5.4. Direct and rapid PCR on mock casework samples
149 mock casework samples, expected to contain single source cell material of known donors, were collected to study the success rate of direct and rapid PCR. The tape lift procedure was used for all stains residing on swabs. For items sampled by tape-lift stubs, 5 mm2 areas were excised from the tape. Chewing gum, hairs and bone powder were added directly to the PCR mixture. Full STR profiles concordant with the donor of the sample were obtained for 60% of the samples representing classical forensic trace evidence types (Table 6). All blood samples and half of the saliva samples yielded full profiles. Hair roots give 65% of full profiles when pulled hairs are used, but the profiles appear overloaded and are best analyzed after injecting less (0.01 μL) PCR product. When hairs are analyzed no sample remains as the hair is fully used during direct and rapid PCR. When items containing contact DNA and bone samples are included, the overall success rate decreases (46%) (Table 6). The bone samples gave full profiles in standard DNA analysis, but no genotyping results using direct PCR. This difference is most likely due to the use of too little bone material for the direct PCR; approximately 10 mL of bone powder was used for DNA isolation of which 1/20th was used as PCR input (this corresponds to 500 μL bone powder), while for direct PCR only 10 μL bone powder was added.