B6.1: Time-resolved Mass Spectrometry

Laser-Induced-Liquid-Bead-Ion-Desorption (LILBID) is a soft ionisation method for the analysis of biological samples via mass spectrometry (MS). This technique makes it possible to analyse oligonucleotides and proteins, especially membrane proteins, in physiological environment containing different kinds and amounts of buffers, detergents and salts. This technique uses piezo-driven droplet generators to produce small droplets (with a diameter of 50 µm) containing the analyte of interest in physiological conditions. Those droplets are transferred into the vacuum of the MS instrument, where they are irradiated by an IR laser at the absorption wavelength of water at 2.94 µm. The absorption of the IR light leads to an explosive expansion of the droplet whereby the analyte of interest is transferred into the gas-phase. The gas-phase ion can now be analysed by conventional time of flight (TOF) MS.

Time-resolved (TR) MS can be achieved by capturing a sample droplet in a droplet trap and starting a reaction inside the droplet. The droplet can then be stored for a well-defined amount of time and then transferred into the MS instrument for analysis. In the trap the droplet will be levitated to store it for various time delays between reaction start and MS analysis. This can be achieved by charging the droplet slightly and levitate it electrodynamicaly inside a Paul trap. High voltage AC potentials make it possible to hold charged droplets inside a small volume in space. The reaction starting the kinetic inside those levitated droplets can be triggered via uncaging photo-labile groups which sets a well-defined start time. This will allow measurement of time resolved mass spectra of biological samples in a physiological environment.