NEUTRALIZATION OF NEGATIVELY CHARGED OLIGONUCLEOTIDES

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