K. Walter

A High-Resolution Time-of-Flight Mass Spectrometer With Laser Desorption and a Laser Ionization Source

ABSTRACT Details of the novel method of laser desorption with a low powered IR-laser and resonance-enhanced multiphoton ionization (REMPI) combined with a high-resolution Reflectron-Time-of-Flight (RETOF) mass spectrometer are explained. Different features of the method are discussed. Some results of mass spectrometric investigations of biomolecules are shown.

Spectroscopy of the benzene cation: Resonance-enhanced multiphoton dissociation spectra of the B̃(E2g)←X̃(E1g) transition

Abstract We present the first well-resolved laser spectra of the lowest, dipole-forbidden electronic transition in the benzene radical cation (B(E 2g )←X(E 1g )). We employ our technique of resonance-enhanced multiphoton dissociation spectroscopy (REMPDS) which is unique in its capability of investigating bound, non-fluorescing ionic states with photofragment detection. The technique of multiphoton ionization permits the preparation of the ions either in the vibrationless ionic ground state or with one quantum of ν 16 excited. We thus obtained two ion spectra, representing vibronic levels of different symmetry. Most of the observed transitions are assignable to combination levels involving the inducing modes ν 16 and ν 17 . Our results support recent theoretical work about Jahn-Teller and pseudo-Jahn-Teller effects in the B(E 2g ) state.

Biomolecules in the gas phase. III. Multiphoton ionization mass spectra of phenylthiohydantoin amino acids and free amino acids

Abstract The multiphoton ionization (MPI) mass spectra of some phenylthiohydantoin (PTH) amino acids and free amino acids are measured and discussed. It is shown that the coupling of laser desorption with MPI allows even highly polar substances to be measured without degradation before ionization. The spatial and temporal separation of the desorption and the ionization step, together with adiabatic cooling of the desorbed neutrals, are main features of the method. As a result, this method yields mass spectra which show the molecular ion as a prominent signal in every case. In some cases, it was found essential to vary the laser wavelength to get better results. One outstanding effect of this feature was the detection of the molecular ion (M+.) of L-arginine for the first time. The influence of the wavelength used in the multiphoton ionization step is discussed, as well as the influence of the laser intensity. The consequences for analytical purposes are reviewed.

Molecular Ion Spectroscopy - Resonance-Enhanced Multiphoton Dissociation Spectra of the Fluorobenzene Cation

We present a well-resolved spectrum of the dipole-forbidden B(B2)←X(B1) transition of the fluorobenzene radical cation. Using a two-color version of our technique of resonance-enhanced multiphoton dissociation spectroscopy (REMPDS), we obtained the first vibrationally resolved spectrum for this molecular ion. Fluorobenzene ions were prepared in the ionic X state by multiphoton ionization; the resulting vibrational population was probed by photoelectron spectroscopy. The most prominent bands of the B←X transition were assigned to combinations of the inducing mode ν16a with a progression of ν6a.

Laser Tandem Mass Spectrometry in a Time of Flight Instrument

Abstract We present a new laser tandem mass spectrometry technique in a reflectron time of flight (TOF) instrument. A first pulsed laser performs the multiphoton ionization and the primary photodissociation. A newly designed ion source permits a high mass resolution in the space focus of the 12 cm long first linear TOF, where then the secondary excitation can take place. For high resolution applications the pure secondary or pure metastable mass spectrum of a preselected precursor ion can be recorded using a new reflectron scanning technique. It is also possible to obtain the whole secondary mass spectrum with one cycle using a new postacceleration method. Several techniques for ejection of interfering ions are shown. The features of our techniques are demonstrated at various primary fragments of benzene.

Biomolecules in the gas phase. V. Small peptides investigated by multiphoton mass spectrometry

Abstract Pulsed laser desorption with a small IR laser is used to evaporate 35 unprotected di- and tripeptides into an expanding supersonic beam. The peptide is then ionized by tunable multiphoton absorption and mass analyzed in a high-resolution time-of-flight mass spectrometer. Multiphoton ionization occurs readily in peptides containing aromatic amino acids, while peptides without an aromatic π-electron system are ionized by non-resonant photon absorption. By optimizing the ionization wavelength the molecular ion is exclusively formed for each investigated peptide at low photon fluxes. At higher photon densities fragment ions are formed yielding structure-characteristic sequence information of the peptide chain.