Characterisation of stationary phases in subcritical fluid chromatography with the solvation parameter model IV. Aromatic stationary phases

The purpose of the present work was to systematically study the chromatographic behaviour of different aromatic stationary phases in a subcritical fluid mobile phase. We attempted to assess the chemical origin of the differences in retention characteristics between the different columns. Various types of aromatic stationary phases, all commercially available, were investigated. The effect of the nature of the aromatic bonding on interactions between solute and stationary phases and between solute and carbon dioxide-methanol mobile phase was studied by the use of a linear solvation energy relationship (LSER): the solvation parameter model. This study was performed to provide a greater knowledge of the properties of these phases in subcritical fluid chromatography, and to allow a more rapid and efficient choice of aromatic stationary phase in regard of the chemical nature of the solutes to be separated. Charge transfer interactions naturally contribute to the retention on all these stationary phases but are completed by various other types of interactions, depending on the nature of the aromatic group. The solvation vectors were used to compare the different phase properties. In particular, the similarities in the chromatographic behaviour of porous graphitic carbon (PGC), polystyrene-divinylbenzene (PS-DVB) and aromatic-bonded silica stationary phases are evidenced.     

Characterisation of stationary phases in subcritical fluid chromatography with the solvation parameter model. III. Polar stationary phases

In this third paper, varied types of polar stationary phases, namely silica gel (SI), cyano (CN)- and amino-propyl (NH2)-bonded silica, propanediol-bonded silica (DIOL), poly(ethylene glycol) (PEG) and poly(vinyl alcohol) (PVA), were investigated in subcritical fluid mobile phase. This study was performed to provide a greater knowledge of the properties of these phases in SFC, and to allow a more rapid and efficient choice of polar stationary phase in regard of the chemical nature of the solutes to be separated. The effect of the nature of the stationary phase on interactions between solute and stationary phases and between solute and carbon dioxide-modifier mobile phases was studied by the use of a linear solvation energy relationship (LSER), the solvation parameter model. The retention behaviour observed with sub/supercritical fluid with carbon dioxide-methanol is close to the one reported in normal-phase liquid chromatography with hexane. The hydrogen bond acidity and basicity, and the polarity/polarizability favour the solute retention when the molar volume of the solute reduces it. As with non-polar phases, the absence of water in the subcritical fluid allows the solute/stationary phase interactions to play a greater part in the retention behaviour. As expected, the DIOL phase and the bare silica display a similar behaviour towards acidic and basic solutes, when interactions with basic compounds are lower with the NH2 phase. On the CN phase, all interactions (hydrogen bonding, dipole-dipole and charge transfer) have a nearly equivalent weight on the retention. The polymeric phases, PEG and PVA, provide the most accurate models, possibly due to their better surface homogeneity.     

Characterization of stationary phases by a linear solvation energy relationship utilizing supercritical fluid chromatography

Supercritical fluid chromatography was utilized in combination with the Abraham model of linear solvation energy relationship to characterize 11 different HPLC stationary phases. System constants were determined at one supercritical fluid chromatography condition for each stationary phase. The results indicate that several types of silica columns, including type B silica, type C silica, and fused core silica, are very similar in their retention behavior. Several aromatic stationary phases were characterized and it was found that, in contrast to the other phases studied, all of the aromatic stationary phases had positive contributions from the dispersion/cavity (v) term of the linear solvation energy relationship. Several aliphatic phases were characterized and there were several linear solvation energy relationship constants that differentiated the phases from each other, mainly the polar terms (dipolarity and hydrogen bonding). One stationary phase, a fused core pentafluorophenyl (PFP) phase, had very poor regression quality. The column volume of this phase was lower than the others in the study, which may have had some impact on the results of the regression.     

Overview of the retention in subcritical fluid chromatography with varied polarity stationary phases

Retention and separation of achiral compounds in supercritical fluid chromatography (SFC) depend on numerous parameters: some of these parameters are identical to those encountered in HPLC, such as the mobile phase polarity, while others are specific to SFC, as the density changes of the fluid, due to temperature and/or pressure variations. Additional effects are also related to the fluid compressibility, leading to unusual retention changes in SFC, for instance when flow rate or column length is varied. These additional effects can be minimised by working at lower temperatures in the subcritical domain, simplifying the understanding of retention behaviours. In these subcritical conditions, varied modifiers can be mixed to carbon dioxide, from hexane to methanol, allowing tuning the mobile phase polarity. With nonpolar modifiers, polar stationary phases are classically used. These chromatographic conditions are close to the ones of normal-phase LC. The addition of polar modifiers such as methanol or ACN increases the mobile phase polarity, allowing working with less polar stationary phases. In this case, despite the absence of water, retention behaviours generally follow the rules of RP LC. Moreover, because identical mobile phases can be used with all stationary phase types, from polar silica to nonpolar C18-bonded silica, the classical domains, RP and normal-phase, are easily brought together in SFC. A unified classification method based on the solvation parameter model is proposed to compare the stationary phase properties used with the same subcritical mobile phase.     

Super/subcritical fluid chromatography chiral separations with macrocyclic glycopeptide stationary phases

The chiral recognition capabilities of three macrocyclic glycopeptide chiral selectors, namely teicoplanin (Chirobiotic T), its aglycone (Chirobiotic TAG) and ristocetin (Chirobiotic R), were evaluated with supercritical and subcritical fluid mobile phases. A set of 111 chiral compounds including heterocycles, analgesics (nonsteroidal antiinflamatory compounds), beta-blockers, sulfoxides, N-protected amino acids and native amino acids was separated on the three chiral stationary phases (CSPs). All separations were done with an outlet pressure regulated at 100 bar, 31 degrees C and at 4 ml/min. Various amounts of methanol ranging from 7 to 67% (v/v) were added to the carbon dioxide along with small amounts (0.1 to 0.5%, v/v) of triethylamine and/or trifluoroacetic acid. The Chirobiotic TAG CSP was the most effective closely followed by the Chirobiotic T column. Both columns were able to separate, partially or fully, 92% of the enantiomers of the compound set. The ristocetin chiral selector could partially or baseline resolve only 60% of the enantiomers tested. All separations were done in less than 15 min and 70% were done in less than 4 min. The speed of the separations is the main advantage of the use of SFC compared to normal-phase HPLC. In addition, SFC is advantageous for preparative separations with easy solute recovery and solvent disposal.     

Characterisation of stationary phases in supercritical fluid chromatography with the solvation parameter model V. Elaboration of a reduced set of test solutes for rapid evaluation

Characterisation of chromatographic systems with the solvation parameter model provides satisfactory information on the main non-ionic interactions developed in a chromatographic system. The procedure requires the analysis of a large number of compounds to warrant the relevance and the accuracy of the calculated models, and even if retention time is lower in supercritical fluid chromatography (SFC) than in HPLC (3-5 times lower), a decrease in the time required for that procedure would favour the use of this model in method development. Consequently, in order to establish a rapid testing procedure that would provide equivalent information, nine key solutes were carefully selected among the hundred we classically use. The separation factors calculated between these key solutes, taken two by two to establish new equations, allow the calculation of the model coefficients. The normal testing procedure is thus reduced from one or two days down to 2h. Precision and accuracy of the models provided are assessed through back-calculation of the coefficients that served for the establishment of the procedure, then through calculation of the coefficients of 13 new SFC systems. The applicability of the rapid testing procedure in SFC is evidenced with three examples: the elaboration of a system map, by varying the modifier concentration in the mobile phase, and the comparison of six ODS phases bonded on the same silica base. The simplified procedure presented here does not pretend to characterize the chromatographic systems as precisely as the complete testing procedure does, but is only aimed at rapidly evaluating the chromatographic retention characteristics when operating parameters are varied.     

Application of the solvation parameter model to poly(methylcyanopropylsiloxane) stationary phases

The solvation parameter model has been applied to the specific retention volumes of 65 solutes of varied polarity on glass capillary columns coated with commercial and synthesized poly(methylcyanopropyl)siloxanes (CNPXX) with eight different percentages of cyanopropyl group (CNP). Their system constants were determined at 75, 90, 105 and 120 degrees C. The polymers examined do not either show any acidity (b = 0) or interact with solute pi/n electrons (e = 0); the prominent constants, dipolarity/polarizability and hydrogen-bond basicity, are of the same order (s approximately a), and the cavity formation/dispersive forces have normal values. Constants s, l and a decrease linearly with temperature for each cyanopropyl percentage. At each temperature, the constants s and a increase with polarity of polymer according to a curve, while the constant l decreases slightly. Cluster analysis shows that six CNPXX with medium to high cyanopropyl substitution integrate into a group with other high-polarity cyano-containing stationary phases taken from the literature, while the other three CNPXX with low CNP percentage form a group with other low-polarity stationary phases of different chemical nature. These clusters are supported by the dendrogram of 52 stationary phases made with the nine polymers presented here and other 43 taken from the literature.     

Chiral separation of amines in subcritical fluid chromatography using polysaccharide stationary phases and acidic additives

The chiral separation of basic compounds by subcritical fluid chromatography (SFC) is often unsuccessful, due possibly to multiple interactions of the analyte with the mobile and stationary phase. Incorporation of a strong acid, ethanesulfonic acid (ESA), into the sample diluent and mobile phase modifier gives a dramatic improvement in these separations. Screening with ethanol containing 0.1% ESA on CHIRALPAK AD-H gave separation of 36 of 45 basic compounds previously not separated in SFC. The mechanism appears to involve the separation of an intact salt pair formed between the basic compound and ESA. Other modifiers, other acids and one additional stationary phase were examined and found to yield additional separations.     

Effect of amine mobile phase additives on chiral subcritical fluid chromatography using polysaccharide stationary phases

Increased retention and selectivity in the subcritical fluid chromatography (SFC) of various amine compounds on polysaccharide chiral stationary phases (CSP) was observed upon incorporation of cyclic amines into the modifier. The retention increases are most pronounced with 2-propanol and are almost absent when methanol is used as modifier. This suggests that the effect may arise from a restriction to the modifier access to the binding site required to effect elution. The effect of the amine additives in SFC does not remain after their removal from the mobile phase. Findings were applied to the development of a 5 min separation of amphetamine and methamphetamine enantiomers.     

Possibility of predicting separations in supercritical fluid chromatography with the solvation parameter model

In order to obtain a selection of optimal chromatographic columns for the separation of chlorotriazine pesticides in packed column supercritical fluid chromatography (pSFC), a multi-criteria approach is applied. For this purpose, prediction of the separations is carried out, based on quantitative structure–retention relationships, then Derringer's desirability function is proposed to determine the stationary phase that will result in the most desirable separation. The best SFC separation obtained was then optimized using a mobile phase gradient. Besides, the accuracy of the solvation parameter model as SFC retention predictive model is assessed.     

Solute-solvent interactions from gas chromatographic activity coefficients and the solvation parameter model for nitrogen-containing stationary phases

Infinite dilution gas-liquid chromatographic activity coefficients (gamma) and excess thermodynamic molar partial magnitudes [Gibbs energy (G(E)), enthalpy (H(E)), and entropy (S(E))] for 37 solutes of varied polarity on four stationary phases with -NH groups are obtained from partition coefficients taken from literature. Relationships between G(E) and S(E) with the 37 solutes' structure in terms of the molecular connectivity index ((1)chi(v)) are investigated. Correlations of solute-solvent interactions calculated in light of the solvation parameter model for selected solutes and stationary phases are tested. The effect of the solute's structure, expressed as the molecular connectivity index, on the nonpolar (cavity formation and dispersion interaction) [c+l. log L(16)] and the effect of the dipole moment and of the activity coefficient on the dipolarity-polarizability interaction (spi(2)(H)) are studied. The correlation between the nonpolar interaction with the athermal activity coefficient on the nonpolymeric stationary phases is also attempted. In addition, the influence of the stationary phase polarity on the solute-stationary phase interactions for a series of solutes is studied.     

Studies on the long-term thermal stability of stationary phases in subcritical water chromatography

The long-term thermal stability of five commercially available reversed-phase columns has been evaluated under subcritical water conditions (100 and 150 degrees C). The five columns included Zorbax RX-C8, Nucleosil C18 AB, Hypersil BDS C18, PRP-1 (poly(styrene-divinylbenzene)), and ZirChrom-PS (polystyrene) columns. Retention factors, plate numbers, and peak areas were monitored over a prolonged period of time. Comparing the three silica-based columns, the Zorbax RX-C, column was the most stable followed by the Nucleosil C18, AB column. The Hypersil BDS C18 column was the least stable under subcritical water conditions. The ZirChrom-PS column was stable at 100 degrees C for at least 7600 column volumes. Of all five columns, the polymeric PRP-1 column was the most stable under subcritical water conditions.     

Liquid crystal stationary phases for gas chromatography and supercritical fluid chromatography

Novel monomeric and polymeric liquid crystalline compounds were synthesized as stationary phases for gas chromatography (GC) and supercritical fluid chromatography (SFC). Monomeric liquid crystalline compounds were used in packed column gas chromatography for the separation of isomeric aromatic compounds and insect sex pheromones. Liquid crystalline polymers possess long nematic ranges and a uniform coating was easily achieved in glass and fused silica capillaries, which could stand temperatures up to 250°C in GC and pressures of 200 MPa at 160°C in SFC. The columns provide excellent selectivity and resolution for fused ring aromatic compounds such as the isomers anthracene and phenanthrene or triphenylene and chrysene.     

Retention of cyanoalkanes and cyanoalkylbenzenes by bonded stationary phases in supercritical fluid chromatography

Summary The retention of cyanoalkanes and cyano-alkylbenzenes during SFC was investigated on alkyl and cyanoalkyl-bonded stationary phases and compared to alkane retention. The particular behaviour of short chain homologues was attributed to a silanophilic interaction with surface-OH groups on the silica and inhibition of specific interactions between solutes and C₁₀CN bonded groups.     

Interactions between 33 solutes and four cyano-containing stationary phases: gas chromatographic activity coefficients and the solvation parameter model

Infinite-dilution gas–liquid chromatographic activity coefficients at 393.15 K (with their thermal and athermal components) and derived excess partial molar Gibbs energies, enthalpies, and entropies have been determined for each of 33 solutes of different polarity on four stationary phases with cyano groups, using retention data taken from the literature. The strongest interactions predicted by the solvation model are the dipolarity/polarizability, the acidic solute–basic stationary phase interaction, and nonpolar cavity formation and dispersion. These interactions were compared with those evaluated from the solute activity coefficients; the effect of the solute connectivity index and dipole moment on nonpolar and polar interactions, respectively, is discussed. The dependence of the thermal activity coefficient on nonpolar interactions, and the influence of stationary phase polarity on the four solute–stationary phase interactions, were evaluated. The nonpolar interaction increases with increasing connectivity and with increasing athermal activity coefficient. The dipolarity/polarizability interaction increases with increasing solute dipole moment. Finally, polar interactions increase with increasing stationary phase polarity whereas the nonpolar interaction is independent of stationary phase polarity.     

多糖类手性固定相超临界流体色谱法分离手性化合物

采用超临界流体色谱法(SFC),在多糖固定相Chiralpak IA、IB、IC、ID、IE和IF上成功拆分了11种手性化合物。分离结果表明,这6支手性色谱柱对这些手性化合物具有良好的手性识别互补性,均可以在10 min之内得到良好的分离结果,具有较好的实用性。改性剂甲醇、乙醇和异丙醇对手性化合物的保留时间以及手性选择性均具有良好的调节作用,需要根据不同手性物质在手性柱上的分离情况加以区别,选择使用,并调节改性剂至合适的比例。针对键合型固定相溶剂通用性的特征,特殊改性剂的应用也有助于优化手性分离。