System Maps for Retention of Small Neutral Compounds on a Superficially Porous Ethyl-Bridged,Octadecylsiloxane-Bonded Silica Stationary Phase in Reversed-Phase Liquid Chromatography

The system constants of the solvation parameter model are used to prepare system maps for the retention of small neutral compounds on an ethyl-bridged, ocatadecylsiloxane-bonded superficially porous silica stationary phase (Kinetex EVO C18) for aqueous mobile phases containing 10–70% (v/v) methanol or acetonitrile. Electrostatic interactions (cation-exchange) are important for the retention of weak bases with acetonitrile–water but not methanol–water mobile phase compositions. Compared with a superficially porous octadecylsiloxane-bonded silica stationary phase (Kinetex C18) with a similar morphology but different topology statistically significant differences in selectivity at the 95% confidence level are observed for neutral compounds that vary by size and hydrogen-bond basicity with other intermolecular interactions roughly similar. These selectivity differences are dampened with acetonitrile–water mobile phases, but are significant for methanol–water mobile phase compositions containing <30% (v/v) methanol. A comparison of a totally porous ethyl-bridged, octadecylsiloxane-bonded silica stationary phase (XBridge C18) with Kinetex EVO C18 indicated that they are effectively selectivity equivalent.     

Insights into the Retention Mechanism of Small Neutral Compounds on Octylsiloxane-Bonded and Diisobutyloctadecylsiloxane-Bonded Silica Stationary Phases in Reversed-Phase Liquid Chromatography

The system constants of the solvation parameter model are used to prepare system maps for the retention of small neutral compounds on an octylsiloxane-bonded (Kinetex C8) and diisobutyloctadecylsiloxane-bonded (Kinetex XB-C18) superficially porous silica stationary phases for aqueous mobile phases containing 10–70% (v/v) methanol or acetonitrile. Electrostatic interactions (cation-exchange) are important for the retention of weak bases with acetonitrile–water but not for methanol–water mobile phases. Compared with an octadecylsiloxane-bonded silica stationary phase (Kinetex C18) retention is reduced due to a less favorable phase ratio for both the octylsiloxane-bonded and diisobutyloctadecylsiloxane-bonded silica stationary phases while selectivity differences are small and solvent dependent. Selectivity differences for neutral compounds are larger for methanol–water but significantly suppressed for acetonitrile–water mobile phases. The selectivity differences arise from small changes in all system constants with solute size and hydrogen-bond basicity being the most important due to their dominant contribution to the retention mechanism. Exchanging the octadecylsiloxane-bonded silica column for either the octylsiloxane-bonded or diisobutyloctadecylsiloxane-bonded silica column affords little scope for extending the selectivity space and is restricted to fine tuning of separations, and in some cases, to obtain faster separations due to a more favorable phase ratio. For weak bases larger differences in relative retention are expected with acetonitrile–water mobile phases on account of the additional cation exchange interactions possible that are absent for the octadecylsiloxane-bonded silica stationary phase.     

Insights into the Retention Mechanism for Small Neutral Compounds on Silica-Based Phenyl Phases in Reversed-Phase Liquid Chromatography

The system constants of the solvation parameter model are used to prepare system maps for the retention of small neutral compounds on phenylhexylsiloxane- and pentafluorophenylpropylsiloxane-bonded superficially porous silica stationary phases (Kinetex Phenyl-Hexyl and Kinetex F5) for aqueous mobile phases containing 10–70% (v/v) methanol or acetonitrile. Electrostatic interactions (cation exchange) are important for the retention of weak bases for acetonitrile–water mobile phases, but virtually absent for the same compounds for methanol–water mobile phases. The selectivity of the Kinetex Phenyl-Hexyl stationary phase for small neutral compounds is similar to an octadecylsiloxane-bonded silica stationary phase with similar morphology Kinetex C-18 for both methanol–water and acetonitrile–water mobile phase compositions. The Kinetex Phenyl-Hexyl and XBridge Phenyl stationary phases with the same topology but different morphology are selectivity equivalent, confirming that solvation of the interphase region can be effective at dampening selectivity differences for modern stationary phases. Small selectivity differences observed for XTerra Phenyl (different morphology and topology) confirm previous reports that the length and type of space arm for phenylalkylsiloxane-bonded silica stationary phases can result in small changes in selectivity. The pentafluorophenylpropylsiloxane-bonded silica stationary phase (Kinetex F5) has similar separation properties to the phenylhexylsiloxane-bonded silica stationary phases, but is not selectivity equivalent. However, for method development purposes, the scope to vary separations from an octadecylsiloxane-bonded silica stationary phase (Kinetex C-18) to “phenyl phase” of the types studied here is limited for small neutral compounds. In addition, selectivity differences for the above stationary phases are enhanced by methanol–water and largely suppressed by acetonitrile–water mobile phases. For bases, larger selectivity differences are possible for the above stationary phases if electrostatic interactions are exploited, especially for acetonitrile-containing mobile phases.     

Insights into the Retention Mechanism on a Pentafluorophenylpropylsiloxane-Bonded Silica Stationary Phase (Discovery HS F5) in RP-LC

The solvation parameter model is used to elucidate the retention mechanism of neutral compounds on the pentafluorophenylpropylsiloxane-bonded silica stationary phase (Discovery HS F5) with methanol-water and acetonitrile-water mobile phases containing from 10 to 70% (v/v) organic solvent. The dominant factors that increase retention are solute size and electron lone pair interactions while polar interactions reduce retention. A comparison of the retention mechanism with an octadecylsiloxane-bonded silica stationary phase based on the same silica substrate and with a similar bonding density (Discovery HS C18) provides additional insights into selectivity differences for the two types of stationary phase. The methanol-water solvated pentafluorophenylpropylsiloxane-bonded silica stationary phase is more cohesive and/or has weaker dispersion interactions and is more dipolar/polarizable than the octadecylsiloxane-bonded silica stationary phase. Differences in hydrogen-bonding interactions contribute little to relative retention differences. For mobile phases containing more than 30% (v/v) acetonitrile selectivity differences for the pentafluorophenylpropylsiloxane-bonded and octadecylsiloxane-bonded silica stationary phases are no more than modest with differences in hydrogen-bond acidity of greater importance than observed for methanol-water. Below 30% (v/v) acetonitrile selectivity differences are more marked owing to incomplete wetting of the octadecylsiloxane-bonded silica stationary phase at low volume fractions of acetonitrile that are not apparent for the pentafluorophenylpropylsiloxane-bonded silica stationary phase. Steric repulsion affects a wider range of compounds on the octadecylsiloxane-bonded than pentafluorophenylpropylsiloxane-bonded silica stationary phase with methanol mobile phases resulting in additional selectivity differences than predicted by the solvation parameter model. Electrostatic interactions with weak bases were unimportant for methanol-water mobile phase compositions in contrast to acetonitrile-water where ion-exchange behavior is enhanced, especially for the pentafluorophenylpropylsiloxane-bonded silica stationary phase. The above results are compatible with a phenomenological interpretation of stationary phase conformations using the haystack, surface accessibility, and hydro-linked proton conduit models.     

Comparison of retention of aromatic hydrocarbons with polar groups in binary reversed-phase high-performance liquid chromatography systems

The retention of aromatic hydrocarbons with polar groups has been correlated as log k1 versus log k2 for reversed-phase high-performance liquid chromatography systems with different binary aqueous mobile phases containing methanol, acetonitrile or tetrahydrofuran as modifiers. Distinct changes in separation selectivity have been observed between tetrahydrofuran and acetonitrile or methanol systems. Methanol and acetonitrile systems show lower diversity of separation selectivity. The changes in retention and selectivity of aromatic hydrocarbons with various polar groups between any two chromatographic systems with binary aqueous eluents (tetrahydrofuran vs. acetonitrile, tetrahydrofuran vs. methanol and methanol vs. acetonitrile) have been interpreted in terms of molecular interactions of the solute with especially one component of the stationary phase region, i.e. extracted modifier, and stationary phase ordering. The ordering of the stationary phase region caused by modifier type influences the chromatographic selectivity of solutes with different molecular shape.     

Influence of solvent effects on retention in reversed-phase liquid chromatography and solid-phase extraction using a cyanopropylsiloxane-bonded,silica-based sorbent

Summary The solvation parameter model is used to characterize the retention properties of a cyanopropylsiloxanebonded, silica-based sorbent with methanol, acetonitrile, tetrahydrofuran, and isopropanol in water as mobile phases. The system constants over the composition range 1 to 50 % (v/v) organic solvent indicate that retention occurs because of the relative ease of cavity formation in the solvated stationary phase compared to the same process in the predominantly aqueous mobile phase as well as from more favorable stationary phase interactions with solutes containing π- and n-electrons. The capacity of the solute for dipole-type interactions is not important whereas all hydrogen-bond-type interactions result in reduced retention. Graphing the system constants as a function of mobile phase composition provides a simple mechanism for interpreting the change in capacity of the chromatographic system for retention in terms of changes in the relative weighting of fundamental intermolecular interactions. A comparison is also made with the retention properties of an octadecylsiloxane-bonded, silica-based sorbent with 30 % (v/v) methanol in water as the mobile phase and the extraction characteristics of a porous polymer sorbent with 1 % (v/v) methanol, acetonitrile, tetrahydrofuran, and isopropanol in water as the sample processing solvent. Changes in sorbent selectivity due to selective uptake of the processing solvent are much smaller for the cyanopropylsiloxane-bonded sorbent than the results found for a porous polymer sorbent.     

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.     

Influence of solvent effects on retention in reversed-phase liquid chromatography and solid-phase extraction using a cyanopropylsiloxane-bonded,silica-based sorbent

Summary The solvation parameter model is used to characterize the retention properties of a cyanopropylsiloxane-bonded, silica-based sorbent with methanol, acetonitrile, tetrahydrofuran, and isopropanol in water as mobile phases. The system constants over the composition range 1 to 50% (v/v) organic solvent indicate that retention occurs because of the relative ease of cavity formation in the solvated stationary phase compared to the same process in the predominantly aqueous mobile phase as well as from more favorable stationary phase interactions with solutes containing - and n-electrons. The capacity of the solute for dipole-type interactions is not important whereas all hydrogen-bond-type interactions result in reduced retention. Graphing the system constants as a function of mobile phase composition provides a simple mechanism for interpreting the change in capacity of the chromatographic system for retention in terms of changes in the relative weighting of fundamental intermolecular interactions. A comparison is also made with the retention properties of an octadecylsiloxane-bonded, silica-based sorbent with 30% (v/v) methanol in water as the mobile phase and the extraction characteristics of a porous polymer sorbent with 1% (v/v) methanol, acetonitrile, tetrahydrofuran, and isopropanol in water as the sample processing solvent. Changes in sorbent selectivity due to selective uptake of the processing solvent are much smaller for the cyanopropylsiloxane-bonded sorbent than the results found for a porous polymer sorbent.     

A simple TLC and HPTLC method for separation of selected steroid drugs

Chromatographic properties of five steroid drugs: cortisone, hydrocortisone, methylprednisolone, prednisolone and norgestrel have been studied by normal-, reversed-phase and hydrophilic neutral cyano-bonded silica stationary phase with five binary mobile phases (acetonitrile-water, acetonitrile-DMSO, acetonitrile-methanol, acetone-petroleum ether, acetone-water) in which the concentration of organic modifier was varied from 0 to 100% (v/v). This study reports the optimization of steroid hormones separation. Chromatographic retention data and possible retention mechanisms are discussed. Separation abilities of mobile and stationary phases were studied using the principal component analysis method. The best separation of methylprednisolone and prednisolone is with a chromatographic system included silica gel as stationary phase and mixture of acetonitrile and DMSO (10:90 v/v). These two anti-inflammatory drugs can be fast separated from norgestrel when CN is used as stationary phase and acetone and water (40:60 v/v) as mobile phase. The highest values of the parameter Δ(ΔG°) and alfa for cortisone and hydrocortisone was observed in case of using CN as stationary phase and water-acetonitryle (40:60 v/v) as mobile phase.      

Evaluation of ternary mobile phases for reversed-phase liquid chromatography: Effect of composition on retention mechanism

The effect of varying mobile phase composition across a ternary space between two binary compositions is examined, on four different reversed-phase stationary phases. Examined stationary phases included endcapped C8 and C18, as well as a phenyl phase and a C18 phase with an embedded polar group (EPG). Mobile phases consisting of 50% water and various fractions of methanol and acetonitrile were evaluated. Retention thermodynamics are assessed via use of the van’t Hoff relationship, and retention mechanism is characterized via LSER analysis, as mobile phase composition was varied from 50/50/0 water/methanol/acetonitrile to 50/0/50 water/methanol acetonitrile. As expected, as the fraction of acetonitrile increases in the mobile phase, retention decreases. In most cases, the driving force for this decrease in retention is a reduction of the enthalpic contribution to retention. The entropic contribution to retention actually increases with acetonitrile content, but not enough to overcome the reduction in the enthalpic contribution. In a similar fashion, as methanol is replaced with acetonitrile, the v, e, and a LSER system constants change to favor elution, while the s and c constants change to favor retention. The b system constant did not show a monotonic change with mobile phase composition. Overall changes in retention across the mobile phase composition range varied, based on the identity of the stationary phase and the composition of the mobile phase.     

Mobile phase effects on retention on a new butylimidazolium-based high-performance liquid chromatographic stationary phase

A new HPLC stationary phase based on n-butylimidazolium bromide has been characterized by a linear solvation energy relationship (LSER) approach in the binary acetonitrile/water mobile phases. The retention properties of the stationary phase were systematically evaluated in terms of intermolecular interactions between 28 test solutes and the stationary phase. The results and further comparisons with conventional reversed phase system confirm that retention properties are similar to phenyl phases in acetonitrile/water mixtures. The results obtained with acetonitrile/water mixtures are also compared with results obtained using methanol/water mixtures.     

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.     

Characterization of some functionalized RP-HPLC phases by the use of linear solvation energy relationships

The linear solvation energy relationship equation developed by Abraham and coworkers was applied to the retention factors k of a series of 20 polar solutes on four chemically different RP-HPLC phases. Three of them were specially synthesized and are functionalized with ether, phenylsulfide or phenylsulfoxide groups. Their retention properties are compared with those of a nonpolar octadecylsiloxane (ODS) phase. The phase properties r, the excess molar refraction; s, the dipolarity; a and b, the hydrogen-bond basicity and acidity; and v, the cavity factor show significant differences on the four phases and are used here to suggest a classification of stationary phases based on the type of interactions that are important for the retention. The hydrophilic system properties r, s, a and b are the reason for different elution orders of a set of solutes on the four phases. The intrinsic hydrophobicity of the system, the v/A ratio (A is the surface coverage in μmol/m²), shows a dependence on the mobile phase composition as do the normalized phase properties r/v, s/v, a/v and b/v. Averaging the constants over a large span of mobile phase composition should be done very carefully. The LSER model is used to predict the elution order on the stationary phases for five phenols which show coelution on ODS. On the phenylsulfide phase they are resolved. Received: 3 December 1998 / Revised: 1 February 1999 / Accepted: 8 February 1999     

Effects of stationary-phase polarity on retention in reversed bonded phase HPLC columns

Summary Variations in retention and selectivity have been studied in cyano, phenyl and octyl reversed bonded phase HPLC columns. The retention of toluene, phenol, aniline and nitrobenzene in these columns has been measured using binary mixtures of water and methanol, acetonitrile or tetrahydrofuran mobile phases in order to determine the relative contributions of proton donor-proton acceptor and dipole-dipole interactions in the retention process. Retention and selectivity in these columns was correlated with polar group selectivities of mobile phase organic modifiers and the polarity of the bonded stationary phases. In spite of the prominent role of bonded phase volume and residual silanols in the retention process, each column exhibited some unique selectivities when used with different organic modifiers.     

Insights into the retention mechanism on an octadecylsiloxane-bonded silica stationary phase (HyPURITY C18) 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 octadecylsiloxane-bonded silica stationary phase HyPURITY C18 with methanol-water and acetonitrile-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. (2), and exhibited normal retention behavior for the full composition range. The solutes in category 2 exhibited normal retention behavior at high organic solvent composition with a discontinuity at low organic solvent 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 solutes 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 not included in the models. The overwhelming number of residual values [log k (experimental) - log k (model predicted)] were negative and could be explained by contributions from steric repulsion, defined as the inability of the solute to insert itself fully into the stationary phase because of its bulkiness (i.e., volume and/or shape). Steric repulsion is shown to strongly depend on the mobile phase composition and was more significant for mobile phases with a low volume fraction of organic solvent in general and for mobile phases containing methanol rather than acetonitrile. For mobile phases containing less than about 20 % (v/v) organic solvent the mobile phase was unable to completely wet the stationary phase resulting in a significant change in the phase ratio and for acetonitrile (but less so methanol) changes in the solvation environment indicated by a discontinuity in the system maps.     

Selectivity equivalence of poly(ethylene glycol) stationary phases for gas chromatography

The solvation parameter model is used to study differences in selectivity for poly(ethylene glycol) stationary phases for packed column (Carbowax 20M) and fused-silica, open-tubular column (HP-20M, AT-Wax, HP-INNOWax and DB-FFAP) gas chromatography. All phases are dipolar, strongly hydrogen-bond basic with no hydrogen-bond acidity and of moderate cohesion. No two phases are exactly alike, however, and selectivity differences identified with cavity formation and dispersion interactions, n- and pi-electron pair interactions, dipole-type interactions and hydrogen-bond interactions are quantified by differences in the system constants at a fixed temperature where retention occurs solely by gas-liquid partitioning. The system constants vary linearly with temperature over the range 60-140 degrees C (except for n- and pi-electron pair interactions which are temperature invariant) facilitating a general comparison of the importance of temperature on selectivity differences for compared phases. From a mechanistic point of view it is demonstrated that selectivity differences can result from chemical differences between the poly(ethylene glycol) stationary phases and from differences in the relative contribution of interfacial adsorption to the retention mechanism. The latter depends on both system properties and solute characteristics.     

Retention characteristics of an immobilized artificial membrane column in reversed-phase liquid chromatography

Retention for a varied group of compounds on an immobilized artificial membrane column (IAM PC DD2) with a methanol-water mobile phase is shown to fit a second-order model for the retention factor (log k) as a function of the volume fraction of organic solvent. The numerical value of the intercept obtained by linear extrapolation to zero organic solvent (log k(w)) is shown to depend on the range of mobile phase composition used for the extrapolation. Each series of intercepts so obtained represents a different hypothetical distribution system as identified by the system constants of the solvation parameter model. Although a linear model is a poor fit for isocratic retention data, the linear solvent strength gradient model provides a reasonable estimate of isocratic retention factor values that are (slightly) larger than experimental values, but provide the same chemical information for the system. These preliminary results suggest that gradient elution may prove to be a rapid and useful method for creating system maps for column characterization and method development. In this work a system map is provided for methanol-water compositions from 0 to 60% (v/v) methanol and additional system constants for acetonitrile-water compositions containing 20 and 30% (v/v) acetonitrile. It is shown that the main factors contributing to retention on the IAM PC DD2 column are favorable cavity formation and dispersion interactions, electron lone pair interactions and the hydrogen-bond basicity of the sorbent. The latter feature more than any other distinguishes the IAM column from conventional chemically bonded phases. Interactions of a dipole-type (weakly) and inability to compete with the mobile phase as a hydrogen-bond acid reduce retention. A comparison of system constant ratios is used to demonstrate that the retention properties of the IAM column are not easily duplicated by conventional chemically bonded phases. The retention characteristics of the IAM column, however, are strongly correlated with the retention properties of pseudostationary phases used for micellar electrokinetic chromatography, which provide a suitable alternative to IAM columns for physical property estimations. By the same comparative method it is shown that retention on the IAM column possesses some similarity to biomembrane absorption processes, allowing suitable correlation models to be developed for the estimation of certain biopartitioning properties.     

Thermodynamic and kinetic characterization of a bridged-ethylene hybrid C18 stationary phase

In this study, a series of polycyclic aromatic hydrocarbons (PAHs) and nitrogen-containing polycyclic aromatic hydrocarbons (NPAHs) is separated on a hybrid stationary phase using methanol and acetonitrile mobile phases. Temperature is varied from 283 to 313 K in order to determine thermodynamic and kinetic parameters of the separation. Thermodynamic behavior is characterized by the retention factor and associated changes in molar enthalpy, whereas kinetic behavior is characterized by the rate constants and associated activation energies. In this study, the retention factors for the NPAHs are smaller than those for the parent PAHs in methanol, while they are more similar to the parent PAHs in acetonitrile. The changes in molar enthalpy are very similar for all solutes, yet are more negative in acetonitrile than in methanol. The rate constants for the NPAHs are smaller than those for their parent PAHs in both mobile phases. Moreover, the rate constants in acetonitrile are one to four orders of magnitude smaller than those in methanol. Based on these thermodynamic and kinetic results, the hybrid stationary phase is compared to traditional silica stationary phases. In addition, the relative contributions from the partition and adsorption mechanisms are discussed.     

Selectivity in reversed-phase separations Influence of the stationary phase

The selectivity difference between 15 different stationary phases was measured using a large number of analytes at 2 or 3 different pH values (3, 7 and 10) with acetonitrile and methanol as the mobile phase modifiers. The packings discussed include standard C(8) and C(18) packings, packings with embedded polar groups, a phenyl packing, a pentafluoro-phenyl packing, an adamantylethyl packing and others. The major selectivity differences observed are discussed in detail. Specific effects such as pi-pi interactions on phenyl packings or hydrogen-bond interactions on phases with embedded polar groups are confirmed.