Capacity Factors Of Solutes Research Articles

Membrane partition coefficients chromatographically measured using immobilized artificial membrane surfaces.

Immobilized artificial membranes (IAMs) are chromatographic surfaces prepared by covalently immobilizing cell membrane phospholipids. IAM surfaces mimic fluid cell membranes. Solute capacity factors (k'IAM) measured on IAM columns correlate very well with solute equilibrium partition coefficients (Km') measured in fluid liposome systems. For 23 structurally unrelated compounds, log-(k'IAM) correlates with log(Km') with a linear correlation coefficient r = 0.907. This indicates that solute partitioning between the IAM bonded phase and the aqueous mobile phase is similar to the solute partitioning between liposomes and the aqueous phase. Although both IAM chromatography and liposome partitioning can be used as in vitro methods to predict solute partitioning into cell membranes, IAM chromatography is experimentally convenient compared to liposome systems. To study the effect of lipid structure on drug binding to IAMs, IAMs were prepared from three different phosphatidylcholine ligands: (i) a diacylated phosphatidylcholine ligand, (ii) a single chain ether phosphatidylcholine ligand, and (iii) a single chain phosphatidylcholine ligand that lacks a glycerol backbone. Solute retention data were identical for all of these IAMs, and consequently, predictions of solute binding to fluid membranes were also identical. This indicates that the structure of the phosphatidylcholine ligand that is immobilized is not critical for the binding of solutes. Since the structure is not important, the binding of solutes to membranes is a bulk phase property, i.e., it is the interface created by the ligands that determines the solute binding properties, not the ligands themselves. Solute partitioning using octanol/water systems does not correlate with k'IAM unless a homologous series of hydrophobic solutes is being evaluated.

Chemical Selectivity in Micellar Electrokinetic Chromatography: Characterization of Solute-Micelle Interactions for Classification of Surfactants

The influence of surfactant type on migration behavior and chemical selectivity in micellar electrokinetic chromatography (MEKC) is investigated through linear solvation energy relationships (LSER) and functional group selectivities. In LSER modeling, solutes' capacity factors are correlated with their structural descriptors such as size, dipolarity, and hydrogen-bonding abilities. Using the LSER methodology, useful information about the nature of solute interactions with different types of surfactant aggregates can be obtained since capacity factor in MEKC is directly related to solute distribution between the bulk aqueous solvent and micelles. High correlations were observed for different LSER models of migration behavior in MEKC for a group of 60 uncharged aromatic compounds of non-hydrogen bonding (NHB), hydrogen-bonding acceptor (HBA) bases, and hydrogen-bonding donor (HBD) acids. In two anionic, hydrocarbon micellar systems of sodium dodecyl sulfate (SDS) and sodium cholate (SC), retention is primarily influenced by the size of molecules and their hydrogen bond accepting basicity. Their dipolarity/polarizability and hydrogen bond donating acidity play minor roles. Capacity factors of solutes in SDS and SC systems increase with their size and decrease for stronger hydrogen bond acceptor bases. These results are similar to those observed for other systems where hydrophobic interactions play a major role, e.g., solute distribution in the 1-octanol-water solvent system or retention in reversed phase LC. In MEKC with an anionic fluorocarbon surfactant, lithium perfluorooctanesulfonate (LiPFOS), however, size and solute HBD acidity are the two predominant factors. The LSER results indicate that compounds find the SDS micellar environments slightly less cohesive (i.e., more apolar) than the SC micelles, while the LiPFOS micelles are the most cohesive among the three surfactant aggregates and 1-octanol provides the least cohesive environment. The fluorocarbon micelles of LiPFOS, on the other hand, are the strongest hydrogen bond donor acids, followed by SDS, SC, and 1-octanol, respectively. The SC micelles have the most hydrogen bond acceptor basic characteristics, followed by 1-octanol, SDS, and LiPFOS micelles. It can be concluded that selectivity differences between these surfactant types in MEKC is primarily due to hydrogen-bonding interactions rather than the dipolar interactions. Comparing the perfluorinated and the hydrocarbon surfactants, even solute size can play a role in selective migration patterns. In addition, information from polar and hydrophobic group selectivities confirm the LSER conclusions about the underlying interactions that control migration behavior and chemical selectivity in MEKC.(ABSTRACT TRUNCATED AT 400 WORDS)

Retention Values of Sulphonic Acids as a Function of the Nature and Concentration of Inorganic Salt in Reversed-Phase Ion-Pair Liquid Chromatography

Abstract The capacity factors of phenylamine and naphthylamine sulphonic acids in reversed-phase ion-pair liquid chromatography (RP-IPC) with different concentration of inorganic anions were measured. The capacity factors of ionic solutes decreases in the order NaF < NaBr < = NaNO3 < Na2SO4, which is the same as their order of anion retention values in anion ion chromatography. The good linear relationship between logarithm of capacity factors at different inorganic salt eluents were observed. The effect of salt concentration (Cs) on retention follows an equation based on the Gouy-Chapman model

Effect of adsorption on the retention values in capillary columns coated with OV-225 and PEG 20M

The effect of adsorption of compounds of different homologous series ( n-alkanes, n-alkenes, n-alkynes, n-alkylcyclohexenes, arenes, alkanones) on capillary columns with different contents of polycyanopropylphenylmethylsiloxane (OV-225) and polyethylene glycol (PEG 20M) on retention indices, relative retention and capacity factors was quantitatively studied. The relationships between the retention characteristics and the stationary phase film thickness or capacity factor of a standard solute were established. These correlations permit the prediction of retention data depending on film thickness. It has been shown that among the compounds studied, the adsorption of n-alkanes and n-alkenes is maximum and comprises up to one third of the relative retention on columns with a thin film of PEG 20M.

Micellar Electrokinetic Capillary Chromatography with Cationic Surfactants

Abstract A series of alkyltrimethylammonium chloride and bromide surfactants were evaluated in micellar electrokinetic capillary chromatography (MECC) of urea herbicides, alkylbenzenes and phenylalkylalcohols. The magnitude of the anodal electroosmotic flow obtained with these cationic micellar phases was largely unaffected by the length of the alkyl chain of the surfactant while the migration time of the micelle increased with decreasing the length of the alkyl tail. The net result was an increase in the retention window as the size of the alkyl tail of the surfactant decreased. The breadth of the retention window stayed almost the same when the micelle counterions were changed from chloride to bromide. At constant micellized surfactant concentration, the capacity factors of neutral solutes increased linearly with increasing alkyl chain length of the surfactant, indicating an increase in the hydrophobic phase ratio of the MECC systems. Under this condition, the value of the methylene group selectivity fo...

Reversed-phase high-performance liquid chromatography of phospholipids with fluorescence detection

Reversed-phase high-performance liquid chromatographic (HPLC) behavior of fluorescent labeled phospholipids (PLs) was studied. Molecular species of phosphatidylethanolamine (PE) derivatized with fluorescein-, thiocarbamoyl-, pyrenesulfonyl-, and dimethylaminonaphthalenesulfonyl (dansyl)-fluorophores were separated on octadecylsilica with a mobile phase of acetonitrilemethanol- water in the presence of tetraalkylammonium phosphates (TAAPs). Under similar HPLC conditions, dansylated phosphatidylserines and PE plasmalogens (ether-linked PLs) were also resolved. Incorporation of the fluorescein moiety to the parent PE appeared to facilitate further resolution of its subcomponents. Subcomponents of dansylated PE derived from egg phosphatidylcholines were quantifiable. Effects of the type and concentration of TAAP on capacity factors of PL solutes were indicative of an ion-pair separation processes. HPLC with high-molecular-mass TAAPs favored the separation of the components that remained unresolved with mobile phases containing low-molecular- mass TAAPS. The HPLC-fluorescence detection method provided a useful approach to quantitative analyses of various PL structures. Compositions of PL subcomponents were determined directly from peak areas.

Separation of glucose polymers by hydrophilic interaction chromatography on aqueous size-exclusion columns using gradient elution with pulsed amperometric detection

Maltooligosaccharides were submitted to hydrophilic interaction chromatography on three aqueous size-exclusion columns. When mobile phase compositions were 0% to 40% (v/v) acetonitrile in water, the chromatographic mechanism was by size exclusion on all three columns; at concentrations ⪰ 50% (v/v) acetonitrile, the carbohydrates were fractionated by partition chromatography (0.88 > k′ > 143; where k′ is the solute capacity factor), and the order of elution was reversed. When maltooligosaccharides were eluted from the three columns using isocratic mobile phases in which the concentration of acetonitrile was varied from 50% to 75% (v/v), a negative linear relationship ( R 2 > − 0.973) existed between retention and solvent strength; retention increased as the polarity of the mobile phase decreased. When the composition of the mobile phase was 65% acetonitrile in water, a correlation ( R 2 > 0.99) was found in all three columns between the degree of polymerization and the retention of the oligosaccharide. With gradient elution, the Protein-Pak 60 column resolved N-acetylneuraminic acid, rhamnose, arabinose and a mixture of commercially available glucose polymers; the between-run precision of the retention times ( n; 16) for the chromatography varied from 0.09 to 0.64% (relative standard deviation). the chromatography was applied to the analysis of enzyme-hydrolyzed starch digests.

Effect of film thickness on the selectivity of cyanosilicone capillary columns

AbstractData presented herein show that the film thickness of a polar stationary phase, SP‐2380, affects column selectivity under isothermal analysis conditions. The expected linear relationship between film thickness and solute capacity factors (k), as predicted by the partition coefficient equation, was confirmed experimentally. However, selectivity, as determined by ECL values for the fatty acid methyl esters, does not remain constant. As the film thickness is increased from 0.10 μm to 0.20 μm, 0.30 μm, and 0.50 μm, there is an increase in effective column polarity. This increase mimics the polarity shift seen when comparing a 90% bis‐cyanopropyl/10% cyanopropyl phenyl polysiloxane capillary column to a 100% bis‐cyanopropyl polysiloxane column of the same film thickness (0.20 μm) under isothermal conditions. The increase in selectivity over the film thickness range studied is revealed by ECL values for an unsaturated C18:3 ester relative to an unsaturated C20:1 ester and saturated esters, C18:0(methyl stearate), (C20:)(methyl arachidate), and C22:0(methyl behenate). As the film thickness increased, the C18:3 ester eluted farther from the C20:0 ester and changed from eluting before the C20:1 ester to eluting after the C20:1 ester and closer to the C22:0 ester.The effective polarity increase also reduced the retention time for all of the saturated FAMEs, compared to their respective unsaturated counterparts. Possible causes of the differences in selectivity among these columns include a lessening of the surface adsorption and increased partitioning with the thicker stationary phase film.

Solute focusing in flow-injection systems: effect of solute capacity factor and position of solute-focusing column

Solute focusing in flow-injection analysis has traditionally been performed for ionic solutes and with a change in carrier stream used to cause elution of the solute. Further investigations of the use of reversed-phase chromatographic materials as a solute-focusing sorbent for non-ionic solutes are presented. By injecting the solute in a chromatographically weaker solvent than the carrier stream, real-time focusing occurs, and no change in carrier stream or second injection is required. The effect of the chromatographic capacity factor and the location of the focusing device in the flow manifold are both important considerations for attaining the maximum enhancement in sensitivity and concentration limits of detection.

Impact of acidic/hydrothermal treatment on pore structural and chromatographic properties of porous silicas : I. The conventional approach

A series of commercial silicas and a laboratory made product were subjected to acidic/hydrothermal treatment with solutions of HCI, HNO 3, H 2SO 4 and HF. The concentration of acid and the temperature and duration of treatment were varied. The specific surface area, content of metal impurities, types and concentration of surface hydroxyl and siloxane groups and crystallinity were determined for native and treated silicas. Only changes in the structural order of the surface could be assessed by means of 19Si cross-polarization magic angle spinning NMR spectrometry and electron diffraction. The native and acid-treated silicas were surface modified to n-octyl derivatives and tested under reversed-phase conditions. A notable improvement in the column plate number and the asymmetry of the peaks was observed, whereas the solute capacity factors remained nearly unchanged. The effect of the conditions of acidic treatment (concentration, temperature and duration) was established by factorial experiments. Acidic/hydrothermal treatment caused a more ordered surface structure with quasi-crystalline domains, which improved the chromatographic mass-transfer kinetics and hence the column performance of acid-treated silicas.

Chiral separation by cyclodextrin-modified micellar electrokinetic chromatography

Chiral separation by micellar electrokinetic chromatography (MEKC), which permits the separation of uncharged or electrically neutral compounds by the electrophoretic technique, was achieved through the addition of cyclodextrins (CDs) to sodium dodecyl sulphate (SDS) micelle solution. In this cyclodextrin-modified MEKC (CD-MEKC), CDs cannot be solubilized to the SDS micelle and migrate with the same velocity as that of the electroosmotic flow. The solutes are distributed among three phases, an aqueous phase, the micelle and the CD. Chiral recognition depended on the type of CD; in particular, γ-CD was effective for the chiral separation in this method. The addition of an organic solvent or a chiral compound such as sodium d-camphor-10-sulphonate or l-menthoxyacetic acid to the SDS micelle solution containing CDs improved the enantioselectivity. The addition of CDs reduced the capacity factors of solutes; in contrast, chiral additives increased them. The resolution was optimized by changing the concentrations of CDs and chiral additives. The chiral separation mechanism is also briefly discussed.

Theoretical and experimental comparison of serially linked and mixed-packing gas-liquid chromatography columns

Theory developed in this laboratory allows accurate prediction of solute capacity factors for serial operation of columns A and B from basic data for A and B alone. Window analysis then identifies the relative and total column lengths required for baseline resolution of all mixture components. The relative lengths required depend on column sequence, A (front)/B (back) or B (front)/A (back), and normally differ significantly although the resulting chromatograms are identical. Provided the theoretical plate height ( H) carrier velocity ( ū) relationships for A and B are not too dissimilar, the total column length required is the same in either mode. It is shown theoretically that an optimised mixed packing (A + B) provides exactly the same chromatogram, though the necessary packing ratio does not correspond to either optimised serial column length ratio. Thus, depending on the packing (length) ratio used, an experimental comparison may indicate that the mixed packing approach is superior, or vice versa. Where the H/ū relationships of A and B differ greatly, total column lengths may vary from one mode to the other to produce the same optimised separation. Generally, the effect is more marked for the serial systems. However, even in this situation, total analysis time will, in certain circumstances, be less for the serial system. The experimental results presented confirm the theoretical arguments.

Supercritical fluid solid chromatography using novel porous glassy carbon. Evaluation of retention mechanisms

AbstractThe use of a porous glassy carbon (PGC) material as a packed‐column SFC stationary phase has been previously demonstrated [1]. The material is further characterized in terms of its retention characteristics. The effects of variations in mobile phase composition, pressure, and temperature conditions are evaluated. Variation of temperature and pressure yielded expected results, specifically, decreased solute capacity factors with increased mobile phase density. The choice of supercritical fluid mobile phase allows the most notable control of solute retention; this was evaluated by adding low percentages of organic modifiers of varying molecular weights to the supercritical carbon dioxide mobile phase. PGC‐SFC provides reversed phase characteristics similar to those found for PGC‐HPLC. Porous glassy carbon has selectivity characteristics previously unavailable in supercritical fluid chromatography. Use of porous glassy carbon in supercritical fluid chromatography may provide distinct advantages in difficult analytical separations, allowing separations of molecules with only slight structural differences.

Computer optimization in ion chromatography : II. A systematic evaluation of linear retention models for anions

Extensive retention for non-suppressed ion chromatography of anions have been acquired for 17 analytes (halides, oxohalides, nitrite, nitrate, sulfite, sulfate, bisulfite, thiosulfate, phosphate, thiocyanate, carbonate, acetate and oxalate) on three stationary phases (Waters IC Pak A, Hamilton PRP-X100 and Vydac 302.IC 4.6) using 7 eluent types (benzoate, phthalate, hydroxide, carbonate/bicarbonate, gluconate/borate, p-toluenesulfonate and phosphate). These retention data are used to assess the validity of retention models which predict a linear relationship between the logarithm of solute capacity factor and the logarithm of the activity of the eluent competing anion. The linearity of these plots is uniformly good, but the slopes differ markedly from those predicted from theory. When the eluent contains two competing anions neither the dominant equilibrium approach nor the effective charge approach give reliable prediction of the slopes. Optimization of one eluent parameter at a time ( e.g. the concentration of the competing anion in the eluent) can be successful if the slope of the retention plot is determined by measurement of analyte retention times at two eluent compositions falling at the extremes of the range of eluent compositions under consideration. An example of this “end points” method is provided, in which the concentration of a phthalate eluent is optimized.

Retention prediction based on the electrostatic model of reversed-phase ion-pair high-performance liquid chromatography: effect of pairing ion concentration

Based on the electrostatic model of reversed-phase ion-pair chromatography, a practical equation was developed to describe the relationship between the capacity factor of ionic solutes and the eluent concentration of the amphiphilic pairing ion. At constant ionic strength and organic modifier concentration, the logarithm of solute capacity factor can be approximated as a linear function of the logarithm of the pairing ion concentration. The slope of this relationship is determined by the sign and number of charges of the solute ion and the pairing ion. The limitations and the practical use of the proposed model are discussed in detail.

Plate height theory for compressible mobile phase fluids and its application to gas, liquid and supercritical fluid chromatography

General expressions for apparent plate height are derived in terms of temporal and spatial average values of local plate height, solute capacity factor and mobile phase density. The general expressions are applied to the appropriate expressions for gas chromatography, liquid chromatography and supercritical fluid chromatography with open tubular and packed columns. For gas chromatography, the equations reduce to the equations presented earlier by Giddings. For liquid chromatography, the equations reduce to those for local plate height. Predicted results for supercritical fluid chromatography are compared to experimental results reported in the literature.

Supercritical fluid-solid chromatography using a carbonaceous stationary phase

Results from a preliminary evaluation of a porous glassy carbon (PGC) material for packed-column supercritical fluid chromatography (SFC) are reported. The material is characterized in terms of its retention characteristics. Increasing temperature at constant pressure causes initial increases in solute capacity factors with subsequent slow decreases when sufficient temperatures ({approx}145{degree}C) are reached. Variation of temperature at constant carbon dioxide mobile phase densities yields enthalpies of interaction for low molecular weight organics (C{sub 8}-C{sub 12}) from {minus}0.5 to {minus}1.5 kcal/mol. PGC is highly retentive; use of a supercritical fluid mobile phase that competes with solutes for adsorption onto the PGC allows significant control of solute retention. PGC offers potential advantages not provided by other materials. PGC-SFC exhibits reverse-phase characteristics similar to those found for PGC-HPLC, retention behavior previously unavailable in SFC. PGC is highly stereospecific and will be useful in SFC applications requiring separations of isomers and molecules with only slight structural differences.

Possibilities of determination and prediction of solute capacity factors in reversed-phase systems with pure water as the mobile phase

A sorption and an elution experimental method were developed and tested for the determination of solute capacity factors in pure water as the mobile phase in reversed-phase systems. The experimental values determined by the sorption method are generally higher than those determined by the elution method, but the two types of values are strongly correlated. These correlations may be used for corrections of the experimental capacity factors to suit either chromatographic or sorption conditions. The log k′ dependencies on methanol concentration are adequately described by quadratic equations for various pesticides tested over a range of mobile phase compositions from 2 to 90% methanol in water, but the extrapolation to zero methanol concentration yields systematically lower extrapolated capacity factors, k′ H 2O . The extrapolated values depend strongly on the range of the experimental k′ and on the range of methanol concentrations used for the determination of the experimental data points, and neither linear nor quadratic extrapolation yield correct k′ H 2O values. The accuracy of predicted extrapolated k′ H 2O improves as the concentration range of methanol approaches zero, but the time of determination and experimental difficulties increase simultaneously. The accuracy of the predicted k' H 2O values from the low methanol concentration range can be improved using correlations with the k′ H 2O values determined experimentally. The correlation equations are determined for a set of (at least three) standard compounds and apply only for a given sorbent. Correlations of log k′ H 2O with solute lipophilic indices, n ce, yield better predictions than similar correlations with logarithms of solute solubilities, with comparable errors in the predicted k′ H 2O values as for the values extrapolated from the data measured in the range of medium methanol concentrations.

Turbulent flow in capillary gas chromatography

The possibilities of turbulent flow capillary gas chromatography for increasing the speed of analysis were examined by use of previously developed sample introduction, detection and registration systems, which are compatible with peak widths in the millisecond range. Existing theoretical models for axial turbulent dispersion in capillary columns were evaluated experimentally. Substantially decreases in reduced plate heights were obtained (h < 1) for unretained components at an average linear velocity of 15 m/s for column diameters of 320 μm. Unfortunately, the plate height increased greatly with increasing solute capacity factor (by a factor 15 from k=0 to k=1). Comparison with theoretical models shows that this effect is mainly due to mobile phase mass transfer. Therefore, the gain in analysis speed is limited to low capacity factors. In addition the pressure drop required is considerably higher than for a comparable improvement in speed obtained by decreasing the column inside diameter.

Comparison of the gas-chromatographic properties of mesomorphic polymeric stationary phases

The gas-chromatographic properties of two mesomorphic polysiloxane (MEPSIL) stationary phases, one smectic (SB) and the other nematic (N), were investigated. The retention indices and separation factors (relative retentions), α, of several pairs of polycyclic aromatic hydrocarbon (PAH) and fatty acid methyl ester (FAME) isomer solutes were used to gauge differences in the selectivity provided by each. The heats of solution of the PAH showed that the smectic MEPSIL is more selective for solutes with different length-to-breadth ratios. Also, five peaks were resolved for sixcis andtrans octadecenoic FAME isomers with this solvent. The nematic MEPSIL, on the other hand, was found to be more useful at higher temperatures because of its higher clearing point. In addition, solute capacity factors were smaller than those observed with the smectic MEPSIL, as evidenced by lower retention indices. The former was therefore more useful for the analysis of high molecular-weight PAH.