Absolute cross-section measurements in XQQ instruments - The NBS round robin

Abstract

Triple-quadrupole (QQQ) tandem mass spectrometry (MS/MS) is used for the analysis of multicomponent mixtures [1]. The analysis makes use of the collisionally activated dissociation (CAD) of ions. A ion may be a molecular radical cation, a protonated molecule, or a progeny fragment ion (daughter, granddaughter, etc., produced by the CAD of a larger precursor parent ion). A ion selected by the first quadrupole (Ql) interacts with a gas within the second quadrupole (Q2). Q2 channels undissociated ions and progeny fragment ions into the third quadrupole (Q3) for mass analysis. The instrument thus produces a CAD spectrum of each initially selected ion. But XQQ instruments (QQQ, BEQQ, etc.) are complex ion-optical devices [2-B]. So the choice of parameter settings and/or of instrument design can provide a distorted view of the molecular dynamics of the CAD process (e.g., if there are scattering losses due to poor ion containment within Q2, fringing fields between Q2/Q3, etc. [2-8]). So one observes instrument-dependent CAD spectra. The key MS/MS parameters are: 1) the target thickness=(actual path length traversed by the ion in its complex oscillatory trajectory through the gas target) X (effective number density of the CAD gas); 2) the type of gas (influences the extent of energy transfer); 3) the center-of-mass interaction energy, Ecm; 4) the energy level of the analyzing quadrupole Q3 relative to that of Q2; 5) the Mathieu parameter q2 (rf voltage of Q2) and restrictive interquadrupole apertures of diameter < 1.4 ra; and 6) differences in mass-dependent conversion gain of ion detectors.