NEUTRALIZATION OF NEGATIVELY CHARGED OLIGONUCLEOTIDES

Negative ions of small oligonucleotides are shown to undergo collisional and collision-free neutralization following matrix assisted laser desorption and ionization (MALDI). Using a modified MALDI time of flight instrument, approximate collisional cross sections and collision-free lifetimes have been measured for negative ions of 3-,4-, 6- and 7-mer oligonucleotides. Collisional cross sections were on the order of 10(-14) cm2, With collision-free lifetimes on the order of 10-100 mus. The evidence suggests that the collision-free lifetime may be decreasing with increasing biopolymer size. Implications of this neutralization process on MALDI-TOF experiments and TOF based DNA sequencing schemes are also discussed.