Testing Minimum Ultraviolet Light Exposures to Effectively Remove Contaminating DNA for Use in Forensics

This study introduces a solar lamp UV light source, for the purpose of removing contaminating DNA in direct relation to forensic testing. The study attempts to demonstrate what level of decontamination occurs from sun lamp exposure at given time intervals of exposure, set at distances from the lamp, and for different types of biological samples. A FS-40 solar lamp was used to irradiate samples of amplified DNA and cellular samples at distances of 5 cm, 10 cm, and 60 cm from the source, with varied exposure times of 15 min, 30 min, 3 hrs, 6 hrs, 12 hrs and 24 hrs. Common forensic DNA typing concerns include contamination by previously amplified DNA products or from transfer of cellular material onto testing materials. Samples exposed included dried PCR products amplified by AmpFlSTR COfiler kit, dried whole blood, and dried saliva. An organic extraction of the blood and saliva samples isolated any remaining genomic DNA. Control blood and saliva samples were quantitated for accurate DNA concentration. All samples were then amplified by AmpFlSTR COfiler kit and analyzed on an ABI 310 Genetic Analyzer, along with an unexposed control PCR product, blood, and saliva samples, reagent blank run alongside each PCR product, blood, and saliva series, and positive and negative PCR controls. Fragment analysis data was analyzed by GeneScan and Genotyper software to obtain any detectable genetic profile from the samples. This experimental design mimics a true forensic casework scenario by following a routine chain of procedures used widely throughout the field. The current standard in forensic DNA testing measures short tandem repeats (STRs), which vary significantly in length between individuals. There are thirteen loci used by the Combined DNA Index System (CODIS), the national DNA index managed by the FBI Laboratory. All thirteen loci are typed in a typical DNA test, with the AmpFlSTR COfiler kit amplifying seven of these loci. For the purposes of this study, successfully decontaminated PCR products, blood, and saliva samples would show no detectable genotype at any of the seven loci. Other DNA testing, such as mitochondrial DNA analysis from hair, bone or teeth, or very low copy number DNA from a small number of cells, require extreme caution to avoid contamination, as these tests have increased sensitivity over standard STR testing. The level of decontamination detected through UV exposure in this study would not provide sufficient information for application to the more sensitive techniques.