Relaxation Processes of Molecules on a Solution Surface Studied by the Two-Color Pump−Probe Ionization Technique

How to make big molecules fly out of liquid water: applications, features and physics of laser assisted liquid phase dispersion mass spectrometry

Applications, features, and mechanistic details of laser assisted liquid phase dispersion mass spectrometry are highlighted and discussed. It has been used in the past to directly isolate charged molecular aggregates from the liquid phase and to determine their molecular weight employing sensitive time-of-flight mass spectrometry. The liquid matrix in this MALDI (matrix assisted laser desorption and ionization) type approach consists of a 10 microm diameter free liquid filament in vacuum (or a free droplet) which is excited with a focused infrared laser pulse tuned to match the absorption frequency of the OH-stretch vibration of bulk water near 2.8 microm. Due to these features we will refer to the approach as free liquid matrix assisted laser dispersion of ions or ionic aggregates (IR-FL-MALDI), although also LILBID ("laser induced liquid beam (bead) desorption and ionization") has been proposed early as a descriptive acronym for the technique and may be used alternatively. Low-charge-state macromolecular adducts are isolated in the gas phase from solution via a yet poorly characterized mechanism which sensitively depends upon the laser intensity and wavelength, and after the gentle liquid-to-vacuum transfer the aggregates are analyzed via time-of-flight (TOF) mass spectrometry (MS). Possible mechanisms for the isolation and charging of biomolecules directly from liquid solution are discussed in the present contribution. Recent technical advances such as minimizing the sample consumption, strategies for high throughput mass spectrometry, and coupling of liquid beam MS with HPLC will be highlighted as well. An interesting feature of IR-FL-MALDI is what we call the linear response, i.e., a surprising linearity of the gas phase mass signal on the solution concentration over many orders of magnitude for a large number of biomolecular systems as well as ions. Due to these features the approach may be regarded as a true solution probing spectroscopy, which enables elegant biokinetic studies. Several experiments in which time resolved IR-FL-MALDI-MS has recently been employed successfully are given. A particular highlight is the possibility to quantitatively detect oxidation states in solution, which clearly distinguishes the present approach from other established MS source concepts. Due to the good matrix tolerance also proteins in complex mixtures can be monitored quantitatively.

Time-Resolved Micro Liquid Desorption Mass Spectrometry: Mechanism, Features, and Kinetic Applications

Liquid water beam desorption mass spectrometry is an intriguing technique to isolate charged molecular aggregates directly from the liquid phase and to analyze them employing sensitive mass spectrometry. The liquid phase in this approach consists of a 10 µm diameter free liquid filament in vacuum which is irradiated by a focussed infrared laser pulse resonant with the OH-stretch vibration of bulk water. Depending upon the laser wavelength, charged (e.g. protonated) macromolecules are isolated from solution through a still poorly characterized mechanism. After the gentle liquid-to-vacuum transfer the low-charge-state aggregates are analyzed using time-of-flight mass spectrometry. A recent variant of the technique uses high performance liquid chromatography valves for local liquid injections of samples in the liquid carrier beam, which enables very low sample consumption and high speed sample analysis. In this review we summarize recent work to characterize the ‘desorption’ or ion isolation mechanism in this type of experiment. A decisive and interesting feature of micro liquid beam desorption mass spectrometry is that — under certain conditions — the gas-phase mass signal for a large number of small as well as supramolecular systems displays a surprisingly linear response on the solution concentration over many orders of magnitude, even for mixtures and complex body fluids. This feature and the all-liquid state nature of the technique makes this technique a solution-type spectroscopy that enables real kinetic studies involving (bio)polymers in solution without the need for internal standards. Two applications of the technique monitoring enzyme digestion of proteins and protein aggregation of an amyloid model system are highlighted, both displaying its potential for monitoring biokinetics in solution.

Structures and dynamics of molecules on liquid beam surfaces

In this review, we describe experimental studies on structures and dynamics of molecules on clean liquid surfaces in a vacuum. These studies use clean-surface preparations in combination with highly sensitive laser-, photoelectron-, and mass-spectroscopic techniques. In particular, we refer to our recent studies on solvation structures and reactions on various solution surfaces using liquid beam-multiphoton ionization-mass spectrometry. These include (a) aggregation of solute molecules on solution surfaces such as formation of solute pairs, clusters, and islands; and (b) chemical reactions induced by photoexcitation and -ionization, such as a nucleophilic reaction of phenyl ketones, polymerization of halo-anilines, and reduction of cations by solvated electrons.