Jon F. Parcher

High-pressure adsorption of carbon dioxide on super-critical-fluid chromatography adsorbents

Abstract Gibbs adsorption isotherms for sub- and supercritical carbon dioxide were measured on four common supercritical-fluid chromatography adsorbents, viz., silica and octadecyl-, cyano- and diol-bonded silica, at 40°C from ambient pressure to 140 bar. All of the isotherms for supercritical carbon dioxide displayed maximum adsorption at pressures close to the critical pressure. Multilayer adsorption occurred at pressures close to the critical pressure in each system, and the type of bonded phase had little or no effect on the CO2 adsorption isotherms. At subcritical pressures, multilayer adsorption of CO2 occurred at relatively low pressures (PR ⪖ 0.4). The adsorbed layer of “mobile” phase persisted to the highest experimental pressures (140 bar). Thus, at least a monolayer of adsorbed CO2 will exist on the surface of adsorbent stationary phases under the conditions normally used for supercritical-fluid chromatography with CO2 as the mobile phase. Both the density and dimensions of the adsorbed layer will vary with temperature and pressure.

Strategies for the determination of the volume and composition of the stationary phase in reversed-phase liquid chromatography

The fundamental difficulties currently impeding the elucidation of retention mechanisms in RPLC are reviewed. The various definitions and conventions concerning void volume and excess adsorption in liquid-solid systems are compared and contrasted. The utility and integrity of various chromatographic methods for the measurement of excess adsorption isotherms of HPLC eluents are discussed. Finally, literature methods for obtaining absolute adsorption data from excess quantities are reviewed and evaluated.

Bioactivity-Guided Investigation of Geranium Essential Oils as Natural Tick Repellents

The evaluation of 10 essential oils of geranium, Pelargonium graveolens (Geraniaceae), were all shown to have repellent activity against nymphs of the medically important lone star tick, Amblyomma americanum (L.). The biological tests were carried out using a vertical filter paper bioassay, where ticks must cross an area of the paper treated with repellent to approach host stimuli. One of the essential oil samples that repelled >90% of the ticks at 0.103 mg/cm(2) was selected for further fractionation studies. The sesquiterpene alcohol, (-)-10-epi-γ-eudesmol, was isolated and identified by spectral methods. (-)-10-epi-γ-Eudesmol at 0.103 and 0.052 mg of compound/cm(2) of filter paper repelled 90 and 73.3% of the ticks, respectively. (-)-10-epi-γ-Eudesmol exhibited similar repellency to the reference standard N,N-diethyl-meta-toluamide (DEET) at concentrations of ≥0.052 mg of compound/cm(2) of filter paper, with (-)-10-epi-γ-eudesmol losing much of its repellency at 0.026 mg of compound/cm(2) and DEET at 0.013 mg of compound/cm(2). Isomenthone and linalool did not repel ticks at the concentrations tested. Most repellents are marketed with much higher concentrations of active ingredient than the concentrations of the natural repellents tested herein; therefore, effective compounds, such as (-)-10-epi-γ-eudesmol, found in geranium oil, have the potential for commercial development.

An integrated approach utilising chemometrics and GC/MS for classification of chamomile flowers, essential oils and commercial products

As part of an ongoing research program on authentication, safety and biological evaluation of phytochemicals and dietary supplements, an in-depth chemical investigation of different types of chamomile was performed. A collection of chamomile samples including authenticated plants, commercial products and essential oils was analysed by GC/MS. Twenty-seven authenticated plant samples representing three types of chamomile, viz. German chamomile, Roman chamomile and Juhua were analysed. This set of data was employed to construct a sample class prediction (SCP) model based on stepwise reduction of data dimensionality followed by principle component analysis (PCA) and partial least squares discriminant analysis (PLS-DA). The model was cross-validated with samples including authenticated plants and commercial products. The model demonstrated 100.0% accuracy for both recognition and prediction abilities. In addition, 35 commercial products and 11 essential oils purported to contain chamomile were subsequently predicted by the validated PLS-DA model. Furthermore, tentative identification of the marker compounds correlated with different types of chamomile was explored.

Tracer pulse chromatography: A review and prospectus

Abstract A review of the tracer pulse form of chromatography is presented along with discussion of the basic procedures and distinguishing features of this unique method for studying phase distribution equilibria. Typical sorbate and sorbent systems are discussed and several practical applications are presented. The basic theory of the method is reviewed; a comparison of this method with other static and chromatographic methods is given; and the review concludes with a discussion of the potential applications of the tracer pulse technique.

Comparative Investigation of Umbellularia californica and Laurus nobilis Leaf Essential Oils and Identification of Constituents Active against Aedes aegypti

Umbellularia californica (California bay laurel) and Laurus nobilis (Mediterranean bay laurel) leaves may be mistaken or used as a substitute on the market due to their morphological similarity. In this study, a comparison of anatomical and chemical features and biological activity of both plants is presented. L. nobilis essential oil biting deterrent and larvicidal activity were negligible. On the other hand, U. californica leaf oil showed biting deterrent activity against Aedes aegypti . The identified active repellents was thymol, along with (-)-umbellulone, 1,8-cineole, and (-)-α-terpineol. U. californica essential oil also demonstrated good larvicidal activity against 1-day-old Ae. aegypti larvae with a LD50 value of 52.6 ppm. Thymol (LD50 = 17.6 ppm), p-cymene, (-)-umbellulone, and methyleugenol were the primary larvicidal in this oil. Umbellulone was found as the principal compound (37%) of U. californica essential oil, but was not present in L. nobilis essential oil. Umbellulone mosquito activity is here reported for the first time.

Adsorption of light hydrocarbons on liquid-modified adsorbents

Abstract Adsorption isotherms of propane and butane were determined at three temperatures on a graphitized carbon black adsorbent (Carbopack C) both uncoated and coated with 0.2% Carbowax 1500. In addition, the specific retention volumes of each alkane were determined as a function of the amount of the other alkane adsorbed on the normal and liquid-modified surfaces. The equilibrium isotherms for both alkanes on the liquid modified adsorbent were almost linear, but segmented into two distinct regions. The low-pressure adsorption was postulated to be adsorption on the portion of the solid not covered by the polymeric modifier. At higher pressures, adsorption occurred on the preadsorbed adsorbate and/or the polymer itself. Cooperative adsorption caused by lateral interactions on the surface was observed for both propane and butane on the uncoated adsorbent, and a similar effect between adsorbate and liquid modifier was used to explain the adsorption data for liquid-modified adsorbent. In general, the effects of volatile and nonvolatile modifiers on the adsorptive properties of a surface are similar and involve concurrent cooperative and competitive adsorption processes. However, volatile modifiers provide much more flexibility and greater potential for control of the physical and chemical activity of adsorbents and catalysts.

Adsorption of acetone and butane on liquid-modified graphitized carbon blacks

Abstract Gas-liquid, gas-solid, and gas-liquid-solid equilibrium isotherms were determined for acetone and butane on graphitized carbon black (Carbopack C) and liquid-modified graphitized carbon black (Carbopack C plus 0.2% Carbowax 1500). The measurements were carried out over the range of temperatures from 30 to 75°C by mass spectrometric tracer pulse chromatography with stable isotopic solutes. Studies of the gas-liquid systems show that the bulk liquid undergoes a phase transition (wax → liquid) at 40°C and that the equilibrium isotherms vary significantly with temperature for the wax form. The temperature of the phase transition is shifted to 50-55°C for the same liquid coated on Carbopack C as a thin film. The adsorption isotherms of acetone on the coated and uncoated Carbopacks were interpreted by means of a model of simultaneous competitive and cooperative adsorption effects. The Carbowax-modified adsorbent generally adsorbed as much, if not more, acetone at any given pressure than the uncoated adsorbent. This effect was attributed to specific lateral interactions between the solute and liquid or wax modifier. The magnitude of the enhanced capacity was much greater than could be accounted for by bulk solubility of acetone in the Carbowax. The effect of preadsorbed acetone on the adsorption and retention of other solutes was also investigated. The specific retention volumes of small samples of butane were measured as a function of the surface coverage by acetone or acetone plus Carbowax 1500. Small amounts of preadsorbed acetone had little or no effect on the retention volume of butane, and the retention volumes were greater on the uncoated adsorbent. However, at surface coverages close to a monolayer, the retention volume of butane decreased dramatically with increased acetone adsorption. Also, at higher surface coverages, the retention volumes were the same for the coated and uncoated adsorbent at any fixed amount of acetone adsorbed. These “interference plots” were also explained in terms of the model previously discussed.

Sorption isotherms of ternary eluents in reversed-phase liquid chromatography.

The experimental technique of mass spectrometric tracer pulse chromatography was used to measure the excess volume of each eluent component for binary and ternary mixtures of water, acetonitrile, and methanol on a C(18)-bonded silica RPLC packing over the full composition range. The tracer pulse method allowed the direct measurement of excess volumes of each eluent component without numerical integration, assumed isotherm equation, detector calibration, or off-line analysis of the eluent composition. Absolute isotherms were estimated from the experimental data for excess volumes by use of various strategies for the estimation of the volumes of the stationary and mobile phases in dynamic equilibrium with eluents of varying composition. The results indicate that all three eluent components interacted with the alkane bonded phase. Some components were selectively taken up as part of the stationary phase while other components were selectively excluded so the composition of eluent in or on the stationary phase often significantly differed from the composition of the bulk eluent. The exact composition of the stationary phase (bonded phase plus immobilized eluent) was dependent upon the type and composition of the bulk eluent.

Mass Spectrometric Tracer Pulse Chromatographic Investigations of Eluent Sorption with Bonded RPLC Packings

The experimental technique of mass spectrometric tracer pulse chromatography was used to investigate the uptake of RPLC eluents by a C 18-bonded packing. The experiments were carried out with eluents consisting of binary aqueous mixtures with acetonitrile, methanol, and tetrahydrofuran over the full range of eluent composition at 25 degrees C. The primary experimental data obtained were excess volumes of sorption for the eluent components. The excess volume data were then used to determine the absolute volume of each eluent component in the stationary phase as a function of composition. The absolute volumes were calculated by utilizing a series of strategies specific to limited eluent composition range. The linear inflection region of the excess volume isotherms was used to calculate the volume and composition of the eluent in the stationary phase for organic-rich eluents. Three different assumptions were used and evaluated for the water-rich eluent compositions. The assumptions were (i) constant volume of the sorbed layer, (ii) constant amount of water sorbed, and (iii) no water sorption. The latter assumption was adopted as the most satisfactory. The calculated void volume data were compared with the retention volume of thiourea and uracil, commonly used dead time markers. Neither thiourea nor uracil proved to be an accurate measure of the kinetic void volume.