Liquid Stationary Phase Research Articles

Preparative separation of a terpenoid and alkaloids from Tripterygium wilfordii Hook. f. using high-performance counter-current chromatography : Comparison of various elution and operating strategies

This paper describes how high-performance counter-current chromatography (HPCCC) was used strategically for the separation of Tripterygium wilfordii Hook. f. Due to the complexity of Chinese herbal medicines, the initial ethanol crude extract was fractionated into seven fractions using medium-pressure liquid chromatography (MPLC). One terpenoid (triptolide) and three alkaloids (peritassine A, wilforgine and wilforine) were further separated from one of the MPLC fractions. This fraction (1.25 g) yielded 8 mg of triptolide and 28 mg of peritassines A after one HPCCC column pass and 30 mg of wilforgine and 120 mg of wilforine after a second column pass with respective purities of 97%, 93.6%, 95.0% and 94.4%, which were determined by high-performance liquid chromatography (HPLC). This was a one-step HPCCC separation, using an n-hexane–ethyl acetate–methanol–water (4:5:4:5, v/v) solvent system, where increases in theoretical plates have been sacrificed in favour of increasing throughput. Structures were identified by electrospray ionization mass spectrometry (ESI-MS), 1H nuclear magnetic resonance ( 1H NMR) and 13C nuclear magnetic resonance ( 13C NMR). Comparison of three different modes of eluting compounds retained in the liquid stationary phase: elution extrusion; dual mode and simple pump-out showed that simply pumping out the column contents at high flow gave better resolution and was eight times faster than the other two well-utilised methods. Triptolide and peritassines A were isolated for the first time from Tripterygium wilfordii Hook. f.

Characterization and utilization of a novel triflate ionic liquid stationary phase for use in comprehensive two‐dimensional gas chromatography

A novel triflate (trifluoromethylsulfonate) ionic liquid (IL) thin film (0.08 microm) stationary phase was implemented for use within the second column of a comprehensive GC x GC configuration. The first column in the configuration had a 5% phenyl/95% dimethyl polysiloxane (DMPS) stationary phase with a 0.4 microm film. The DMPS x IL column configuration was used to separate a mixture of 32 compounds of various chemical functional classes. The GC x GC results for the IL column were compared with a commercially available polar column (with a 0.1 microm PEG stationary phase film) used as the second column instead. Additional studies focused on the rapid and selective separation of four phosphorous-oxygen (P-O) containing compounds from the 32-compound matrix: dimethyl methylphosphonate (DMMP), diethyl methylphosphonate (DEMP), diisopropyl methylphosphonate (DIMP), and triethyl phosphate (TEP). van't Hoff plots (plots of ln k vs. 1/T) demonstrated the difference in retention between the P-O containing compounds (with DMMP reported in detail) and other classes of compounds (i. e., 2-pentanol and n-dodecane as representative) using either the IL column or the commercial PEG column. The selectivity (alpha) of the triflate IL column and the commercially available PEG column were also compared. The IL column provided significantly larger selectivities between DMMP and the other two compounds (2-pentanol and n-dodecane) than the commercial PEG column. The alpha for DMMP relative to n-dodecane was 3.0-fold greater for the triflate IL column, and the alpha for DMMP relative to 2-pentanol was 1.7-fold greater for the triflate IL column than for the PEG column.

The thermodynamic characteristics of retention of tricyclo[5.2.1.02,6]decane isomer molecules on different sorbents in gas chromatography

The thermodynamic characteristics of sorption of the isomeric tricyclo[5.2.1.02.6]decane (tetrahydrodicyclopentadiene, TDCPD) molecules were for the first time determined experimentally and by molecular statistical methods under the conditions of gas-adsorption chromatography on graphitized thermal carbon black and gas-liquid chromatography on stationary liquid phases of different polarities (Apiezon L and Carbowax 20M). The effects of the chemical nature of sorbents on the retention of the TDCPD isomers are considered. A procedure for calculating the thermodynamic characteristics of adsorption of molecules with a complex structure is suggested within the framework of the atom-atom approximation of the semiempirical molecular statistical theory of adsorption for the example of isostructural norbornane molecules. The procedure involves simultaneous variation of geometric parameters and refinement of the parameters that determine the special features of the electronic structure. This method was used to determine several geometric parameters of the TDCPD molecules for the first time. The results were in close agreement with the quantum-chemical data.

Characterization of a novel pyridinium bromide surface confined ionic liquid stationary phase for high-performance liquid chromatography under normal phase conditions via linear solvation energy relationships

Utilizing linear solvation free energy relationship methodology, a novel pyridinium bromide surface confined ionic liquid (SCIL) stationary phase was characterized under normal phase high-performance liquid chromatographic conditions. A limited set of neutral aromatic probe solutes were utilized to rapidly assess the utility of the LSER model, using mobile phases of hexane modified with 2-propanol. The excellent correlation of the global fit across the mobile phase composition range used in this study for the experimental and calculated retention values ( R 2 = 0.994) indicates that the LSER model is an appropriate model of characterizing this polar bonded phase under normal phase conditions. For a limited subset of compounds, retention on the pyridinium bromide SCIL stationary phase is more highly correlated with that obtained on a cyano column than on a diol column under NP conditions.

Characterizations for Vinylimidazolium Based Ionic Liquid Polymer Stationary Phases for Capillary Gas Chromatography

Six polyvinylimidazolium based ionic liquids were prepared for use as stationary phases for gas chromatography. The influences of the attached side-chains (hexyl-, octyl- and phenylpropyl-) on the vinylimidazolium cations and of different counter ions (bromide, hexafluorophosphate, bis-trifluoromethanesulfonlyimide, and bis-trifluoroethanesulfonylimide) were studied. Linear solubility parameter experiments were conducted to characterize the specific interactions of these stationary phases. Some of the polymerized ionic liquid stationary phases exhibited unique structural selectivity. m-, and p-xylenes could be distinguished. The preparation of the polymerized ionic liquid column is simple and reliable. This work provides detailed information for designing polymerized ionic liquids, and shows that these materials have great potential extending the range of options for stationary phases in gas chromatography.

Fullerene-impregnated ionic liquid stationary phases for gas chromatography

A new method has been developed to facilitate the use of fullerenes as stationary phases (SPs) in gas chromatography (GC). In this method, ionic liquids (ILs) are used as solvents to coat fullerenes (C(60), amino-C(60) and hydroxy-C(60)) onto GC columns. However, the ILs serve not just as coating solvents but also act synergistically with fullerenes to provide unique properties as stationary phases, namely dual modal characteristics. They act as non-polar SPs when separating non-polar analytes (aromatic hydrocarbon mixtures and alkane mixtures), and as polar SPs for polar analytes (e.g., alcohol mixtures). The polarity of the stationary phase can be adjusted by changing either the type of the IL and/or by adding either C(60) (or its amino or hydroxy derivatives) to the IL. It was found that C(60) and its derivatives produce not only a change in the polarity of the SP but also substantial enhancement in separation efficiencies for both non-polar and polar analytes. More importantly, when added to the IL SP, C(60) improves separation efficiencies not just for non-polar analytes (aromatic hydrocarbon mixtures and alkane mixtures) but also for polar analytes (mixtures of ortho-, meta- and para-xylene and alcohol mixtures) as well. Moreover, C(60) SP provides higher efficiencies than amino-C(60) and hydroxy-C(60) for separation of polar analytes. This is rather surprising considering that not only are amino-C(60) and hydroxy-C(60) more polar than C(60), but that the IL used to coat the amino- and hydroxy-C(60) (i.e., N-ethylpyridinium trifluoroacetate, [EtPy(+) CF(3)COO(-)]) is more polar than the IL used to coat the C(60) (i.e., octylmethylimidazolium bis(trifluoromethyl)sulfonyl)amide], [OMIm(+) (CF(3)SO(2))(2)N(-)]). Moreover, compared to its amino and hydroxy derivatives, the concentration of C(60) in the column was 10 times lower.

Rapid screening of bioactive components from Zingiber cassumunar using elution-extrusion counter-current chromatography

Elution-extrusion counter-current chromatography (EECCC) takes full advantages of the liquid nature of the stationary phase. It effectively extends the solute hydrophobicity window that can be studied and renders the CCC technique particularly suitable for rapid analysis of complex samples. In this paper, EECCC was used to screen the crude ethanol extract of Zingiber cassumunar and to isolate milligram-amounts of bioactive components. The two column volume (2 V C) EECCC method was applied to rapidly optimize the composition of the biphasic liquid system in both reversed- and normal-phase separation mode. With the n-hexane/ethyl acetate/methanol/water 1/1/1/1 (v/v) system, 100 mg of crude Z. cassumunar extract were fractionated on a 140 mL-capacity semi-preparative hydrodynamic CCC column and 0.5 g on a 1600 mL column for large-scale preparation. Satisfactory separation efficiency was achieved in both cases, producing milligram-amounts of four phenylbutenoids over 90% pure and of a mixture of diastereoisomers (phenylbutenoid dimers). However, the global throughputs of the two columns were 8 and 11 mg/h, not very different. This is due to the fact that the 1600 mL column could not retain the liquid stationary phase as well as the smaller 140 mL column. It was necessary to work at much lower flow rate than calculated. Methanol was added as a post-column clarifying reagent for stable continuous UV detection. A lipophilic biphasic liquid system composed of n-hexane/acetonitrile/water (5/3/2, v/v) allowed to resolve the pair of diastereoisomers with the larger preparative instrument producing 35 mg of the (±)-trans form 99.1% pure and 28 mg of the (±)-cis isomer 98.1% pure. Compared with classical elution, the EECCC approach exhibits strong separation efficiency and great potential to be a high-throughput separation technique in the case of complex samples.

Using the liquid nature of the stationary phase in counter-current chromatography: V. The back-extrusion method

The main feature of counter-current chromatography (CCC) is that the stationary phase is a liquid as well as the mobile phase. The retention volumes of solutes are directly proportional to their distribution coefficients K D in the biphasic liquid system used in the CCC column. Solutes with high K D coefficients are highly retained in the column. The back-extrusion method (BECCC) uses the fact that the liquid stationary phase, that contains the retained solutes, can be easily moved. Switching the column inlet and outlet ports without changing the liquid phase used as the mobile phase causes the rapid collapse of the two immiscible liquid phases inside the column, the previously stationary phase being gathered at the new column outlet. Then this previously stationary liquid phase is extruded outside the CCC column carrying the retained solutes. The back-extrusion method is tested with a standard mixture of five compounds and compared with the recently described elution–extrusion method. It is shown that the chromatographic resolution obtained during the back-extrusion step is good because the solute band broadening is minimized as long as the solute is located inside the “stationary” phase. However, a major drawback of the BECCC method is that all solutes are split between the liquid phases according to their distribution ratios when the CCC column equilibrium is broken. The change of flowing direction should be done after a sufficient amount of mobile phase has flushed the column in the classical mode, eluting solutes with small and medium distribution ratios. Otherwise, a significant portion of the solutes will stay in the mobile phase inside the column and produce a broad peak showing after the stationary phase extrusion.

Allosteric Modifiers of Neuronal Nicotinic Acetylcholine Receptors: New Methods, New Opportunities

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Gas Chromatography-Mass Spectrometry Analysis of Regioisomeric Ring Substituted Methoxy Methyl Phenylacetones

The methoxy methyl phenylacetones share an isobaric relationship (equivalent mass but different elemental composition) to the controlled precursor substance 3,4-methylenedioxyphenylacetone. The 10 methoxy methyl phenylacetones as well as the methylenedioxyphenylacetones show essentially equivalent mass spectra with major fragment ions at m/z 135 and 43. Those methoxy methyl phenylacetones with the methoxy group substituted ortho to the benzylic cation in the m/z 135 ion show a further fragmentation to lose formaldehyde (CH2O) and yield a significant ion at m/z 105. The loss of formaldehyde from the ortho methoxy benzyl cation was confirmed using commercially available regioisomeric 2-, 3-, and 4-methoxyphenylacetones. The 10 regioisomeric methoxy methyl phenylacetones were prepared from the appropriately substituted benzaldehydes. Complete gas chromatographic resolution of all ten regioisomeric ketones was obtained on a stationary phase containing modified beta-cyclodextrin. Using the cyclodextrin containing phase, the ortho methoxy-substituted ketones (K1-K4) eluted before the meta-methoxy-substituted ketones (K5-K8) and the para-methoxy-substituted ketones (K9-K10) showed the greatest affinity for the stationary liquid phase and eluted last. Complete separation of the 10 ketones was not obtained on Rtx-1 and Rtx-200 columns.

Study of the mass transfer of elements in their dynamic leaching from soils and bottom sediments

Effective mass-transfer coefficients of elements in their dynamic extraction from soils, silts, and bottom sediments in a rotating coil column were calculated with the use of the deterministic model of sorption/desorption dynamics describing the behavior of a microcomponent in the system stationary solid sorbent-mobile liquid phase. It was demonstrated that on-line fractionation of element forms in a rotating coil column allows not only a more correct (in comparison with successive extraction in the static mode) determination of heavy metals and arsenic in the most mobile and biologically accessible fractions, but also the prediction of the dynamics of the element release with changing environmental conditions.

Dicationic ionic liquid stationary phase for GC-MS analysis of volatile compounds in herbal plants

The seeming "dual nature" of ionic liquids (ILs) for separating both apolar and polar compounds suggests that ILs may have a great potential for complex samples like essential oils from herbal plants that contain a great variety of compounds. In the present work, a geminal dicationic IL, 1,9-di(3-vinylimidazolium)nonane bis[(trifluoromethyl)sulfonyl]imidate, was investigated for this purpose. To find the best way to achieve satisfactory separations simultaneously for the compounds in essential oils, the dicationic IL was used as the stationary phase for capillary gas chromatography (GC) in two ways, either in its pure state or as a mixed stationary phase with monocationic ILs and a polysiloxane diluent. Interestingly, it was found that the mixed stationary phase exhibited a much better selectivity for polar and nonpolar compounds than either the dicationic IL or the polysiloxane, suggesting that a kind of synergistic effect occurred when these stationary phases were combined in the way described. A comparison with two commercial stationary phases (polar and nonpolar) indicated that this novel mixed stationary phase behaved in a way closer to a polar stationary phase in terms of selectivity and elution order. The present work demonstrates that the mixed stationary phase is efficient and selective and can be an alternative choice for the GC analysis of samples of complex composition.

Research Profile: Revisiting liquid stationary phases

Research Profile: Revisiting liquid stationary phases

Countercurrent Chromatography and the Journal of Liquid Chromatography: A Love Story

Countercurrent chromatography (CCC) is a separation technique that works with a support‐free liquid stationary phase. Since the mobile phase is also a liquid, biphasic solvent systems are used. The problem with CCC is due to its nature: it is difficult to maintain a stable liquid stationary phase when a liquid mobile phase is pushed through it. All modern CCC columns use centrifugal fields. The hydrodynamic CCC columns use open tubes coiled on spools that rotate in a planetary manner. The hydrostatic CCC columns use channels interconnected by ducts, mounted in a centrifuge rotor. It is difficult to work with a support‐free liquid stationary phase, but the advantages of this chromatographic configuration are so important that it is worth finding better ways and to invent better CCC columns. The main advantage is that all injected compounds can access the whole volume of the liquid stationary phase, compared to the necessary solid‐liquid interphase contacts with classical solid stationary phases. Saturation of the stationary phase, or column overload, is much more difficult in CCC than in classical preparative LC with silica gel based stationary phases. CCC is definitely a preparative technique. The other advantages of a liquid stationary phase include the wide range of polarities that can be obtained using the numerous available solvents and mixing them. No irreversible adsorption is possible in CCC, since the liquid stationary can always be recovered. These advantages and the modern commercially available CCC columns are presented in this article celebrating the 30th birthday of the Journal of Liquid Chromatography that always, with its editor, strongly supported the technique by publishing, at certain occasions, half of the whole scientific CCC production of the world.

Photoremediation of natural leachate from a municipal solid waste site in a pilot-scale bubble column reactor

An 18-litre pilot-scale bubble column photoreactor has been used to carry out the mineralisation of a municipal solid waste dirty brown leachate. Rate data collected at 300 K and atmospheric pressure with titania particles suspended in upward flowing oxidising air through a stationary liquid phase suggested that the mineralisation of humic compounds was accompanied by a parallel hydrolytic photodecomposition of nitrogenous matter to NH4+ ions. The total organic carbon (TOC) reduction rate followed a Langmuir–Hinshelwood kinetics, while NH4+ ions production is competitively inhibited at high TOC concentration (>35 mM). Photoremediation was also attended by significant (50–70%) decolourisation. The treated leachate may be recycled for agricultural use.

Rapid determination of complex mixtures by dual-column gas chromatography with a novel stationary phase combination and spectrometric detection

Fast GC separations of a broad range of analytes are demonstrated using a capillary column coated with a novel immobilized ionic liquid (IIL) stationary phase. Both completely cross-linked and partially cross-linked columns were evaluated, yielding ∼1600 and ∼2000 theoretical plates per meter, respectively. Enhanced separation is demonstrated using a dual-column ensemble comprised of an IIL column, a commercially coated Rtx-1 column, and a pneumatic valve connecting the inlet to the junction point between the two columns. Enhanced separation of 20 components, with two sets of co-eluting peaks is shown in ∼150 s, while sacrificing only a length of time equivalent to the sum of the stop flow pulses, or about 15.5 s. A novel application of a band trajectory model that shows band position as a function of analysis time as analytes move through the column ensemble is employed to determine pulse application times. The model predicts component retention times within a few seconds. Another method of selectivity enhancement of the IIL stationary phase-coated columns is demonstrated using a differential mobility spectrometer (DMS) that provides a second dimension separation based on ion mobility in a high-frequency electrical field. The DMS is able to separate all but one set of co-eluting components from the IIL column. The separation of 13 components found in the headspace above U.S. currency is demonstrated using the IIL column in a dual-column ensemble as well as with the DMS.

Further consideration of the gas–liquid chromatographic method for determining the intramolecular hydrogen bond energies with the example of 2-substituted phenols

The GLC method for determining the intramolecular hydrogen bond (intra-HB) energy as the difference between the heats of dissolution of two isomers, one of which forms an intra-HB and the other has no such bond for steric reasons, in a polar stationary liquid phase was further considered by taking account of a correction for the difference between the energies of nonspecific interactions of the isomers with a polar phase. With 4 - and 2 -substituted phenols as an example, the differences between their heats of dissolution in polyethylene glycol and in poly-1,4-butanediol succinate were measured. As a first approximation, corrections for the contribution of the energy of nonspecific interactions with a liquid phase to the total magnitude of the difference between the heats of isomer dissolution were found. These corrections were determined as the difference between the heats of dissolution for 1,4 - and 1,2 -disubstituted benzene derivatives—analogs of the compounds under study that either do not have hydroxyl groups altogether or, in the case of 1,2 -isomers with a hydroxyl group, cannot form an intra-HB. On the basis of the obtained results, the average intra-HB energies of 2-fluoro- ( 1), 2-chloro- ( 2), 2-bromo- ( 3), 2-iodo- ( 4), 2-methoxyphenol ( 5), salicylaldehyde ( 6), methyl ether of salicylic acid ( 7) and 2-nitrophenol ( 8) were estimated to be 7.4 ± 0.3, 13.3 ± 0.8, 14.4 ± 0.4, 12.8 ± 0.3, 17.6 ± 0.1, 24.6 ± 1.2, 29.8 ± 0.3 and 32.5 ± 4.7 kJ/mol, respectively. The values were compared with those found in the literature.

Investigation of the Determination of Diffusion Coefficients of Gases into Liquids from Reversed‐Flow Gas Chromatography

The interaction of vinyl chloride (VC) with water is used as the model system for the investigation of the determination of diffusion coefficients of gases into liquids from revered‐flow gas chromatography (RF‐GC). A thorough, full investigation of the initially suggested methodologies of RF‐GC [Anal. Chem. 1989, 61, 2231–2237 and Adv. Chromatogr. 2000, 40, 231–273] is carried out. The appropriate experimental conditions for the measurement of accurate liquid diffusivities (e.g., quiescent or stirred liquids) are investigated, and an insight into the physical meaning of the measured physicochemical parameters is attempted. In order for liquid diffusion to be the main broadening factor, it is necessary to ensure the regularity of the liquid stationary phase, which can be achieved by stirring the liquid substance under study. Furthermore, the experimentally obtained diffusion coefficients, the respective mass transfer coefficients, as well as the activation energy for the diffusion of VC in water are of the same order of magnitude and are very close to the values obtained by other techniques or calculated from empirical equations, ascertaining the potential of the presented methodology.

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, σ, inside the column is simply σ = x H . 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 2 h 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.

Effects of nonideal adsorption on the surface of a liquid stationary phase in gas chromatography. The degree of freedom restriction

Based on the concept that the properties of the surface of liquids applied onto solid supports in gas-liquid chromatography differ from those of an ideal flat adsorbent, the effects of the fine structure of the surface of a nonpolar liquid (microscopic protrusions formed by the end atoms) on the freedom of motion and the steric factor of the adsorbate molecule were considered.