RECOGNITION OF QUATERNARY AMMONIUM-COMPOUNDS USING MASS-SPECTROMETRY

Liquid secondary-ion mass spectrometry (LSIMS) experiments were perfor med on a series of model quaternary ammonium chlorides [M(Q)+Cl-] in o rder to develop a strategy, via an understanding of cluster-ion format ion, for recognition of quarternary ammonium structures. Model structu res contained a single quaternary site and various other structural fe atures (for example, conjugated quaternary systems and carboxylic acid groups), using amine hydrochlorides as controls. Cluster ions were ex amined with both positive- and negative-ion LSIMS, using a range of ma trix modifiers. Precursor scans, or 'molecular weight confirmation sca ns', of M(Q)+ (the charged molecular ion of quaternary ammonium compou nds) and [M+H]+ ions were used to assist in the observation of cluster ions and to unambiguously demonstrate precursor/product relationships . Positive-ion experiments typically showed dimeric clusters of [2M(Q) + A]+ (A = anion) and [2M(Q) - H]+ ions, while negative-ion experimen ts produced [M(Q) + 2A]- ions. Observation of these cluster ions coupl ed with the lack of an [M-H]- ion are a distinguishing feature of quat ernary amines. Additional cluster ions were targeted for distinguishin g between protonated amines and quaternary amonium compounds containin g a carboxylic acid group. A strategy for recognizing and distinguishi ng quaternary ammonium structures using LSIMS is proposed, and some in itial experiments with thermospray and electrospray are discussed.