A TANDEM SELECTED-ION FLOW TUBE TRIPLE QUADRUPOLE INSTRUMENT

The design, operation and calibration of a selected ion flow tube (SIF T)-triple quadrupole instrument are described. A detailed examination of the gas-phase reaction between ClCH2+ and C-H3Cl has been carried o ut in order to demonstrate some of the unique experimental capabilitie s of the new instrument. The primary reactions at room temperature and 0.6 Torr total pressure are thermoneutral Cl-isotope exchange and ter molecular association to yield [ClCH2ClCH3]+, with apparent bimolecula r rate coefficients of 6.6 x 10(-11) and 4.1 X 10(-11) cm3 S-1, respec tively. Double-labelling experiments with CICD2+ as the reactant ion i dentify 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 p roduced by CID of the mass-selected (Cl-35, Cl-37) isotopomeric adduct have been measured as a function of the CH3Cl concentration in the fl ow reactor. Analysis of these data with a simple kinetic model indicat es that approximately one third of the adduct-forming collisions are a ccompanied by Cl-exchange via hydride transfer within the collision co mplex. 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-isoto pe in the ClCH2+ fragment ion produced by CID.