Crossed Beam Research Lab

Mass Spectrometry Division at the University of Delaware

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CURRENT RESEARCH

A central theme of the Futrell Research Group for the past decade has been the application of reaction dynamics methods--particularly the use of crossed molecular beams-- to investigate the detailed mechanism of ion activation in tandem mass spectrometry, MSn. This technique is very important in analytical applications of mass spectrometry, and is increasingly important as new "soft" ionization methods are rapidly adapted to the analysis of complex molecular and quasi-molecular ions. An important question addressed in these studies is whether the fundamental mechanisms of collisional activation change dramatically as ion collision energy is varied from the low energy regime typical of ion traps and triple quadrupole instruments to high energy in four-sector tandem mass spectrometers.

Several unique instruments have been constructed at the University of Delaware to address these questions. These include three ion-neutral crossed-beam appratuses, namely:

which enable the measurement of velocity vectors of reactants and products over a broad range of collision energies. These differential scattering instruments are complemented by a which permits high resolution kinetic energy measurement of product ions and by a recently commissioned In addition to these specialized instruments we utilize several commercial tandem mass spectrometers, including a recently acquired Collectively these instruments provide us with unique capabilities to determine reaction mechanisms and characterize energy transfer in collisional activation of ions as a function of translational energy.

This research has radically altered our understanding of energy deposition in tandem mass spectrometry. The widely-held general concept that high energy collisions largely involve electronic excitation and low energy collisions involve only vibrational excitation have been demonstrated to be completely incorrect. In special circumstances the conversion of translational energy into internal energy to fragment molecular ions is more facile by five orders of magnitude than is predicted by simple theories for energy transfer. Further it appears that MS/MS mechanisms are generally impulsive and that angular scattering is a common characteristic in tandem mass spectrometry. This is a major reason that experimental strategies--choice of collision gas and collision energy, for example--are often quite different for solving the same structural problem using different types of mass spectrometers.

Most of our research has involved molecular cations of small molecule model compounds chosen as representative of classes of molecules which have been widely investigated in mass spectrometry. Recently we have begun investigation of multiply-charged cations for which the coulomb explosion of collisionally-activated ions is an important reaction channel. The relative kinetic energy of the product ions directly measures the average distance between the charges when the bond between the two fragments is broken. We are testing whether this approach can provide information on sites of protonation in model compounds with highly basic groups and in small peptides.

An overview of this research is contained in the abstract of a talk given in 1995 when Dr. Futrell received the Delaware ACS Research Award.

Recent research is described in abstracts for the  1996 ASMS and  1997 ASMS conferences.

REPRESENTATIVE PUBLICATIONS

K. Qian, A. Shukla and J. Futrell, "Electronic Excitation in Low Energy Collisions: A Crossed-Beam Tandem Mass Spectrometric Study of the Collision-Induced Dissociation of Nitromethane Ion," J. Am. Chem. Soc. 113, 7121 (1991).

R. Tosh, A. Shukla and J. Futrell, "Elaboration of an Impulsive Model for Collision-Induced Dissociation: Application to CS2+ + Ar --> S+ + CS + Ar," J. Phys. Chem. 99, 15488 (1995).

A. K. Shukla, R. E. Tosh, Y. B. Chen and J. H. Futrell, "Molecular Dynamic Mass Spectrometric Study of the Collision-Induced Dissociation of CS2+ Ions at Low and Intermediate Collision Energies," Int. J. Mass Spectrom. Ion Proc. 146/147, 323 (1995).

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