A New Electrostatic Transfer Line for Improved Transmission in Fourier Transform Laser Microprobe Mass Spectrometry with External Ion Source

Abstract

The efficient extraction and transfer of ions from an external ion source outside the magnet and their injection through the inhomogeneous fringing field becomes a key factor in the micro-analytical application of Fourier transform mass spectrometry (FTMS) instruments with an external ion source. This paper reports on a new ion transfer line with static electrical fields for a source at a distance of 131 cm from the cylindrical ion cyclotron resonance (ICR) “Infinity” cell in an unshielded 4.7 Tesla superconducting magnet. Basically, ions are accelerated to 2–3 kV, transported through a time-of-flight (TOF) system and decelerated at the entrance of the magnet. The size of the system, the magnetic stray field and the significant electrical field gradients require careful optimisation of the simulations with the SIMION software. According to our calculations, the new ion transfer line should allow transport and trapping of ions initially emitted with an angle of up to 35° with the normal, as opposed to 5° with the former commercial transfer line with improved cylindrical front optics. The design has been experimentally verified using focused laser desorption/ionisation of solids. The data on reference compounds have demonstrated that the sensitivity with the new transfer line increases by a factor of 10. A substantial gain of sensitivity is also expected for other ionisation methods producing ions with a more isotropic angular emission than focused laser irradiation. The new ion optics are fully compatible with the existing hardware so that conversion becomes a minor operation.