Electrospray ionisation (ESI) mass spectrometry has also been used for direct detection of nucleic acids (13) (14) (15) . ESI-MS has some advantages over MALDI TOF MS particularly the availability of lower cost instrumentation. Moreover, sample handling can be simpler since most molecular biology assays are carried out in solution and such liquid samples are injected directly into the instrument. Furthermore, very high molecular weight species can be analysed due to the propensity for large molecules, such as PCR products, to form multiply charged ions that have relatively low overall mass-to-charge ratios under electrospray ionisation conditions. However, direct analysis of nucleic acids by ESI-MS still suffers from the same problems as MALDI TOF MS such as cation adduct formation (14) . In addition, the multiply charged ion spectra that are generated for large nucleic acid fragments can be very complicated reducing sensitivity and making multiplexing difficult (16) .