No abstract

Water-soluble organic compounds (WSOC) often represent a large fraction of the total organic mass found in the atmospheric aerosol. They play a very important role in determining the ability of aerosol particles to act as cloud condensation nuclei (CCN), influencing cloud and fog formation and cloud albedo. Molecular characterization of WSOC in fogwater samples was achieved using a twostage ion-trap mass spectrometer equipped with electrospray ionization (ESIMS/MS). Negative ionization conditions in the electrospray interface finalized our characterization of the acidic fraction of WSOC that comprises both monoand di-carboxylic acids and polycarboxylic acids for which a similarity was suggested with naturally occurring humic (or fulvic) acids, and which are sometimes referred to in the literature as humic-like substances (HULIS). Molecular structure elucidation was accomplished using several model compounds and exploiting mass spectral resolution for compound separation. Single compound identification was attempted by recording typical MS/MS fragmentation pathways of model substances and comparing them with actual sample pathways in order to establish specific correspondences. Besides this spectrum-matching identification process, MS/MS interpretation led to several hypothetical structures for HULIS, extending the comprehension of their chemical nature. Suwannee River fulvic acid, proposed as a suitable model for representing the complex mixtures of HULIS in cloud and water aerosol extracts, was also analyzed, and the data obtained were compared with those from WSOC.

No abstract

A new, simple device generates accurate nano- and microflow rate gradients from any conventional HPLC system. The core of the new device is represented by an electric-actuated, computer-controlled, multiposition HPLC valve. The valve hosts six reservoirs for as many different mobile-phase compositions of increasing strength. A low flow rate stream pushes the weakest solvent through the column as long as required and at the desired flow rate, until the chromatographic run is started. From this time on, the electric actuation allows one to select which reservoir will be on-line with the column and for how long, thus generating a specific solvent gradient, through a sequence of controlled segments of precise mobile-phase composition. This permits one not only to exactly reproduce the programmed slope but also to achieve different gradient shapes (i.e., linear, convex, concave) for different separation needs. The new device has proven to be reliable and reproducible even at the lowest flow rate tested (250 nL x min(-1)) and in different chromatographic conditions.