Transition metal complex stationary phases for the CGC analysis of ethers, thioethers and ketones

Summary The use of polysiloxanes with cyano or thiol groups as stationary phases for complexation gas chromatography is discussed. The polymers were obtained via polycondensation followed by polymerisation of the corresponding dichloro- or dimethoxy-silanes. Both the phases were modified by bonding transition metal chlorides with were modified by bonding transition metal chlorides with cyano (CuCl₂ or CoCl₂) and thiol groups (NiCl₂ or CoCl₂). The phases were examined in order to determine their application to the analysis of ethers, thioethers and ketones. Due to the presence of lone electron pairs on oxygen or sulphur atoms, the compounds should be capable of specific interacting with the electron-withdrawing centre of the liquid stationary phases. A number of retention parameters (retention indices, molecular retention indices and specific retention volume) were determined, which allowed characterisation of specific interactions between the bonded metal and the compound analysed. The results also enabled assessment of the influence of the structure of the compounds on their retention. The measurements were also carried out for the phases with free SH and CN groups as reference phases. The work demonstrates that the phases obtained may be useful for effective separations.     

Comparison of the Separation Characteristics of the Organic–Inorganic Hybrid Stationary Phases XBridge C8 and Phenyl and XTerra Phenyl in RP-LC

Differences in the system constants of the solvation parameter model and retention factor correlation plots for varied solutes are used to study the retention mechanism on XBridge C₈, XBridge Phenyl and XTerra Phenyl stationary phases with acetonitrile–water and methanol–water mobile phases containing from 10 to 70% (v/v) organic solvent. These stationary phases are compared with XBridge C₁₈ and XBridge Shield RP₁₈ characterized in an earlier report using the same protocol. The XBridge stationary phases are all quite similar in their retention properties with larger difference in absolute retention explained by differences in cohesion and the phase ratio, mainly, and smaller changes in relative retention (selectivity) by the differences in individual system constants and their variation with mobile phase type and composition. None of the XBridge stationary phases are selectivity equivalent but XBridge C₁₈ and XBridge Shield RP₁₈ have similar separation properties, likewise so do XBridge C₈ and XBridge Phenyl, while the differences between the two groups of two stationary phases is greater than the difference within either group. The limited range of changes in selectivity is demonstrated by the high coefficient of determination (>0.98) for plots of the retention factors for varied compounds on the different XBridge phases with the same mobile phase composition.     

Separation of Dietary Folates by Gradient Reversed-Phase HPLC: Comparison of Alternative and Conventional Silica-Based Stationary Phases

A study of ten silica-based stationary phases and gradient elution conditions to separate dietary folates by reversed-phase HPLC was performed. Alkyl-bonded stationary phases (both conventional and alternative) were found to be the most promising for the separation of different folate monoglutamates in terms of selectivity and peak shape. These phases were better than phenyl-bonded phases which lacked selectivity when separating 10-formyl-folic acid and 5-formyl-tetrahydrofolate. Polar-bonded (cyano) stationary phase showed similar retention characteristics as the conventional alkyl-bonded phases, but ranked below those in terms of peak shape. Overall, alternative stationary phases exhibited slightly higher retention of late-eluted folates and greater retention variability for early-eluting tetrahydrofolate and 5-methyl-tetrahydrofolate. Best selectivity was achieved on alternative polar endcapped Aquasil C₁₈ followed by conventional Synergy MAX C₁₂ and Genesis C₁₈ stationary phases.     

Comparative evaluation of retention of hydrocarbons present in the C5-petroleum fraction on methylsilicone and squalane phases

Summary The retention of hydrocarbons present in the C₅ pyrolysis fraction of gasolines on the stationary phases squalane and methylsilicone oil JXR at 30, 40 and 50°C was investigated by capillary gas chromatography. The unified retention indices of the hydrocarbons were also calculated on squalane. The retention indices obtained on these two phases were interrelated and the quantitative relationship with the structure of the solutes was studied. Equations based on the unified retention indices calculated on squalane and some selected structural elements of the solutes permit the calculation of their retention on methylsilicone with sufficient accuracy.     

Insights into the Retention Mechanisms on Perfluorohexylpropylsiloxane-Bonded (Fluophase-RP) and Octadecylsiloxane-Bonded (Betasil C18) Stationary Phases Based on the Same Silica Substrate in RP-LC

The solvation parameter model is used to elucidate the retention mechanism on a perfluorohexylpropylsiloxane-bonded (Fluophase RP) and octadecylsiloxane-bonded (Betasil C18) stationary phases based on the same silica substrate with acetonitrile–water and methanol–water mobile phase compositions. Dewetting affects the retention properties of Fluophase RP at mobile phase compositions containing less than 20% (v/v) acetonitrile or 40% (v/v) methanol. It results in a loss of retention due to an unfavorable change in the phase ratio as well as changes in specific intermolecular interactions. Steric repulsion reduces retention of bulky solutes on fully solvated Betasil C18 with methanol–water (but not acetonitrile–water) mobile phase compositions but is not important for Fluophase RP. The retention of weak bases is affected by ion-exchange interactions on Fluophase RP with acetonitrile–water, and to a lesser extent, methanol-water mobile phases but these are weak at best for Betasil C18. The system constants of the solvation parameter model and retention factor scatter plots are used to compare selectivity differences for Fluophase RP, Betasil C18 and a perfluorophenylpropylsiloxane-bonded silica stationary phase Discovery HS F5 for conditions where incomplete solvation, steric repulsion and ion-exchange do not significantly contribute to the retention mechanism. Lower retention on Fluophase RP results from weaker dispersion and/or higher cohesion moderated to different extents by polar interactions since solvated Fluophase RP is a stronger hydrogen-bond acid and more dipolar/polarizable than Betasil C18. Retention factors for acetonitrile–water mobile phases are highly correlated for Fluophase RP and Betasil C18 except for compounds with a large excess molar refraction and weak hydrogen-bonding capability. Selectivity differences are more significant for methanol–water mobile phases. Retention factors on Fluophase RP are strongly correlated with those on Discovery HSF5 for acetonitrile–water mobile phases while methanol–water mobile phases retention on Fluophase RP is a poor predictor of the retention order on Discovery HS F5.     

π-Selective stationary phases: (III) Influence of the propyl phenyl ligand density on the aromatic and methylene selectivity of aromatic compounds in reversed phase liquid chromatography

The retention characteristics of phenyl type stationary phases for reversed phase high performance liquid chromatography are still largely unknown. This paper explores the retention process of these types of stationary phases by examining the retention behaviour of linear PAHs and n-alkylbenzenes on a series of propyl phenyl stationary phases that have changes in their ligand density (1.23, 1.31, 1.97, 2.50 μmol m⁻²). The aromatic and methylene selectivities increased with increasing ligand density until a point where a plateau was observed, overall the propyl phenyl phases had a higher degree of aromatic selectivity than methylene selectivity indicating that these columns are suitable for separations involving aromatic compounds. Also, retention characteristics relating to the size of the solute molecule were observed to be influenced by the ligand density. It is likely that the changing retention characteristics are caused by the different topologies of the stationary phases at different ligand densities. At high ligand densities, the partition coefficient became constant.     

Comparison of four cholesterol‐based stationary phases for the separation of steroid hormones

The chromatographic behavior of steroid hormones on four cholesterol‐bonded stationary phases with different structures in binary methanol/water mobile phases was studied. Of the stationary phases tested, the commercially available stationary phases Cogent UDC cholesterol? and COSMOSIL cholester? provided better separations of steroid hormones in comparison to homemade aminocholesterol and diaminocholesterol stationary phases. The results show that the temperature has a significant influence on the retention and selectivity for steroid hormones separation. The temperature increase may cause changes in the elution order. From the dependences of the retention (ln k) on temperature (1/T), the standard partial molar enthalpy and standard partial molar entropy were calculated and their enthalpic and entropic contributions to the retention were compared. The enthalpic effects principally control the retention mechanism.     

一种混合模式反相强阳离子交换色谱固定相的制备及其保留机理的考察

采用一种简单的“混合配体”方法将辛基和磺酸基键合到硅胶表面,制备了辛基-磺酸基共同键合硅胶(OSS)材料。通过元素分析和吸附容量检测对OSS材料进行表征,证明辛基和磺酸基已成功键合到硅胶表面上。将该OSS硅胶材料作为混合模式反相强阳离子交换(RP/SCX)色谱固定相,在反相液相色谱(RPLC)流动相条件下,采用几种碱性探针分子定量地考察了该固定相的混合模式RP/SCX保留机理。通过改变流动相中缓冲盐的浓度,考察了溶质保留因子和盐浓度的对数和倒数的关系,得到了几种碱性探针分子在该混合模式OSS固定相上的单点和两点保留机理的数学模型。对两种数学模型的方程进行了线性拟合,结果表明两点保留机理更加符合实验的结果。此外,根据混合模式两点保留机理的数学方程,可以得到单一的疏水或离子交换作用力对总保留的影响,对混合模式色谱的实际分离应用可提供有价值的参考。     

Chromatographic Behaviour and Lipophilicity of Estradiol Derivatives

The retention behaviour of estradiol derivatives has been studied by HPLC on polar chemically bonded stationary phases: C₃CN, DIOL and C₃NH₂, commercially available columns. The mobile phases used were: methanol-water and acetonitrile-water in various proportions. Reversed-phase chromatography occurred on polar chemically bonded stationary phases. Correlation between the retention constants of estradiol derivatives obtained on polar chemically bonded phases and log P calculated via different methods was examined too.     

Separation of xylose and glucose on different silica-confined ionic liquid stationary phases

Xylose and glucose, as the main hydrolyzed products of plant cell wall, were separated by silica-confined ionic liquid (IL) stationary phases. Five different stationary phases were synthesized and characterized. Instead of using the traditional NH₂ column, the imidazolium stationary phases exhibit excellent retention to the xylose and glucose. The retention factor and resolution of the monosaccharides decreased with decreasing acetonitrile concentration. In addition, the effects of the IL cations and anions on the retention of xylose and glucose were studied and the adsorption behavior of these two monosaccharides on the stationary phases was investigated. Then the mobile phase and temperature were optimized to improve the performance for the separation of xylose and glucose.     

Prediction of gas chromatographic retention time via an additive thermodynamic model

A straightforward group contribution model based on thermodynamic parameters was developed to predict retention times for a series of alcohols and ketones on three different stationary phases. Thermodynamic parameters determined from gas chromatographic retention data for structurally similar compounds via a three-parameter model were used to predict the retention times of test molecules consisting of ketones and alcohols. The model worked well for the compounds tested with a root mean square error of prediction of 5.50 s across all compounds, phases, and temperature ranges studied. Considering just the alcohols, the error of prediction was 2.79 s across all phases and temperatures.     

Characterization of surface-confined ionic liquid stationary phases: impact of cation and anion identity on retention

A series of surface-confined ionic liquid (SCIL) stationary phases for high-performance liquid chromatography were synthesized in-house. The synthesized phases were characterized by the linear solvation energy relationship (LSER) method to determine the effect of residual linking ligands and the role of the cation and the anion on retention. Statistical analysis was utilized to determine whether the system coefficients returned from multiple linear regression analysis of chromatographic retention data for a set of 28 neutral aromatic probe solutes were significantly different. Examination of the energetics of retention via κ−κ plots agrees with the results obtained from the LSER analysis. Residual linking ligands were determined to contribute reversed-phase-type retention character to the chromatographic system. Furthermore, retention on the SCIL phases was observed to be more profoundly affected by the identity of the anion than by that of the cation.     

Retention behavior of large polycyclic aromatic hydrocarbons on metalloprotoporphyrin-silica stationary phases

The retention behavior of large polycyclic aromatic hydrocarbons (LPAHs) (> or = 7 rings) on newly developed metalloplotoporphyrin (MProP)-silica stationary phases is examined and the results are compared to previously reported data for retention of the same solutes on commercially available phases. HPLC columns packed with FeProP-silica are shown to exhibit unique shape selectivity for LPAH retention, with the planar LPAHs always retained much longer than corresponding non-planar solutes. Solute planarity, length to breadth ratio (L/B value), and number of carbon atoms within the LPAHs are all demonstrated to contribute to the retention sequence observed. Further, the retention of LPAH solutes on FeProP-silica phases is shown to be more predictable than on other reversed-phase columns, with the elution sequence constant regardless of the mobile phase composition. Due to the extremely high planar selectivity of FeProP-silicas with respect to LPAH retention, it is envisioned that columns packed with these phases could be used in conjunction with existing commercial columns to devise Inethods for more efficient separation of complex mixtures of LPAHs in environmental and other samples.     

Stationary phase effects in reversed-phase chromatography II. Substituent selectivities for retention on various hydrocarbonaceous bonded phases

Summary Linear free-energy relationships are manifest in the linear dependence of the logarithmic retention factor on the carbon number for homologous series in reversedphase chromatography and allow the comparison of the free-energy increments of substituents for retention of a given elute by different stationary phases. Three methods were used for such comparison. In the first two, methylene group selectivities obtained from retention data obtained with homologous series on different hydrocarbonanceous bonded phases were compared by using different approaches. In the third method the energetics of retention of the end groups such as phenyl-, bromo- and disulfide-moieties of homologous series were examined. The results show that at fixed mobile phase composition and temperature the free energy increments due to methylene groups upon retention by alkyl-silica stationary phases are identical. Only hydrocarbonaceous bonded phases with short (C₁–C₂) alkyl chains or functional groups of unusual shape show consistent differences from octyl- or octadecyl-silicas. The results broaden the theoretical basis for the development of quantitative structure-retention relationships to be used for the prediction or interpretation of retention data in reversed-phase chromatography.Part I: ref. [23]Dedicated to Professor István Halász for his six tieth birthday.     

Selectivity of calixarene‐bonded silica phases in HPLC: Description of special characteristics with a multiple term linear equation at different methanol concentrations

Retention and selectivity characteristics of different calixarene‐, resorcinarene‐ and alkyl‐bonded stationary phases are examined by analyzing a set of test solutes covering the main interactions (hydrophobic, steric, ionic, polar) that apply in HPLC. Therefore Dolan and Snyder's multiple term linear equation has been adapted to fit the properties of calixarene‐bonded columns. The obtained parameters are used to describe retention and selectivity of the novel Caltrex^® phases and to elucidate underlying mechanisms of retention. Here, differences of stationary phase characteristics at different methanol concentrations in the mobile phases are examined. Both selectivity and retention were found to depend on the methanol content. Differences of these dependencies were found for different stationary phases and interactions. The differences between common alkyl‐bonded and novel calixarene‐bonded phases increase with increasing methanol content.     

Description of retention characteristics of calixarene-bonded stationary phases in dependence of the methanol content in the mobile phase

Calixarene-bonded stationary phases in HPLC are known to support additional interactions compared to conventional alkyl-bonded phases (π–π interactions, complex-building interactions). Thus it cannot be presumed that the same mechanisms of retention apply and that retention can be predicted in similar ways. Here 31 solutes of highly various molecular structures have been analysed at different mobile phase compositions (0–98% (v/v) methanol) in order to characterise the chromatographic behaviour of the novel stationary phases and to test the applicability of established models predicting retention factors. The influence of a change of the methanol content is discussed for non-polar, polar and ionic solutes and differences of their behaviour on the differing column types are shown. Additionally estimates about underlying retention mechanisms are given.     

Retention mechanism of the multifunctional solute on columns with different coverage densities using highly aqueous reversed‐phase conditions

A series of 11 homemade octadecyl bonded phases with different coverage densities were tested to determine the influence of the stationary phase on the retention in highly aqueous mobile phases. The concentrations of the organic modifiers (methanol and ACN) were in the range of 0–20%_v/v. The coverage density of bonded ligands and the presence of the end‐capping have strong influence on the solute retention. Amoxicillin (AMO) was chosen as the test compound. Dual properties of AMO, which contain hydrophobic skeleton and polar groups (amino, hydroxyl and carbonyl), cause irregular changes of the retention over the stationary phase hydrophobicity and silanol activity at given mobile phase composition. Presented data show that application of non‐standard low coverage density C18 phases allow to determine AMO in the RPLC condition with high retention.     

Capillary complexation gas chromatography in analysis of cyclic and aromatic hydrocarbons

Summary Results of studies on stationary phases containing copper(II), nickel(II) and cobalt(II) chloride chemically bonded via cyano—or thiol groups are presented. The retention parameters—i.e. retention factor (k), retention index (I) and molecular retention index (ΔMe) and specific retention volume (Vg)-enabled the characteristics of specific interaction between aromatic and cyclic hydrocarbons and metal complexes chemically bonded to stationary phases to be determined. Presented at Balaton Symposium on High Performance Separation Methods, Siófok, Hungary, September 1–3, 1999     

Solvation parameter models for retention on perfluorinated and fluorinated low temperature glassy carbon stationary phases in reversed-phase liquid chromatography

The retention of solutes on two fluorinated low temperature glassy carbon (F-LTGC) stationary phases under reversed-phase liquid chromatographic conditions was studied by employing the solvation parameter model. The two fluorinated glassy carbon stationary phases were produced by slowly heating zirconia particles that were encapsulated with oligo[1,3-dibutadiyne-1,3-(tetrafluoro)phenylene] precursor polymer to two different final temperatures (200 and 400 degrees C). The resulting carbon particles had different amounts of fluorine after thermal processing. The solvation parameter models indicated that different intermolecular interactions are important in describing retention on the two stationary phases. The interactions that are important for describing retention on the 200 degrees C processed F-LTGC stationary phase are hydrogen bond basicity> or =dispersion>hydrogen bond acidity>dipolarity/polarizability. The interactions that describe the retention on the 400 degrees C processed F-LTGC are hydrogen bond basicity>dispersion>excess molar refraction> or =hydrogen bond acidity. The solvation parameter model for the 200 degrees C processed F-LTGC showed similar trends in the relative importance of intermolecular interactions as previously found for octadecyl-polysiloxane stationary phases, while the 400 degrees C processed F-LTGC had similar intermolecular interactions with solutes as found with porous glassy carbon in that pi-pi interactions with the carbon surface contribute more so to the retention.