ON LINE MICRODIALYSIS SAMPLE CLEANUP FOR ELECTROSPRAY-IONIZATION MASS-SPECTROMETRY OF NUCLEIC-ACID SAMPLES

A major limitation of electrospray ionization mass spectrometry (ESI-M S) for oligonucleotide analysis arises due to sodium adduction, a prob lem that increases with molecular weight. Sodium adduction can preclud e useful measurements when-limited sample sizes prevent off-line clean up. A novel and generally useful on-line microdialysis technique is de scribed for the rapid (similar to 1-5 min) DNA sample cleanup for ESI- MS. Mass spectra of oligonucleotides of different size and sequence sh owing no significant sodium adduct peaks were obtained using the on-li ne microdialysis system with sodium chloride concentrations as high as 250 mM. Signal-to-noise ratios were also greatly enhanced compared to direct infusion of the original samples. By using ammonium acetate ga the dialysis buffer, it was also found that the noncovalent associati on of double-stranded oligonucleotides could be preserved during the m icrodialysis process, allowing analysis by ESI-MS.