Membrane desolvation

Kunze J, Koelling S, Reich M and Wimmee MA (2000) Use of ultrasonic nebulizer with desolvator membrane for the determination of titanium and zirconium in human serum by means of inductively coupled plasma-mass spectroscopy. Eresenius J Anal Chem 366 165-166. 

Aridus (Cetac, Omaha, NE) and the Apex (Elemental Scientific, Omaha, NE). The transport efficiency can be so great that the Aridus desolvation membrane becomes clogged with solids. The Apex is not clogged but causes the accumulation of the efficiently transported solids from urine samples in the injector and interface area even when the urine samples are diluted 1/10. An example of an interior of a 1.8-mm injector partially clogged by solids from diluted urine (axial view) after using an Apex is shown in Figure 23.3. 

Kunze.J., Koelling, S., Reich, M., andWimmer, M. A. (1998). ICP-MS Determination of titanium and zirconium in human serum using an ultrasonic nebulizer with desolvator membrane./4f. Spectrosc. 19(5), 164. 

A second form of desolvation chamber relies on diffusion of small vapor molecules through pores in a Teflon membrane in preference to the much larger droplets (molecular agglomerations), which are held back. These devices have proved popular with thermospray and ultrasonic nebulizers, both of which produce large quantities of solvent and droplets in a short space of time. Bundles of heated hollow polyimide or Naflon fibers have been introduced as short, high-surface-area membranes for efficient desolvation. 

One of the most remarkable results from the molecular simulation studies of aqueous electrolyte solutions was that no additional molecular forces needed to be introduced to prevent the much smaller ions (Na has a molecular diameter of less than 0.2 nm) from permeating the membrane, while permitting the larger water molecules (about 0.3 nm in diameter) to permeate the membrane. This appeared to be due to the large ionic clusters formed. The ions were surrounded by water molecules, thus increasing their effective size quite considerably to almost 1 nm. A typical cluster formed due to the interaction between the ions and a polar solvent is shown in Fig. 7. These clusters were found to be quite stable, with a fairly high energy of desolvation. The inability of the ions to permeate the membrane is also shown... 

Flow-injection (FI) on-line analyte preconcentration and matrix removal techniques greatly enhance the performance of atomic spectrometry [348], By using USN with membrane desolvation (MDS) as the interface, FI sorbent extraction can be directly coupled with ICP-MS for the analysis of organic solutions [349]. 

In series with a desolvation energy barrier required to disrupt aqueous solute hydrogen bonds [14], the lipid bilayer offers a practically impermeable barrier to hydrophilic solutes. It follows that significant transepithelial transport of water-soluble molecules must be conducted paracellularly or mediated by solute translocation via specific integral membrane proteins (Fig. 6). Transcellular permeability of lipophilic solutes depends on their solubility in GI membrane lipids relative to their aqueous solubility. This lumped parameter, membrane permeability,... 

In this model, one can argue that a peptide must have both an affinity for the interface (favorable n-octanol partition coefficient) and small desolvation energy (favorable A log PC) in order to efficiently cross a cell membrane. On the other hand, this model also predicts that a peptide with a large n-octanol/water partition coefficient and large desolvation energy, due to a significant number of polar groups, should adsorb and remain at the membrane interface. Both of these predicted events have been observed in the laboratory. 

To move through the membrane (change sides or transverse diffusion), a molecule must be able to pass through the hydrophobic portion of the lipid bilayer. For ions and proteins, this means that they must lose their interactions with water (desolvation). Because this is extremely difficult, ions and proteins do not move through membranes by themselves. Small molecules such as C02, NH3 (but not NH ). and water can diffuse through membranes however, most other small molecules pass through the lipid bilayer very slowly, if at all. This permeability barrier means that cells must develop mechanisms to move molecules from one side of the membrane to the other. 

However, as the number of H-bonding functions in a molecule rises, octanol/ water distribution, in isolation, becomes a progressively less valuable predictor. For such compounds desolvation and breaking of H-bonds becomes the rate-limiting step in transfer across the membrane [7]. 

The steroid cyclophane also provides a sizable and well-desolvated hydro-phobic cavity in aqueous media in a manner as observed for the octopus cyclophane. The molecular recognition ability of the steroid cyclophane is inferior to that of the octopus cyclophane in aqueous solution due to the structural rigidity of steroid segments of the former host. When the steroid cyclophane is embedded in the bilayer membrane to form a hybrid assembly, however, the steroid cyclophane becomes superior to the octopus cyclophane with respect to functions as an artificial cell-surface receptor, performing marked guest discrimination. 

A frequently used micronebulizer with heated spray chamber and membrane desolvator is the Aridus from CETAC Technologies, Ohama, NE. The experimental setup of the Aridus II microconcentric nebulizer is shown in Figure 5.16. 

Figure 5.17 Setup of ultrasonic nebulizer with membrane desolvator (USN 6OOOAT, CETAC Technologies, Ohama). (Reproduced by permission of CETAC Technologies.)...

Figure 6.19 Schematic setup of IA-ICP-MS with solution based calibration using microconcentric or ultrasonic nebulizer with membrane desolvator. (j. S. Becker et ai, Int. /. Mass Spectrom. 237, 13 (2004), reprinted by permission of Elsevier.)...

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