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Molecular dynamics simulations were used to study charge reduction in electrosprayed liquids through the formation of slender nanojet intermediates. The dynamics of shape relaxation and disintegration were followed as a function of charge in cylindrical water nanojets containing protonated diglycine molecules. Depending on the overall charge, simulations showed three basic scenarios for nanojet evolution. Moderately charged nanojets reduced to spheres, whereas nanojets charged close to the Rayleigh limit divided into two offspring droplets. Due to the large Coulomb interaction between ions, highly charged nanojets suffered repeated fission until the resulting droplets were charged below the Rayleigh limit. We demonstrated the role of surface fluctuations and Maxwell stress distributions in the disintegration process. The relaxation dynamics of the moderately charged systems to spherical geometry followed a damped oscillator behavior. Compared to neutral water jets, the presence of charges in subcritical nanojets resulted in a stiffer system with longer relaxation times to spherical geometry. Interparticle forces acting between the separating offspring droplets in nanojet breakup were also determined. Due to the increased role of fluctuations in nanojets, the Rayleigh limit was shown to overestimate the maximum charge on stable systems indicating higher nanodroplet production efficiency than one would expect from macroscopic theories alone.

The kinetics of guest ion formation in matrix-assisted laser desorption ionization was studied for substance P and insulin from 2,5-dihydroxybenzoic acid (DHB), α-cyano-4-hydroxycinnamic acid (CHCA), 9-anthracenecarboxylic acid (9-ACA), and sinapinic acid (SA) matrixes. The results show two very different responses. Although guest ion yields from DHB and SA follow a simple fast exponential decay (with decay times of 4.3 ± 0.7 ns and 4.6 ± 0.8 ns, respectively), the ions from the other two matrixes show extended maxima (at 6.1 ns for CHCA and 3.2 ns for 9-ACA) with only a moderate decline in their yield. Possible explanations for the fast exponential decay include the relaxation of electronically and/or vibrationally excited matrix precursors for DHB and SA. The explanation of extended maxima in the case of 9-ACA and especially CHCA can be based on competition for the protons between matrix and guest species. CHCA has significantly lower proton affinity than SA, thus, resulting in lower deprotonation rates...

Individually subthreshold, time-delayed 355 nm laser pulses were used to investigate matrix-assisted laser desorption/ionization (MALDI) ion formation time scales for the matrix 2,5 dihydroxybenzoic acid. With 30 ps pulses, it was possible to identify a prompt, subnanosecond, mechanism and a slow, nanosecond, process involving at least two steps.

Protonation reactions of Glyn homologues were studied in order to elucidate the effect of peptide chain length and matrix materials on the matrix-assisted laser desorption ionization (MALDI) mechanism. At 337 nm nitrogen laser excitation the relative ion yield−chain length relationship showed considerable variability in 3,5-dimethoxy-4-hydroxycinnamic acid (SA), 2,5-dihydroxybenzoic acid (DHB), 6-aza-2-thiothymine (ATT), and α-cyano-4-hydroxycinnamic acid (CHCA), the four matrices studied. For SA and ATT, a monotonic increase of relative ion yields (RIYs) was observed with increasing peptide length. Similar increasing pattern and significantly higher RIYs were found for the homologues of the Glyn series in the CHCA matrix with the exception of Gly. The particularly high MALDI ion yield of this amino acid in CHCA may be attributed to preferential embedding and/or more efficient condensed-phase proton transfer. To unveil the nature of the proton-transfer reactions in MALDI, the peptide length dependence of ...