Staci Bennett, Minnesota Bureau of Criminal Apprehension, St. Paul
Determining "whodunit," based on DNA evidence.
It's sometimes difficult to isolate DNA amenable to PCR from stains in dark denim.
Forensic labs do everything by the book. They follow an exhaustively validated and detailed standard operating procedure (SOP) in order for evidence to be admissible in court. The SOP even includes directions on how much wiggle room Bureau scientists have when diluting a sample; such variation is often needed to overcome PCR inhibitors.
For saliva or blood stains on clothing, for example, the SOP calls for a 3 x 3 mm denim cutting to be placed in extraction buffer with Proteinase K, vortexed, incubated at 56°C, and precipitated with phenol-chloroform. Yet denims are not all dyed the same way,...