Lilian Mónica Romero

Thermodynamic consideration of the retention mechanism in a poly(perfluoroalkyl ether) gas chromatographic stationary phase used in packed columns

Abstract Retention volumes of fifteen hydrocarbons were measured in columns containing several concentrations of a commercial poly(perfluoroalkyl ether), Fomblin Y HVAC 140/13, as the stationary phase. Two different types of packing were studied: one of them employed pre-silylated Chromosorb P AW DMCS as the solid support and the other type was prepared by coating the stationary phase on Chromosorb P AW and silylating on-column. On-column silylated columns showed unequivocal symptoms of partial deactivation; retention volumes changed regularly with the content of stationary phase in pre-silylated columns. Analysis of retention in pre-silylated columns indicates that a mixed mechanism (partition and adsorption onto the gas-liquid interface) is operative. The systems hydrocarbon-perfluorocompound show pronounced positive deviations from the ideal behaviour that can be attributed to repulsion between the hydrocarbon and the perfluorocompound segments.

Reliable and simple analytical methods for determination of citrulline and metabolically related amino acids by liquid chromatography after derivatization: comparison between monolithic and core–shell columns

We describe the development of a high-performance liquid chromatography (HPLC) method for the determination of citrulline and other amino acids relevant to intestinal diseases. The amino acids were derivatized with 9-fluorenylmethylchloroformate (FMOC-Cl) and their derivatives were separated on two different columns, a core–shell column (Halo C18) and a silica-based monolith (Chromolith Performance RP-18). The derivatization reaction was optimized with respect to pH, buffer concentration and reproducibility. The optimal derivatization conditions were achieved with 0.4 M borate buffer at pH 9.20, a constant ratio of FMOC-Cl/total amino acids (10 : 1) and 75 mM tyramine after 1 min (quenching reaction). The separation conditions with both chromatographic supports were also optimized. The chromatographic performance (peak capacity and global resolution) of these two columns was compared. This proposed HPLC-UV method was satisfactorily applied to the analysis of a real plasma sample.

Alkanol gas-liquid partition coefficients in squalane measured with packed columns. A revision of measurement methods

Abstract Retention volumes of 13 alkanols were measured at five temperatures between 30 and 60°C in a series of six columns packed with different squalane percentages on Chromosorb W previously modified with Carbowax 20M according with Aue et al.’s method [J. Chromatogr. 77 (1973) 299]. While alkanol retention times are independent of sample size when chromatographed in these packings, an important variation is demonstrated in columns packed with squalane coated on Chromosorb W DMCS under otherwise identical conditions. It is demonstrated that the intercepts of plots VN/VL vs. 1/VL (VN is retention volume per gram of packing; VL is stationary phase volume per gram of packing) cannot be identified with the partition coefficient, and a method to calculate the gas–liquid partition coefficient is proposed.

Determinations of gas-liquid partition coefficients using capillary chromatographic columns. Alkanols in squalane.

This study focused on an investigation into the experimental quantities inherent in the determination of partition coefficients from gas-liquid chromatographic measurements through the use of capillary columns. We prepared several squalane - (2,6,10,15,19,23-hexamethyltetracosane) - containing columns with very precisely known phase ratios and determined solute retention and hold-up times at 30, 40, 50 and 60°C. We calculated infinite dilution partition coefficients from the slopes of the linear regression of retention factors as a function of the reciprocal of the phase ratio by means of fundamental chromatographic equations. In order to minimize gas-solid and liquid-solid interface contributions to retention, the surface of the capillary inner wall was pretreated to guarantee a uniform coat of stationary phase. The validity of the proposed approach was first tested by estimating the partition coefficients of n-alkanes between n-pentane and n-nonane, for which compounds data from the literature were available. Then partition coefficients of sixteen aliphatic alcohols in squalane were determined at those four temperatures. We deliberately chose these highly challenging systems: alcohols in the reference paraffinic stationary phase. These solutes exhibited adsorption in the gas-liquid interface that contributed to retention. The corresponding adsorption constant values were estimated. We fully discuss here the uncertainties associated with each experimental measurement and how these fundamental determinations can be performed precisely by circumventing the main drawbacks. The proposed strategy is reliable and much simpler than the classical chromatographic method employing packed columns.

Gas chromatographic study of the hydrogen bonding of aliphatic alcohols to tri-n-octylphosphine oxide.

Retention volumes of 21 aliphatic alcohols were measured at five temperatures between 30 and 60 degrees C in columns packed with different percentages of squalane or with solutions of tri-n-octylphosphine oxide (TOPO) in squalane coated on previously deactivated Chromosorb W. Experimental data fit to a 1:1 alcohol-TOPO association model, with association constants ranging from 26 to 59 dm3 mol(-1) at 45 degrees C. Association constants follow the trend primary alcohols>secondary alcohols>tertiary alcohols, with minor differences between the members of each of these three groups. The association enthalpy for the 21 alcohols averages -21.8 kJ mol(-1), with a standard deviation of -1.3 kJ mol(-1).