Extrathermodynamic relationships in reversed-phase liquid chromatography

Enthalpy-entropy compensation (EEC) and linear free energy relationships (LFER) are extrathermodynamic correlations frequently used to discuss the mechanistic similarities of chemical equilibria and reaction kinetics. Although empirical, they are widely applied, proving the substantial effectiveness of fundamental studies based on them. Many attempts have been made to interpret theoretically the necessary conditions (or preconditions) of EEC and LFER. LFER is known to rest on the existence of EEC. However, the intimate correlations between EEC on one hand and LFER and the temperature dependence of LFER on the other hand were insufficiently discussed from the viewpoint of molecular structure contributions. We present a simple LFER model relating the slope and intercept of LFER to the compensation temperatures, themselves derived from EEC analyses, and to several parameters characterizing the molecular contributions to the changes in enthalpy and entropy associated with the passage from one phase of the chromatographic system to the other. A theoretical explanation is supplied for the intimate correlation between the two types of extrathermodynamic relationships, EEC and LFER. We demonstrate also that the characteristics of EEC and LFER depend on the structural parameters. This new model allows a proper interpretation of the temperature dependence of LFER. It should permit further progress of fundamental studies of chemical reaction mechanisms based on extrathermodynamic relationships.     

Electrogenerated chemiluminescence detection in reversed-phase liquid chromatography

Electrogenerated chemiluminescence (ECL) has been developed as a detection method for liquid chromatography. The radical cation of tri-p-tolylamine (TPTA) is used as a common electron acceptor for the electrogenerated radical anions of a variety of organic analytes. ECL is accomplished with a high-frequency potential pulse program applied to a microelectrode immersed in the column eluent. ECL detection is demonstrated with reversed-phase liquid chromatography. Selectivity at the ECL detector is shown to be tunable based on differing electrochemical conditions and excited-state energetics. Low minimum detection limits in ECL are attributed to the dependence on the photon detector shot noise, allowing a limit of detection of 0.14 nM for perylene in the presence of 0.1 mM TPTA. A derivatization agent useful for ECL detection is demonstrated by the use of naphthalene-2,3-dicarboxaldehyde. This reagent, which does not itself result in ECL, forms ECL candidates following reaction with primary amines.     

Estimation of ipratropium bromide from aerosols by reversed-phase liquid chromatography

Abstract

Ipratropium bromide, a derivative of N-isopropyl noratropine, was estimated from the samples of aerosols. An isocratic, reversed-phase liquid chromatographic separation method was developed by using an ODS, 5 μm column with the mobile phase acetonitrile-sodium dihydrogen orthophosphate (0.05M)-diethylamine (50 : 50 : 0.1, v/v) pH adjusted to 4.5 with phosphoric acid. Recoveries obtained were in the range 98% to 102% of ipratropium bromide from aerosol. Fluoxetine hydrochloride was used as an internal standard for quantitation.     

Estimation of ipratropium bromide from aerosols by reversed-phase liquid chromatography

Ipratropium bromide, a derivative of N-isopropyl noratropine, was estimated from the samples of aerosols. An isocratic, reversed-phase liquid chromatographic separation method was developed by using an ODS, 5 μm column with the mobile phase acetonitrile-sodium dihydrogen orthophosphate (0.05M)-diethylamine (50 : 50 : 0.1, v/v) pH adjusted to 4.5 with phosphoric acid. Recoveries obtained were in the range 98% to 102% of ipratropium bromide from aerosol. Fluoxetine hydrochloride was used as an internal standard for quantitation.     

Quantification in comprehensive two-dimensional liquid chromatography

Although the use of comprehensive two-dimensional liquid chromatography (LCxLC) as a powerful separation technique is continuously increasing, its employment in quantification experiments is rather limited. The present research is focused on the quantification of a series of standards, as well as real-world sample compounds, by using dedicated laboratory-constructed LCxLC software, developed through a novel approach. Moreover, the difficulties encountered during software operation, in various elution conditions, are described and discussed. The results attained were compared with those observed in conventional LC, and no statistically significant differences were observed in the determination of aurapten in grapefruit oil. However, a loss in sensitivity was observed when using LCxLC (limit of detection = 0.10 ppm) compared to conventional LC (limit of detection = 0.05 ppm) as a consequence of the sample dilution in comprehensive two-dimensional liquid chromatography.     

Characterization of currently marketed heparin products: reversed-phase ion-pairing liquid chromatography mass spectrometry of heparin digests

Here we report results from the analyses by enzymatic digestion and reversed-phase ion-pairing liquid chromatography mass spectrometry (RPIP-LC-MS) of active pharmaceutical ingredient (API) unfractionated heparins (UFHs) from six different manufacturers and one USP standard sample. We employed a reverse phase ion-pairing chromatography method using a C(18) column and hexylamine as the ion-pairing reagent with acetonitrile gradient elution to separate disaccharides generated from the digestion of the heparins by lyase I and III (E.C. 4.2.2.7 and 4.2.2.8) before introduction into an ion-trap mass spectrometer by an electrospray ionization (ESI) interface. Extracted ion chromatograms (EICs) were used to determine the relative abundance of the disaccharides by mass spectrometry. Eight disaccharides were observed and a similar composition profile was observed from digests of 20 UFH samples. The compositional profile determined from these experiments provides a measure of the norm and range of variation in "good" heparin to which future preparations can be compared. Furthermore, the profile obtained in the RPIP-LC-MS assay is sensitive to the presence of the contaminant, oversulfated chondroitin sulfate A (OSCS), in heparin.     

Shape selectivity for constrained solutes in reversed-phase liquid chromatography

In reversed-phase liquid chromatography (RPLC), the separation of compound mixtures of similar polarity can present a significant challenge for the analyst. Examples of such compounds include geometric isomers present in environmental samples (e.g., polycyclic aromatic hydrocarbons, polycyclic aromatic sulfur heterocycles, and polychlorinated biphenyl congeners) and compounds of biological significance (e.g., carotenoids and steroids). In general, compounds with rigid, well-defined molecular shape are best separated using a column with enhanced shape selectivity characteristics. This perspective presents an overview of column properties that influence shape selectivity for constrained solutes. Approaches to the characterization of stationary-phase structure are described, and the findings are correlated with chromatographic performance. Finally, retention models of shape discrimination are presented that are consistent with observed retention behavior. An appreciation for shape recognition effects in RPLC will facilitate method development for certain classes of difficult to resolve compounds.     

Overload for ionized solutes in reversed-phase high-performance liquid chromatography

Overloading occurs for submicrogram quantities of ionized solutes particularly when using low ionic strength mobile phases at low pH (e.g., formic acid), even with highly inert silica RP-HPLC columns of normal dimensions. Much higher loads can produce a sharp L-shaped peak with retention above the column void volume, in line with the hypothesis that a small number of high-energy sites fill first and are rapidly overloaded, followed by a much larger number of weaker sites. However, charged acids and bases show identical overloading behavior; overloading is reduced as the mobile-phase ionic strength is increased. These findings raise questions about the physical nature of the strong sites. The rapid overloading of silica and purely polymeric phases could be explained by mutual repulsion of ionic species or their inability to fully penetrate the hydrophobic structure of the phase. However, these alternative hypotheses cannot readily explain the high total saturation capacities obtained using frontal analysis. Ion pairing with trifluoroacetic acid may reduce overload, while the effect is less important for formate or phosphate buffers. A surface layer of acetonitrile is not a prerequisite for rapid overloading, as shown by studies using purely aqueous buffers.     

Improved protein recovery in reversed-phase liquid chromatography by the use of ultrahigh pressures

The effect that elevated pressure used in ultrahigh-pressure liquid chromatography (UHPLC) has on protein recovery was investigated. Specifically, protein carryover ("ghosting") and recovery were examined. Four model proteins (ribonuclease A, ovalbumin, myoglobin, BSA) were separated by gradient RPLC at both conventional (160 bar) and ultrahigh pressures (>1500 bar). A custom gradient UHPLC system was used to generate conventional pressures on 5-microm diameter reversed-phase supports and ultrahigh pressures on identical 1.4-microm supports. The results indicate that, by increasing the pressure, protein carryover from run to run is reduced and in some cases eliminated above a certain threshold pressure for the model proteins studied. Further work indicates that recovery was enhanced for each of the proteins studied, even approaching 100% for certain proteins.     

Selective enrichment of glycopeptides from glycoprotein digests using ion-pairing normal-phase liquid chromatography

Detailed structural analysis of glycoproteins requires methods capable of isolating glycopeptides from tryptic digests of purified glycoproteins and complex protein mixtures. Here, we describe the selective and reproducible isolation of glycopeptides from a peptide mixture using ion-pairing normal-phase chromatography (IP-NPLC). The addition of inorganic monovalent ions in normal-phase chromatography appears to increase the hydrophobicity difference between peptides and glycopeptides, allowing for more efficient separation. Our data show that IP-NPLC effectively enriches glycopeptides from a tryptic digest of ribonuclease B, bovine fetuin, and a complex mixture of glycoproteins, when compared with normal-phase chromatography alone. The results of the IP-NPLC experiments can be explained using the Wimley-White water/octanol free energy scale to illustrate the hydrophobicity difference of nonglycosylated peptides with and without ion-pairing. We believe that IP-NPLC will be an important tool in glycoprotein characterization and glycoproteomic studies.     

Retention mechanism in reversed-phase liquid chromatography: a molecular perspective

A detailed, molecular-level understanding of the retention mechanism in reversed-phase liquid chromatography (RPLC) has eluded analytical chemists for decades. Through validated, particle-based Monte Carlo simulations of a model RPLC system consisting of dimethyloctadecylsilanes at a coverage of 2.9 micro mol/m2 on an explicit silica substrate with unprotected residual silanols in contact with a water/methanol mobile phase, we show that the molecular-level retention processes for nonpolar and polar analytes, such as alkanes and alcohols, are much more complex than what has been previously deduced from thermodynamic and theoretical arguments. In contrast to some previous assumptions, the simulations indicate that both partitioning and adsorption play a key role in the separation process and that the stationary phase in RPLC behaves substantially different from a bulk hydrocarbon phase. The retention of nonpolar methylene segments is dominated by lipophilic interactions with the retentive phase, while solvophilic interactions are more important for the retention of the polar hydroxyl group.     

Restricted diffusion model for surface diffusion in reversed-phase liquid chromatography

The analysis of experimental results in reversed-phase liquid chromatography (RPLC) allows further discussion of the restricted diffusion model of surface diffusion formulated on the basis of the absolute rate theory. Chromatographic data were acquired on different RPLC systems with two series of homologous compounds, several stationary phases having different alkyl ligand densities and ligands of various lengths, and methanol/water mobile phases of different compositions. The enthalpy-entropy compensation observed and the linear free energy relationships found for surface diffusion suggest that the surface diffusion mechanism remains probably the same in all RPLC conditions studied. Whereas the isosteric heat of adsorption approaches zero with decreasing retention, the activation energy of surface diffusion tends toward a finite limit and the surface diffusion coefficient tends toward a value near the corresponding molecular diffusivity. These results support the validity of the restricted diffusion model. The influence of different factors on the validity of this model (i.e., the activation energy and the frequency factor of surface diffusion, and the surface tortuosity) was also considered.