Gas chromatographic stationary phase properties of two room-temperature liquid organi salts

The gas chromatographic properties of two room-temperature liquid organic salts, triethyl-n-hexylammonium triethyl-n-hexylboride (TEHAB) and stearylmethyldipolyoxethyl(15)ammonium chloride (Ethoquad $\text{18}\text{25}$, are described. Triethyl-n-hexyl-ammonium triethyl-n-hexylboride could be used up to temperatures of 130°C but showed poor stability towards air and undesirable reactivity towards some dipolar and proton donor/acceptor solutes when used as a column packing material. In contrast, Ethoquad $\text{18}\text{25}$ had a maximum column operating temperature of 280°C, or 300°C after vacuum conditioning. From a calculation of mcReynolds' phase constants and the molar free energy, enthalpy, and entropy of solution for polarity test probes, it was established that Ethoquad $\text{18}\text{25}$ showed intermediate selectivity for dipolar and proton-donor solutes compared to results for conventional non-ionic phases. Ethoquad $\text{18}\text{25}$ is an excellent phase for the profiling of essential oils.     

The selectivity of the stationary liquid in vapour phase chromatography

Vapour phase chromatography is a highly efficient analytical technique for the separation of gases and volatile liquids. The stationary phase in this percolation process is a liquid supported by an inert solid carrier material of a suitable particle size. The important role played by this liquid in the separation of a mixture into its components is discussed.     

Characterisation of the chromatographic properties of a silica-polypyrrole composite stationary phase by inverse liquid chromatography

Silica-polypyrrole particles have been used as a composite stationary phase for liquid chromatography. Determination of capacity factors (k') of a wide number of polycyclic aromatic hydrocarbon (PAH) molecular probes allows the characterisation of the chromatographic properties of the silica-polypyrrole stationary phase. Capacity factors in the range of 0.10 up to 6.1 were determined, thus demonstrating the high affinity of the PAH probes towards the stationary phase. The selectivity of the composite stationary phase was also evaluated as a function of the planarity of the molecular probes injected. Capacity factors determined for PAHs (two-dimensional molecular probes) are higher than those measured for phenyl-substituted PAHs (phenyl-PAHs, three-dimensional molecular probes). Determination of capacity factors, dependence on the composition of the mobile phase demonstrates the reverse alpha-phase properties of the composite stationary phase. The acid-base properties of the composite silica-polypyrrole stationary phase were investigated using benzene derivative molecular probes (i.e., toluene, phenol, benzoic acid and aniline). Capacity factors in the range of 0.45 to 1.0 were determined. This study clearly demonstrates that this composite stationary phase exhibits selective interactions towards PAHs and phenyl-substituted PAHs and strong acid-base properties depending on the structure, the geometry and the acid-base properties of the molecular probes eluted.     

Carrier gas as a new factor influencing the selectivity of the gas-stationary liquid phase chromatographic system

This paper generalizes studies on the influence of carrier gas on relative and absolute retention values. This line of research is also of importance due to the fact that, in the opinion of many chromatographers, the role of the carrier gas is limited only to transporting analyzed compounds along the column. However, even under conditions of the conventional capillary gas-liquid chromatography (i.e. at column pressures under 5 atm) carrier gas (its nature and pressure) significantly influences retention and separation of the analyzed compounds. First, carrier gas (N2 and CO2, for example) dramatically affects relative retention values. For this reason, one should use limit values of alpha(ij) (0) = lim alpha(ij)(P(av)) and I(i)(0) = lim Ii(Pav) I(0) = limI(i) (Pav) with Pav-->0 as chromatographic constants, rather than traditional relative retention values alpha(ij)(P(av)) and I(i)(P(av)). Second, the average pressure Pav of the carrier gas in a column and the nature of the carrier gas influence the selectivity of the gas-stationary liquid phase chromatographic system. Third, wishing to maximize the role of the carrier gas as a factor that improves separation of analyzed compounds, we should design a special gas chromatograph that would allow work with pressures in the column up to 30-50 atm.     

Band broadening inside the chromatographic column: the interest of a liquid stationary phase

Band broadening inside chromatographic columns was studied by Giddings 40 years ago. This theory is revisited pointing out that the band width depends only on the band position, x, inside the column and the height equivalent to a theoretical plate, H, and not on the solute affinity for the stationary phase. The band standard deviation, sigma, inside the column is simply sigma = square root [xH]. This property can be used in countercurrent chromatography (CCC), a chromatographic technique that works with a liquid stationary phase. Two possibilities are presented: 1-extrusion of the liquid stationary phase called elution-extrusion method, and 2-slow motion of the stationary phase in the same direction as the mobile phase, called cocurrent CCC method. A mixture of five steroids, prednisone, prednisolone acetate, testosterone, estrone and cholesterol, with partition coefficient varying from 0.1 to 40, is used with a 53 mL CCC column to show the method capabilities. The elution-extrusion method is discontinuous; however, it allows saving dramatic amounts of solvent and time. Cholesterol could be fully resolved in 2h and 120 mL instead of 7 h and 1.2 L using the classical elution way. The cocurrent CCC method is continuous and was able to resolve cholesterol at baseline in 40 min using 110 mL. Detection is difficult due to the fact that two immiscible liquid phases enter the detector.     

新型苯基桥键色谱固定相的热力学性质

将制备的球形苯基桥键型杂化介孔色谱固定相与商品化的C18和苯基键合硅胶固定相对比,研究其热力学性质。以稠环芳烃为例,探讨了溶质在固定相和流动相之间的迁移焓变、迁移熵变等热力学参数的变化。结果表明,与两种商品化的固定相不同,实验制备的苯基桥键固定相不存在明显的焓-熵补偿效应,证实新型桥键固定相分离机理是疏水作用、π-π作用、包结作用等协同作用的结果。     

Isocratic high-performance liquid chromatographic separation and multiple-wavelength ultraviolet detection of aldicarb and its soil degradation products : Optimization of stationary phase selectivity

The unique selectivities which can be generated through optimization of stationary/mobile phase combinations have been applied to the development of an isocratic, reversed-phase high-performance liquid chromatographic method for the separation of aldicarb and its primary soil degradation products. The method utilizes a cyanopropyl bonded stationary phase and water-acetonitrile mobile phase and is capable of separating aldicarb and its various sulfoxide, sulfone and oxime derivatives in less than 10 min. When combined with multiple-wavelength UV detection and an appropriate preconcentration step, this method can in principle be applied to the routine monitoring of aldicarb and its soil derivatives in water at concentrations less than 1 microgram/l.     

Molecular-dynamics simulation for the characterization of liquid chromatographic stationary phase: effect of temperature.

The influence of temperature on the surface structure of the octadecylsilica (ODS) bonded phase was investigated with a molecular dynamics (MD) simulation. The MD simulation was applied to a molecular model consisting of three parts: amorphous silica base, dimethyloctadecylsilyl ligands and n-hexane as a mobile phase solvent. More detailed information on the effect of temperature was obtained at the low temperature region than that reported in our previous study. The motion of ODS ligands could be estimated by the mean square displacement (MSD) of the terminal carbon atoms of ODS ligands. The gauche fraction in the ODS ligand conformation can also be estimated to obtain the ligand conformation for each simulation condition in detail. It can be seen that an elevated temperature induced the more bent ligand conformation. The trend has a good agreement to that of the results experimentally observed by using various spectroscopic techniques such as nuclear magnetic resonance (NMR), Fourier-transform infrared spectroscopy (FT-IR), and Raman spectroscopy.     

Solute retention in the stationary phase of a liquid-solid chromatographic system

Summary Utilizing the UNIFAC group model of activity coefficients the retention behaviour of a solute in the stationary phase of a liquid-solid chromatographic system is studied. By comparison of experimentally observed capacity ratios and calculated activity coefficients of solutes in the mobile phase, varying the concentration of a polar moderator, it is shown that the calculated activity coefficients in the stationary phase fit very well the equation formally identical with the Langmuir function. Comparison of activity coefficients in the mobile and the stationary phase proves equivalence between the solvent interaction and the competition theory.     

The chromatographic properties of a mixed-bed stationary phase combining reversed-phase and strong anion exchange properties

Summary The feasibility of using columns prepared by mixing together ion exchange and reversed-phase packing materials, for the simultaneous separation of ionised and unionised molecules, has been examined using a range of test solutes. Columns prepared in this way showed chromatographic properties which were intermediate between those of the individual phases. The dual nature of the retention mechanism allowed the retention of ionisable molecules to be adjusted using pH whilst that of uncharged compounds was unaffected. The simultaneous chromatography of model compounds and their glucuronide, sulphate and glycine conjugates, under a variety of conditions, was demonstrated.     

Studies on the special selectivity of the liquid crystal stationary phase used in gas chromatography: II. A new parameter for characterization of the liquid crystal stationary phase

In this paper, the special effect of liquid crystal stationary phase is expressed in terms of ΔI, the difference of the retention index of probed solute between mesomorphic phase and isotropic phase. A series of compounds with different structures and polarities are selected as probed solutes for the investigation of their retention behavior on liquid crystal PBOB and MPBOB. According to the differences of ΔI of various solutes, the rule of special selectivity on liquid crystal is explained.     

The theory of gas-liquid chromatography : The influence of the thickness of the stationary liquid film

The thermodynamic theory of gas-liquid chromatography has been developed, taking into account specific properties of thin films of the liquid phase expressed in terms of the “disjoining pressure”. Equations were derived to describe the dependence of the retention volume on the thickness of the stationary phase over the whole range of thickness. The results are in good agreement with experimental values.A chromatographic method for determining the constants of disjoining pressure is suggested.     

Optimization of the selectivity of a cyanopropyl stationary phase for the gas chromatographic analysis of trans fatty acids

The quantification of monounsaturated and polyunsaturated trans fatty acids in partially hydrogenated fats by gas-liquid chromatography on a CP-Select CB-FAME capillary column was optimized using equivalent chain length values of fatty acids methyl esters that could coelute in the temperature range from 155 to 200 degrees C. The most appropriate temperature for the simultaneous determination of these trans isomers is around 197 degrees C. It is proposed a system to discriminate trans from cis octadecenoic fatty acid methyl esters based on the angular coefficient values of the equivalent chain length curves. The limits of detection and quantification were 0.28 and 0.93 mg g(-1). Quantification was performed in the range from 0.93 to 280 mg g(-1). Quantification accuracy was estimated by spike recovery of elaidic and trans-13-octadecenoic acids in hydrogenated fat samples. The obtained levels were from 97.60 to 103.28% for elaidic acid and from 98.12 to 99.27% for trans-13-octadecenoic acid. These results indicate that the accuracy of the methodology proposed for the quantification of monounsaturated and polyunsaturated trans fatty acids in hydrogenated fats is adequate.     

KF-Silica as a stationary phase for the chromatographic removal of tin residues from organic compounds

Through the simple expedient of using a mixture of KF and silica as the stationary phase in column chromatography, levels of organotin impurities from tributyltin hydride mediated reductions have been reduced from stoichiometric levels to approximately 30 ppm.     

Effects of stationary phase structure on retention and selectivity in reversed-phase liquid chromatography of flavonoid compounds with methanol as organic modifier

Summary Retention of an extended set of flavonoid compounds by octyl, octadecyl, phenyl and cyanopropyl-bonded, reversed-phase columns, with methanol, as the organic mobile phase modifier and acetic acid as the acid modifier is reported. Solvent strengths and useful ranges for both isocratic and gradient elution are determined. Relative retention is found to be independent of methanol volume fraction in the mobile phase for all the columns examined. Correlations between retention on different columns, specific selectivity effects and their dependence on molecular structure are analyzed. Practical applications for separation and identification are discussed.     

New chiral crown ether stationary phase for the liquid chromatographic resolution of alpha-amino acid enantiomers

A new chiral stationary phase (CSP) for the liquid chromatographic separation of enantiomers was prepared by bonding a novel enantiopure (diphenyl-substituted 1,1'-binaphthyl) crown ether to 5 microm silica gel. The resulting CSP was applied to the separation of the enantiomers of various natural and unnatural alpha-amino acids. All alpha-amino acids tested were resolved very well on the new CSP, with the exception of proline, which does not contain a primary amino group. The resolution of alpha-amino acid enantiomers on this new CSP was found to be dependent on the type and amounts of organic and acidic modifiers, and on column temperature.