Evidence Collection And Preservation

The importance of proper DNA evidence collection cannot be overemphasized. If the DNA sample is contaminated from the start, obtaining unambiguous information becomes a challenge at best and an important investigation can be compromised (see D.N.A. Box 3.2). Samples for collection should be carefully chosen as well to prevent needless redundancy in the evidence for a case. The following suggestions may be helpful during evidence collection to preserve it properly (see page 38).

Importance of carefully collecting DNA evidence: the O.J. Simpson case

On the night of 12 June 1994, Nicole Brown Simpson and Ronald Goldman were found brutally murdered at Nicole's home. A few days later Nicole's ex-husband, Orenthal James (O.J.) Simpson, was picked up by Los Angeles police officers and became the chief suspect in the murder investigation. Due to O.J. Simpson's successful football career and popularity, the case immediately drew the public's attention. Over 100 pieces of biological evidence were gathered from the crime scene consisting primarily of blood droplets and stains. DNA samples were sent to three laboratories for testing. Over the summer months of 1994, the Los Angeles Police Department (LAPD) DNA Laboratory, the California Department of Justice (CA DOJ) DNA Laboratory in Berkeley, and a private contract laboratory from Maryland named Cellmark Diagnostics performed the DNA testing using both RFLP and PCR techniques. A number of RFLP and PCR markers were examined in this high-profile case. However, no STRs were typed.

The so-called 'Trial of the Century', People of the State of California v. Orenthal James Simpson, began in the fall of 1994. O.J. Simpson hired a legal 'dream team', which worked hard to acquit their client. O.J.'s defense team knew that the DNA evidence was the most powerful thing going against the football star and vigorously attacked the collection of the biological material from the crime scene. Through accusations of improper sample collection and handling as well as police conspiracies and laboratory contamination, the defense team managed to introduce a degree of 'reasonable doubt'. After a lengthy and exhausting trial, the jury acquitted O.J. Simpson on 3 October 1995.

There were seven sets of bloodstains collected by the LAPD and analyzed by the three DNA laboratories mentioned above. These sets of samples are reviewed below along with the challenges put forward by the defense team. For each sample, the statistics for the odds of a random match ranged from 1 in 40 when only PCR testing with the DQ-alpha marker was evaluated to more than 1 in 40 billion when all RFLP markers were examined.

To gain a better understanding of the magnitude of the DNA testing conducted in the O.J. Simpson case, 61 items of evidence were received by CA DOJ from LAPD (Sims et al. 1995). From these evidence items, 108 samples were extracted in 22 sets and tested alongside 21 quality control samples that were co-extracted and 24 extraction reagent blanks. These extraction reagent blanks were performed to verify that no contamination was introduced in the CA DOJ laboratory.

From a scientific point of view, the results from the three testing laboratories agreed and more than a score of DNA markers were examined with no exclusions between the crime scene samples and Mr. Simpson. The acquittal verdict goes to show that DNA evidence is not always understood and can be quite complex to explain to the general public. Expert witnesses have the challenge of presenting the difficult subjects of DNA biology, technology, and genetics and jury members must make sense of concepts such as contamination and mixture analysis that can be fairly complex.

To their credit, the defense team focused on the evidence collection and preservation as the most important issues in the trial rather than attacking the validity of DNA testing. They implicated the LAPD in planting some of O.J. Simpson's liquid blood reference sample collected on 13 June- the day after the murders took place. Furthermore, the defense attacked the manner in which the evidence was handled in the LAPD DNA laboratory and alleged that contamination of the evidence samples by O.J.'s reference blood sample resulted from sloppy work and failure to maintain sterile conditions in the laboratory.

The contamination allegation became the focus of their arguments because much of the evidence had been handled, opened, and supposedly contaminated in the LAPD lab before it was packed up and sent to other laboratories for further testing. Thus, according to the defense, no matter how carefully the samples were handled by the California Department of Justice DNA Laboratory or Cellmark Diagnostics their testing results would not reflect the actual evidence from the crime scene. Since the samples were supposedly tainted by the LAPD laboratory, the defense argued that the evidence should not be considered conclusive. However, the sheer number of DNA samples that typed to O.J. makes it hard to believe that some random laboratory error made it possible to obtain such overwhelming incriminating results.

Since the conclusion of the O.J. Simpson trial in 1995, forensic DNA laboratories have improved their vigilance in conducting DNA evidence collection and performing the testing in a manner that is above reproach. Because PCR is an extremely sensitive technology, laboratories practicing the technique need to take extraordinary measures to prevent contamination in the laboratory. Hence, the value of laboratory accreditation and routine proficiency tests to verify that a laboratory is conducting its investigations in a proper and professional manner is clear (see Chapter 16).

The issuance of the DNA Advisory Board Quality Assurance Standards (see Appendix IV) has helped raise the professional status of forensic DNA testing. It is noteworthy that in a systematic analysis of circumstances normally encountered during casework, no PCR contamination was ever noted according to a recent study (Scherczinger et al. 1999). Significant contamination occurred only with gross deviations from basic preventative protocols, such as those outlined in the DAB Standards, and could not be generated by simple acts of carelessness. Arguably the most important outcome of the O.J. Simpson trial was the renewed emphasis placed on DNA evidence collection.


Number of Samples

DNA Match

Defense Challenge

(Date collected)


Blood drops at Nicole Brown's home (13 June)

5 drops leading away from house


Heavy degradation of the 'real' killer's DNA; tampering with evidence 'swatches'; sample contamination during laboratory investigation

Stains on rear gate at Brown's home (3 July)

3 stains


Samples planted by rogue police officers prior to collection

5 stains around vehicle; bloody footprint; stain on center console

Simpson in 5 stains; Brown in footprint; Simpson/Goldman mixture on console

Simpson's DNA present for reasons unrelated to the crime; Detective Mark Fuhrman planted the blood footprint; Laboratory controls failed on console mixture analysis

Second collection of stains in O.J.'s Bronco (26 August)

3 stains

Mixture of Simpson, Brown, and Goldman

Blood planted in the vehicle between the crime and the collection

Stains at Simpson's home (13 June)

2 drops in driveway, 1 in foyer, 1 in master bedroom


Simpson bled at these locations for reasons unrelated to the crime

Socks found in Simpson's bedroom (13 June)

Multiple stains

Simpson and Brown

Blood planted after the socks were collected

Bloody glove found on the grounds of Simpson's home (13 June)

15 stains identified

Goldman, Simpson, and Brown alone or as mixture

Glove was removed from murder scene and planted by Detective Mark Fuhrman; Simpson's DNA was present because of laboratory contamination


Levy, H. (1996) O.J. Simpson: what the blood really showed. In Levy, H. (ed) And the Blood Cried Out, pp. 157-188. New York: Basic Books; Weir, B.S. (1995) Nature Genetics, 11, 365-368.

■ Avoid contaminating the area where DNA might be present by not touching it with your bare hands, or sneezing and coughing over the evidence.

■ Use clean latex gloves for collecting each item of evidence. Gloves should be changed between handling of different items of evidence.

■ Each item of evidence must be packaged separately.

■ Bloodstains, semen stains, and other types of stains must be thoroughly air-dried prior to sealing the package.

■ Samples should be packaged in paper envelopes or paper bags after drying. Plastic bags should be avoided because water condenses in them, especially in areas of high humidity and water can speed the degradation of DNA molecules. Packages should be clearly marked with case number, item number, collection date, and initialed across the package seal in order to maintain a proper chain of custody.

■ Stains on unmovable surfaces (such as a table or floor) may be transferred with sterile cotton swabs and distilled water. Rub the stained area with the moist swab until the stain is transferred to the swab. Allow the swab to air dry without touching any others. Store each swab in a separate paper envelope.

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  • rina
    How to collect evidence and prevent contamination forensic?
    5 months ago

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