Stationary Phases Of Different Polarity Research Articles

Use of reversed-phase liquid chromatography for determining the lipophilicity of α-aryl- N-cyclopropylnitrones

The relationship between a reversed-phase high-performance liquid chromatography (RP-HPLC) retention parameter and various calculated log P-values of our previously synthesized α-aryl- N-cyclopropyl-nitrone derivatives was investigated. The RP-HPLC experiments were carried out with acetonitrile–water and methanol–water mixtures as mobile phases and with two kinds of stationary phases of different polarity. The retention parameter, log k w was obtained by linear extrapolation of the log k retention to pure water as the mobile phase. The calculated log P-values were C log P, ACD/log P, R log P, A log P, LogKow, X log P and M log P. Statistically, highly significant correlations were found between log k w and the calculated log P-values with squared correlation coefficients ranging from 0.771 (with A log P) to 0.956 (with C log P). In addition, the comparative molecular similarity indices analysis (CoMSIA) method was also applied to correlate the log k w retention parameter of the compounds with their molecular fields. Statistically significant CoMSIA models were obtained between log k w and the hydrophobic and steric molecular fields of our compounds. The CoMSIA models describe how the structure of the nitrone derivatives influences (through hydrophobic and steric interactions with the stationary phase) the chromatographic retention of the compounds.

Kovats retention index-boiling point relationship of 2-phenyl-2-alkyl-acetonitriles on stationary phases of different polarity

Linear and reciprocial Kovats retention index-boiling point relationships known from the literature were used to study the Kovats retention index-boiling point dependence of 2-phenyl-2-alkylacetonitriles on stationary phases of different polarity (OV-17, OV-210 and OV-225). The standard chemical potential of the partitioning of one methylene group of an n-alkane for the stationary phase was calculated and compared with available literature data.

Application of the semi-empirical topological index in quantitative structure-chromatographic retention relationship (QSRR) studies of aliphatic ketones and aldehydes on stationary phases of different polarity

The semi-empirical topological index, previously developed by Heinzen and Yunes, has been applied to predict the chromatographic retention of aliphatic ketones and aldehydes on stationary phases of different polarities (HP-1, HP-50, DB-210 and Innowax). Simple linear regressions between the retention indices and the semi-empirical topological indices (RI = a + bI ET) were established for each stationary phase separately, showing satisfactory statistical parameters. The polarity of the stationary phases, indicated by McReynolds polarity (P R), is reflected in the coefficients of the equations (a and b) obtained for HP-1, HP-50 and DB-210, which linearly vary as the polarity of the stationary phase increases (intercept). Statistical analyses showed that the quantitative structure-chromatographic retention relationship (QSRR) models obtained on stationary phases of low-to-medium polarity (HP-1 and HP-50) have higher stability and predictive ability than those on polar stationary phases (DB-210 and Innowax). Thus, it can be concluded that the semi-empirical topological method, using only one descriptor, yielded better results on low-to-medium polarity stationary phases than methods that used multiple descriptors.

Relationships Between Retention Data of Benzene and Chlorobenzenes with their Physico-chemical Properties and Topological Indices

Relationships between the retention data of benzene and chlorobenzenes, obtained in a number of stationary phases of different polarity, and three topological indices (first-order connectivity index, Wiener's index and Balaban index) as well as several physico-chemical properties (freezing point, boiling point, refraction index, dipole moment, density, molecular mass and vapor pressure) of the solutes were studied. The effect of column temperature on the retention data was studied, and the dependence of the retention data on the connectivity indices was investigated. Retention indices were fit with a multiple linear regression analysis for one to six physico-chemical properties giving r 2=0.9952 and 0.9996, and F=1971 and 2471, respectively. The ability of some of these equations to predict the retention index was checked. Principal component analysis supports the obtained results. Excess thermodynamic magnitudes were also evaluated for n-alkanes, benzene and chlorobenzenes, investigating the influence of column temperature and of the solute's structure on the thermodynamic values.

The prediction for gas chromatographic retention index of disulfides on stationary phases of different polarity

Quantitative structure–retention relationship (QSRR) models for the gas chromatographic (GC) Kaváts indices of disulfides on four different polarity stationary phase have been developed. Semi-empirical quantum chemical method (AM1) implemented in hyperchem 4.0 was employed to calculate a set of molecular descriptors of 50 disulfides. The four stationary phases in the research were: Apiezon M, OV-17 , Triton X-305 and PEG-1000 . By using multiple linear regression (MLR), we obtained four empirical functions with high correlation coefficient ( R 1=0.995, R 2=0.994, R 3=0.990, R 4=0.976). At the same time, using Thin Plat Spline the Radial Basis Function neural networks models were obtained with root mean squared error (RMS) of training set (RMS T1=0.013351, RMS T2=0.012973, RMS T3=0.023228, RMS T4=0.020755) and RMS of validation set (RMS V1=0.007626, RMS V2=0.005897, RMS V3=0.005109, RMS V4=0.007377) and RMS of testing set (RMS X1=0.016676, RMS X2=0.016704, RMS X3=0.017162, RMS X4=0.014755). The results indicated that the QSRR models proposed were very satisfactory.

Prediction of the chromatographic retention of saturated alcohols on stationary phases of different polarity applying the novel semi-empirical topological index

The novel semi-empirical topological index ( I ET), previously developed by Heinzen and Yunes, was extended to predict the chromatographic retention of saturated alcohols on low polarity stationary phases (OV-1). The predictive ability of I ET was also verified on stationary phases of different polarity (SE-30, OV-3, OV-7, OV-11, OV-17 and OV-25). Simple linear regressions between the retention indices and the semi-empirical topological index (RI= a+ bI ET) were established for each stationary phase separately, showing good statistical parameters. Statistical analysis showed that the QSRR model used on stationary phases of low polarity (OV-1) has high internal stability and good predictive ability for external groups. The polarity of the SE-30, OV-3, OV-7, OV-11, OV-17 and OV-25 stationary phases, indicated by retention polarity ( P R) given by Tarján et al., is reflected in the ‘ a’ (intercept) and ‘ b’ (slope) coefficients of the equations obtained for each of these phases. The linear relationship between the ‘ a’ coefficient and P R showed satisfactory statistical quality. Thus, it was possible to generate a single combined model of QSRR, including a parameter that represents the property of the stationary phase P R. The combined model also has a satisfactory predictive quality, as shown by the plot of calculated versus experimental retention indices for saturated alcohols on six stationary phases of different polarity ( r 2=0.9956; S.D.=9.54).

The prediction for gas chromatographic retention indices of saturated esters on stationary phases of different polarity

A quantitative structure-retention relationship (QSRR) model has been developed for the gas chromatographic Kováts indices of 98 saturated esters on seven different polar stationary phases by multiple linear regression analysis (MLR). The seven stationary phases are: SE-30, OV-7, DC-710, OV-25, 100% phenyl, DC-230 and DC-530. Chemical descriptors were calculated from the molecular structures by PM3 of Hyperchem 4.0. Principal component analysis (PCA) was applied to extract the data structure. Multiple linear regression was made in forward stepwise manner to select suitable variables in the model. The proposed model had a high multiple square correlation coefficient R 2 and low standard error S.E. The result proved the strong predictive power of the model.

GC–MS analysis of essential oil of some commercial Fennel teas

Abstract Fennel teas were prepared by classical infusion or microwave decoction of unbroken and crushed fruits, three pre-packaged teabags and two instant teas. Their volatile constituents were obtained by extraction with n-hexane and analysed by gas chromatography (GC) and gas chromatography/mass spectrometry (GC–MS), using two columns with stationary phases of different polarity. Of the constituents 85–95% were identified on the basis of their GC retention times and their mass spectra in relation to authentic compounds. No volatile constituents were detected in one sample of instant tea. Conventional teas from crushed fruits and teas prepared from the other instant tea showed the highest levels of volatile constituents. Anethole (30–90%) and/or anisaldehyde (0.7–51%) were the main constituents of all the samples. Methychavicol (0.8–4.1%), eugenol (1.5–11.3%) and fenchone (0.5–47%) were detected in most samples. Carvone (2.1–6.1%) was presenting only some teabags and camphor (2.3–2.6%) in others. The volatile constituents of only one instant tea included limonene (1.4%) and α-terpineol (0.4%).

Correlation between retention indices and quantum-chemical descriptors of ketones and aldehydes on stationary phases of different polarity

Kováts retention indices determined on four different capillary columns (OV-1, HP-50, DB-210 and HP-Innowax) were correlated with molecular structural parameters calculated by a semiempirical quantum-chemical method PM3. Multivariate techniques: principal component analysis and cluster analysis were applied to extract the data structure. Multiple linear regression was made in forward stepwise manner to select suitable variables in the model. Basic correlations were found between the retention indices on different columns and the molecular surface, energies of highest occupied and lowest unoccupied molecular orbitals { E(HOMO) and E(LUMO)}, polarizability, and dipole moments. These correlations provide insights into the mechanism of chromatographic retention on a molecular level. They support the view: the more polar is the stationary phase, the greater is the effect of the polarity (polarizability and dipole moment) of solute molecules on the retention phenomena.

Relationship between GLC retention data and topological indices for a wide variety of solutes on five stationary phases of different polarity

Numerical correlations between the specific retention volumes of several dozen solutes (hydrocarbons and derivatives containing oxygen, nitrogen and halogens) and between the retention indices, from the literature, of 26 chemical derivatives of benzene on several stationary phases have been studied.

High-performance liquid chromatography of chalcones: Quantitative structure-retention relationships using partial least-squares (PLS) modeling

In this study, the multivariate partial least squares projections to latent structures (PLS) technique was used for modeling the RP-HPLC retention data of 17 chalcones, which were determined with methanol-water mobile phases of different compositions. The PLS model was based on molecular descriptors which can be calculated for any compound utilizing only the knowledge of its molecular structure. The PLS analysis resulted in a model with the following statistics: r=0.976, Q=0.933, s=0.076, and F=43.63. The adequacy of the developed model was assessed by means of crossvalidation and also, by PLS modeling of the retention data of several chalcones reported by Walczak et al. [J. Chromatogr. 353, 123, (1986)], which were obtained using stationary phases of different polarity (-NH2, DIOL,-CN, ODS, C8). The structural interpretation of the developed PLS model was accomplished by means of comparative correlations between the nonempirical descriptors used in the model and the solvation parameters developed by Abraham. The results obtained in this work provides evidence for the great potential of the topological approach for the development of quantitative structure-retention relationship (QSRR) models.

Partial least squares modeling of retention data of oxo compounds in gas chromatography

Partial least squares modeling of latent structures were carried out on a data matrix consisting of Kovats retention indices of 35 aliphatic ketones and aldehydes and their physical characteristics. The retention indices were determined on capillary columns with 4 different stationary phases, namely bonded methyl-{HP-1}, methyl-phenyl- {HP-50} and trifluoropropyl-methyl siloxane {DB-210}, as well as polyethylene glycol {HP-Innovax} at four different temperatures. It was found that ketones and aldehydes cannot be classified on the basis of retention data solely, whereas the physical characteristics (boiling point, molar volume, molecular mass, molar refraction, octanol-water partition coefficient) contain the necessary information for differentiation of the two classes of compounds. The retention index of but-2-enal does not fit into the general trend of aliphatic aldyhydes and ketones. A predictive PLS model was built to estimate retention data of oxo compounds at different temperatures and various stationary phases of different polarity. Cross-validation suggests a good reliability of the results. Characteristic plots (PLS weights, scores) show the similar retention behavior of oxo compounds (Figures 3 and 4).

Determination of heats of vaporization and gibbs free energies of alkylbenzenes on GC stationary phases of different polarity

Summary In our paper we propose a new method for the determination of heats of vaporization (Anvap(i)) and the Gibbs free energies of vaporization (Ap.p(i)) for individually selected alkylbenzenes, chromatographed on stationary phases of low and medium polarity. The method is based on a new thermodynamic description of the Ko%ts retention index (I(i)) making use of Trouton's rule. In fact, we attribute physical significance to two empirical relationships, well known from the literature. We can describe thermodynamically the magnitude of I(i) using two relations, each of them potentially useful for further determination of Anvap(i) and A~p(i ). A comparison was made of the results obtained with the use of our new approach and of those already existing in the literature attained in a completely different way. The comparison suggests that the calculated AHvap(i) and Al.tp(i) values are thermodynamically acceptable and hence correct, and the data suit into the series of similar values determined on stationary phases of different polarity.

Peculiar features of sorption of positional isomers of formyl-, acetyl-, and aminopyridines in capillary gas-liquid chromatography

The GLC retention parameters of α-, β-, and γ-isomers of formyl-, acetyl-, and aminopyridines were determined on two columns with stationary phases of different polarities. The contributions of formyl, acetyl, methyl, and amino groups located in the α-, β-, and γ-positions of the ring to the retention indices were calculated. The contributions of these groups to the retention were found to depend on their positions with respect to the N atom of the ring and are the smallest for α-substituents. The difference between pyridines containing electron-withdrawing substituents and those containing electron-donating substituents is manifested as different orders of elution of α-, β-, and γ-derivatives. The distinctions between the GLC behaviors of pyridines, benzenes, and furans with the same substituents were identified.

Investigation of gas chromatographic interaction mechanism on permethylated cyclodextrins by molecular modelling

The capillary gas chromatographic retention behavior of α-pinene and tricyclene has been investigated on stationary phases of different polarities. On all but one of the columns employed, tricyclene eluted before α-pinene; only permethylated β-cyclodextrins dissolved in moderately polar polisiloxanes gave a reversed elution order. The intermolecular interactions which caused the unexpected retention behavior were investigated in detail, applying methods of computer simulation. To achieve this, we have developed a calculation algorithm on the basis of molecular mechanical optimizations and programmed it in a macro. This makes it possible to systematically investigate a given configuration space in which all the possible interactions can take place. It was shown that permethylated β-cyclodextrin as host molecule for both guest molecules offers an optimum cavity size. As a result the number of energetically favorable contacts between host and guest molecules as well as the strength of the interactions in this stationary phase were larger. As a consequence the elution order, normally only influenced by the vapor pressure of the compounds at a given temperature, was changed. Nonspecific interactions played an especially important role for these kinds of substances.

Derivation of solubility parameters of chlorinated dibenzofurans and dibenzo[ p]dioxins from gas chromatographic retention parameters via SOFA

Molar heat of vaporization, molar liquid volume and Scatchard-Hildebrand solubility parameters of chlorinated dibenzofurans (PCDFs) and dibenzo[ p]dioxins (PCDDs) were derived from (semi-)empirical values of heat of vaporization, (subcooled) liquid density and gas chromatographic Kovats retention indices. To this end a thermodynamic prediction model (SOFA), recently developed, was extended by an exchange entropy contribution representing molecular orientational disorder. The parameters derived were validated using experimental values of relative retention times on stationary phases of different polarities. Accurate to extremely accurate results were obtained with standard errors of regression close to experimental inaccuracy and high correlation coefficients comparing favourably to existing methods of prediction. The parameters derived performed quite well for apolar to moderately polar stationary phases. For polar phases inaccuracy increased significantly. The method has several advantages compared to other methods used for the prediction of chromatographic retention.

Influence of temperature on the mechanism by which compounds are retained in gas-liquid chromatography

The influence of temperature on the retention mechanism and solvation interactions of 46 varied solutes in 10 representative stationary phases of different polarity within the temperature range of 60 to 140°C is discussed. Gas-liquid partition is shown to the dominant retention mechanism for most solutes with inrterfacial adsorption of increasing importance at low phase loadings, low temperatures and for solutes of different polarity to that of the stationary phase. Guidelines are presented for predicting those conditions for which interfacial adsorption is likely to be a significant retention mechanism. A cavity model is used to characterize the solvation process in terms of the free energy contributions to solvation from the cavity-dispersion interactions and the sum of the remaining polar interactions. As a function of temperature it is shown that the contribution from polar interactions are only weakly temperature dependent over the temperature range studied while the cavity-dispersion interactions term shows a much more significant variation becoming less favorable for solute transfer at higher temperatures. In all cases, the contribution of the cavity-dispersion interactions term is favorable for solute transfer from the gas phase to the liquid phase. Principal component analysis is used to identify the factors contributing to the solvation process and their individual temperature dependence. In the case of the cavity-dispersion interactions term one factor accounts for more than 99.7% of the total variance. Three factors are identified as contributing to the polar interactions term. The first principle component accounts for more than 95% of the total variance at all temperatures and by correlation with other independent scales of dipolarity/polarizability is identified as representing the contribution from orientation and induction interactions. The two remaining principal components are shown to represent hydrogen-bond formation and charge-transfer complexation involving systems with π-electrons. The temperature dependence of the principal components provides insights into the general role of polar intermolecular interactions on the solvation process and their temperature variation.

Variation in the sorption energy of esters ofn-perfluoroalkanoic acids on chromatographic phases of different polarity

The contributions of methylene and difluoromethylene units of six homologous and six pseudohomologous series of esters ofn-perfluoroalkanoic acids to the differential molar free energy, enthalpy, and entropy of sorption on stationary phases of different polarities in glass capillary columns under isothermal conditions were calculated. The features observed characterize the peculiarities of the sorption of esters of polyfluorinated carboxylic acids on phases of different polarities and make it possible to estimate the enthalpy and entropy contributions to the energy of sorption of the esters, the effect of the temperature of the analysis and the length of the alkyl or polyfluoroalkyl chain on the retention parameters of the compound analyzed. The inversion of the retention order of trifluoroacetates and pentafluoropropionates in XE-60 nitrile-containing phase was found.

Analysis of vegetable and fish oils by capillary supercritical fluid chromatography with flame ionization detection

Capillary SFC with supercritical CO2 as mobile phase and flame ionization detection was adopted to separate and determine triacylglycerols in vegetable and fish oils. The effect on separation of many experimental variables has been checked along with the selectivity of several stationary phases of different polarity. A standard mixture of 13 triacylglycerols was used to optimize the procedure and their retention times were used as reference to identify peaks in real samples. In spite of the fact that many peaks remained unidentified, the chromatograms obtained were useful as fingerprint analyses of the real samples examined.

Cyclodextrin derivatives for the GC separation of racemic mixtures of volatile compounds. Part VI: The influence of the diluting phase on the enantioselectivity of 2,6‐Di‐O‐methyl‐3‐O‐pentyl‐β‐cyclodextrin

AbstractAs a continuation of previous studies on the use of cyclodextrin derivatives (CD) for the separation of volatile compounds by capillary GC, the influence of diluting phases other than OV‐1701 or OV‐1701‐OH has been investigated.2,6‐Di‐O‐methyl‐3‐O‐pentyl‐β‐cyclodextrin (2,6‐DiMe‐3‐Pe‐β‐CD) was taken as the reference CD derivative, because of the large number of volatile racemates it is able to separate; OV‐1701 or OV‐1701‐OH was chosen as the reference diluting phase.The performance of a column coated with a 0.15 μm film of 10 % 2,6‐DiMe‐3‐Pe‐β‐CD in OV‐1701 was compared with that of analogous columns coated with films of the same thickness containing the same percentage of the CD derivative diluted with stationary phases of different polarity, i.e. PS‐086, PS‐347.5, and OV‐225. Resolution values and separation factors of thirty racemates were used to evaluate the effect of different diluting phases on column performance.