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• “The fi ngerprint of powder is a rare form of black powder that was last manufactured 27 years ago, and is not in current production and rarely seen today”
• “The gunshot residue indicates that the defendant had fi red a similar or the same gun within the last XY hours.”
Analysis of DNA with quantitative
PCR DNA analysis with rtPCR (real-time poly- merase chain reaction, or quantitative PCR) is exceptional detection sensitivity—detection of a single molecule is possible. Analysis of specimens obtained from motel rooms or steering wheels of rental cars could wind up in the databases, with the wrong attributes or origin. Indeed, the article in Ref. 1 depicted a half-page picture of a man swabbing the steering wheel of a car to collect a sample for DNA identification. A match to the car may be all a prosecutor needs to elicit a confession from an innocent person who had the misfortune to have used the car days or weeks earlier, such as a parking attendant. The Innocence Project is working on hundreds of cases involving the role of modern DNA analysis in overturning mistaken convictions.
Lab contamination Laboratory contamination is usually among the
most often cited sources of error, particularly false positives. A false positive can erroneously place the suspect at the scene of the rape or murder, etc., simply if that suspect’s DNA is found in the assay that purports to have only DNA recovered at the crime scene.
Laboratory contamination is a common prob- lem. A simple Google search on “laboratory contamination” lists many examples describing how contamination can creep in and lead to incorrect results. Some of the most interesting cases deal with contamination of heavy metal samples. For example, mercury can be reduced to Hg0
, which makes it soluble in plastic con-
tainers. Ionized mercury can be retained on the surfaces of glass by ion exchange.
Several articles discuss contamination of cell cultures with microbes. One talks about infec- tion of inoculated specimens with live polio
virus. Many examples read like a murder mys- tery. Few of the articles are recent, however; laboratory contamination is not a topic with a high impact factor and is hence avoided by the editors of “white” journals.
One reference presents a fl ow chart for ra- dioactive contamination. It lists pipetting as a likely source of contamination. Glass pipets are diffi cult to clean. Air-driven pipets can lead to contamination since, during the aspirate phase, the velocity of the load cycle, including repeated cycles of aspirate and dispense, can create aerosols that contaminate the pipet body. This can transfer to subsequent samples. Overly forceful dispensing is also implicated in creating an aerosol that can spread contamina- tion to the surrounding area during dispense. Think this is crazy? Why do we have Biosafety Level 4 (BSL-4) facilities?
The future of forensic DNA analysis Some will be quick to point out that forensic DNA analysis is based on old technology that looks for variations, often simple repeats, in specifi c loca- tions. It is not whole genome sequencing (WGS). But, the price of WGS is declining by a factor of 10 every fi ve years, so we can anticipate that soon huge databases of WGS of Americans will be compiled, primarily for precision medicine. Will law enforcement have access to these records? You can bet they will try.
References 1. Goldstein, J. Police Agencies are Assem- bling Records of DNA. The New York Times, June 13, 2013, p 1A.
2. Stevenson, R.L. The Time has Come for a Forensic Laboratory Improvement Act;
http://www.americanlaboratory.com/913- Technical-Articles/128817-The-Time-has- Come-for-a-Forensic-Laboratory-Improve- ment-Act.
Robert L. Stevenson, Ph.D., is a Consultant and Editor of Separation Science for American Laboratory/Labcompare; e-mail: rlsteven@
yahoo.com.
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