Dense plumes

It must be emphasized that most MALDI experiments are conducted with an evacuated ion source, so that cluster (droplet) evaporation might not seem to be possible in the same way as in ESI, but there are many more (xlO -lO ) neutral species than ions in the dense plume ejected from the solid surface so the local pressure is high. Although the cluster ionization mechanism is consistent... 

It is important to examine the energetics of MALDI ion generation. Breaking a covalent C-H or 0-H bond to yield and H" requires about 14 eV or 1350kJ/mol. An example is the 0-H bond energy in phenol 14.65 eV. At 355 nm (tripled Nd YAG laser), this would be a 4.2-photon process and hence is not very probable. However, MALDI ionization occurs in the condensed phase or dense plume. Neutral matrix is present in abundance, allowing protonated matrix to be readily formed in the above phenol example (m = neutral matrix). The total ionization process then requires only around 5eV or 480kJ/mol ... 

These similarities are once again a consequence of the plume expansion. During the expansion, collisions become less frequent and the rates of ion-molecule reactions decrease. Because matrix is normally in considerable excess, the last reactions of any ion wUl nearly always be with matrix neutrals. This is fortunate for understanding MALDI spectra, because simple bimolecular matrix-analyte reactions are the limiting reactions. Quantitative understanding of complex processes in dynamic cluster or dense plume environments is not necessary, at least to a good first approximation. 

The ionization typically proceeds in two steps. In the first step (primary ion formation), the matrix absorbs the laser energy. Together with intact macromolecules, the formed matrix ions desorb into the gas phase. This process is very fast and happens in a few nanoseconds. A dense plume is formed in which the second step, the charge transfer from the matrix ions to the maaomolecules, occurs. This is mostly done by a gas phase cation (H, Na, K ) transfer. A quantitative two-step rate equation model of the ionization process was developed by Knochenmuss. This approach was extended by introducing a quantitative molecular dynamics model. According to Karas et al.. ..single charged ions are the lucky survivors.... These ions are accelerated in an electric field of several kilovolts and introduced into the mass analyzer. 

Initial plume volume flux for dense gas dispersion, voliime/time Continuous release rate of material, mass/time Instantaneous release of material, mass Release duration, time T Absolute temperature, K... 

SLAB calculates chemical concentrations at positions downwind and heights above the ground. Tlic plume may be denser-than-air, neutrally-buoyant, or less dense than air. Thermodynamics effeci.s are accounted for, including latent heat exchanges due to the condensation or evaporation ot liquids, Time averaged results may be calculated. SLAB is the easiest of the publicly-available dense gas models to set up and mn. It has been extensively validated against large-scale field data. 

The atmospheric dispersion model for dense ammonia vapor evolves a slice of the plume, from the source to receptor (Kaizer, 1989 ... 

Dc is the characteristic source dimension for continuous releases of dense gases (length), q0 is the initial plume volume flux for dense gas dispersion (volume/time), and u is the wind speed at 10 m elevation (length/time). 

Figure 5-13 Britter-McQuaid dimensional correlation for dispersion of dense gas plumes.

Anderson, M. R., Johnson, R. L., and Pankow, J. F., 1992, Dissolution of Dense Chlorinated Solvents into Groundwater. Modeling Contaminant Plumes Form Fingers and Pools of Solvent Environmental Science and Technology, Vol. 26, No. 5, pp. 901-907. 

Internal structure of the tube worm Riftia pachyptila. (a) Oxygen, sulfide, and carbon dioxide are absorbed through the plume filaments and transported In the blood to the cells of the trophosome. (b) The chemicals are absorbed into these cells, which contain dense colonies of sulfur bacteria, where they are converted to organic compounds and (c) passed back into the circulatory system to act as an energy source for the worms. Source-. From Childress, J. J., et al. (1987). Scientific American, 256, 114-121. 

The behavior of nonaqueous phase liquids (NAPLs) as they enter the partially saturated subsurface from a land surface source follows two well-defined scenarios in one case, the physical properties of the NAPL remain unchanged, while in the second case, NAPL properties are altered during transport. In the case of dense NAPLs, the contaminant plume reaches the aquifer and is subject to longterm, continuous, slow local redistribution due to groundwater flushing-dissolution processes. These plumes become contamination source zones that evolve over time, often with major negative impacts on groundwater quality. 

The Waterloo Barrier does not remediate wastes. The contaminant plume must be small enough for enclosure to be practical. The vibration and noise associated with pile driving equipment may be a problem in densely populated areas. Funnel-and-gate system can be problematic because they alter groundwater flow. In bouldered terrain and very dense unconsolidated sediments, the use of sheet piling may not be possible. Steel sheet pile applications are generally restricted to depths of less than 30 m. At some sites it is necessary to seal the barrier system to bedrock. 

Solutions containing ammonium sulfate, with or without the addition of ammonium hydroxide, have been widely used. The ammonium system can operate effectively only within a pH range of 4.0 to 7.0. As the pH value increases above 7.0, progressively more gaseous ammonia is liberated and this reacts in the gaseous phase with water vapor and SO2 to produce a dense aerosol (white plume) which is difficult for scrubbers to remove. In an ammonia system, in order to regenerate the scrubbing solution, the ammonium bisulfite and sulfite mixture is heated... 

---