Normal-phase stationary phases characterization

Liquid-solid chromatography (LSC), sometimes referred to as normal phase or straight phase chromatography, is characterized by the use of an inorganic adsorbent or chemically bonded stationary phase with polar functional groups and a nonaqueous mobile phase... 

The predominant mode of HPLC, reversed phase, involves the separation of material based on the partitioning between a relatively polar mobile phase and a nonpolar stationary phase. Normal phase HPLC—nonpolar mobile phase and polar stationary phase—is considered an orthogonal technique to reversed-phase HPLC when qualifying reference standards. In fact it is common for the elution order to be entirely reversed when switching an analysis from reversed to normal phase. Therefore, highly nonpolar impurities can be easily characterized by normal phase separations. 

The mobile phase should be chosen carefully to fit certain criteria it must completely dissolve the polymer sample in a continuous solution phase (non-0 condition), it must be low enough in viscosity for the SEC system to operate in a normal pressure range, and it must effectively prevent the polymer molecules from interacting energetically with the stationary phase (e.g., adsorption). Failure to achieve even one of these criteria results in the inability of the system to characterize the sample properly. Temperature is a useful parameter to adjust when one or more of these conditions has not been met but when one is constrained to use a particular mobile phase. Certain polymers (e.g., polyesters and polyolefins) may achieve dissolution only at elevated temperatures. The viscosity of inherently viscous mobile phases may also be lowered by raising the temperature. 

Jandera P, Petranek L, and Kucerova M (1997) Characterization and prediction of retention in isocratic and gradient-elution normal-phase high-performance liquid chromatography on polar bonded stationary phases with binary and ternary solvent systems. Journal of Chromatography A 791 1-19. 

Thus for a given stationary phase the relative retention is independent of the column and represents a thermodynamic quantity that characterizes the degree of separation of two components it can be used for the identification of one component provided that the other component is a standard substance. Normal hydrocarbons, h, w hose retention times are close to thslt of the component f tmder consideration, are used as standard substances n =... 

The use of this retention index to characterize a component has the following advantages normal paraffins as standard are easily obtainable at high purity, cover the whole retention data interval and feature a linear dependence of the logarithm of retention parameters on the number of carbon atoms in the molecule for almost aU stationary phases. However some cases require secondary standards. If the relative retention of secondary standards against n-paraffins are known, the retention indices of unknown components are obtained by calculation [20]. 

The consequence of the inevitable adaptation of a catalyst to the reac tion medium was formulated by G. Boreskov in 1947 as a particular rule In stationary conditions, the specific catalytic activity, SCA, of a heteroge neous catalyst (i.e., the rate of the catalytic reaction per unit of exposed surface of the catalyticaUy active phase) is approximately constant at a given temperature and the reaction medium composition, thus SCA being dependent only on the chemical composition of the active catalyst phase. At present, turnover frequency (TOF) of the active center is normally used instead of SCA to characterize the activity of the center, TOF being in direct proportion to SCA. The Boreskov rule is evidently valid for the TOF value, too. 

Liquid-liquid system (LLC) The solute concentrations in both phases will be expressed in mole fractions, a hypothetical pure solute at infinite dilution in the solvent at the temperature and mean pressure of the system will be chosen as a standard concentration and standard physical state for the solute in both phases, and the activity coefficient of the solute in both phases will be normalized by the convention according to which y 1 as Xj - 0. The fugacities of the solute in the stationary and mobile phases are then /jg = Yis isJCjs and f,M = YiM iM- iM where is the activity coefficient characterizing the deviation from Henry s law, is the Henry law constant, and X is the molar fraction of the solute in a given phase. The standard fugacities (x" = l and y = 1) will then be fs = h,s and = By substituting from the above relations into equation 43 the relation... 

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