Charge ionization

Most drugs are ionized in aqueous solution (Table 2.1), and can therefore exist in a neutral or a charged state, depending on the pH of the local environment. Molecules are more lipophilic when neutral than when charged. Ionization is expressed by the aqueous ionization constant, pKa. As pointed out below, log D is a p Independent term for ionizable drugs. Permeability and aqueous solubility are also pKa-dependent. Lipophilicity, pKa, permeability through artificial membranes and... 

On the other hand, the ionized forms, which tend to be less lipid soluble, cannot diffuse across tire lipid phase of the cell membrane. Ionized molecules may also repelled from the cell surface by groups with similar charge, or may be attracted to it and held there by groups with opposite charge. Ionized drug forms are, sometimes, unable to be filtered even through the aqueous pores of the membranes due to their own size or to the size they attain after the attraction of water molecules. 

A sphere with a tolerance radius of 1.5 A represents an ionizable feature and is added for the case that a reversely charged ionizable feature can be found on the macromolecule side within a plausible distance range. This interval is set to 1.5-5.6 A and is user-adjustable. 

Electrovalent. That type of chemical linkage in which elements are bound by the coulombic force between ions of opposite charge ionized the opposite of covalent. 

The concept of ion pairing refers to the ionic association between ions added to the mobile phase and oppositely charged ionized groups of the sample molecule, i.e.. 

Effective nuclear charge Ionization energy Electronegativity Ionic radius Oxidation state Lanthanide contraction... 

A perfect stoichiometric crystal with complete absence of defects is considered neutral, which implies that the surface will be neutral too, thus there is no net surface charge. This is true for diamond. As soon as the surface is oxidized, hydroxyl and other groups form that develop a charge as soon as it is immersed in a protic liquid. Eor any material, there are four major sources of this charge ionization or dissolution of surface groups, specific ion adsorption, ion exchange, and solution of specific ions out of the surface. 

The K values in the table reveal some interesting trends. First, the binding affinities are higher at pH 3.2, where the carboxylic acid groups on the dendron periphery are protonated and, thus, uncharged. At neutral pH, the negatively charged, ionized... 

Electrospray Generation of charged Ionization directly from sample solution... 

Ionic interactions arise from electrostatic attraction between two groups of opposite charge. These bonds are formed between positively charged (o -ammonium, -ammonium, guanidinium, and imidazolium) side chains and negatively charged (ionized forms of a-carboxyl, j6-carboxyl, y-carboxyl, phosphate, and sulfate) groups. 

Increases electronegativity, nuclear charge, ionization energy, electron affinity, atomic number (number of protons, number of electrons), acidity of oxides... 

TCD = Thermal Conductivity D., MDID = MicroD is charge Ionization D., FD = Fluorescence D., CID = Chemi-Impedance D. (polymer film), MDOD = MicroDischarge Optical D., ITMS = Ion Trap MS,... 

The long-chain organic cations of the salts are attracted by electrostatic forces [24] and combine with a negatively charged (ionized silanol) site which becomes neutralized, while another cation (which may be a ion) is released from the diffuse Gouy layer. 

Thermospray and, more recently, electrospray ionization have found wide application as an interface technology between HPLC instruments and mass spectrometers. They represent powerful techniques for the analysis of complex lipids directly from solutions (Henion and Lee, 1990 Murphy, 1993). In most instances, the total HPLC eluant can be sent directly into the heated thermospray ion source. Here, the combination of heat and eluant velocity creates a plume of small-diameter particles suspended in a vapor (nebulization). A strong electric charge forms on the surface of the liquid particles and as the droplets evaporate the increase in charge ionizes analyte molecules that are discharged directly from the droplet into the gas phase. From here, they may enter the mass spectrometer directly. 

Utilizing a space-charge ionization-detection technique, discovered independently by Kingdon and Hertz, Foote and Mohler were able to study the photoionization of cesium vapor with a much greater... 

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