Photodissociation of ions generated by soft ionization techniques

Abstract

Photodissociation wlth vlslble light of Ions generated by thesoft Ionization techniques ceslum ion bombardment, fleld desorption, and fleld lonlratlon has been demonstrated. An argon Ion laser was used to irradiate ions In the flrst fleld-free region of a Mattauch-Herrog geometry mass spectrometer. Ions that dlssoclated In this region were detected by means of a linked scan at a constant ratlo of the magnetlc field to the electrlc fleld. Detection of only those ions created by photodlssoclation was accomplished by use of a lock-in amplifier trlggered by a mechanlcal chopper In the laser beam.A variety of compounds were tested for evldence of photodissociation activity. Only compounds with chromophores capable of absorbing visible llght were found to photodlssoclate. Several peptides tested showed no photodissoclation actlvlty. The photodlssoclations of (M + H)+ Ions from methyl red and bilirubin are used to illustrate the potential of the technlque for providlng structural Information. For methyl red the photodlssoclatlon mass spectrum exhlblts several prominent Ions that are very weak or absent In the normal mass spectrum. Improvements, Including use of shorter wavelengths, wlli be needed before thls technlque can become a useful tool for structure determinations.Photodissociation has frequently been applied as a tool for studying the structure of gaseous ions and the thermody-namics and kinetics of gaseous ion processes ( I ) . Most of the studies involved ions consisting of only a few atoms. Ion cyclotron resonance instruments (ICR) and sector instruments equipped with lasers have been used to study the fragmentation of somewhat more complex ions generated by electron impact ionization (EI). These studies have been limited to compounds of low molecular weight, generally less than 200, and low polarity, which are amenable to ionization by EI.They have also been directed, for the most part, to determining the structure of the ions generated rather than to the development of a means for the determination of the structures of organic molecules. E1 mass spectra show extensive fragmentation which provides considerable information on structure. Soft ionization techniques such as chemical ionization (CI), field ionization (FI), field desorption (FD), fast atom bombardment (FAB), and secondary ion mass spectrometry (SIMS) often give little fragmentation, and therefore, the spectra from these techniques contain less information on structure. These last three techniques are generally the methods of choice for obtaining mass spectral information on polar molecules for which E1 is not suitable. Fragmentation can be induced by addition of energy to the ions after their formation through collisions with neutral species. Collision induced dissociation (CID), both at low energies in quadrupole mass spectrometers and high energies in sector mass spectrometers, has been extensively used as a tool for structure determinations. An alternative way to add energy to ions thereby inducing fragmentation is through the absorption of Thi...

The biosynthesis of porphyrins, chlorophylls, and vitamin B12

“…Chemical ionization, with ammonia as the reagent, is reported to give a useful amount of fragmentation of porphyrins into tri-, di-, and mono-pyrroles, allowing some sequencing of the substituents.280 For chlorophylls, the use of laser-desorption Fourier-transform mass spectrometry has been demonstratedzs1 and so has resonance-enhanced multiphoton ionization.282 Laser desorption and multi-photon ionization have been combined in a new technique which gives very good molecular ions even from the extremely sensitive chlorophyll^.^^^ Bilirubin has been used as an example to demonstrate the photodissociation of molecules in the first field-free region of a mass spectrometer that can be induced by laser irradiation. 284 The various ionization methods that have been used in the mass spectrometry of corrins and vitamin B,, have been compared in a recent review.2s5…”