Figure 2. Ion/surface collision spectrum recorded after collision of 30 eV benzene molecular ions at an H-SAM surface. The ion at m/z 91 corresponds to the addition of a methyl group followed by loss of H 2 and that at m/z 65 to further loss of C 2 H 2 . The peak at m/z 79 is due to a H atom abstraction reaction. The inset shows the collision process and some reaction products. Reprinted from ref 72.

Mass-selected polyatomic cations and anions, produced by electrosonic spray ionization (ESSI), were deposited onto polycrystalline Au or fluorinated self-assembled monolayer (FSAM) surfaces by soft landing (SL), using a rectilinear ion trap (RIT) mass spectrometer. Protonated and deprotonated molecules, as well as intact cations and anions generated from such molecules as peptides, inorganic catalysts, and fluorescent dyes, were soft-landed onto the surfaces. Analysis of the modified surfaces was performed in situ by Cs ϩ secondary ion mass spectrometry (SIMS) using the same RIT mass analyzer to characterize the sputtered ions as that used to mass select the primary ions for SL. Soft-landing times as short as 30 s provided surfaces that yielded good quality SIMS spectra. Chemical reactions of the surfaces modified by SL were generated in an attached reaction chamber into which the surface was transferred under vacuum. For example, a surface on which protonated triethanolamine had been soft landed was silylated using vapor-phase chlorotrimethylsilane before being returned still under vacuum to the preparation chamber where SIMS analysis revealed the silyloxy functionalization. SL and vapor-phase reactions are complementary methods of surface modification and in situ surface analysis by SIMS is a simple way to characterize the products produced by either technique. (J Am Soc Mass Spectrom 2009, 20, 949 -956)

In this work, we have examined the diffusive mixing of chloromethanes in different molecular solids H 2 O, D 2 O, and CH 3 OH by monitoring their chemical sputtering spectra due to the impact of Ar + ions in the collision energy range of 3-60 eV, focusing on amorphous solid water. The chemical sputtering spectra have been monitored over the temperature window accessible by liquid nitrogen, and the coverages of the molecules of interest and ice have been varied from one to several hundred monolayers. Instrumentation and sensitivity of the technique have been discussed. It is found that while the diffusion of CCl 4 in the molecular solids investigated is hindered, other choloromethanes such as CHCl 3 and CH 2 Cl 2 undergo diffusive mixing over the same temperature range. Quantitatively, while ∼4 monolayers (ML) of ice are found to block CCl 4 diffusion, the numbers are ∼250 and ∼600 ML for CHCl 3 and CH 2 Cl 2 , respectively. Crystallinity of ice does not have any effect on the diffusivity of water molecules when it is deposited below the chloromethanes. The effect of substrate was insignificant, and the rise in temperature increased diffusive mixing wherever the process was observed at a lower temperature.

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Doped fluorescent carbon dots (CDs) have drawn widespread attention because of their diverse applications and attractive properties. The present report focusses on the origin of photoluminescence in nitrogen-doped CDs (NCDs), which is unraveled by the interaction with Cu(2+) ions. Detailed spectroscopic and microscopic studies reveal that the broad steady-state photoluminescence emission of the NCDs originates from the direct recombination of excitons (high energy) and the involvement of defect states (low energy). In addition, highly selective detection of Cu(2+) is achieved, with a detection limit of 10 μm and a dynamic range of 10 μm-0.4 mm. The feasibility of the present sensor for the detection of Cu(2+) in real water samples is also presented.

Distributions of charge deposited on surfaces in desorption electrospray ionization mass spectrometry (DESI-MS) were investigated using a static charge measurement apparatus, which gives an output voltage proportional to the local surface charge density. By scanning the probe along the surface and taking measurements at fixed intervals, a contour image of relative charge density reflecting the charge distribution on the surface can be plotted. Through the measured charge distribution and the derived charge density gradient, the motion of charged droplets in the DESI experiment can be inferred. Measurements taken under various DESI operating conditions, including spray pressure, angle, flow rate, and sprayer tip-to-surface distance, show that charge is spread over an area of a few square centimeters under typical conditions; effective desorption occurs from a much smaller area (∼1 mm2) of highest charge density. Higher sheath gas pressures and smaller sprayer tip-to-surface distances lead to concentration of charge distribution into a smaller area, whereas smaller spray angles favor charge distribution over a larger area. The appearance of the highest charge density in front of the DESI sprayer tip and near the MS inlet suggests that charged droplets are moved toward the MS inlet by pneumatic forces and by the vacuum suction, in agreement with results of earlier simulations. The present observations are consistent with previous studies using other techniques and support the accepted droplet splashing DESI mechanism.