Purification of STR-multiplex-amplified microsamples can enhance signal intensity in capillary electrophoresis

In this study, the effect of sample purification on total signal intensities of samples amplified with genRES MPX-2 (nine-locus multiplex) prior to capillary electrophoretic analysis has been investigated. Sample purification with the Qiaquick PCR purification kit led to an increase of the relative fluorescent signal intensity by a factor of 3.8 +/- 0.8. In contrast, the application of larger sample volumes led to a decrease of signal intensities from 20% to 80%, depending on whether the samples were purified or not. In addition, increase of injection time showed a linear increase of signal intensity between 3 s and 10 s. Increasing the number of PCR cycles from 30 to 33 also led to a significant increase of signal intensities. Nevertheless, this increase greatly depended on the fragment lengths and was sometimes accompanied by the appearance of non-specific signals. In combination, optimisation of sample preparation and increase of injection time may intensify signals up to 12-fold, thereby increasing the overall sensitivity of the assay. This may be of special interest for forensic analysis of microspecimens containing limited amounts of DNA.