Elimination technique for alkali metal ion adducts from an electrospray ionization process using an on-line ion suppressor.

The effects of an on-line ion suppressor device on alkali metal ion adduct formations of the model compound tacrolimus were investigated. The base peak ion in the positive ion ESI-MS spectrum of tacrolimus was a sodium ion adduct, [M+Na](+). On the other hand, an ammonium ion adduct, [M+NH(4)](+), was the base peak ion in the full-scan mass spectrum of tacrolimus with a cation-exchange suppressor resin, and both [M+Na](+) and [M+K](+) were eliminated. These results indicate that the combination of an on-line ion suppressor with ESI-MS is a simple and effective technique that eliminates undesirable alkali metal ion adduct formations in the positive-ion mode.