Measurement of desorption rates from octadecylsilyl bonded-phase HPLC particles and its characterization in terms of pore, surface, and film diffusion.

An instrument is developed to measure rates of desorption of solutes from particulate HPLC packing materials for processes that are quantitatively complete in a few tenths of a second. The instrument is a modified, pressure-driven, stopped-flow device. The major modifications include positioning a very short (0.

Continuous desorption rate measurement from a shallow-bed of poly(styrene-divinylbenzene) particles with correction for experimental artifacts.

A 0.50 mm high bed, containing ca. 3 mg of the nominally non-porous poly(styrene-divinylbenzene) (PS-DVB) sorbent Hamilton PRP-infinity, is located in a valve.

Stationary-phase contribution of 1-propanol organic modifier to changes in sorption of 1-hexanol on an ODS-bonded phase.

Using the reversed-phase bonded-phase HPLC packing Partisil-10 ODS-3, sorption isotherms have been measured for the alcohols 1-propanol (PrOH) and 1-hexanol (HexOH), and as well, a simultaneous sorption curve for the two alcohols has been measured from solutions containing a low and constant concentration of HexOH as sample with increasing concentrations of PrOH as organic modifier. The mobile-phase effect of PrOH is quantified by solution-phase activity coefficients obtained either from vapor/solution equilibrium measurements or from cloud point measurements. Since sorbed alcohols are located at the ODS/solution interface, the stationary-phase effect of PrOH is modeled in terms of three processes: (i) competition for space; (ii) decrease of space required per mole with increasing concentration of sorbed PrOH; and (iii) change of free energy of sorption with increasing concentration of sorbed PrOH.

Chain Unfolding in an ODS-Bonded Phase Caused by the Sorbed Tetra-n-butylammonium Ion.

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.

Effect of Stationary-Phase Sorption of Organic Modifier from a Water-Rich Mobile Phase on Solute Retention by an ODS Bonded Phase.

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.

Solvent microextraction as a speciation tool: determination of free progesterone in a protein solution.

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.

Solvent microextraction into a single drop.

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.

Electrical Double-Layer Effects and the Origin of Indirect UV Detection in the Liquid Chromatography of a Cationic Sample Using a Cationic Probe.

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.

Electrical double-layer models of ion-modified (ion-pair) reversed-phase liquid chromatography.

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.

Measurement of acidity constants of benzothiadiazines by solvent extraction with use of a membrane phase-separator.

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.

Retention model for ion-pair chromatography based on double-layer ionic adsorption and exchange.

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).

Determination of salicylic acid and aspirin in multicomponent tablets by liquid chromatography on a nonionic resin.

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.