Preliminary study of mixed traces on cigarette butts and non-self DNA transfer, persistence, prevalence and recovery in different forensic scenarios

Recent studies focus on the different ways in which DNA can be transferred from and individual to another or to a surface. This work investigates how mixed DNA profiles can be generated on items commonly collected at the crime scene, specifically on cigarette butts. The aim is to determine how biological material exchanged during intimate contact (kissing or sharing a cigarette) can be secondarily transferred onto cigarettes, and how long non-self DNA persists and remains recoverable under different conditions and time intervals. The preliminary experimental study involved one investigative couple: each volunteer smoked a cigarette at fixed intervals after kissing (from 0 to 120 min) and both partners smoked a shared cigarette. DNA was extracted from cigarette butts immediately and after 30 days using the QIAamp DNA Investigator Kit on the QIAcube Connect. Quantification was performed with PowerQuant® System Kit and genetic profiles were obtained using PowerPlex® Fusion and Y23 Kits. Results showed non-self DNA alleles up to 2 h after kissing, even in non-immediate extractions, partial Y-STRs profiles in female butts after 1 h: individual variability and degradation made interpretations harder. Based on preliminary findings, the study will be expanded including more partner pairs, more smoking devices and longer processing timeframes. Our findings suggest the possibility of reconstructing transfer scenarios leading to mixed DNA traces on cigarette butts and of assessing a time window within which non-self DNA can be secondarily transferred to the cigarette, highlighting the impact of processing times and smoking device on DNA results.