Concentration Of Neutral Species Research Articles

A numerical model of ion concentration profiles in the lower ionosphere

A one-dimensional numerical model has been developed which gives the vertical profiles of the electron and ion concentrations at altitudes between 50 and 100 km. The model has been constructed for day-time ionization conditions in midlatitudes and yields a slightly abbreviated scheme of ion-molecular reactions. Neutral species concentrations have been compiled from various authors. Seasonal variations of temperature and the most important neutral species have been taken into account. For the purpose of this paper moderate solar fluxes in all required radiation bands have been considered.

The role of ions in soot formation

Abstract

Ion-molecule reactions in sooting acetyleneoxygen flames

Ion concentration profiles up to mass 557 amu were measured in a sooting acetyleneoxygen flame at an equivalence ratio of 3.0, a total pressure of 2.67 kPa, and an unburned gas velocity of 50 cm s −1. The mass spectrometer was calibrated for mass by seeding flames with isotopes of several metals and by using deuterated acetylene, which also allowed us to measure the number of hydrogen atoms in each hydrocarbon ion. The ion concentration sensitivity of the mass spectrometer was calibrated by comparing the individual ion currents with the ion current obtained in operation as a high-pass mass filter (that is, transmitting only those ions whose masses were greater than a specific mass) and by relating these currents to the total ion concentrations determined using a Langmuir probe. Ion-molecule reactions, including those with large ions, were demonstrated to be rapid under these flame conditions. The question of whether the large ions are produced by reactions of small ions with large neutral molecules or by reactions of large ions with small molecules was resolved in favor of the latter reactions. From the sequential changes in the number of carbon and hydrogen atoms and the carbon-to-hydrogen ratio in ions of increasing mass, we deduced that the types of ion-molecule reactions occurring included not only acetylene addition to growing ions, but also polyacetylene addition (with or without acetylene elimination) and the addition of allene and propyne. From reaction rate considerations, using literature values for neutral species concentrations in the same flame, we concluded that many of the small hydrocarbon molecules observed in this flame, up to and including benzene, could add to large ions to increase the ion mass. This accounts for the extremely rapid growth observed from small ions to very large ions, that is, from 39 amu to about 4500 amu (3 to about 370 carbon atoms) in 1 ms.

Ion-etching of discharge-produced tetrafluoroethylene film with hydrogen, oxygen and noble gases. A mass spectrometric analysis

Freshly deposited discharge-produced tetrafluoroethylene films were ion-etched with either helium, neon, argon, oxygen or hydrogen. The ions C +, CF +, CF 2 + and CF 3 + comprised most of the positive ions in rare gas discharges, with CF + always dominant. Sputtered fragments containing two or more carbon atoms were rare. These findings are compatible with the ion-etching of a highly crosslinked polymer film. Residual background gases were contributed to 1–3% of the total ion flux even though their actual partial pressures were very low. The concentration of neutral species corresponding to the ions observed was less than one part in ten thousand of the etching gas. With pure hydrogen, very little etching occurred and the degree of ionization relative to the rare gases was low. The principal reaction was the abstraction of fluorine from the polymer to give hydrogen fluoride and a more highly crosslinked film. Oxygen containing discharges produced the largest total yield of all the systems studied and the most evidence of chemical attack on the polymer. The ions observed were CO +, CO 2 +, COF +, COF 2 + as well as C +, CF +, CF 2 + and CF 3 +. Thus oxygen etches the polymer by preferentially attacking the carbon-carbon framework.

The physicochemical and pharmacokinetic bases for the biopharmaceutical evaluation of drug biological availability in pharmaceutical formulations.

Abstract: The operational determination of the bioavailability of a vital drug from its pharmaceutical formulation by the extent and rate of its appearance in the blood and tissues of an intact biological organism is termed biopharmaceutics. Such bioavailability is a complex function of the physicochemical factors that determine its stability in the formulation and in the biological fluids; the rate of release of drug particles from, or disintegration of, the formulation matrix; the rate of dissolution of the dispersed drug particles as affected by the pH, volume, and agitation of the dissolution media; the solubility of the drug in the solvent; its transport to the absorption site; the pKa of the dissolved substance that determines the concentration of neutral species at the pH at the site; the partition of dissolved drug into the lipid‐like membranes and the subsequent rate of diffusion into the body compartments. Pharmacokinetics determines the amounts of drug absorbed, the rate of absorption and release from the pharmaceutical formulation in the intact biological system. Experimental data may be obtained from amounts of drug and/or metabolites monitored in the urine, faeces and blood as functions of time. Total amounts excreted and areas under blood level‐time curves with their defined limitations are one set of criteria of bioavailability. Rates of absorption, deduced from rates of change of blood level and excretion, form another set of criteria whose various limitations are also defined. Dissolution rate studies have limited a priori value in predicting bioavailability but may be useful in quality control procedures when correlated with biopharmaceutical data. Although many mathematical functions may be operationally useful in characterizing in vitro dissolution rates of drugs from dosage forms, the most valid presentation of data is the percentage og drug released at various time intervals.