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The keys to understanding reversed-phase liquid chromatography (LC) are provided at the molecular mechanism level as determined by high accuracy molecular simulation. The essential features of [C.sub.18] stationary-phase chains in [...]

Stationary phases with embedded polar groups possess several advantages over conventional alkylsilane phases, such as reduced peak tailing, enhanced selectivity for specific functional groups, and the ability to use a highly aqueous mobile phase. To gain a deeper understanding of the retentive properties of these reversed-phase packings, molecular simulations were carried out for three different stationary phases in contact with mobile phases of various water/methanol ratios. Two polar-embedded phases were modeled, namely, amide and ether containing, and compared to a conventional octadecylsilane phase. The simulations show that, due to specific hydrogen bond interactions, the polar-embedded phases take up significantly more solvent and are more ordered than their alkyl counterparts. Alkane and alcohol probe solutes indicate that the polar-embedded phases are less retentive than alkyl phases for nonpolar species, whereas polar species are more retained by them due to hydrogen bonding with the embedded groups and the increased amount of solvent within the stationary phase. This leads to a significant reduction of the free-energy barrier for the transfer of polar species from the mobile phase to residual silanols, and this reduced barrier provides a possible explanation for reduced peak tailing.

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/[m.sup.2] 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.

Various driving forces have been suggested to explain retention and selectivity in reversed-phase liquid chromatography (RPLC). To provide molecular-level information on the retention mechanism in RPLC, configurational-bias Monte Carlo simulations in the Gibbs ensemble were carried out for model systems consisting of three phases: an n-hexadecane retentive phase, a mobile phase with varying water-methanol composition, and a helium vapor phase as reference state. Liquid n-hexadecane functions as a model of a hydrophobic stationary phase, and a wealth of experimental data exists for this system. Gibbs free energies for solute transfers from gas to retentive phase, from gas to mobile phase, and from mobile to retentive phase were determined for a series of short linear alkanes and primary alcohols. Although the magnitude of the incremental Gibbs free energy of transfer for a methylene segment is always larger for the gas- to retentive-phase transfer than the gas- to mobile-phase transfer, it is found that the partitioning of alkanes and alkyl tail groups is mostly affected by the changes in the aqueous mobile phase that occur when methanol modifiers are added. In contrast, the partitioning of the alcohol headgroup is sensitive to changes in both the n-hexadecane and the mobile phases. In particular, it is found that hydrogen-bonded aggregates of methanol are present in the n-hexadecane phase for higher methanol concentrations in the mobile phase. These aggregates strongly increase alcohol partitioning into the retentive phase. The simulation data clearly demonstrate that due to modification of the retentive-phase hydrocarbons by solvent components, neither the solvophobic theory of RPLC, advocated by Horvath and co-workers, nor the lipophilic theory of RPLC, advocated by Carr and co-workers, can adequately describe the separation mechanism of the hexadecane model system of a retentive phase studied here nor the more complex situation present in actual RPLC systems.

A computer simulation of a section of the interior region of a liquid chromatographic column is performed. The detailed fluid flow profile is provided from a microscopic calculation of low Reynolds number flow through a random packed bed of nonporous spherical particles. The fluid mechanical calculations are performed on a parallel processor computer utilizing the lattice Boltzmann technique. Convection, diffusion, and retention in this flow field are calculated using a stochastic-based algorithm. This computational scheme provides for the ability to reproduce the essential dynamics of the chromatographic process from the fundamental considerations of particle geometry, particle size, flow velocity, solute diffusion coefficient, and solute retention parameters when retention is utilized. The simulation data are fit to semiempirical models. The best agreement is found for the 'coupling' model of Giddings and the four-parameter Knox model. These models are verified over a wide range of particle sizes and flow velocities at both low and high velocity. The simulations appear to capture the essential dynamics of the chromatographic flow process for nondimensional flow velocities (Peclet number) less than 500. Since the same packing geometry is utilized for different particle size studies, the interpretation of the parameter estimates from these models can be extended to the physical column model. The simulations reported here agree very well with a number of experiments reported previously.

The importance of adsorption at the gas-liquid interface on retention in gas-liquid chromatography has been controversial since the pioneering work of Martin in the 1960s. In particular, experimental studies using chromatographic and static techniques to quantify partitioning and adsorption of polar analytes on nonpolar liquid phases yielded conflicting results. In this work, Monte Carlo simulations were carried out for a free-standing liquid slab of squalane surrounded by a helium vapor to investigate interfacial adsorption effects for n-pentane, n-hexane, n-heptane, 1-butanol, and benzene solutes at infinite dilution. The simulations indicate preferential adsorption for the flexible alkane and alcohol solutes in a narrow region just inside the Gibbs dividing surface, but no such effect was observed for the rigid benzene solute. Nevertheless, the extent of the interfacial enrichment is small, as measured by the partition coefficient between the bulk liquid and the interfacial region ([K.sub.bulk-interface] [approximately equal to] 1.5). In addition, a region that is slightly depleted for all solute molecules is found to separate the interfacial and bulk regions of the squalane slab. Thus, adsorption at the gas-liquid interface should not contribute significantly to the retentive behavior observed in gas-liquid chromatography on nonpolar capillary columns but might play a role in packed-bed columns with low bonded-phase loadings. The origin for the small enrichments and more favorable free energies for solutes at the interface is that the enthalpies of solvation decrease to a smaller relative extent than the entropies of solvation compared to the bulk liquid.

The coupling of field-flow fractionation (FFF) and multiangle light scattering (MALS) detectors is complementary in that the MALS system allows particle characterization when a narrow dispersity particle population is present in the detector. The fractionation process provides this narrow dispersity. Utilizing discrete particle simulations of FFF and optical calculations based on both the Mie theory of particle scattering and Rayleigh-Gans-Debye (RGD) scattering theory, the extent of polydispersity that can be tolerated for accurate particle quantitation is explored. It is found that flow, electrical, and sedimentation FFF provide adequate separation for accurate particle quantitation by MALS. The Mie theory is more accurate than the RGD theory, which is known to deviate at higher particle size. Low error in the measurement of mean diameters is found when only the particle diameter is of interest. It is shown that the reconstruction of the particle size distribution from time slice data is distorted due to errors in concentration, which result from finite polydispersity and other effects. A number of procedures are evaluated in restoring the size distribution to higher accuracy. None of these procedures is deemed of general purpose and none of these is reliable. The best results are obtained when fractionation is conducted under the minimal possible outlet polydispersity and when steric effects are minimized. In addition, best results are had for inherently narrow dispersity colloidal materials.

In an attempt to elucidate the molecular basis for concentration (isotherm) effects on retention in gas-liquid chromatography, configurational-bias Monte Carlo simulations in the Gibbs ensemble were carried out to investigate changes in analyte partitioning caused by overloading a model chromatographic system with either an alkane or an alcohol. Squalane was used as the stationary-phase material, and the analytes included n-pentane, n-hexane, n-heptane, 1-butanol, and 1-pentanol. Three systems were studied that differed in the mobile-phase composition: (i) a helium vapor, (ii) a n-hexane vapor, and (iii) a 1-pentanol-saturated helium vapor. While the amount of helium that partitions into the stationary phase is very small, both n-hexane and 1-pentanol partition strongly into and thereby swell the stationary phase. Although the swelling of the stationary phase leads to a reduction in the partition coefficients for the alkane solutes for both the n-hexane-and 1-pentanol-swollen stationary phases, the effects on the alcohol solutes differ markedly. Whereas saturation by n-hexane causes a decrease of the alcohol partition contants (to an extent similar to that for the alkane solutes), the saturation by 1-pentanol causes a dramatic increase of the alcohol partition coefficients; e.g., the Kovats index of 1-butanol increases by more than 150 Kovats units. The formation of hydrogen-bonded alcohol aggregates in the liquid phase is the microscopic origin for the dramatic effect of 1-pentanol saturation on the retention of alcohols.

It is with a heavy heart that we remember our friend and colleague Roy Eksteen. Roy passed away unexpectedly on September 21, 2015, on a plane returning home from a [...]

Two-dimensional liquid chromatography (2DLC) is an increasingly popular technique which has the potential to provide a far more detailed separation and characterization of alcohol ethoxylates (AE) than has been shown by previously utilized separation techniques. The AE are unique in that these molecules have distributions in both alkyl and ethylene oxide chain lengths. In this paper, we compare the single-column techniques of open-tubular SFC, normal- and reversed-phase HPLC, and the multiple column technique of 2DLC in terms of the efficacy of separation and characterization of the alkyl and ethoxylate distributions in a select group of AE. The combination of normal- and reversed-phase HPLC in a 2DLC system accomplishes the simultaneous alkyl and ethylene oxide distribution analysis. The advantage of using 2DLC over one-dimensional chromatographic techniques is clearly demonstrated in the increased selectivity resulting in the ability to produce the ethylene oxide distributions of each alkyl component in an AE. In addition, 2DLC chromatograms are easier to interpret due to ordering of the chromatograms.

In 'comprehensive' two-dimensional liquid chromatography, the column effluent from the first separation system (the first dimension) is sequentially sampled by the second dimension separation system. The total analysis time is largely determined by the speed of the second dimension separation system; the most retained component must elute before the least retained component of the next second dimension separation. Optimization of multidimensional separation systems requires that one understand the relationship between system resolution and the number of second dimension samples across a first dimension peak. In this paper, we study the theoretical and experimental aspects of this sampling process. To obtain high two-dimensional resolution, each peak in the first dimension should be sampled at least three times into the second dimension when the sampling is in-phase. ff the sampling is maximally out of phase, there should be at least four samples per peak for high-fidelity separation. The sensitivity of the resolution with respect to the sampling phase is discussed in detail and shown to be insignificant when four or greater samples are taken across the first dimension peak width. These results suggest optimal criteria for method development with multidimensional chromatography.

Carbon adsorbents for RPLC separations are greatly underutilized due to the poor chromatographic properties of the earliest commercially available materials and our limited understanding of solute interactions with the solid surface. Previously, we reported on the properties of a carbon surface prepared by vapor deposition on porous zirconia microspheres. The resulting material is a new type of carbon sorbent with considerably improved chromatographic properties. Here we present a fundamental study of the intermolecular interactions influencing solute retention on these novel carbon phases under RPLC conditions. Retention on seven unique carbon phases has been correlated with solute descriptors of dispersion, dipolarity/polarizability, and hydrogen bond basicity through the use of linear solvation energy relationships (LSERs). In stark contrast, conventional bonded phases do not show the large contribution from dipolarity/polarizability that is observed on these types of carbon. The presence of this interaction indicates a distinct difference between carbon and conventional bonded RPLC phases. Other results suggest that solvent sorption plays a significant role in controlling solute retention on carbon. In addition, we investigated the temperature dependence of retention on carbon and found typical RPLC-like behavior.

Simple theoretical models are utilized to assess the extent of zone broadening and relative decrease in resolution that takes place when an adsorptive wall is included in a model of capillary electrophoresis. One of the theoretical models is derived from plate height theory, and the other results from a modification of the kinetic zone shape theory of Lapidus and Amundson, used in chromatography. Both are shown to agree favorably with a stochastic computer simulation used to test the long-time assumptions implicitly present in these theories. The theories predict that small levels of retention drastically reduce resolution from that expected from broadening due solely to axial diffusion. Considerations of capillary diameter demonstrate that, when adsorption is present, small capillaries are advantageous for small molecule solutes (large diffusion coefficients) to minimize the resolution loss. For larger molecules (small diffusion coefficients), small diameter capillaries offer little advantage in performance when adsorption is present as compared to larger capillaries; hence, in this case larger capillaries are suggested for use because the larger path length gives greater sensitivity for on-column detection.

Polymeric additives like hydroxyethyl cellulose enhance the separation of polystyrenesulfonates by capillary electrophoresis (CE). To reduce wall effects and electroosmotic flow, the capillary walls are coated with an organic phase. A comparison with size-exclusion chromatography shows that CE needs shorter analysis time, applies to a wider molecular range and provides better fractionating power. The mechanism of CE separation is comparedto that of biopolymers by slab gel electrophoresis, except that the stronger electric fields used affect the mobility behavior in a different way.

On October 21, 2014, University of Tennessee Distinguished Scientist Georges Andre Guiochon succumbed to neuromuscular failure caused by post-polio syndrome. Here, we pay tribute to his remarkable career and life. [...]

On October 21, 2014, University of Tennessee Distinguished Scientist Georges Andre Guiochon succumbed to neuromuscular failure caused by post-polio syndrome. Here, we pay tribute to his remarkable career and life. [...]

A computational study of the packing of various bonded-phase ligands bound to chromatographic silica is presented. This is done with the intention of examing the type of surface structures that are typically found in real chromatographic systems. Utilizing the surface structure of the (111) face of the [Beta]-cristobalite crystal, it is shown that the maximum surface coverages of dimethyloctylsilane, dimethyloctadecylsilane, triisopropylsilane, diisopropyloctylsilane, and diisopropyloctadecylsilane can be calculated that are in good agreement with experiment. The maximum surface coverages are also calculated for the (100) face of the [Beta]-cristobalite crystal and for a set of random silica surfaces. The coverages for the latter two surfaces types are found to be significantly lower than the experimental values for chromatographic silica surfaces. These results further suggest that chromatographic silica surfaces may resemble crystalline surface sites similar to the (111) face of [Beta]-cristobalite, as has been previously suggested in the literature. Hence, these structures can be reliably utilized in molecular simulations of bonded-phase chromatography where the atomic-level detail of the silica surface has been previously lacking.

The theoretical development of the limit of detection (LOD) concept is extended from one separation column or channel used in gas, liquid, and size-exclusion chromatography, capillary electrophoresis, and field-flow fractionation to the case where multiple columns and channels are utilized in the so-called 'comprehensive' and 'heart cutting' modes of operation. Simple equations show that the column dilution factors and the split ratios used to compute the LOD are multiplicative per dimension. Thus, when more than one separation dimension is utilized in a sequential column mode of operation, a larger overall dilution of the original injected solute concentration will occur. The dilution of the initial zone in multidimensional geometrical separation systems such as planar two-dimensional thin-layer chromatography and two-dimensional slab gel electrophoresis is also considered. The practical aspects of this dilution are discussed with respect to the types of separation techniques that can be used to implement multidimensional separation systems.

Capillary gel electrochromatography is evaluated with an entangled polymer solution which is pumped into the capillary and run under fritless conditions. The polymer used has an acid backbone with grafted hydrophobic segments, the polyacid giving the electroosmotic flow and the hydrophobe segments providing the retentive component. Experimental evaluation of this type of system reveals performance similar to capillary electrophoresis and other forms of electrochromatography. The analysis of plate height data demonstrates that zone broadening is primarily due to diffusion with little contribution from nonequilibrium zone broadening. Hence, operation at high velocities (high voltages) is most desirable as opposed to most chromatographic methods. Some of the advantages of this type of experiment include being able to replace the retentive media in a few minutes, fast and reproducible high-performance separation, and having a retention mechanism similar to reversed-phase liquid chromatography. Disadvantages include a low retentive phase concentration and hence low sample loadability and limited solvent compatibility of the polymer. A number of different separations are demonstrated including separation of alkyl benzoates, alkylphenones, alkylbenzenes, oxidation inhibitors, and PAHs.