Individuals in the laboratory must follow the standard operating procedures for analyzing samples. Laboratories require that no more than one piece of evidence should be opened at a time. Limiting the number of samples opened at a time will prevent cross contamination. The work station or area and utensils should also be cleaned between use with 10% bleach or 70% ethanol. An argument the defense may make during trial is that the DNA sample from the suspect somehow got into the evidence sample. To eliminate any doubt regarding contamination, the known standards should be handled in a separate time and place than the evidence samples.
Instrumentation protocols for DNA analysis also integrate a number of secondary assurances. The use of reagent blanks with positive and negative controls are included with each set of samples. The positive control will ensure that the test is performing correctly for a given substance being tested. The negative control will ensure that the reagents are not reacting within the test or instrument. Negative controls however will not determine whether evidence samples are contaminant free but to detect the general level of contamination within the lab. Other control mechanisms include extraction and amplification methods to occur in separate areas and that post amplified DNA does not return to the extraction area.5
As part of quality assurance standards, a chain of custody and evidence log must accompany all objects that are collected pertaining to a crime. Chain of custody establishes who has possessed or examined evidence items and must be present when evidence is accepted into court. The record will show the collector’s initials, location of the evidence and the date of collection. All the individuals documented in connection with the evidence may be called to testify in court. The chain of custody should be kept to a minimum to maintain control of the items and avoid confusion. Use of a proper seal as noted in the procedure for using chain of custody will decrease the likelihood of contamination. The probability of altering a finger print or contaminating DNA is reduced if an evidence item is enclosed in the proper container (i.e. paper bag or envelop).
For documentation purposes an evidence log is important because it will aid the laboratory departments in prioritizing the significance of evidentiary items. Investigators tend to pick up too much evidence in hopes of it being important when it may not pertain to the case at all. An evidence log will also describe what the article is, where it was found and by whom it was found. In the case of mislabeling an evidence item on a sealed container, the analyst may refer to the evidence log to determine what the item may be (i.e. a black sock mistakenly labeled as a wash cloth). Training and education on the importance of these methods help reduce the possibility of contamination when performed correctly and maintain reliability of the...