A New Technique for Decomposition of Selected Ions in Molecule Ion Reactor Coupled with Ortho-Time-of-flight Mass Spectrometry A Dodonov, 1 * V. Kozlovsky, 1 A. Loboda, 1 V. Raznikov, 1 I. Sulimenkov, 1 A. Tolmachev, 1 A. Kraft 2 and H. Wollnik 2 1 Institute of Energy Problems in Chemical Physics, Russian Academy of Science, Chernogolovka 142432, Russia 2 II. Physikalisches Institut, Universität Giessen, 35392 Giessen, Germany A molecule ion reactor (MIR), i.e. a gas filled radio-frequency only quadrupole with a longitudinal electrical field (RFQLEF), is used as an atmospheric pressure ionization interface for an orthogonal time-of-flight mass spectrometer (O-TOFMS). A new phenomenon of selective ion ‘heating’ in a MIR near Mathieu’s instability threshold was found and confirmed by computer simulation. The ‘heating’ in collisions with buffer gas molecules leads to ion decomposition. In the case of multicharged ions, fragments with an m/z value higher than that of the parent ion have a stable motion and can be analysed by an O-TOFMS. Fragmentation of bradykinin and -endorphin molecular ions having a selected charge state is demonstrated. The spectra show clear ‘ladder’ structure. The phenomenon may be used as an alternative to tandem mass spectrometry (MS/ MS) for molecule structure analysis. © 1997 by John Wiley & Sons, Ltd. Received 5 A ugust 1997; A ccepted 22 A ugust 1997 Rapid. Commun. Mass Spectrom. 11, 1649–1656 (1997) No. of Figures: 9 No. of Tables: 0 No. of Refs: 9 Tandem mass spectrometry (MS/MS) with electro- sprayed ions is being increasingly applied to determine molecule structure, for example for the high-sensitivity sequencing of short peptides. 1 In this method collision- ally induced dissociation of selected ions yields infor- mation about the composition and structure of these ions. As an alternative we suggest here to use a molecule–ion reactor (MIR), based on a gas filled radio frequency quadrupole (RFQ) with a superimposed longitudinal electrical field, to investigate the composi- tion and structure of selectivity ‘heated’ ions. The molecule–ion reactor described previously 2,3 has two modes of operation. In the first mode the motion of parent and fragment ions is chosen to be stable. The inhomogeneous RF electric field forces the ion to oscillate around the quadrupole axis while the collision- ally induced fragmentation of the ion is controlled by the strength of the longitudinal electric field. In the second mode the amplitude and frequency of the RF electric field are chosen to be near Mathieu’s instability threshold for the parent ion. This leads to a resonant increase of the ion velocity and thus to a collisional ‘heating’ and fragmentation of the selected parent ion. A combination of both methods can be used also. The present study explores this fragmentation technique and the possibility of using it for a structure analysis of short peptides. DESIGN OF THE MIR A molecule–ion reactor, shown in Fig. 1, is a part of an electrospray ionization interface. Electrosprayed ions pass through a gas curtain and through a nozzle (dia. 0.15mm) into the MIR chamber and through a skimmer (dia. 0.35 mm) to the O-TOF mass analyser. The MIR itself is an RF-only quadrupole with a longitudinal electric field. Each of the four quadrupole rods consists of a set of metal rings (dia. 4 mm, length: 2.1 mm) separated by thin insulators. Longitudinal and RF voltages are applied through series of resistors and capacitors shown in Fig. 1. The full length of the quadrupole is 25mm and the distance between oppo- site rods is 2  = 3.6mm. The pressure in the MIR chamber is maintained at 0.1–4 mbar. EXPERIMENTAL RESULTS Experiments were performed using the home-built MIR-TOFMS. This system has been described in detail elsewhere. 3,4 Our TOF mass analyzer provides a rou- tine mass resolving power ~ 10 000 full width at half maximum (FWHM) and an accuracy of the mass measurement of 10ppm. As an illustration of the new method for a fragmentation of selected ions two examples are presented, bradykinin (Sigma) and -endorphin (Beckman), with the monoisotopic molec- ular masses 1059.651 Da and 3462.822 Da, respectively. These peptides were dissolved in methanol at a concentration of 10 –5 M; the -endorphin solution contained 2% acetic acid. A fused silica capillary with i.d. of 0.05 mm was used in the electrospray ion source with a flow rate of 0.18 µL/min. The electrospray voltage between the capillary and gas curtain electrode was about 3kV. Pressure in the MIR chamber was 0.93mbar at room temperature. Dry air was used for the gas curtain and as the MIR buffer gas. Ion masses and charge states in mass spectra were analysed by measuring an accurate m/z value for monoisotopic peaks and by an m/z difference between neighbouring peaks in the isotopic distribution. In mode 1 the fragmentation of the bradykinin ions was performed using a constant RF amplitude, U RF = 85 V, and frequency v RF = 1.001 MHz. A few *Correspondence to: A. Dodonor, Institute of Energy Problems in Chemical Physics, Russian Academy of Science, Chernogolovka, 142432, R ussia Contract/grant sponsor: Russian Foundation of Basic Research; Contract/grant number: N-96-03-34254 Contract/grant sponsor: Volkswagen Shftung RAPID COMMUNICATIONS IN MASS SPECTROMETRY, VOL. 11, 1649–1656 (1997) CCC 0951–4198/97/151649–08 $17.50 © 1997 by John Wiley & Sons, Ltd.