Molar enthalpy and molar volume of methylene and benzene homologues in reversed-phase liquid chromatography

In this study, thermodynamic properties are measured for methylene and benzene homologues in reversed-phase liquid chromatography using octadecylsilica stationary phases and methanol mobile phase. The change in molar enthalpy (delta H degree) is determined from graphs of the logarithm of the capacity factor versus the inverse temperature (15 to 60 degrees C), whereas the change in molar volume (delta V degrees) is determined from graphs of the logarithm of the capacity factor versus pressure (830 to 5000 p.s.i.). For octadecylsilica phases with low bonding density (2.7 mumol m-2), delta H degree and delta V degree are small and are relatively unaffected by temperature and pressure. These thermodynamic parameters are linearly related to the homologue number for the methylene homologues, but not for the benzene homologues. For the ethylene group, delta delta H degree and delta delta V degree are in the order of -0.41 kcal mol-1 and -1.0 cm3 mol-1, respectively, at 30 degrees C. As the bonding density increases (5.4 mumol m-2), the molar volume and molar enthalpy decrease in a significant and nonlinear manner with the homologue number. Moreover, these thermodynamic parameters are markedly affected by temperature and pressure. For the ethylene group, delta delta H degree and delta delta V degree are in the order of -3.65 kcal mol-1 and -14.1 cm3 mol-1, respectively, at 30 degrees C. The theoretical and practical implications of these measurements are discussed with respect to the retention mechanism in reversed-phase liquid chromatography.

Determination of fatty acids in fish oil dietary supplements by capillary chromatography with laser-induced fluorescence detection

The 4-bromomethyl-7-methoxycoumarin derivatives of 14 saturated and unsaturated fatty acids, including the omega-3 fatty acids, were separated by reversed-phase liquid chromatography and detected by laser-induced fluorescence. Baseline resolution was obtained by using a high-efficiency packed capillary column with 240,000 theoretical plates, together with a systematic optimization of the mobile phase composition. The retention indices of the fatty acid derivatives correlated well with a predictive empirical model, showing accuracy better than 0.46% relative error and reproducibility better than +/- 0.1% relative standard deviation. The physiologically important fatty acids with 12-22 carbon atoms and 0-6 double bonds were determined at the femtomole level in fish oil dietary supplements by using this methodology.

Femtomole analysis of prostaglandin pharmaceuticals

An analytical method is described whereby the major classes of prostaglandins are fully resolved by microcolumn liquid chromatography and detected at the subfemtomole level by laser-induced fluorescence. The prostaglandins are labeled with the fluorescent reagent 4-bromo-methyl-7-methoxycoumarin and are subsequently separated on a high-efficiency fused-silica microcolumn (0.2 mm i.d., 1.06 m length, 150,000 theoretical plates). The optimal chromatographic conditions consist of a 3-micron octadecylsilica packing material and an isocratic mobile phase of 47.6% methanol, 23.8% acetonitrile, and 28.6% water. The prostaglandin derivatives are detected directly on the microcolumn by laser fluorimetry, using a helium/cadmium laser (325 nm, 15 mW) as the excitation source together with a simple filter/photo-multiplier optical detection system. In real sample matrices, the prostaglandin PGF2 alpha is readily quantifiable from the detection limit (0.3 fmol) to the formulation strength of the therapeutic agent Lutalyse (Upjohn), spanning more than six orders of magnitude in concentration. The simplicity and general applicability of the present analytical methodology and instrumentation suggest that this technique can be used to attack a wide variety of biomedically important problems with exceptional sensitivity and selectivity.

Direct measurement of antigens in serum by time-resolved fluoroimmunoassay

A long-lived fluorescence label (Tb3+) has been attached to the antigen of interest by using a bifunctional chelating agent 1-(p-benzenediazonium)-EDTA. A nonequilibrium competitive-binding immunoassay protocol, in conjunction with time-resolved detection of the long-lived fluorescence label, allows the antigen to be analyzed directly in samples containing diluted human serum. Results obtained for immunoglobulin G with this simple and rapid procedure correlated well (r = 0.93) with those by a commercially available fluorescence immunoassay method.

Direct measurement of antigens in serum by time-resolved fluoroimmunoassay

A long-lived fluorescence label (Tb3+) has been attached to the antigen of interest by using a bifunctional chelating agent 1-(p-benzenediazonium)-EDTA. A nonequilibrium competitive-binding immunoassay protocol, in conjunction with time-resolved detection of the long-lived fluorescence label, allows the antigen to be analyzed directly in samples containing diluted human serum. Results obtained for immunoglobulin G with this simple and rapid procedure correlated well (r = 0.93) with those by a commercially available fluorescence immunoassay method.