Charged water droplets generated by electrospray, sonic spray, and a vibrating orifice aerosol generator (VOAG) have been studied by digital macrophotography and image charge detection mass spectrometry. Image charge detection mass spectrometry provides information on the droplet size, charge, and velocity after transmission through a capillary interface. The digital images provide the droplet size distribution before they enter the capillary. Droplets with 10-100 microm radii generated by sonic spray and VOAG are reduced to 2-3 microm radii by transmission through the capillary interface. The droplets from sonic spray and VOAG are much more highly charged than expected for random charging, and positive droplets are much more prevalent than negative. For positive mode electrospray, >99% of the detected droplets carry a positive charge, whereas for negative mode electrospray, <30% of the detected droplets carry a negative charge (i.e., >70% carry a positive charge). These observation can all be accounted for by the aerodynamic breakup of the droplets in the capillary interface. This breakup reduces the droplets to a terminal size at which point further breakup does not occur. Charge separation during droplet breakup is responsible for the relatively high charges on the sonic spray and VOAG droplets and for the preference for positively charged droplets. The charge separation can be explained using the bag mechanism for droplet breakup and the electrical bilayer at the surface of water.

We examined complexes between cucurbit[6]uril and each of ortho-, meta-, and para-phenylenediamine using computational methods, Fourier transform ion cyclotron resonance mass spectrometry, and ion mobility spectrometry. These fundamental gas phase studies show that the lowest energy binding sites for ortho- and meta-phenylenediamine are on the exterior of cucurbit[6]uril, whereas para-phenylenediamine preferentially binds in the interior, in a pseudorotaxane fashion. This conclusion is based on reactivity of each of the complexes with tert-butylamine, where the ortho- and meta-phenylenediamine complexes exchange with tert-butylamine, whereas the para-phenylenediamine complex undergoes two slow additions without displacement. Further, under sustained off-resonance irradiation conditions, the ortho- and meta-phenylenediamine complexes fragment easily via losses of neutral phenylenediamine, whereas the para-phenylenediamine complex fragments at higher energies primarily via cleavage of covalent bonds in the cucurbituril. Finally, ion mobility studies show ion populations for the ortho- and meta-phenylenediamine complexes that primarily have collision cross sections consistent with external complexation, whereas the para-phenylenediamine complex has a collision cross section that is smaller, the same as that of protonated cucurbit[6]uril within experimental error. In agreement with experiment, computational studies indicate that at the HF/6-31G* and B3LYP/6-31G*//HF/6-31G* levels of theory external complexation is favored for ortho- and meta-phenylenediamine, whereas internal complexation is lower in energy for para-phenylenediamine. In contrast, MP2/6-31G*//HF-6-31G* calculations predict internal complexation for all three isomers.

We describe a new approach to measuring the masses of individual macroions. The method employs a pulsed acceleration tube located between two sensitive image charge detectors. The charge and velocity of the macroion are recorded with the first image charge detector. The ion is pulse accelerated through a known voltage drop, and then the charge and velocity are remeasured using the second image charge detector. The mass of the ion is deduced from its charge and its initial and final velocities. The approach has been used to measure masses in the 10(10)-10(14) Da range with z = 10(3)-10(6) and m/z = 10(6)-10(9). It should be extendable to masses of <10(6) Da. We have used the method to determine the size and charge of water droplets transmitted through a capillary interface and an aperture interface. The droplets detected from the aperture interface are approximately 1 order of magnitude smaller in mass than those detected from the capillary interface. The droplets from both interfaces have relatively low charges, particularly with the capillary interface where they are only charged to a small fraction of the Rayleigh limit. These results suggest that the aerodynamic breakup of the droplets plays a significant role in the mechanism of electrospray ionization.

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