R.K. Gilpin

Effect of temperature on precision of retention measurements in liquid chromatography

Abstract The effect of temperature on precision of retention measurements in liquid chromatography for a number of chromatographic systems has been examined. These have included both reversed-phase systems used in combination with aqueous-organic mobile phases, as well as bonded phases used in combination with a completely aqueous mobile phase. The effect of varying degrees of surface coverage (amount of bonded carbon) on these measurements also has been evaluated. In addition to these systems, several normal-phase systems have been studied. These experiments have been carried out by varying surface polarity, as well as mobile phase composition.

Studies of physicochemical and chromatographic properties of mixed amino-alkylamide bonded phases☆

Abstract Mixed amino-alkylamide chemically bonded phases have been prepared via two-step reaction which involved the initial modification of silica with aminopropyldimethylsilyl groups under environmentally sealed conditions followed by a secondary derivatization with different acid chlorides (i.e., acetyl, hexanoyl and stearoyl chlorides). In addition, for comparative purposes two conventional C18 phases with different coverage densities have been prepared. Subsequently, the physical properties of the mixed phases have been studied by liquid chromatography as well as by elemental analysis and solid-state nuclear magnetic resonance spectrometry. The resulting data have provided information about how the hydrophobic chains and the specific interaction sites in the bonded phases influence solute retention under reversed-phase conditions. A potentially important application of the mixed amino-alkylamide materials is for separating basic compounds such as amines and amides.

Comparison of the retention of organic acids on alkyl and alkylamide chemically bonded phases

Abstract Retention of pyridine and indole carboxylic acids was studied on alkylamide bonded phases under reversed-phase conditions at 35°C. The alkylamide phases were synthesized by a two-step process, in which an initial amino phase was prepared under environmentally isolated conditions and subsequently reacted with alkanoyl chlorides. The retention measurements were carried out using acetonitrilewater eluent at different volume compositions and different pH values. For the comparative purposes analogous retention data were collected for conventional octyl phases bonded to various silicas. A comparison of the retention data measured on alkyl and alkylamide packings have demonstrated that the later, polymeric or monomeric, are better for separating pyridine and indole carboxylic acids. The comparative analysis of the chromatographic systems studied suggests that retention of these acidic solutes on alkylamide phases is governed by a mixed partition-displacement and ion-exchange mechanism.

Comparative studies of chromatographic properties of silica-based amide-bonded phases under hydro-organic conditions

The basic chromatographic properties of silica-based amide-bonded phases possessing unsubstituted or substituted aromatic rings were evaluated using retention measurements carried out for simple probe molecules such as alkylbenzenes and aromatic hydrocarbons. An octylamide phase, the chromatographic properties of which have been investigated previously, was used as a reference material in the current work.

Chromatographic and related studies of alkylamide phases

SummaryA series of chemically bonded phases has been prepared with amide groups localized by means of hydrophobic ligands. The physicochemical and chromatographic properties of such phases with chain lengths ranging from C5 to C8 have been examined by porosimetry, elemental analysis, solid-state NMR, and liquid chromatography. Subsequently, the conformational dynamics of these phases have been investigated by analyzing the dependence of the capacity factor (k) on the reciprocal of temperature for different organic compounds. Special emphasis has been given to the reproducibility of retention data obtained before and after temperature-dependent measurements.

Reordering/resolvation of silica immobilized hydrogen bonding ligands used as stationary phases in high-performance liquid chromatography

Abstract The thermally induced reordering/resolvation of ω-haloalkyl modified surfaces are studied as a function of increasing temperature. The thermal on-set, To, determined from plots of ln k′ (capacity factor) vs. 1/T varies with chain length and functionality of the immobilized groups. A direct correlation between To and boiling point of corresponding non-immobilized compound is observed. These data are explained by a three term model that accounts for energy differences which occur with reordering/resolvation in terms of: (1) cohesive interaction between the bonded chains; (2) hydrophobic interaction between the bonded chains and the contact solvent, water; and (3) specific interaction resulting from hydrogen bonding between aqueous mobile phase and unreacted surface silanols.

Reordering / resonation studies of alkylamide phases

Abstract A series of chemically bonded liquid chromatographic packings containing a specific interaction site localized in the interior region of the attached ligands has been prepared by a two-step synthetic route. In the first-step an amino phase is formed and then it is converted to an alkylamide group via reaction with an acid chloride. Using this procedure materials have been prepared with immobilized terminal alkyl groups with chain lengths ranging from C 5 to C 8 . Subsequently, the conformation of these modified surfaces have been investigated as a function of temperature under totally aqueous mobile phase conditions. Plots of ln k′ vs. 1 / T curves have been found to be similar to those reported for alkyl phases. However, the onset of reordering/ resolvation of the attached ligands is shifted to lower temperatures and the incremental change per methylene group is smaller.

Influence of bonding chemistry on the reordering/resolvation of silica immobilized alkyl chains

Abstract The reordering/resolvation of silica modified with mono-, di-, and trireactive alkylsilanes has been studied. Under totally aqueous conditions, the onset for reordering has been found to begin at approximately the same temperature irrespective of the attachment chemistry. These results support the idea that bonding occurs heterogeneously in patches and that within a patch spacing is controlled by silanol concentration.

Application of a new numerical method for characterizing heterogeneous solids by using gas-solid chromatographic data

Abstract A new numerical method for calculating the adsorption energy distribution function from retention volume data obtained by gas-solid adsorption chromatography is introduced. The method, which considers the ill-posed character of the problem, is applicable to all physico-chemical models that describe the local adsorption on one kind of sites. Using Langmuir and Jovanovic local isotherms the distribution functions are determined for n -hexane and 1-hexene on a glass and for cyclohexane and cyclohexene on two silica gels with different pore sizes.

Thermoanalytical studies of water on aluminum oxides with different porosities

Abstract The thermal desorption of water from five aluminum oxide samples with different porosities and specific surface areas was studied under dynamic and quasi-isothermal conditions. The thermogravimetric (TG) curves obtained show multi-step weight losses associated with the initial evaporation of excess water, the removal of the water retained in small pores and the desorption of physisorbed and chemisorbed water from active surface sites with different energies. Further, it appears that the weight losses at the inflection points of the TG curves can be correlated with the surface free energies and porosities of the samples studied.