Nuclear magnetic resonance and electron spin resonance spectroscopic investigations of reversed-phase liquid chromatographic retention mechanisms: stationary phase structure

Microscopic Origins of Band Broadening in Chromatography. Polarity Distribution in C18 Stationary Phase Probed by Confocal Ratiometric Imaging of Nile Red

Band broadening is a major factor that influences the efficiency and resolution of chromatographic separations. Studies of microscopic origins of band broadening, such as the micropolarity distribution of chromatographic stationary phase, can provide a better understanding of many chromatographic phenomena and retention behavior. In this work, we probe the chemical environments of C18 chromatographic stationary phase with quantitative confocal fluorescence microscopy under real reversed-phase liquid chromatography conditions. Ratiometric imaging of C18 interface is achieved by loading the stationary phase with a polarity-sensitive dye, Nile red, and optical sectioning with confocal microscopy. The results reveal that there are uniform micropolarity distributions inside individual chromatographic beads, but the polarity may differ between stationary-phase particles. The homogeneity of micropolarity of individual beads suggests that there are not any spatially large exposed silica sites beyond the optical resolution in C18 stationary phase. The strong adsorption sites are smaller in size than the optical resolution of a few hundred nanometers. The heterogeneity between chromatographic beads indicates that the interactions of Nile red with C18 bonded phase are different between beads. This contributes to the broad overall polarity distribution of the C18 stationary phase and can be one of the factors that cause band broadening in separations. With its high spatial resolution and optical sectioning capabilities, confocal fluorescence imaging is shown to be an ideal method to probe the chromatographic stationary phase. The distribution of micropolarity sheds light on the microscopic heterogeneity in chromatographic processes and its influence on chemical separations.

Solvents effects on the conformational order of triacontyl modified silica gels as evaluated by Fourier transform infrared spectroscopy

C30 alkyl modified silica gels have attracted much attention because of their enhanced shape selectivity for various types of analytes, which for instance cannot be separated with conventional C8 and C18 stationary phases. Since the retention processes strongly depend on the nature of solvents and composition of the stationary phases, a FTIR study was conducted to evaluate the influence of solvents on the conformational order of the alkyl chains in C30 alkyl modified silica gels. Variable temperature IR measurements are carried out between 273 and 353 K in the presence of polar and nonpolar solvents. Information about the conformational behavior of the tethered alkyl chains is derived from the analysis of the symmetric and antisymmetric CH2 stretching band regions. Polar solvents show both enhanced conformational order and disorder of the alkyl chains - irrespective of temperature - when compared to dry C30 alkyl modified silica gels, while nonpolar solvents in general give rise to enhanced conformational disorder in the alkyl chain region. Moreover, for polar solvents a correlation exists between the stretching band position, reflecting alkyl chain conformational order, and the solvent solvatochromic parameter pi*. Finally, both partition and adsorption models are considered to play an important role for the solvent-alkyl chain interactions which in turn determines the conformational order of the alkyl chains and thus the chromatographic properties of these phases.

Effect of temperature on the adsorption behavior of tryptophan in reversed-phase liquid chromatography

Single-component adsorption isotherm data of l-tryptophan on a C(18)-bonded silica column were acquired by frontal analysis (FA), with aqueous mobile phases containing 5% of acetonitrile at five different temperatures between 23 and 62 degrees C. The non-linear fitting of these data provided the bi-Moreau model for all temperatures as the best isotherm model. The inverse method (IM) was used to derive the parameters at these temperatures from the parameters of the 25 degrees C isotherm. The adsorption constants and the saturation capacities of the low and high-energy sites decreases by increasing the temperature, while the adsorbate-adsorbate parameters of both sites increase. An excellent agreement was found between the experimental and calculated overloaded band profiles at all the temperatures used. The breakthrough curves obtained and the overloaded band profiles obtained were found to have different shapes according to the range of concentration studied and the temperatures. At low concentration 0.05-0.5 g/L the breakthrough curves and the overloaded band profiles have a front shock and diffuse rear, which indicates langmuirian behavior, but at intermediate 1-2 g/L and high concentration 8 g/L they start to have diffuse fronts and shocks at the rear or more than one shock at the rear which indicates non-langmuirian behavior. At 23 degrees C the isotherm has another langmuirian part, which appears at high concentration. The behavior of the breakthrough curves is explained by the shape of the isotherm in which all of the isotherms have a langmuirian part (the isotherm is concave upward) and an antilangmuirian part (the isotherm is concave downward). The temperature affected the breakthrough curves by decreasing the time of the appearance of the fronts for all concentration ranges studied, and by decreasing the time difference between the highest concentration and lowest concentration of the fronts, especially the low concentration range at 0.5 g/L. The fronts of the breakthrough curves at high concentration seems to be the most affected by temperature.

Structure−Function Relationships in High-Density Docosylsilane Bonded Stationary Phases by Raman Spectroscopy and Comparison to Octadecylsilane Bonded Stationary Phases

Raman spectroscopy is used to investigate the effects of temperature, surface coverage, polymerization method (surface or solution polymerized), and nature of the alkylsilane precursor on alkyl chain conformational order in a series of high-density docosylsilane (C22) stationary phases at surface coverages ranging from 3.61 to 6.97 mumol/m(2). The results of this study contribute to an enhanced understanding of the shape-selective retention characteristics of these phases at a molecular level. Conformational order is evaluated using the intensity ratio of the antisymmetric and symmetric nu(CH2) modes as well as the frequency at which these modes are observed. Alkyl chain order is shown to be dependent on surface coverage, alkyl chain length, and polymerization method. In general, alkyl chain order increases with surface coverage. Temperature-induced changes are observed between 250 and 350 K for the three phases with surface coverages between 3.61 and 4.89 mumol/m(2). These changes occur over a broad range of temperatures characteristic of two-dimensional systems, but in general adhere to the behavior predicted for a simple first-order transition. This change is not believed to be an abrupt cooperative disassociation characteristic of a phase transition in a bulk phase, but instead is thought to involve significant changes in conformational order in segments of the surface-tethered molecules, mostly those segments at the outer edge of the alkylsilane. In contrast to the changes observed in coverages below 5 mumol/m(2), a first-order change is not observed for the stationary phase with coverage of 6.97 mumol/m(2). A molecular picture of the temperature-induced disorder is proposed with disorder originating at the distal carbon and propagating only a short distance toward the proximal carbon. A comparison is made between these C22 stationary phases and similar high-density octadecylsilane (C18) bonded phases.

Probing the interaction of solvents with the stationary phase of C18 high-performance liquid chromatographic column material by variable-temperature dependent 129Xe nuclear magnetic resonance spectroscopy

VT (129)Xe NMR was applied to probe the interactions of solvents having different polarity indices with the stationary phase of a RP-C18 HPLC column material. It was observed that the highly polar ethylene glycol molecules do not mix with the alkyl chains of the RP-C18 stationary phase and the solvent is unable to enter the pores and the spaces between the particles. Three phases in this sample are defined as stationary/xenon phase, xenon gas phase (in the pores and the spaces between the particles) and ethylene glycol/xenon phase. In contrast to ethylene glycol, the nonpolar solvent cyclohexane was observed to be well mixed with the RP-C18 stationary phase. The capillary rise effect allows the solvent to enter the pores and the spaces between the particles. Two phases in this sample are defined as stationary/cyclohexane/xenon phase and cyclohexane/xenon phases. The properties of ethyl acetate are between those of ethylene glycol and cyclohexane. The (129)Xe NMR results show that the rational reversed phases should be conditioned from highly solvating to more polar solvents to remove the trapped air. The (129)Xe NMR results also show that pure stationary phase exists only when a highly polar solvent is used in reversed-phase chromatography. For a solvent with lower polarity, a stationary/solvent phase actually forms. This, together with the mobile phase, determines the selective factor for separating mixtures.

Influence of synthetic routes on the conformational order and mobility of C18 and C30 stationary phases

Silica gels modified with n-alkyl chains (n = 18, 30) are prepared by two different synthetic routes and are examined by variable temperature FTIR and solid-state NMR spectroscopy. HPLC measurements of SRM 869, cis/trans ss-carotene isomers and xanthophylls isomers confirm the dependence of the separation mechanism on the alkyl chain length and the synthetic routes. The determination of the silane functionality and degree of cross-linking of silane ligands on the silica surface is achieved by 29Si CP/MAS NMR measurements. The structural order and mobility of the alkyl chains are investigated by means of variable temperature 13C CP/MAS NMR measurements. Variable temperature FTIR studies are performed where conformational order and flexibility of the alkyl chains in C18 and C30 phases are monitored through conformational sensitive CH2 symmetric, anti-symmetric stretching and wagging modes. In addition, the chromatographic properties of the C18 and C30 phases are determined. The results derived from the FTIR, NMR and HPLC measurements are discussed in the context of the applied synthetic routes and alkyl chain lengths.

Effect of surface coverage on the conformation and mobility of C18-modified silica gels

C18-modified silica gels with surface coverages of 2 to 8.2 micromol m(-2), were prepared by different synthetic pathways and characterized by Fourier Transform infrared spectroscopy (FTIR), solid-state nuclear magnetic resonance (NMR) spectroscopy, and chromatographic measurements. The effects of temperature and bonding density on the conformational order of C18-modified silica gels were studied in detail by FTIR spectroscopy. The silane functionality and degree of cross-linking of silane ligands on the silica surface were evaluated by 29Si cross-polarization magic-angle spinning (CP/MAS) NMR and the structural order and mobility of the alkyl chains were investigated by 13C CP/MAS NMR spectroscopy. CH2 symmetric and anti-symmetric stretching bands and CH2 wagging bands were used as IR probes to monitor the conformational order and flexibility of the alkyl chains in the C18 phases. Qualitative information about the conformational order was obtained from frequency shifts of the CH2 symmetric and anti-symmetric stretching bands. The relative amounts of kink/gauche-trans-gauche, double-gauche, and end-gauche conformers in the alkyl chains were determined by analysis of CH2 wagging bands. These results indicate that surface coverage plays a dominant role in the conformational order of C18-modified silica gels. The FTIR and NMR data are discussed in the context of the chromatographic shape-selectivity differences.

Conformational order of n-alkyl modified silica gels as evaluated by Fourier transform infrared spectroscopy

The conformational behaviour of non-deuterated and selectively deuterated alkyl modified silica gels in the dry state is examined by variable temperature FT IR spectroscopy. In the present study, three systems are considered, which are distinguished by the length of the tethered alkyl chains (C9Hl9-, C18H37-, C22H45-). The desired information is obtained by the analysis of various conformational-sensitive IR bands, including CH2 wagging, CD2 stretching and CD2 rocking bands. The analysis of the CH2 wagging bands provides the relative amounts (i.e., integral numbers over the whole chain) of the kink/gauche-trans-gauche, double-gauche and end-gauche conformers in the tethered alkyl chains. From the analysis of the CD2 stretching and CD2 rocking bands information about the conformational order at a specific deuterated methylene segment is available. Here, the CD2 rocking band data are used to determine the amount of gauche conformers at the deuterated carbon positions C-4 and C-6, and C-12. It is found that the conformational order critically depends on the actual alkyl chain length, chain position and sample temperature. Particular emphasis is given to the impact of the external pressure during sample preparation on the alkyl chain conformations, about which so far no information is available. It is observed that the samples prepared as KBr pellets, which experienced a pressure of about 10 kbar, are characterised by a lower amount of gauche conformers. This substantial increase of conformational order is attributed to better alkyl chain packing along with a gain of intermolecular chain interactions.

Influence of the pressure on the properties of chromatographic columns

The influence of the average column pressure (ACP) on the elution volume of thiourea was measured on two RPLC columns, packed with Resolve-C18 (surface coverage 2.45 micromol/m2) and Symmetry-C18 (surface coverage 3.18 micromol/m2), and it was compared to that measured under the same conditions on an underivatized silica (Resolve). Five different methanol-water mixtures (20, 40, 60, 80 and 100% methanol, v/v) were used. Once corrected for the compressibility of the mobile phase, the data show that the elution volume of thiourea increases between 3 and 7% on the C18-bonded columns when the ACP increases from 50 to 350 bar, depending on the methanol content of the eluent. No such increase is observed on the underivatized Resolve silica column. This increase is too large to be ascribed to the compressibility of the stationary phase (silica + C18 bonded chains) which accounts for less than 5% of the variation of the retention factor. It is shown that the reason for this effect is of thermodynamic origin, the difference between the partial molar volume of the solute in the stationary and the mobile phase, Delta V, controlling the retention volume of thiourea. While Delta V is nearly constant for all mobile phase compositions on Resolve silica (with Delta V approximately equal to -4 mL/mol), on RPLC phases, it significantly increases with increasing methanol content, particularly above 60% methanol. It varies between -5 mL/mol and -17 mL/mol on Resolve-C18 and between -9 mL/mol and -25 mL/mol on Symmetry-C18. The difference in surface coverage between these two RP-HPLC stationary phases increases the values of Delta V by about 5 mL/mol.

Order and disorder in alkyl stationary phases

Covalently modified surfaces represent a unique state of matter that is not well described by liquid or solid phase models. The chemical bond in tethered alkanes imparts order to the surface in the form of anisotropic properties that are evident in chromatographic and spectroscopic studies. An understanding of the structure, conformation, and organization of alkyl-modified surfaces is requisite to the design of improved materials and the optimal utilization of existing materials. In recent years, the study of alkyl-modified surfaces has benefited from advances in modern analytical instrumentation. Aspects of alkyl chain conformation and motion have been investigated through the use of nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, fluorescence spectroscopy, and neutron scattering studies. Chromatography provides complementary evidence of alkyl chain organization through interactions with solute probes. Computational simulations offer insights into the structure of covalently modified surfaces that may not be apparent through empirical observation. This manuscript reviews progress achieved in the study of the architecture of alkyl-modified surfaces.

Effects of alcohol mobile-phase modifiers on the structure and chiral selectivity of amylose tris(3,5-dimethylphenylcarbamate) chiral stationary phase

Following a previous publication, the present paper reports additional results on the effects of alcohol mobile-phase modifiers on the structure and chiral selectivity of amylose tris(3,5-dimethylphenylcarbamate) (Chiralpak AD) chiral stationary phase (CSP). Solid-state NMR (1H/13C CPMAS) was utilized to identify and compare structural differences in Chiralpak AD caused by the various alcohol mobile-phase modifiers, many of which were not studied in the previous publication. The influences of the various alcohol modifiers (in hexane-based mobile phase) on the structure and chiral selectivity of the CSP were studied and compared. CPMAS spectra of Chiralpak AD flushed with the mobile phases displayed clear evidence of solvent incorporation into the CSP. When alcohol modifiers with varying size and bulkiness were used in the mobile phase, differences in structure and chiral selectivity were observed on Chiralpak AD based on solid-state NMR and chromatographic data. The change of t-butanol concentration in the t-butanol/hexane mobile phase caused changes of structure and chiral selectivity of the Chiralpak AD. These data further support our belief that the different chiral selectivities of the CSP associated with the use of different alcohol modifiers are due to different alterations of the steric environment of the chiral cavities in the CSP by the different mobile-phase modifiers.

Characterization of a Thermally Induced Irreversible Conformational Transition of Amylose Tris(3,5-dimethylphenylcarbamate) Chiral Stationary Phase in Enantioseparation of Dihydropyrimidinone Acid by Quasi-Equilibrated Liquid Chromatography and Solid-State NMR

A thermally induced irreversible conformational transition of amylose tris(3,5-dimethylphenylcarbamate) (i.e., Chiralpak AD) chiral stationary phase (CSP) in the enantioseparation of dihydropyrimidinone (DHP) acid racemate was studied for the first time by quasi-equilibrated liquid chromatography with cyclic van't Hoff and step temperature programs and solid-state ((13)C CPMAS and (19)F MAS) NMR using ethanol and trifluoroacetic acid (TFA)-modified n-hexane as the mobile phase. The conformational transition was controlled by a single kinetically driven process, as evidenced by the chromatographic studies. Solid-state NMR was used to study the effect of the temperature on the conformational change of the solvated phase (with or without the DHP acid enantiomers and TFA) and provided some viable structural information about the CSP and the enantiomers.

Probing transport and microheterogeneous solvent structure in acetonitrile-water mixtures and reversed-phase chromatographic media by NMR quadrupole relaxation

Mixtures of CH(3)CN and H(2)O are the predominant solvent systems used in reversed-phase liquid chromatographic (RPLC) separations, as well as in a multitude of other applications. In addition, acetonitrile is the simplest model for an amphiphilic molecule possessing both organic and polar functional groups. Although many studies have focused on this solvent system, the general nature of the intermolecular interactions are not fully understood, and a microscopic description of the proposed microheterogeneity that exists is still not clearly established. In the present study, we measure the spin-lattice relaxation times (T(1)) of (14)N to determine reorientational correlation times (tau(c)) of CH(3)CN-H(2)O solvent mixtures over the entire binary composition range and at temperatures ranging from 25.0 to 80.0 degrees C. At all compositions, the microscopic observable, tau(c), is found to be directly proportional to the macroscopic solution viscosity when scaled for temperature (eta/T). This indicates that for a constant composition, this system's dynamics are well described by hydrodynamic theory on a microscopic level. These results suggest that under appropriate conditions, the measurement of changes in quadrupolar relaxation times is a reliable means of determining changes in solution viscosity. We stress the importance of this approach in systems not amenable to traditional viscosity measurements, such as those having species in interfacial regions. This approach is used to examine the changes in the interfacial solution viscosity of CH(3)CN-H(2)O mixtures in contact with a commercially available C(18)-bonded stationary phase. The measurements indicate that CH(3)CN is motionally hindered at the stationary phase surface. The surface affected CH(3)CN has a larger dependence of tau(c) on temperature than the bulk CH(3)CN, indicating greater changes in the interfacial viscosity as a function of temperature. Additionally, the bulk relaxation data show direct correlations to existing models of proposed regions of structure for CH(3)CN-H(2)O mixtures. Using a microscopic hydrodynamic approach, we show that, quite unexpectedly, each of the experimentally determined parameters in the viscosity correlation plots change simultaneously, and we propose that these are indicative of changes in the distribution of species for this microheterogeneous liquid system. Although distinct regions for the onset of microheterogeneity have previously been proposed, within the framework of a microscopic hydrodynamic model and the recently proposed model of Reimers and Hall,(1) the present data support the existence of a microheterogeneous solvent structure that varies continuously over the full range of temperatures and compositions examined.

Deuterium nuclear magnetic resonance spin-lattice relaxation of analytically relevant solvent systems

We previously reported that the nuclear magnetic resonance (NMR) 14N spin-lattice relaxation times (T1) of CH3CN in CH3CN-H2O mixtures directly correlate with the solution viscosity when scaled for temperature (eta/T) in this common chromatographic mobile phase system.' Here, we demonstrate that the 2HT1 relaxation times also correlate with viscosity, contrary to a previous report. (2) This establishes 2HT1 relaxation times as a useful means of measuring changes in solution viscosity in CH3CN-H2O mixtures. We show thermal convection to result in grossly decreased, apparent T1's, by as much as approximately 40%, in nonspinning samples. This effect can be eliminated by moderate sample rotation or confinement of the sample to within the rf-irradiated region. The problem of thermal convection is revealed in systems having long Ti's and has implications in T1 experiments employing nonspinning samples at elevated temperatures, including inherently nonspinning systems, such as those used in high-pressure studies.

Column selectivity from the perspective of the solvation parameter model

The solvation parameter model is a useful tool for delineating the contribution of defined intermolecular interactions to retention of neutral molecules in separation systems based on a solute equilibrium between a gas, liquid or fluid mobile phase and a liquid or solid stationary phase. The free energy for this process is decomposed into contributions for cavity formation and the set up of intermolecular interactions identified as dispersion, electron lone pair, dipole-type and hydrogen bonding. The relative contribution of these interactions is indicated by a series of system constants determined by the difference of the defined interaction in the two phases. The interpretation of these system constants as a function of experimental factors that affect retention in the chromatographic system provides the connection between relative retention (selectivity) and the control variables for the separation system. To aid in the understanding of these processes we perform an analysis of system constants for gas chromatography, liquid chromatography, supercritical fluid chromatography and micellar electrokinetic chromatography as a function of different experimental variables as a step towards gaining a theoretical understanding of selectivity optimization for method development.

Raman spectroscopy of octadecylsilane stationary phase conformational order. Effect of solvent

The effect of solvent on the conformation of alkyl chains of two octadecysilane-based stationary phases is probed using Raman spectroscopy. Spectral data indicate that the alkyl chains of commercially available polymeric and monomeric solid-phase extraction stationary phases are disordered to a varying extent by solvents of different polarity. For the polymeric octadecylsilane stationary phase, the polar solvents water, acetonitrile, methanol, acetone and isopropanol have little impact on the conformational order of the octadecylsilane bonded phase relative to air. However, the alkyl portion of this stationary phase is substantially disordered in the low-polarity solvents tetrahydrofuran, chloroform, benzene, toluene and hexane. The monomeric octadecylsilane stationary phase is less susceptible to disordering by solvents, although more disorder in the less polar solvents is also observed for this system. These results are interpreted in terms of the local surface bonding density and interchain spacing of these two stationary phases, and the ability of the solvent to penetrate the chains as a function of polarity. The results clearly demonstrate the ability of Raman spectroscopy to precisely indicate subtle changes in conformational order of alkylsilane stationary phases.

Solid-state NMR characterization of amylose tris(3,5-dimethylphenylcarbamate) chiral stationary-phase structure as a function of mobile-phase composition

Solid-state NMR (1H/13C CPMAS) was utilized to identify structural differences in amylose tris(3,5-dimethylphenylcarbamate) chiral stationary phase (Chiralpak AD), as a function of mobile-phase composition. Dry Chiralpak AD stationary phase displayed an amorphous CPMAS NMR spectrum. However, CPMAS spectra of Chiralpak AD flushed with organic mobile phases clearly displayed evidence of solvent complexes. Chiralpak AD flushed with nonpolar hexane exhibited solvent complexes with minimal structural perturbation. For Chiralpak AD flushed with hexane containing alcohol modifiers, however, solvent incorporation caused significant difference in conformation distribution as evidenced by increased resolution of 13C peaks in the CPMAS spectrum of the stationary phase. 2-Propanol modifier displayed more efficient displacement of incorporated hexane while forming relatively more distinct/ordered solvent complexes with Chiralpak AD in comparison to ethanol modifier. Reversed elution order and unusual retention behavior on Chiralpak AD as a function of mobile-phase modifier was reported earlier. These chromatographic behaviors are believed to be due to different alterations of the steric environment of the chiral cavities in the CSP by the different mobile-phase modifiers. In addition, on the basis of the chemical shift of C-1 carbon on the amylose backbone, it is possible that Chiralpak AD's structure is a helix with a number of fold less than six.

Characterization of C18-bonded liquid chromatographic stationary phases by Raman spectroscopy: the effect of temperature

This study represents the first time that both the mobile phase composition and the temperature are simultaneously controlled to examine silica-bonded octadecylsilyl (C18) ligands spectroscopically at typical liquid chromatographic (LC) mobile phase flow-rates and back-pressures. Raman spectroscopy is used to characterize the behavior of the C18 bonded ligands equilibrated at temperatures from 45 to 2 degrees C in neat, single-component, mobile phase solvents including: water, acetonitrile, methanol, and chloroform. In addition, the effect of stationary phase ligand bonding density is examined by using two different monomeric reversed-phase liquid chromatographic (RPLC) stationary phases, a 2.34 and a 3.52 micromol m(-2) Microporasil C18 stationary phase, under identical conditions. The direct, on-column, spectroscopic analysis used in this study allows direct evaluation of the temperature-dependent behavior of the bonded C18 ligands. The temperature-dependent ordering of the stationary phase ligands is examined to determine if the ligands undergo a phase transition from a less-ordered "liquid-like" state at higher temperatures to a more-ordered "solid-like" state at lower temperatures. A discrete phase transition was not observed, but rather a continual ordering as temperature was lowered.

Characterization of C18-bonded liquid chromatographic stationary phases by Raman spectroscopy: the effect of mobile phase composition

Raman spectroscopy is used to examine the effect of mobile phase composition on the orientation of octadecyl-bonded silica-based reversed-phase liquid chromatographic (RPLC) stationary phase ligands. The effect of ligand bonding density is also investigated. The present experimental set-up utilizes a direct, noninvasive, on-column approach to examine the solvent dependent conformational behavior of the bonded ligands under flow-rate and back pressure conditions similar to those used during conventional RPLC measurements. Neat, single-component, mobile phase solvents including water, acetonitrile, methanol and chloroform are used to investigate the hypothesized collapsing and extension of stationary phase ligands with changes in mobile phase composition. No evidence of phase collapse was observed upon changing the mobile phase composition from an organic to an aqueous content. Also, Raman spectroscopic measurements allowed the differentiation between associated and free acetonitrile solvent.

Stationary-phase contribution of 1-propanol organic modifier to changes in sorption of 1-hexanol on an ODS-bonded phase

Using the reversed-phase bonded-phase HPLC packing Partisil-10 ODS-3, sorption isotherms have been measured for the alcohols 1-propanol (PrOH) and 1-hexanol (HexOH), and as well, a simultaneous sorption curve for the two alcohols has been measured from solutions containing a low and constant concentration of HexOH as sample with increasing concentrations of PrOH as organic modifier. The mobile-phase effect of PrOH is quantified by solution-phase activity coefficients obtained either from vapor/solution equilibrium measurements or from cloud point measurements. Since sorbed alcohols are located at the ODS/solution interface, the stationary-phase effect of PrOH is modeled in terms of three processes: (i) competition for space; (ii) decrease of space required per mole with increasing concentration of sorbed PrOH; and (iii) change of free energy of sorption with increasing concentration of sorbed PrOH. The model yields excellent fits to the isotherms and to the simultaneous sorption curve. Comparison of the model-fitting parameters for the simultaneous sorption curve with those for the PrOH isotherm confirms that the stationary-phase effect of PrOH on HexOH is due exclusively to processes i and ii. Sorbed PrOH causes rearrangement of the C18 chains of the ODS phase. For volume percent PrOH less than 15% in the mobile phase, the effect of PrOH on sample sorption is nearly exclusively in the stationary phase. Between 15 and 30%, both mobile- and stationary-phase effects are important.

Shape-selective separation of polycyclic aromatic hydrocarbons on protoporphyrin-silica phases. Effect of surface porphyrin distribution on column efficiency

The chromatographic performance of various metalloprotoporphyrin-silica (MProP-silica) packing materials prepared using different porphyrin immobilization schemes is examined. Column efficiency and solute resolution for the shape-selective separation of polycyclic aromatic hydrocarbons (PAHs) can be improved significantly by preparing phases with lower porphyrin coverages and with a more homogeneous distribution of the porphyrin species on the surface. The latter is accomplished by spreading/diluting the number of aminopropyl reactive sites on the silica surface via mixing an inert methyltrimethoxysilane with 3-aminopropyltriethoxysilane during this preliminary reaction step. Subsequent covalent attachment of the ProP via amide bonds to the pendant amine sites results in a more even distribution of the porphyrins on the surface. Band shapes and retention times as a function of injected solute concentration as well as HPLC separation of various test mixtures of PAHs (including standard reference material SRM 869) are used to confirm the enhanced performance of these so-called "spread" phases. Changes in the nature of the immobilized porphyrin distribution on the silica surface are further probed by a coupled redox/UV-Vis absorbance method, and results suggest a decrease in the number of ProP species immobilized as aggregates on the surface.

Characterization of liquid chromatographic stationary phases by Raman spectroscopy. Effect of ligand type

This study represents the first Raman spectroscopic characterization of conventional chemically-bonded liquid chromatographic (LC) stationary phases under typical flow-rate and pressure conditions. Raman spectra were obtained for amino propyl (NH2), cyano propyl (CN), phenyl (Ph), octadecyl (C18), octyl (C8), and methyl (C1) chemically-bonded silica-based stationary phases in 100% aqueous mobile phases. The present experimental set-up has allowed Raman spectra of various stationary phase ligands, present in sub-monolayer coverages on the siliceous supports, to be obtained. This study: (1) demonstrates that conventional Raman spectroscopic techniques can be used to study LC stationary phases; (2) presents the experimental set-up, conditions, and approaches utilized to obtain Raman spectra of conventional stationary phases; (3) examines the spectroscopic differences observed for a variety of different types of bonded ligands that are typically used in reversed-phase (RPLC) and normal-phase (NPLC) liquid chromatographic separations; and (4) considers other future studies that are possible with this experimental approach, including mobile phase composition and temperature studies.