples having all these factor is just 0.44%.

O.J’s guilt was also suggested by DNA tests carried out on his blood, which revealed the probability of his being innocent was 1 in 57 billion. The main reason he wasn’t convicted was due to the seed of doubt the defence sowed in the minds of the jurors about possible interference with the evidence by the FBI.

Using the many DNA databases now available, such as CODIS, for instance, DNA samples from a crime scene can be compared with the DNA from suspects and victims related to that crime. Chester Dewayne Turner , for instance, was linked by DNA evidence to deaths in the U.S. as far back as March, 1987.

About 25% of violent crime cases in the U.S. since 1989, however, have in fact resulted in the exoneration of suspects because of DNA profiling procedures. For example, after spending 21 years in an Indiana prison for rape, DNA tests now indicate Larry Mayes was innocent. He was subsequently released in 2002. By 1996, over 108 post-conviction exonerations had in fact occurred in the USA using DNA profiling.

The anions of inorganic salts and acids are often found in gunshot and explosive residues, and also in foodstuffs that have been adulterated with poisons. Capillary electrophoresis can rapidly profile and often quantify these chemicals, and this method therefore has merit as collaborative evidence in forensic science.

Future Directions in Electrophoresis

Research is currently being undertaken in the US to develop portable microchip DNA profiling devices that can be used in the field. In this method, STR analysis of a small DNA sample can be achieved on the surface of microchips in much less time than traditional techniques. ‘Pulsed field electrophoresis’ is another innovation being investigated – here, the direction of the electric field is alternated, allowing for the separation of DNA molecules up to several million base pairs in length.

These, and other advances in electrophoretic technology, will ensure faster and more effective analysis of crime scene evidence in the years ahead.

References

Petricevic, S., 2010, "DNA Profiling in Forensic Science", nzic.org.nz, accessed 1/2/2010

Saferstein, R., 2004, "Criminalistics, an Introduction to Forensic Science", Pearson Education, New Jersey

Tissue, B.M, 2010, "Electrophoresis", The Chemistry Hypermedia Project, chem vt.edu, accessed 1/2/2010