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The following are the most important developments during the final months of this PRF project, now completed. All are included in the Ph.D. Thesis of Carlos Larriba, successfully defended in Aug/2010. All are still pending journal publication, but submission of at least 3 journal articles is expected to take place through the fall of 2010.

1.  Injection of ions and nanodrops into hydrocarbons from immersed Taylor cones of ionic liquids (ILs)

We have succeeded at analyzing the ions and nanodrops formed by a Taylor cone of a salt of the cation Ethyl-methylimidazolium (EMI⁺) paired with BF₄^-, immersed in several hydrocarbons including heptane and decane. The experimental approach involves (i) forming a coaxial (compound) electrospray by the methods described in our earlier report (in press in Phys. Fluids); (ii) providing time for complete evaporation of heptane, and (iii) analyzing the resulting dry EMI-BF₄ clusters in an ion mobility spectrometer of the DMA type. We have also attempted this analysis via DMA followed in series by mass spectrometry (MS), but the signal obtained was insufficient to succeed. The mobility spectra show the presence of several singly charged (EMI-BF₄)_nEMI⁺ clusters, dominated by the n=1 cluster (the dimer), as confirmed via DMA-MS. The mobility peaks associated to larger multiply charged clusters are too many to be separately identifiable by the DMA alone. However, their corresponding size and charge has been obtained under the assumption that the IL nanodrops are at a fixed fraction of the Rayleigh limit. This shows that drops with diameters as small as 3-4 nm are abundantly produced along with the ions. The behaviour is in many ways similar to what is found in vacuum. However, the purely ionic regime observed in vacuo has not been obtained in a hydrocarbon bath, although we still encounter high levels of ionic emissions. The new discovery of ion evaporation from an electrified conducting liquid surface into an insulating liquid confirms one of the main hypothesis contained in our original proposal.

2. Electrospraying polymers and the surface free energy of single molecules

We have successfully electrosprayed polar and nonpolar polymers, the latter from 50/50 toluene-methanol mixtures. These ions have been studied by tandem ion mobility (IMS) and mass spectrometry (MS), from which we infer the shape of all the ions formed at various charge states and sizes. This has been achieved over an unprecedented range of masses. Besides the near spherical and fully stretched linear (beads on a string) polymer ion shapes previously identified, we discover intermediate shape families formed by a single multiply charged globule tied to one or two bead on a string structures. No other shape families form in the polymers studied (PDMS and PEG).

The first transition away from a near sphere arising at fixed charge upon reducing the chain length occurs at a Rayleigh-like limit, from which we can infer an effective polymer surface tension g_e. This quantity is independent of size to a first approximation, but shows a small in line with the Tolman effect. g_e was known to be substantially smaller than the bulk surface tension for PEG, and this anomaly had previously been tentatively attributed to the fact that PEG is solid at room temperature. However, we have now shown that the anomaly applies also to PDMS, which is liquid at room temperature. The still pending theoretical explanation for this effect may perhaps be associated to the fact that the material composing the globule is tied in a single chain.

3. The mechanism of charge reduction of electrospray ions by triethylammonium formate

In the work just described we have controlled the charge on the polymer ions by adding a buffer salt formed by the reaction of an amine with formic acid, including ammonium acetate (AA) and formates of dimethylammonium (Met₂AF), and triethylammonum (Eth₃AF). The analytical advantage of charge reduction is avoiding fully stretched configurations, at which mobility and m/z are highly correlated and the DMA is not very effective in simplifying the crowded mass spectra. In addition, the DMA-MS study of the effect of the buffer has revealed some other most interesting facts. The DMA performs its analysis at atmospheric pressure and reveals that much of the charge reduction usually observed in the MS in the presence of Et₃AF takes place not during the electrospray process, but after the ions enter the mass spectrometer. We can as a result reconstruct the original charge state distribution as produced by the electrospray process, from the final distribution due to RF heating in the ion guides leading to the MS. We are still analyzing these data, but they evidently contain unique new information on the direct and indirect charge-reducing mechanisms of agents of amines.