Instrument-Independent CAD Spectral Databases: Absolute Cross-Section Measurements In QQQ Instruments

The energy dependence of the cross section, σ(E), for the symmetric (resonant) charge transfer reaction Ar+(Ar,Ar)Ar+ was measured in our triple quadrupole (QQQ) tandem mass spectrometer.1 Our σ(E), for P≃0.04–0.43 mtorr and E≃5 – 60 eV (LAB) [the range of collision energies used for collisionally activated dissociation (CAD)], agrees to within 10% with the Rapp-Francis theory (impact parameter method in the two-state approximation), as corrected by Dewangan. We measured identical σ(E) from both the rate of reactant ion decay and the rate of product ion formation; i.e., our instrument is kinetically well behaved. The measurement of these σ(E) in other QQQ instruments can be used to validate whether or not a QQQ instrument has been properly designed to be kinetically well behaved. This is essential if generic, instrument-independent CAD spectral databases are to be developed on the basis of the absolute cross sections for the CAD of known ionic substructures. That is, since tandem mass spectrometry (MS/MS) exploits the ion fragmentation patterns characteristic of ionic substructures, the characteristic profiles [“breakdown curves”] of ion abundance versus target thickness (or collision energy) correspond uniquely to the sequence: (parent)i σij→ (daughter), σjk→ (granddaughter)k, etc. Hence, computer simulation of experimentally observed breakdown curves enables the structure of an unknown species to be assigned on the basis of the absolute cross sections σij, σjk, etc. for CAD of known ionic substructures i, j, k, etc. Thus, if the calculated and experimental breakdown curves agree, the structure would be characterized.