The identification of binding partners of proteins by mass spectrometry following specific capture on a biosensor surface is a promising tool for proteomics research and the identification and characterization of protein-protein interactions. Previous approaches include the direct ionization of analyte from the biosensor chip on a matrix assisted laser desorption ionization time-of-flight mass spectrometer (MALDI-TOFMS) apparatus and the on-chip digestion followed by elution, chromatographic concentration of the fragments, and electrospray mass spectrometry. In the present paper, using the small-volume microfluidic sample manipulation technique with oscillatory flow reported recently (Abrantes et al. Anal. Chem. 2001, 73, 2828-2835), analyte is shown to be eluted from the sensor surface into a small volume of buffer that promotes dissociation from the capture surface and delivery to the mass spectrometer. Both the incubation of the sensor surface with the sample and the recovery of analyte can be achieved with a few microliters and conducted until steady-state is attained. Because the procedure is non-destructive for the sensor surface, multiple cycles of capture and elution allow the transfer and concentration of analyte into the elution buffer. The eluted analyte can be studied directly by MALDI-TOFMS, or subjected to proteolytic digestion for protein identification. Transfer into the elution buffer and MALDI-TOFMS detection was achieved from 5 microL of starting samples containing <50 fmol of analyte. Examples are presented for the specific detection and recovery of a protein from a complex mixture of cytosolic proteins.

Reaction mechanisms of protonated water clusters [D3O+(D2O) n , n = 4−30] with hydrogen bromide (HBr) and nitric acid (DNO3) were elucidated using a fast-flow reactor operated at thermal conditions over a temperature range of 143−173 K. The dissolution of these acid species is found to occur at critical cluster sizes as reported herein. Rate coefficients of the thermal energy reactions of DNO3 and D3O+(D2O) n (n = 6−22) were investigated and found to display a dependence on cluster size. We also report findings on the minimum cluster size necessary for the first uptake of nitric acid and hydrogen chloride by hydrated sodium and potassium cluster ions and compare the values with those for protonated water clusters. The present work is considered along with previous literature results obtained using surface sensitive and mass spectrometric techniques, with the objective of providing complementary information about the uptake of acid molecules on ice/aqueous surfaces.

Electrospray ionization-high-pressure mass spectrometry was used to determine the enthalpy, entropy, and Gibbs free energy for the equilibrium gas-phase ion-molecule association reactions of C m H 2m+1 NH 3 + (m ) 1-8), (CH 3 ) 2 NH 2 + , and (CH 3 ) 3 NH + with one to three water ligands. A subset of these ions has been studied previously, and the thermochemical properties determined in this investigation compare favorably with the previously published results, providing validation of our experimental technique and method. In addition, the present measurements show that the addition of a water ligand to the monohydrated alkylammonium ion results in an increased stabilization in its structure and binding.

Results of studies of the uptake of HCl by the deuterated analogue of protonated water clusters are reported. The successive uptake of nHCl n = 1−4 is observed, n = 2−4 appearing in a stepwise manner with a ratio of 6:1 D2O/HCl for the bimolecular reaction products. This primary uptake scheme is observed over a range of pressures (0.24−0.46 Torr) and temperatures (130−170 K). However, for increased flows of HCl, enhanced uptake is observed at a lower ratio of D2O/HCl, a trend that is effected by an increased buffer gas pressure. Two distinctly dominant mechanisms of HCl uptake are operative:  the bimolecular uptake of HCl in a 6:1 ratio with water and a subsequent association mechanism of HCl binding to water in a 3:1 ratio. The atmospheric implications are discussed along with a proposed molecular activation by surface coordination (MASC) model for HCl uptake and subsequent reactivity on polar stratospheric clouds.

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