Notizen: Fast Pulsed Laser Induced Electron Generation for Electron Impact Mass Spectrometry

Abstract

A new ultra fast electron impact (EI) ion source is pre sented that produces a very short, high intensity electron beam, allowing medium resolution mass spectra to be re corded without pulsing the ion accelerating voltages in a time-of-flight mass spectrometer (TOF-MS). The ion source requires minimum modification of any TOF-MS equipped with an electrostatic ion reflector and UV-laser. El-spectra are presented for comparison with literature spectra.The time-of-flight mass spectrometer with laser induced multiphoton ionization (MUPI-TOF-MS) used in our laboratory has already been described [1]. The unique features of this instrument include high mass range, ionization in a collisionless environment (no adduct ions, no influence of matrix on fragmenta tion patterns), selective and very efficient ionization by resonant MUPI, the possibility to change the frag mentation from soft to hard by merely adjusting the ionizing photon density and simultaneous collection of all ions.A desirable feature for the instrument would be a simple electron impact (El)-source with sufficient sen sitivity to monitor neutral molecules introduced into the supersonic jet [2]. Electron impact ionization com plements the selective resonant MUPI, in the sense that all compounds are ionized, ionization efficiencies do not vary greatly from one compound to the next, large libraries of EI spectra are readily available and unknown spectra can be interpreted by well estab lished rules.These features are of utmost importance to the qualitative and quantitative analysis of unknown mix tures with coupled chromatography-MS techniques. The belief that our instrument could provide the extraordinary sensitivity required for the new genera tion of high resolution chromatographic techniques (capillary LC [3,4], capillary zone electrophoresis [5] and micellar electrokinetic capillary chromatography [6]) gave the final impetus for the design of this novel combined MUPI/EI-source.The ion source is shown schematically in Figure 1. The modification required for EI-ionization consists merely of a 0.25 mm Ta wire held in place between the repeller and the first extraction lens of the ion source at an electric potential intermediate to that of the two mentioned plates. The wire crosses both the molecular beam and the laser beam normally used for MUPI and is perpendicular to both these beams.When photons with an energy higher than the work function of Ta (4.25 eV) hit the wire, a swarm of elec trons instantaneously leave the surface according to the photo-electric effect. These electrons are accelerat ed to the repeller and ionize incoming sample mole cules in the supersonic jet. Molecules closer to the wire at the time of the electron pulse are ionized with less energetic electrons than those nearer the repeller. However, the point of incidence not only defines the electron energy, but also the initial electric potential of the ion formed in the accelerating field. By adjusting the voltage on the ion reflector to selectively reflect ions of limited kinetic energy...

Laser mass spectrometry for environmental and industrial chemical trace analysis

“…Similar arguments concerning sensitivity and the problems addressed in Fig. 10 are valid for laser‐induced electron ionization 47. Electron ionization is very useful as a non‐selective ionization method delivering a general survey about the main components of a gas sample.…”

Electrospray ionization/atmospheric pressure photoionization multimode source for low‐flow liquid chromatography/mass spectrometric analysis

“…Mechanisms to generate dopant ions or solvent ions are numerous, but include direct thermal ionization (thermospray),21 photoionization (e.g., dopant,22 solvent or solvent cluster4), or electron ionization via thermal electrons originating from metal surfaces exposed to VUV light (observed often with low pressure ion sources23). While both Ar and Kr lamps used with APPI can directly ionize the target molecule and/or dopant, light from the Kr lamp often cannot directly ionize the solvent molecule due to the high ionization potential (IP) of solvents (e.g., IP = 10.85 eV for methanol) 24.…”

Application of laser induced electron impact ionization to the deposition chemistry in the hot-wire chemical vapor deposition process with SiH4-NH3 gas mixtures

“…These photoelectrons are then accelerated in the electric field of the ion source of the TOF MS to ionize the sample molecules. Previous experiments have reported on directing the laser beam to the repeller plate [22], the extraction plate [23], or to a metal target placed in between the two plates [24,25] in an existing TOF MS. Since the photoelectron energies typically exceed the energy of a single 118 nm photon (10.5 eV), they should be suitable for ionizing all species with IP of either lower or greater than 10.5 eV.…”