Separation of pesticides on an octadecyl-coated alumina column

The retention times of 33 commercial pesticides were determined on an octadecyl alumina HPLC column using water-methanol mixtures. The retention of the pesticides decreased with an increasing concentration of methanol in the eluent proving the regular retention behaviour of the solutes. The retention characteristics of pesticides did not correlate to their hydrophobicity parameters indicating that the absorption centers of alumina not covered by the hydrophobic octadecyl ligand considerably influence the retention. The opposite linear correlation between the retention of pesticides on alumina and octadecyl alumina supports proves that octadecyl alumina supports behave as real reversed-phase supports, however, their retention characteristics differ considerably from those of traditional octadecylsilica supports. Received: 2 August 1996 / Revised: 12 December 1996 / Accepted: 19 December 1996     

Separation of pesticides on an octadecyl-coated alumina column

The retention times of 33 commercial pesticides were determined on an octadecyl alumina HPLC column using water-methanol mixtures. The retention of the pesticides decreased with an increasing concentration of methanol in the eluent proving the regular retention behaviour of the solutes. The retention characteristics of pesticides did not correlate to their hydrophobicity parameters indicating that the absorption centers of alumina not covered by the hydrophobic octadecyl ligand considerably influence the retention. The opposite linear correlation between the retention of pesticides on alumina and octadecyl alumina supports proves that octadecyl alumina supports behave as real reversed-phase supports, however, their retention characteristics differ considerably from those of traditional octadecylsilica supports.     

Retention models for isocratic and gradient elution in reversed-phase liquid chromatography

One- and multi-variable retention models proposed for isocratic and/or gradient elution in reversed-phase liquid chromatography are critically reviewed. The thermodynamic, exo-thermodynamic or empirical arguments adopted for their derivation are presented and discussed. Their connection to the retention mechanism is also indicated and the assumptions and approximations involved in their derivation are stressed. Special attention is devoted to the fitting performance of the various models and its impact on the final predicted error between experimental and calculated retention times. The possibility of using exo-thermodynamic retention models for prediction under gradient elution is considered from a practical point of view. Finally, the use of statistical weights in the fitting procedure of a retention model and its effect on the calculated elution times as well as the transferability of retention data among isocratic and gradient elution modes are also examined and discussed.     

反相毛细管电色谱中中性化合物的保留及有机改性剂对其保留值的影响

以硫脲作为电渗流标记物测定的死时间和以苯同系物线性回归方法测定的死时间为基础，将液相色谱中溶质保留值方程应用到电色谱中，得到了容量因子与二元流动相体系中有机改性剂含量之间的关系曲线；通过实验数据说明了一些极性的电中性化合物在电场作用下也会发生迁移而引起保留值的变化，且这种变化还受有机改性剂含量的影响。     

Gas-chromatographic analysis of alkylated derivatives of carane and of cycloheptane in capillary columns

The retention indices of synthesized alkyl derivatives of carane and of cycloheptane in capillary columns with tricresyl phosphate and squalane at 70°C have been determined. The influence of the structure of the hydrocarbons studied on the values of their retention parameters is considered. An estimate is given of the nature of the interrelationship between the physical properties of these hydrocarbons and their retention indices on a nonpolar stationary phase.     

Chromatographic characteristics of α-alkynols

Chromatographic retention indices on a standard nonpolar poly(dimethylsiloxane) phase were determined for 33 α-alkynols and their 27 dehydration products (conjugated alkenynes). The resulting data are proposed to be verified using the retention indices of acetylenic alcohols estimated by the additive scheme from the retention indices of isostructural alkynes with the inclusion of increments for secondary and tertiary hydroxy groups. The analogous approach to data the verification of data for alkenynes is based on correlation of their retention indices with normal boiling points.     

Prediction of gas chromatographic retention indices of polychlorinated dibenzothiophenes on non-polar columns

Polychlorodibenzothiophenes (PCDTs) have been found in several kinds of environmental samples. The lack of reference compounds has meant that very little is known about their gas chromatographic behavior. Here we discuss the retention of 19 authentic PCDTs and their sulfones on the widely used gas chromatographic stationary phases DB-5 and DB-5ms. The retention order is different from that of the polychlorodibenzofurans. The data generated allowed us to carry out a multiple linear regression to generate parameters for predicting the retention indices of unknown congeners based only on their structural features.     

Fluoroalcohols as novel buffer components for basic buffer solutions for liquid chromatography electrospray ionization mass spectrometry: retention mechanisms

Two fluoroalcohols--1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and 1,1,1,3,3,3-hexafluoro-2-methyl-2-propanol (HFTB)--were evaluated as volatile buffer acids in basic mobile phases for LC-ESI-MS determination of acidic and basic compounds. HFIP and HFTB as acidic buffer components offer interesting possibilities to adjust retention behavior of different analytes and expand the currently rather limited range of ESI-compatible buffer systems for basic mobile phases. Comparing with commonly used basic buffer components the fluoroalcohols did not suppress the ionization of the analytes, for several analytes ionization enhancement was observed. RP chromatographic retention mechanisms were evaluated and compared to traditional buffer system. All trends in retention of the acidic and basic analytes can be interpreted by the following model: the neutral fluoroalcohols are quite strongly retained by the stationary phase whereas their anions are less retained, thus their amount on the stationary phase is dependent on mobile phase pH; the anions of the fluoroalcohols form ion pairs in the mobile phase with the basic analytes; the fluoroalcohols on the stationary phase surface compete with acidic analytes thereby hindering their retention; the fluoroalcohols on the stationary phase bind basic analytes thereby favoring their retention.     

Application of target factor analysis of gas chromatography. Reproduction,prediction and classification

Summary Using the method of target factor analysis (TFA) described by Malinowski and Howery a computer program has been developed to study different sets of gas chromatographic retention data. Physico-chemical, topological and uniqueness parameters have been found to be basic factors to describe solute behaviour problems. Factor analytical solutions have been used to reproduce the data matrices and to make predictions based on best sets of basic factors. The mean absolute error in the reproduction step is between 1.72 retention index units (i.u.) for a relatively simple matrix consisting of retention indices of alcohols and 7.36 i.u. for a combined data matrix of alcohol, aldehyde and ketone retention indices. TFA has also been used to classify solutes based on their retention behaviour. Alkanes have been classified from cycloalkanes, alkanes from alkenes, and alcohols from aldehydes and ketones using only their retention data and a special kind of uniqueness vector.     

Mechanism of TiO2 pigment retention on pulp fiber with polyelectrolytes

Pulp fibers and pigment dispersed in water are both negatively charged and, therefore, when a paper sheet is formed on a screen the pigment particles, which are of colloidal dimension, are not retained. Cationic polyelectrolytes are employed to promote pigment retention but the mechanism by which they operate is not well understood.Two cationic polyelectrolytes, based on polyethylenimine and polyacrylamide and used commercially as retention aids were examined to clarify their role in TiO₂ retention. From the comparison of polymer adsorption on both fiber and pigment, their electrophoretic mobility, and the extent of pigment retention it was concluded that the main mechanism is mutual attraction between oppositely charged fiber and pigment. In general, the fibers became positive at a polymer concentration where the pigment was still negative and maximum retention was realized when these negative pigment particles deposited on the already positive fibers. Homoflocculation of the pigment (and mechanical entrapment of pigment flocs in the forming sheet) also contributed to the overall retention and was more pronounced with the polyacrylamide polymer.     

Interaction of amino acids, N-acetyl amino acid esters, thymine and adenine with sephadex LH-20 gel.

N-Acetyl aromatic amino acid esters, tryptophan, adenine and thymine show strong retention in Sephadex LH-20 gel in an aqueous phase. The decreased retention in 6 M urea, the absence of retention in absolute methanol and the increased retention at higher temperatures in the aqueous phase indicate that hydrophobic interaction is responsible for the observed retention of the amino acids and their esters in the gel. Adenine was found to be retained by polar interaction in the gel. The increased retention of the solutes in the presence of different electrolytes suggests that the lyotropic effect is more important than the ionic strength effect. The relevance of the results obtained with amino acids and esters to the conformational aspects of proteins in aqueous solution is discussed.     

Contribution of Steric Repulsion to Retention on an Octadecylsiloxane-Bonded Silica Stationary Phase in Reversed-Phase Liquid Chromatography

Plots of the retention factor against mobile phase composition were used to organize a varied group of solutes into three categories according to their retention mechanism on an octadecylsilioxane-bonded silica stationary phase, Ascentis ^TM C18, with acetonitrile-water and methanol-water mobile phase compositions containing 10–70% (v/v) organic solvent. The solutes in category 1 could be fit to a general retention model, Eq. (1), and exhibited normal retention behavior for the full composition range. The solutes in category 2 exhibited normal retention behavior at high organic solvent compositions with a discontinuity at low organic solvent mobile phase compositions. The solutes in category 3 exhibited a pronounced step or plateau in the middle region of the retention plots with a retention mechanism similar to category 1 at mobile phase compositions after the discontinuity and a different retention mechanism before the discontinuity. Selecting solutes and appropriate composition ranges from the three categories where a single retention mechanism was operative allowed modeling of the experimental retention factors using the solvation parameter model. These models were then used to predict retention factors for solutes excluded from the models. The overwhelming number of residual values, here defined as the difference between experimental and model predicted retention factors for the excluded solutes, could be explained by contributions from steric repulsion. The latter defined as the inability of solutes to fully insert themselves into the solvated stationary phase because of their size or conformation. Steric repulsion resulted in a systematic reduction in retention compared with predicted values for the fully inserted solute. The bonding density of the stationary phase; the type and composition of the mobile phase; and the size, conformation, type and number of functional groups on the solute are shown to affect the contribution of steric repulsion to the retention mechanism.     

Migration behavior of weakly retained, charged analytes in voltage-assisted micro-high performance liquid chromatography

The application of voltage in micro-high performance liquid chromatography (micro-HPLC) creates a system where separation is governed by a hybrid differential migration process, which entails the features of both HPLC and capillary zone electrophoresis (CZE), i.e., chromatographic retention and electrophoretic migration. In this paper, we use our previously published approach to decouple these two mechanisms via analysis of the input data for estimation of electrokinetic parameters, such as conductivity, equivalent lengths, mobilities and velocities. Separation of weakly retained, charged analytes was performed via voltage-assisted micro-HPLC. Contrary to conclusions from data analysis using the conventional definitions of the retention factor, it is shown that our approach allows us to isolate the "chromatographic retention" component and thus, investigate the "modification" of the retention process upon application of voltage in micro-HPLC. It is shown that the traditional approaches of calculating retention factor would erroneously lead to conclusion that the retention behavior of these analytes changes upon application of voltage. However, the approach suggested here demonstrates that under the conditions investigated, most of the charged analytes do not show any significant retention on the columns and that all the changes in their retention times can be attributed to their electrophoretic migration.     

The chromatographic behaviour of cycloolefins on stationary phases of different polarity. Prediction of their retention indices and boiling points

Summary The retention indices of mono and polycycloolefins with endo or exo double bonds, on squalane or polypropylenglycol 425 are related to their physico-chemical properties such as boiling point, molar volume, molar refraction, and refractive index. Equations have been empirically obtained, from which accurate retention indices of these compounds can be predicted. Unknown boiling points of some of these compounds can also be calculated from their lineal relation with the retention index on squalane.     

Modelling of n-alkane retention and optimization of the conditions for analysis in gas adsorption chromatography with temperature programming

Summary The possibility of constructing a mathematical model of n-alkanes retention upon their separation by gas solid chromatography with temperature programming has been studied. The functional dependence between the number of the carbon atoms in n-alkane molecules, their retention in isothermal conditions and temperature of chromatographic column was used for constructing this mathematical model. It showed necessary to take into account the variance in the process temperature programming of both the carrier gas volume velocity and the column inlet pressure to obtain the adequate mathematical model of the chromatographic retention. With the use of the specific retention parameters of substances i.e. relative to the surface or the mass of sorbents the proposed model can be used not only for Silochrom C-80 but for the whole class of macroporous silica sorbents.     

The effect of chaotropic mobile phase additives on the separation of selected alkaloids in reversed-phase high-performance liquid chromatography

The retention behavior of selected alkaloids from different classes was studied. The effect of chaotropic salts additives to the mobile phase on chromatographic parameters of protonated basic analytes was investigated on Zorbax Extend-C18 column. The influence of the type of salts and their concentration on retention, efficiency, peak symmetry and separation selectivity of investigated alkaloids was established. Buffered acetonitrile-water mobile phase was chosen because of significant retention of added liophilic ions due to strong dispersive pi-pi interactions. These conditions are responsible for great contribution of electrostatic forces in the retention of protonated bases. The addition of salt, such as hexafluorophosphate, perchlorate, trifluoroacetate leads to the increase in retention, efficiency and separation selectivity of examined analytes. The influence of added salts on increase in retention parameters could be expressed as follows: H2PO4- < CF3COO- < ClO4- < PF6-. This order is in agreement with ability of salts to "salting-in" effect according to Hofmeister series. Obtained chromatograms of alkaloids mixture illustrate suitability of chaotropic effect to improve their separation selectivity.     

A quantitative peptide structure vs. retention relationship study

The quantitative relationship between the TLC retention parameters of 28 short peptides and their physicochemical characteristics was calculated using linear regression analysis. It was established that surface parameters exert a significant impact on the retention of peptides and the ratio of non-polar surface area/polar surface area exerts the highest influence on the retention. This result may be due to the fact that peptides turn towards the apolar surface of the stationary phase with their non-polar substructures while the polar molecular parts point into the hydrophilic mobile phase.     

Use of correlation between retention indices on low-polarity phases for screening complex mixtures by gas chromatography-mass spectrometry

The reliable identification of numerous compounds in biological fluids by chromatography-mass spectrometry requires the knowledge of their retention on the phase used under specific temperature conditions. A simple method was proposed for calculating retention indices on a 5% phenyl dimethyl polysiloxane phase based on a correlation between the retention values observed for methyl- and phenyl dimethyl polysiloxane phases at different temperatures.