In-Source Fragmentation Technique for the Production of Thermalized Ions

Our electrospray ionization-ion funnel-rf hexapole (ESI-IF-6P) source is designed to produce ions for threshold collision-induced dissociation (TCID) studies in a guided ion beam mass spectrometer. This ion source forms an initial distribution of Ca2+(H2O)(x) ions where x is 6-9. A new in-source fragmentation technique within the hexapole ion guide of the source is described, which is easy to implement and of modest machining and electrical costs, and is able to generate smaller Ca2+ (H2O)(x) complexes, where x = 2-5. Fragmentation is achieved by biasing an assembly of six 0.25 in. long electrodes that are inserted between the hexapole rods. The assembly is positioned in the high-pressure region of the source such that newly formed Ca2+ (H2O)(x) ions undergo enough collisions to become thermalized, as verified by TCID studies. From the initial distribution of ions, fragmentation proceeds along the lowest energy pathway, which corresponds to sequential water loss for most complexes. However, the Ca2+ (H2O) complex cannot be formed using this method because charge separation into CaOH+ and H3O+ becomes the lowest energy pathway from the Ca2+ (H2O)(2) complex. Therefore, this fragmentation technique can be used to identify the critical size complex for M2+(H2O)(x) systems, which we define as the complex size (x) at which charge separation becomes a lower energy pathway compared with simple ligand loss. (J Am Soc Mass Spectrom 2009, 20, 2312-2317) (C) 2009 American Society for Mass Spectrometry