Repeat silica extraction: a simple technique for the removal of PCR inhibitors from DNA extracts

Polymerase chain reaction (PCR) inhibitors are often co-extracted with ancient DNA (aDNA) and when present make the analysis of aDNA difficult, if not impossible. In this study we review previous research on PCR inhibitors and techniques that address their co-extraction with DNA from sub-optimal samples. Additionally, we introduce a simple extraction technique, “repeat silica extraction,” that effectively removed PCR inhibitors from extracts of 7000–8000-year-old human skeletal remains from the Windover archaeological site in Florida and 700–2000-year-old human coprolites excavated from Fish Slough Cave in southern California. A series of tests on these same samples demonstrates that N-phenacylthiazolium bromide is largely ineffective, despite previously reported success using this compound as part of the DNA extraction process. We also describe a method for demonstrating the presence as well as successful removal of PCR inhibitors by use of a “positive aDNA control,” a test necessary to conclude that negative PCR amplification is the result of the absence of preserved DNA.