ABSTRACT
D. Dialysis-ESI-MS A very effective and fast method to remove cation adducts from oligonucleotides by dialysis was developed by Liu et al. [ 48,49] and a downscaling of the system to chip size was described by Xu et al. [52]. Although several other desalting procedures described in this review give satisfactory results, their salt concentration tolerance is generally very low (<25 mM) [49]. The on-line microdialysis procedure developed by Liu et al. [ 49] provided complete on-line salt removal in less than 5 min from sodium chloride concentrations as high as 250 mM. This is, by far, the most rapid method to remove cation adducts from oligonucleotide samples. In a more recent publication, Liu et al. [ 48] further optimized the method by the addition of piperidine and imidazole to the dialysis buffer, enhancing detection sensitivity. By performing the dialysis in an off-line mode and by decreasing the concentration of the dialysis buffer (0-2.5 mM NH40Ac), sensitivity could be further enhanced by more than 10-fold (to less than 10 pmol) [48].
