RESUMEN
Sorption isotherms for 1-butanol (BuOH) and eucalyptol (Eu) on Partisil 10 ODS-3 are plotted as concentration sorbed versus activity in aqueous solution. The former follows the Langmuir equation and the latter follows the equation for an "associative bilayer isotherm with limited solubility in the aqueous phase". Two series of simultaneous sorption experiments of BuOH and Eu were also performed. One series involved measuring the influence of BuOH, as an organic modifier solvent in water, on the sorption of Eu as solute. In the second series, the roles of BuOH and Eu were reversed. These studies show that when sorption is treated as partitioning between two bulk liquid phases, the stationary phase behaves as an ideal solution which is composed of C(18) chains, sorbed organic modifier, and sorbed solute. Combined with the Langmuir isotherm behavior of BuOH, this ideal solution behavior supports the view that BuOH is sorbed at the stationary-phase/mobile-phase interface, with its hydrocarbon tail in the stationary phase.
RESUMEN
The simultaneous sorption of the tetra-n-butylammonium ion (TBA(+)) and butanol on the bonded phase sorbent Partisil-10 ODS-3 from an aqueous mobile phase, at the two different ionic strengths 0.50 and 0.050 mol/L, is studied by the column equilibration technique. When the TBA(+) concentration in the mobile phase is kept constant while the butanol concentration is varied, the plots of moles of TBA(+) sorbed versus moles of butanol sorbed decrease linearly. This indicates that butanol simply competes with TBA(+) for sorption space. In contrast, when the butanol concentration in the mobile phase is kept constant while the TBA(+) concentration is varied, the plots of moles of butanol sorbed versus moles of TBA(+) sorbed decrease nonlinearly. This indicates that, in addition to competing with butanol for space, sorbed TBA(+) also causes an unfolding of the originally collapsed C(18) chains. At 0.050 mol/L ionic strength, chain unfolding causes an increase in the total space that is available for sorption of butanol, a decrease in overlap between sorbed TBA(+) and butanol, and a decrease in the sorbent strength of the ODS phase for butanol. The last effect is likely due to a reduction in contact area between a sorbed butanol molecule and the C(18) chains. At 0.50 mol/L ionic strength, chain unfolding decreases both the TBA(+)/butanol overlap and the sorbent strength but does not increase the total space available for butanol sorption. A physicochemical model is developed which fits well to the experimental data.
RESUMEN
A 1-microL drop of n-octane suspended from the tip of a microsyringe needle in 500 mL of stirred aqueous solution is used to extract unbound progesterone in the presence of 1% (w/v) bovine serum albumin (BSA) for analysis by gas chromatography (GC). The observed extraction kinetics are in good agreement with a proposed convective-diffusive mass transfer model which accounts for diffusion of both free and bound species in the aqueous Nernst diffusion film adjacent to the interface. The equilibrium binding constant, which can be measured both at equilibrium and at nonequilibrium extraction times, is in good agreement with literature values. The very small phase ratio employed (i.e., 2 x 10(-6) mL of organic/mL of water) avoids perturbation of the aqueous solution equilibria.
Asunto(s)
Progesterona/análisis , Albúmina Sérica Bovina/química , Adsorción , Cromatografía de Gases , Cinética , Modelos Químicos , Progesterona/aislamiento & purificación , Soluciones , SolventesRESUMEN
Two cations, tetra-n-butylammonium (TBA(+)) and (4-nitrobenzyl)trimethylammonium (NBTA(+)), are simultaneously sorbed on the bonded phase sorbent Partisil-10 ODS-3 from water at various ionic strengths. The non-UV-absorber TBA(+) is thought of as the sample ion and the UV-absorber NBTA(+) as the probe ion in an indirect detection system. In the sorbed state, the two ions reside at different charge planes: NBTA(+) is very close to the ODS-water interface, and TBA(+) is about 11 Å deeper into the ODS phase. Both ions contribute to the electrical potential ψ(I) at the NBTA(+) charge plane, which can be calculated from an electrical triple-layer model. The sorption of TBA(+) influences the amount of NBTA(+) sorbed both by raising ψ(I) and by altering the amount of space available for NBTA(+). The former effect is greater than the latter at ionic strengths up to at least 0.5 mol/L. For the latter effect, the direction and magnitude of the influence of TBA(+) on available space both change with ionic strength (c). At low c, sorbed TBA(+) increases the amount of space available for NBTA(+) by disrupting the conformation of the alkyl chains of the ODS phase. At high c, sorbed TBA(+) decreases the amount of space available by competing with NBTA(+) for space. This dramatically different effect of TBA(+) on available space at low and high c occurs because higher c produces both a weaker interaction of the ODS alkyl chains with water and a reorientation of NBTA(+) from flat to perpendicular in the ODS phase.
RESUMEN
An analytical technique is described which combines solvent extraction with gas chromatographic (GC) analysis in a simple and inexpensive apparatus involving very little solvent consumption. A small drop (8 µL) of a water-immiscible organic solvent, containing an internal standard, is located at the end of a Teflon rod which is immersed in a stirred aqueous sample solution. After the solution has been stirred for a prescribed period of time, the probe is withdrawn from the aqueous solution, and the organic phase is sampled with a microsyringe and injected into the GC for quantification. The observed rate of solvent extraction is in good agreement with a convective-diffusive kinetic model. Analytically, the relative standard deviation of the method is 1.7% for a 5.00-min extraction of the analyte 4-methylacetophenone into n-octane.
RESUMEN
Stoichiometric models of ion-modified reversed-phase liquid chromatography are based on chemical equilibria between ionic modifiers and analyte. These are briefly discussed. Non-stoichiometric models portray the ionic solute as being under the summed influence of all of the ions in the system. Chromatographic theories have been developed that are based on the Poisson-Boltzmann equation, which quantitates the summed influence of the ions in the system on the solute. These ideas and quantitative predictions are described and are critically discussed.
Asunto(s)
Cromatografía Liquida , Electroquímica , Intercambio Iónico , Modelos QuímicosAsunto(s)
Feniramina/análisis , Fenilefrina/análisis , Administración Intranasal , Aerosoles , Fotometría , SolventesRESUMEN
For extraction studies a "filter-probe" porous Teflon membrane phase-separator is used in conjunction with a spectrophotometer fitted with a small-volume flow-cell and a peristaltic pump to measure absorbance of the organic phase as a function of aqueous phase pH. Acidity constants of monoprotic (HA) and diprotic (H(2)A) acid benzothiadiazines have been measured with the apparatus. Accurate pK(a), values for these compounds are difficult to measure by other techniques. Mixtures of n-octanol and chloroform in various proportions can be used as the organic phase in order to adjust the distribution coefficient of the neutral species to give optimum precision and accuracy in measurement of the pK(a) values.
RESUMEN
Previous work has demonstrated the validity of an electrical double-layer model for sorption of sample ions onto low capacity ion exchangers. In the present work it is shown how this model can be used in describing the chromatographic retention of sample ions in so-called reversed-phase ion-pair chromatography. The pairing ion added to the mobile phase is sorbed onto the reversed-phase sorbent, creating an electrical double layer and a surface electrical potential psi(o). Sample counterions undergo both ion exchange for electrolyte ions in the diffuse part of the double layer and surface adsorption. The latter depends on the magnitude of psi(o), which can be calculated from the Stern-Gouy-Chapman theory. The model predicts virtually all of the phenomena that have been described in the literature on ion-pair chromatography.
RESUMEN
Aspirin and free salicylic acid were determined in combinations containing caffeine, phenacetin, salicylamide, and acetaminophen by liquid chromatography on poly(methyl methacrylate) resin. Precision and accuracy were +/- 2% for salicylic acid, the latter at levels corresponding to 0.02% of the aspirin content.