ABSTRACT
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.
ABSTRACT
A commercially available airbrush gun as a new source for spray ionisation is presented. It is best operated employing moderate stagnation pressures, resulting in a sonic gas flow. A mass spectrometric investigation on the amino acid Lysine and several peptides reveals that this inexpensive approach results in reproducible mass spectra. The ion patterns strongly resemble the results from other studies obtained with custom-made sonic spray vaporisers. The patterns also resemble the mass spectra recorded with electrospray devices. For a better understanding of the vaporisation process, the mass spectrometry experiments are accompanied by laser-induced fluorescence experiments. Inverse Abel Transform of the obtained fluorescence maps allows the determination of the full three-dimensional distribution of the spray cone. Furthermore, via exploitation of the solvatochromism of the used dye the solvation-state distribution can be visualised. In addition, expansion parameters, such as droplet size and velocity, are obtained by laser stroboscopy. The experiments demonstrate that the analyte hardly desolvates throughout the expansion. This indicates a subsequent vaporisation of the residual solvent in the intermediate pressure region of the mass spectrometer.
ABSTRACT
One major drawback of matrix-assisted laser desorption/ionization (MALDI) is still the relatively poor pulse-to-pulse reproducibility of the signal intensity. This problem, caused by insufficient homogeneity in the matrix/analyte co-crystallization, is usually circumvented by averaging the detected ion intensity over several shots. However, during the consecutive laser pulses, the applied matrix gets depleted and only a number of subsequent experiments can be done on the same sample spot. In order to achieve the desired long-term stability in combination with a sufficient pulse-to-pulse reproducibility, recently liquid MALDI matrices have been introduced. This contribution demonstrates the promising combination of liquid matrices with high repetition rate lasers for atmospheric pressure MALDI (AP-MALDI). To demonstrate the robustness of the new approach, two different kinds of liquid matrices were used in combination with both a typical flashlamp pumped 15 Hz laser and a diode pumped solid state laser operated at 5 kHz. The latter showed a stable ion signal over more than 3,500,000 consecutive laser pulses.
ABSTRACT
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.
