‘Bubble chamber model’ of fast atom bombardment induced processes

Abstract

A hypothesis concerning FAB mechanisms, referred to as a 'bubble chamber FAB model', is proposed. This model can provide an answer to the long-standing question as to how fragile biomolecules and weakly bound clusters can survive under high-energy particle impact on liquids. The basis of this model is a simple estimation of saturated vapour pressure over the surface of liquids, which shows that all liquids ever tested by fast atom bombardment (FAB) and liquid secondary ion mass spectrometry (SIMS) were in the superheated state under the experimental conditions applied. The result of the interaction of the energetic particles with superheated liquids is known to be qualitatively different from that with equilibrium liquids. It consists of initiation of local boiling, i.e., in formation of vapour bubbles along the track of the energetic particle. This phenomenon has been extensively studied in the framework of nuclear physics and provides the basis for construction of the well-known bubble chamber detectors. The possibility of occurrence of similar processes under FAB of superheated liquids substantiates a conceptual model of emission of secondary ions suggested by Vestal in 1983, which assumes formation of bubbles beneath the liquid surface, followed by their bursting accompanied by release of microdroplets and clusters as a necessary intermediate step for the creation of molecular ions. The main distinctive feature of the bubble chamber FAB model, proposed here, is that the bubbles are formed not in the space and time-restricted impact-excited zone, but in the nearby liquid as a 'normal' boiling event, which implies that the temperature both within the bubble and in the droplets emerging on its burst is practically the same as that of the bulk liquid sample. This concept can resolve the paradox of survival of intact biomolecules under FAB, since the part of the sample participating in the liquid-gas transition via the bubble mechanism has an ambient temperature which is not destructive for biomolecules. Another important feature of the model is that the timescale of bubble growth is no longer limited by the relaxation time of the excited zone ( approximately 10(-12) s), but rather resembles the timescale characteristic of common boiling, sufficient for multiple interactions of gas molecules and formation of clusters. Further, when the bubbles burst, microdroplets are released, which implies that FAB processes are similar to those in spraying techniques. Thus, two processes contribute to the ion production, namely, release of volatile solvent clusters from bubbles and of non-volatile solute from sputtered droplets. This view reconciles contradictory views on the dominance of either gas-phase or liquid-phase effects in FAB. Some other effects, such as suppression of all other ions by surface-active compounds, are consistent with the suggested model.

A Mechanism for Ionization of Nonvolatile Compounds in Mass Spectrometry: Considerations from MALDI and Inlet Ionization

“…He suggested that superheating a surface under vacuum conditions by an energy sudden event resulted in bubbles bursting and forming possibly a supersonic jet. Sunner et al [67] and Kosevich et al [68] suggested superheating below the liquid surface as a model for desorption/ionization processes in FAB. Kosevich et al [68] compared the formation of bubbles which burst releasing analyte and charged droplets under vacuum conditions to a bubble chamber.…”

“…Sunner et al [67] and Kosevich et al [68] suggested superheating below the liquid surface as a model for desorption/ionization processes in FAB. Kosevich et al [68] compared the formation of bubbles which burst releasing analyte and charged droplets under vacuum conditions to a bubble chamber. Karas et al proposed such a process as a possible charging mechanism in the 'lucky survivor' model for MALDI [10].…”

“…Under vacuum conditions used in MALDI, it is also possible for the ablated molten matrix/analyte droplets to experience superheating in the expanding plume while traversing the pressure gradient from solid to vacuum similar to that described for AP-LSI. Only a nucleation event [68] is required to violently release the excess energy in bubble formation and bursting leading to highly charged droplets.…”

“…Of course, the processes occurring in the laser plume are complex and this represents just one possible ionization mechanism, but one that may be important in ionization of nonvolatile compounds. As noted above, MALDI is not the only ionization method for which singly charged ions are proposed to be formed from a charged droplet or cluster [68]. Vestal's unified ionization model suggests that charged droplets are responsible for ionization of nonvolatile compounds in a variety of ionization methods which produce substantial abundances of singly charged ions [29].…”

Monomer/dimer dependent modulation of reduction of the cationic dye methylene blue in negatively charged nanolayers as revealed by mass spectrometry

“…This assumption as to structure of the nanocomposite was conrmed by molecular dynamics simulation and quantum chemical calculations. 43 In the framework of this model bombarding particles initiate boiling, i.e. [33][34][35][36][37] Such nanostructure as a layer of surfactants at the liquid-gas interface can be formed at the surface of liquid glycerol used as a liquid matrix compound in FAB and liquid SIMS (lSIMS) techniques.…”