A TANDEM SELECTED-ION FLOW TUBE TRIPLE QUADRUPOLE INSTRUMENT

The design, operation and calibration of a selected ion flow tube (SIFT)-triple quadrupole instrument are described. A detailed examination of the gas-phase reaction between ClCH2+ and C-H3Cl has been carried out in order to demonstrate some of the unique experimental capabilities of the new instrument. The primary reactions at room temperature and 0.6 Torr total pressure are thermoneutral Cl-isotope exchange and termolecular association to yield [ClCH2ClCH3]+, with apparent bimolecular rate coefficients of 6.6 x 10(-11) and 4.1 X 10(-11) cm3 S-1, respectively. Double-labelling experiments with CICD2+ as the reactant ion identify hydride transfer as the mechanism for the observed Cl-isotope exchange. Collision-induced dissociation (CID) of the addition product yields ClCH2+ with a threshold energy of 31.1 +/- 3.0 kcal mol-1. The relative yields of the (ClCH2+)-C-35 and (ClCH2+)-C-37 product ions produced by CID of the mass-selected (Cl-35, Cl-37) isotopomeric adduct have been measured as a function of the CH3Cl concentration in the flow reactor. Analysis of these data with a simple kinetic model indicates that approximately one third of the adduct-forming collisions are accompanied by Cl-exchange via hydride transfer within the collision complex. When the [ClCH2ClCH3]+ ions are formed in the flow tube by a ''switching'' reaction between ClCH2+ (SCO) and CH3Cl, Cl-exchange does not occur, as shown by the complete retention of the original Cl-isotope in the ClCH2+ fragment ion produced by CID.