Temporal Evolution of a Laser Desorption Plume

The desorption of ions and neutrals into the vacuum upon irradiation of a laser pulse proceeds as a jet-like supersonic expansion [39] a small, but initially hot and very rapidly expanding plume is generated [15,52]. The description of MALDI as an energy-sudden method [42] nicely expresses the explosive character of the plume formed by the nanosecond laser pulse. As the expansion is adiabatic, the process is accompanied by fast cooling of the plume [39]. 

Example Laser flash light photographs of the temporal evolution of a laser desorption plume are highly illustrative [55,56]. The plume shown in Fig. 11.5 was generated from neat glycerol by an EnYag (2.94 pm) laser of 100 ns pulse width. Such laser pulse durations are typical for IR-MALDI. Glycerol was employed as a 

Although the initial velocity of the desorbed ions is difficult to measure, reported values generally are in the range of 400-1200 m s The initial velocity is almost independent of the ionic mass but dependent on the matrix. [33,36-38,46,50,51] On the other hand, the initial ion velocity is not independent of the compound class, i.e., peptides show a behavior different from oligosaccharides. [51] 

The essential independence of mean ion velocities on the molecular weight of the analyte leads to an approximate linear increase of the mean initial kinetic energies of the analyte ions with mass. High-mass ions therefore carry tens of elec-tronvolts of translational energy before ion acceleration. [33,41,50] The initial velocity of the ions is superimposed onto that obtained from ion acceleration, thereby causing considerable losses in resolution with continuous extraction TOP analyzers, in particular when operated in the linear mode. 

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Fig. 10.5. Temporal evolution of a laser desorption plume generated by a 100 ns Er Yag (2.94 pm) laser pulse from neat glycerol. [53] By courtesy of F. Hillenkamp and A. Leisner, University of Munster.

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