The chemical interpretation and practice of linear solvation energy relationships in chromatography

This review focuses on the use of linear solvation energy relationships (LSERs) to understand the types and relative strength of the chemical interactions that control retention and selectivity in the various modes of chromatography ranging from gas chromatography to reversed phase and micellar electrokinetic capillary chromatography. The most recent, widely accepted symbolic representation of the LSER model, as proposed by Abraham, is given by the equation: SP=c + eE + sS + aA + bB + vV, in which, SP can be any free energy related property. In chromatography, SP is most often taken as logk' where k' is the retention factor. The letters E, S, A, B, and V denote solute dependent input parameters that come from scales related to a solute's polarizability, dipolarity (with some contribution from polarizability), hydrogen bond donating ability, hydrogen bond accepting ability, and molecular size, respectively. The e-, s-, a-, b-, and v-coefficients and the constant, c, are determined via multiparameter linear least squares regression analysis of a data set comprised of solutes with known E, S, A, B, and V values and which span a reasonably wide range in interaction abilities. Thus, LSERs are designed to probe the type and relative importance of the interactions that govern solute retention. In this review, we include a synopsis of the various solvent and solute scales in common use in chromatography. More importantly, we emphasize the development and physico-chemical basis of - and thus meaning of - the solute parameters. After establishing the meaning of the parameters, we discuss their use in LSERs as applied to understanding the intermolecular interactions governing various gas-liquid and liquid-liquid phase equilibria. The gas-liquid partition process is modeled as the sum of an endoergic cavity formation/solvent reorganization process and exoergic solute-solvent attractive forces, whereas the partitioning of a solute between two solvents is thermodynamically equivalent to the difference in two gas/liquid solution processes. We end with a set of recommendations and advisories for conducting LSER studies, stressing the proper chemical and statistical application of the methodology. We intend that these recommendations serve as a guide for future studies involving the execution, statistical evaluation, and chemical interpretation of LSERs.     

Characterization of sorption mechanisms of solid-phase microextraction with volatile organic compounds in air samples using a linear solvation energy relationship approach

The linear solvation energy relationship (LSER) model was used to characterize interactions responsible for sorption of volatile organic compounds (VOCs) in air samples on six different solid-phase microextraction (SPME) fibers at 296K and zero relative humidity. The polydimethylsiloxane and polyacrylate fibers sorption data were also modeled at different relative humidities in the range of 10-90% and influence of water vapors on the extraction process is discussed. The LSER equations were obtained by a multiple regression of the distribution coefficients of 14 probe solutes on an appropriate SPME fiber against the solvation parameters of the solutes. The derived LSER equations successfully predicted the VOC distribution coefficients and the selectivity of individual SPME fibers for the various volatile solutes. The LSER approach coupled with SPME is a relatively simple and reliable tool to rapidly characterize the sorption mechanism of VOCs with various stationary phases and may potentially be applied to design and test new chromatographic materials for sampling or separation of VOCs.     

Polymeric sulfated surfactants with varied hydrocarbon tail: II. Chemical selectivity in micellar electrokinetic chromatography using linear solvation energy relationships study

The effect of hydrocarbon chain length on chemical selectivity in micellar electrokinetic chromatography (MEKC) was investigated using polymeric sulfated surfactants: poly-(sodium 7-octenyl sulfate), poly(sodium 8-nonenyl sulfate), poly(sodium 9-decenyl sulfate), and poly(sodium 10-undecenyl sulfate). Linear solvation energy relationships (LSERs) and free energy of transfer studies were conducted to predict the selectivity differences between the four polymeric surfactants. The overall nature of the solute/ polymeric micelle interactions was found to be different despite the fact that all polymeric surfactants have the same head group. The polar character and acidic strength of the polymeric surfactant are found to decrease as the hydrocarbon chain length of the surfactant is increased. On the other hand, the polarizability of the polymeric sulfated surfactants increases (upon interacting with solute lone-pair electrons) with increasing hydrocarbon chain length. The LSER results show that the solute size and hydrogen bond accepting ability play the key roles in MEKC retention.     

用线性溶剂化能相关(LSERs)方法评价聚合物包覆钛胶固定相并与相关固定相比较

利用线性溶剂化能相关(LSERs)方法对聚(乙基苯乙烯-二乙烯基苯)包覆钛胶固定相(ES-DVB-TiO₂)的保留行为进行评价,并与聚丁二烯涂覆钛胶固定相(PBD-TiO₂),键合硅胶固定相(ODS)和树脂固定相(PR-1)作了比较,计算出各变最对logk'的百分方差数,发现V₂、Σα₂^H,和Σβ₂^H对logk'有较大的贡献,与聚合物固定相PRP-1近似,因此它们有相似的保留机理,即吸附机理大于分配机理.     

Rapid estimation of octanol-water partition coefficients using synthesized vesicles in electrokinetic chromatography

Vesicle electrokinetic chromatography (VEKC) using vesicles synthesized from the oppositely charged surfactants cetyltrimethylammonium bromide (CTAB) and sodium octyl sulfate (SOS) and from the double-chained anionic surfactant bis(2-ethylhexyl)sodium sulfosuccinate (AOT) was applied to the indirect measurement of octanol-water partition coefficients (log Po/w). A variety of small organic molecules with varying functional groups, pesticides, and organic acids were evaluated by correlating log Po/w and the logarithm of the retention factor (log k') and comparing the calibrations. A linear solvation energy relationship (LSER) analysis was conducted to describe the retention behavior of the vesicle systems and compared to that of octanol-water partitioning. The solute hydrogen bond donating behavior is slightly different with the vesicle interactions using CTAB-SOS vesicles as compared to the octanol-water partitioning model. The AOT vesicle and octanol-water partitioning systems showed similar partitioning characteristics. VEKC provides rapid separations for determinations of log Po/w in the range of 0.5 to 5 using CTAB-SOS vesicles and 0 to 5.5 using AOT vesicles.     

Study of the Effect of Mobile Phase Additives on Retention in Reversed Phase HPLC Using Linear Solvation Energy Relationships

Linear solvation energy relationships (LSERs) were used to delineate which specific intermolecular interactions are responsible for changes in retention for a variety of well characterized analytes when acidic and basic additives were used in reversed phase HPLC. The effects of trifluoroacetic acid, triethylamine and a combination of trifluoroacetic acid and triethylamine on the LSERs were compared to those observed in the absence of additives. These effects were examined using four different mobile phase modifiers and five different stationary phases. Trifluoroacetic acid alone and in combination with triethylamine produced LSER regression coefficients nearly identical to those obtained with no additive present in the mobile phase. Triethylamine alone produced different LSER regression coefficients from the other systems unless the mobile phase contained trifluoroethanol as the mobile phase modifier, or the stationary phase consisted of a polymeric support.     

Statistical evaluation of linear solvation energy relationship models used to characterize chemical selectivity in micellar electrokinetic chromatography

Characterization of retention and selectivity differences between surfactants in micellar electrokinetic chromatography (MEKC) using linear solvation energy relationships (LSERs) has been given a significant amount of attention in the last four years. This report evaluates the validity of using the two LSER models that have been used to fit retention in MEKC in the literature. The results and the fit of the revised model and parameters developed by Abraham and coworkers are compared to the original model developed by Kamlet, Taft, and coworkers. LSERs can generally only be used as a comparative tool to describe the selectivity differences between surfactant systems used in MEKC. With this in mind, it was determined that the results of both models essentially provide the same information about these differences. However, the revised model and parameters have been found to yield a statistically better fit of the MEKC retention data as well as providing more chemically sound LSER coefficients.     

Influence of ester and amide-containing surfactant headgroups on selectivity in micellar electrokinetic chromatography

The selectivity differences between six anionic surfactants in micellar electrokinetic chromatography (MEKC) are presented and the structural influence of the surfactant head-group is investigated. It was determined that the surfactant structure can have a significant impact on retention and selectivity. Linear solvation energy relationships (LSERs) are used to study the role of solute size, polarity/polarizability, and hydrogen bonding characteristics in determining retention and selectivity. While both the solute size and hydrogen bond accepting ability were found to be the most important factors in solute retention, the hydrogen bonding characteristic of the solutes have the largest influence on selectivity differences between surfactants.     

Self-doped polyaniline as new polyaniline substitute for solid-phase microextraction

This work is a first study on extraction efficiency and thermal stability of nano-structured self-doped polyaniline (SPAN) as a coating of solid-phase microextraction (SPME) fibers. SPAN-based fibers were prepared using electrochemical deposition on platinum wires. The particle sizes of prepared nano-structure were in the range of 50–100 nm. Extraction properties of the fiber to 1,4-dioxane were examined using headspace solid-phase microextraction (HS-SPME) coupled to gas chromatography-flame ionization detection (GC-FID). The results have proved higher thermal stability of the proposed fiber compared to common PANI fiber. The SPAN coating was proved to be very stable at relatively high temperatures (up to 350 °C) with high extraction capacity and long lifespan (more than 50 times). Therefore, it can be a good substitute of polyaniline (PANI) as a SPME coating. The extraction procedure was optimized by selecting the appropriate extraction parameters including extraction time, extraction temperature, salt concentration, stirring rate and headspace volume. Calibration graph was linear in the concentration range of 1–100 ng mL⁻¹ (R² > 0.993) with detection limit of 0.1 ng mL⁻¹. Single fiber and fiber-to-fiber repeatability were lower than 6.0% and 10.4%, respectively. Different water samples were analyzed as real samples and good recoveries (98–120%) were obtained.     

Binary mixed micelles of chiral sodium undecenyl leucinate and achiral sodium undecenyl sulfate: I. Characterization and application as pseudostationary phases in micellar electrokinetic chromatography

Sodium 10-undecenyl sulfate (SUS), sodium 10-undecenyl leucinate (SUL) and their five different mixed micelles at varied percent mole ratios were prepared. The critical micelle concentration (CMC), C₂₀, γ_CMC, partial specific volume, methylene group selectivity, mobilities and elution window were determined using a variety of analytical techniques. These surfactant systems were then evaluated as novel pseudostationary phases in micellar electrokinetic chromatography (MEKC). As a commonly used pseudostationary phase in MEKC, sodium dodecyl sulfate (SDS) was also evaluated. The CMC values of SUS and SUL were found to be 26 and 16 mM, respectively, whereas the CMC of mixed surfactants was found to be very similar to that of SUL. The C₂₀ values decreased dramatically as the concentration of SUL is increased in the mixed micelle. An increase in SUL content gradually increased the methylene group selectivity making the binary mixed surfactants more hydrophobic. Linear solvation energy relationships (LSERs) and free energy of transfer studies were also applied to predict the selectivity differences between the surfactant systems. The cohesiveness and the hydrogen bond acidic character of the surfactant systems were found to have the most significant influence on selectivity and MEKC retention. The SUS and SDS showed the strongest while SUL showed the weakest hydrogen bond donating capacity. The basicity, interaction with n and π-electrons of the solute and dipolarity/polarizability were the least significant factors in LSER model for the surfactant systems studied. Free energies of transfer of selected functional groups in each surfactant systems were also calculated and found to be in good agreement with the LSER data.     

Effects of organic modifiers on retention mechanism and selectivity in micellar electrokinetic capillary chromatography studied by linear solvation energy relationships

The effects of six organic modifiers (urea, methanol, dioxane, tetrahydrofuran, acetonitrile and 2-propanol) on the retention mechanism and separation selectivity of the bulk buffer in micellar electrokinetic capillary chromatography (MECC) with sodium dodecyl sulfate (SDS) micelles as pseudo-stationary phase have been investigated through linear solvation energy relationships (LSERs). It is found that the retention value in MECC systems with or without organic modifier is primarily dependent on the solvophobic interaction and the hydrogen bonding interaction with the solute as proton acceptor, while the dipolar interaction and the hydrogen bonding interaction with the solute as proton donor play minor roles. The effects of the organic modifiers on the solvophobic, dipolar and hydrogen bonding interactions are evaluated in terms of the relationship between regression coefficient of the LSER equations and the modifier concentration. The variations of the solvophobic interaction and the dipolar interaction with change of the modifier concentration can be approximately explained using the solubility parameter and the dipolarity/polarizability parameter of the organic modifier, respectively. However, the relationships between the hydrogen bond acidity and basicity of the bulk buffer and the organic modifiers are rather complicated. Those results may be caused from the displacement of organic modifiers to the water adsorbed on the micellar surface as well as changes in the acidity and basicity of the bulk buffer with the addition of organic modifiers. In addition, it is found that the phase ratio is influenced significantly by the use of organic modifier.     

Quantitative structure-retention relationship studies using immobilized artificial membrane chromatography I: amended linear solvation energy relationships with the introduction of a molecular electronic factor

Linear solvation energy relationships (LSERs) amended by the introduction of a molecular electronic factor were employed to establish quantitative structure-retention relationships using immobilized artificial membrane (IAM) chromatography, in particular ionizable solutes. The chromatographic indices, log k(IAM), were determined by HPLC on an IAM.PC.DD2 column for 53 structurally diverse compounds, including neutral, acidic and basic compounds. Unlike neutral compounds, the IAM chromatographic retention of ionizable compounds was affected by their molecular charge state. When the mean net charge per molecule (delta) was introduced into the amended LSER as the sixth variable, the LSER regression coefficient was significantly improved for the test set including ionizable solutes. The delta coefficients of acidic and basic compounds were quite different indicating that the molecular electronic factor had a markedly different impact on the retention of acidic and basic compounds on IAM column. Ionization of acidic compounds containing a carboxylic group tended to impair their retention on IAM, while the ionization of basic compounds did not have such a marked effect. In addition, the extra-interaction with the polar head of phospholipids might cause a certain change in the retention of basic compounds. A comparison of calculated and experimental retention indices suggested that the semi-empirical LSER amended by the addition of a molecular electronic factor was able to reproduce adequately the experimental retention factors of the structurally diverse solutes investigated.     

Prediction of internal standards in reversed-phase liquid chromatography. 1. Initial study on predicting internal standards for use with neutral samples based on linear solvation energy relationships

This paper describes the results of an initial study on the application of linear solvation energy relationships (LSERs) to the prediction of internal standard compounds in reversed-phase liquid chromatographic (RPLC) method development. Six neutral samples are separated on an Inertsil ODS(3) column by either acetonitrile-water or methanol-water mobile phases under either isocratic or linear gradient conditions. After the separation conditions are optimized, the desired positions for internal standard candidates are selected based on the "open windows" of the chromatograms. The compounds with the desired retention range are then predicted based on LSERs from a database consisting of more than 700 compounds with defined physicochemical properties. The prediction requires the use of LSER coefficients under the separation conditions for each sample. They are determined a priori by performing multivariable linear regression on the retention of 20 reference solutes against their physicochemical properties. It can be concluded from the study that LSER is an excellent approach to the selection of internal standard compounds for RPLC under either isocratic or gradient elution. The average prediction error is usually within 10%, but no more than 20%. Finally, LSER approach is fast and systematic, and will save a significant amount of time and resources during RPLC method development.     

Solid-phase microextraction for the evaluation of partition coefficients of a chlorinated dioxin and hexachlorobenzene into humic substances.

We developed a method for the evaluation of the partition coefficients (K(oc)) of hexachlorobenzene (HCB) and 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (HpCDD) into humic substances (HSs) by using solid-phase microextraction (SPME). In the aqueous solution containing HCB or HpCDD and HS, the unbound species of HCB or HpCDD were accumulated on the SPME fiber. Subsequently, HCB or HpCDD on the SPME fiber was directly analyzed by GC-ECD. When the concentration of organic carbon in HS ([OC]) was plotted against the ratio of [HCB] or [HpCDD] in the absence of HS to that in the presence of HS, linear relationships were observed. The slope of the line corresponded to the K(oc) value. The log K(oc) values for HCB and HpCDD evaluated were in the ranges of 3.9 - 4.9 and 5.9 - 7.2, respectively. These values were the same order as those in the literature, which were evaluated by other methods (e.g., solubility enhancement, solid-phase extraction and dialysis). The relative standard deviations of the log K(oc) values evaluated in this study were within 5%.