The role of ion-bound cluster formation in negative ion mass spectrometry

The ionization mechanisms operative in negative ion atmospheric pressure mass spectrometry are far from being properly understood. In an excess of oxygen superoxide (O(2)(-)) is generally the primary charge-carrying species that is generated. However, subsequent reactions leading to the finally detected ion signals remain obscure.

Since adiabatic expansion induced cluster growth and collision-induced dissociation (CID) processes rendered a representative sampling of ion distributions present in the source difficult, a custom-built thermally sampling time-of-flight mass spectrometer was used for the investigations. Using atmospheric pressure laser ionization of toluene as the reagent gas, high yields of thermal electrons were observed, but only negligible amounts of by-products. Ab initio calculations for individual ion/molecule reaction pathways were performed.

Electron capture by molecular oxygen resulted in the formation of subsequent superoxide water clusters as well as distinct analyte-adduct ions. By adjusting the extent of CID within the ion optical stages of the mass spectrometer, the cluster distribution changes to smaller cluster sizes and the analyte signals strongly shifted towards M(-) or [M-H](-). The observed superoxide water cluster distribution was close to thermal. The theoretical results confirmed the experimental findings.

In negative atmospheric pressure mass spectrometry the water concentration in the ion source (determining the ionization efficiency) and the CID energy provided through electrical fields (determining the ion distribution) are primary, critical parameters for the observed overall ionization mechanism and efficiency.