NEGATIVE-ION FORMATION IN ELECTROSPRAY MASS-SPECTROMETRY

Negative ion electrospray (ES) operating on a single quadrupole mass s pectrometer for the detection of low-molecular-weight molecules is dis cussed. The ES interface was operated at a positive cylindrical electr ode potential to produce negative ions, and the results obtained were compared to the positive ion mode. As in the case of operation in the more common positive mode, negative ions with varying degrees of solva tion and structurally relevant fragments can be obtained from a variet y of solute species, including beta-lactam antibiotics, aminoglycoside s, aminocyclitols, tetracyclines, sulfonamides, nucleotides, peptides, and explosives. No fragmentation of parent species, except those from some labile explosives, was provided because low potential difference s are applied between the capillary and the first skimmer, and electri cal discharge is avoided in the gas phase. An increase in the capillar y voltage resulted in collision-induced decomposition to produce struc turally relevant fragment ions in both operation modes. An evaluation of representative chromatographic solvents indicated that 2-propanol a dded with oxygen in the ES bath gas is best suited to suppress electri cal (corona) discharge phenomena in negative ion operation, whereas it aids in solution nebulization, desolvation, and transfer of ions in s olution to the gas phase. For positive ion mode, no such precaution wa s necessary. Conditions that promote the formation of ions in solution usually improve ES response. Therefore, an increase in the solvent pH can increase the sensitivity in negative ion ES ionization. Negative ion ES offers the advantage of providing complementary structural info rmation to help in the characterization of an unknown compound or to c onfirm a certain tentatively proposed structure. Nucleotides and explo sives were best characterized in negative ion mode owing to the ease w ith which they form anions in solution, and they could be detected dow n to the 1-pg/muL level.