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For many analytical purposes, direct laser ionization of liquids is desirable. Several studies on supported droplets, free liquid jets, and ballistically dispensed microdroplets have been conducted, yet detailed knowledge of the underlying mechanistics in ion formation is still missing. This contribution introduces a simple combination of IR-MALDI mass spectrometry and an acoustical levitation device for contactless confinement of the liquid sample. The homebuilt ultrasonic levitator supports droplets of several millimeters in diameter. These droplets are vaporized by a carbon dioxide laser in the vicinity of the atmospheric pressure interface of a time of flight mass spectrometer. The evaporation process is studied by high repetition rate shadowgraphy experiments elucidating the ballistic evaporation of the sample and revealing strong confinement of the vapor by the ultrasonic field of the trap. Finally, typical mass spectra for pure glycerol/water matrix and lysine as an analyte are presented with and without the addition of trifluoracetic acid, and the ionization mechanism is briefly discussed. The technique is a promising candidate for a reproducible mass spectrometric detection scheme for the field of microfluidics.
An ionization scheme for fast online mass spectrometric interrogation of levitated droplets is presented. That renewed method comprises the output of an a Er:YAG laser at λ = 2.94 µm which is in resonance with the OH stretch vibration band of solvents like water and alcohols. A temporal pulse width larger than the time needed for pressure redistribution and also above the temperature redistribution time constant was found to lead to soft evaporation/ionization. Despite these mild desorption conditions, no additional postionization is found to be needed. Accordingly, the ionization is found to be very soft resulting in entirely intact analyte ions and concentration dependent cluster ions. Resulting mass spectra of small amino acids and large antibiotics are presented showing the versatility of the introduced technique. Above a critical mass of m ≈ 1 kDa, the formed ions carry multiple charges as it is typical for thermospray or electrospray ionization. The detection technique enables fast contactless analysis of the chemical composition of levitated microreactors and, thus, paves the way for future contactless reaction monitoring.