On the mechanism of solute retention in RP-HPLC systems with B+AB1+AB2 type eluents

Summary Intermolecular interactions between the solute and the moieties constituting the mobile phase significantly contribute to the overall retention pattern of a given solute in a given chromatographic system. In this paper retention of solute is discussed in the case of the B+AB₁+AB₂ type mobile phase, which, in a quasithermodynamic way, can be divided into seven individual moieties. One evaluates the influence of each moiety on solute retention, and refers these regularities to the polarity of the solutes.     

Selectivity comparison of acetonitrile-methanol-water ternary mobile phases on an octadecylsiloxane-bonded silica stationary phase

The solvation parameter model was used in this study to investigate various intermolecular interactions that influence retention on the standard C18 stationary phase for the solvent system acetonitrile:methanol (ACN:MeOH, 1:1). In comparison to the organic mobile phase modifiers acetonitrile, acetone, methanol, 2-propanol, and tetrahydrofuran, the solvent strength for the ACN:MeOH (1:1) solvent system was evaluated. To facilitate the interpretation of various intermolecular interactions that contribute to retention on a standard C18 stationary phase for the solvent system ACN:MeOH (1:1), system maps were constructed and compared with those of acetone, tetrahydrofuran, acetonitrile, 2-propanol, and methanol. The solvation parameter models were constructed for the ternary solvent system ACN:MeOH (1:1)-water, and in the models constructed, the coefficient of determination values were from 0.998 to 0.999, the Fisher statistic values for the models were from 1687 to 4015, and the standard error of the estimate values ranged from 0.022 to 0.029. The solvent system ACN:MeOH (1:1) has retention properties more similar to methanol than acetonitrile, indicating methanol's influence is more dominant.     

Lewis acid-base titrations employing megacycle-frequency oscillators : Part III. Preparation,isolation and characterization of some adducts predicted from titration curves

Adducts of stannic chloride with thirteen oxygen bases, isolated as solids from benzene or heptane solution, were characterized by analysis and melting point behavior; several of these are new compounds. Diethyl ether, n-dibutyl ether and tetrahydrofuran form AB₂ adducts; p-dioxane, in conformity with its being a diacid base, forms only a 1 : 1 species. Methyl, ethyl, n-propyl and n-butyl alcohols form simple AB₂ adducts at room temperature which on recrystallization from boiling solvent give SnCl₃(OR)ROH; iso-propyl alcohol gives only the AB₂ adduct; tert.-butyl alcohol only forms SnCl₃. OC₄H₉ (steric hindrance probably prevents formation of the AB2 adduct). Water gives a AB₄ or AB₅ adduct.Adducts of aluminum chloride with three nitrogen bases were similarly prepared from acetonitrile solution. Pyridine forms a 1 : 1 species. Acetonitrile itself forms a AB₂ species. Piperidine forms a AB₃ species. The latter two adducts support the view that AlCI₃ can form other than 1 : 1 adducts with nitrogen bases.     

Hyperbranched aryl polycarbonates derived from A2B monomers versus AB2 monomers

Hyperbranched aryl polycarbonates were prepared via the polymerizations of A₂B and AB₂ monomers, which involved the condensation of chloroformate (A) functionalities with tert‐butyldimethylsilyl‐protected phenols (B), facilitated by reactions with silver fluoride. The polymerization of the A₂B monomer gave hyperbranched polycarbonates bearing fluoroformate chain ends, which were hydrolyzed to phenolic chain‐end moieties and further elaborated to tert‐butyldimethylsilyl ether groups. The polymerization of the AB₂ monomer gave tert‐butyldimethylsilyl ether‐terminated hyperbranched polycarbonates. The polymerizations were conducted at 23–70 °C in 20% acetonitrile/tetrahydrofuran in the presence of a stoichiometric excess of silver fluoride for 20–40 h to afford hyperbranched polycarbonates with weight‐average molecular weights exceeding 100,000 Da and polydispersity indices of typically 2–3. The degrees of branching were determined by a reductive degradation procedure followed by high‐performance liquid chromatography. Alternatively, the degrees of branching were measurable by solution‐state ¹H NMR analyses and agreed with the statistical 50% branching expected for the polymerization of A₂B and AB₂ monomers not experiencing constructive or destructive electronic effects on the reactivity of the multiple functional groups. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 823–835, 2002; DOI 10.1002/pola.10167     

Retention Time of Unretained Compound Calculation and Determination of Retention Indices of Some Monosubstituted Benzenes Using a Multiparametric Method in Binary, Ternary and Quaternary Solvent RP-LC Systems

In reversed phase liquid chromatography (RP-LC), the validity of a multiparametric (MP) non-linear least-squares regression iterative method has been evaluated for 14 different aqueous mobile phases modified with one, two or three organic solvents [acetonitrile (ACN), methanol (MeOH), or tetrahydrofuran (THF)] for calculating the retention time of unretained compound t _M and the regression parameter (slope b), based on the use of alkan-2-ones and alkyl aryl ketones homologous series. The determination of t _M and b has been studied for eight binary (ACN?CH₂O or MeOH?CH₂O), 3 ternary (ACN?CMeOH?CH₂O) and 3 quaternary (ACN?CMeOH?CTHF?CH₂O) mobile phase systems on an Omnispher C₁₈ column. The multiparametric calculated t _M and b values were compared with those obtained by Guardino??s, and Grobler??s methods. The MP retention indices (RI) of ten monosubstituted benzenes with different functionality (hydroxyl, carbonyl, nitro, etc.) based on the alkan-2-ones retention index standards have been determined and compared for the different mobile phase compositions studied. The influence of organic modifier type, the nature of mobile phase system and water content on the variation of retention parameter studied in this work were discussed.     

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.     

Multiple Linear Regression Analysis Of The Retention Data For Several Polynuclear Aromatic Hydrocarbons

Abstract

The retention data for benzene, toluene, naphthalene, cumeme, biphenyl, and durene in ternary (methanol, tetrahydrofuran, water) and quaternary (methanol, tetrahydrofuran, acetonitrile, water) solvent systems has been reduced to linear equations relating the retention volume to solvent composition. In particular, attention is focused on the biphenyl, naphthalene, cumene system and comments are made as to the optimum separation conditions.     

Retention and selectivity of aromatic hydrocarbons with mono-substituted polar groups in ternary RP-HPLC systems

Summary The variation in selectivity for aromatic hydrocarbons with mono-substituted polar groups is investigated in ternary mobile-phase systems on C₁₈ stationary phases. The dependence of log k′ on the proportions of two modifiers was determined for the solutes within the concentration range of ternary systems obtained by mixing two binary eluents: methanol + water and tetrahydrofuran + water or acetonitrile + water and tetrahydrofuran + water. The nature of the relationships is explained in terms of molecular shape, molecular interactions between solute and extracted modifier and the ordering of solvated ligands on the stationary phase.     

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.     

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.     

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.     

Combined column–mobile phase mixture statistical design optimization of high-performance liquid chromatographic analysis of multicomponent systems

A statistical approach for the simultaneous optimization of the mobile and stationary phases used in reversed-phase liquid chromatography is presented. Mixture designs using aqueous mixtures of acetonitrile (ACN), methanol (MeOH) and tetrahydrofuran (THF) organic modifiers were performed simultaneously with column type optimization, according to a split-plot design, to achieve the best separation of compounds in two sample sets: one containing 10 neutral compounds with similar retention factors and another containing 11 pesticides. Combined models were obtained by multiplying a linear model for column type, C8 or C18, by quadratic or special cubic mixture models. Instead of using an objective response function, combined models were built for elementary chromatographic criteria (retention factors, resolution and relative retention) of each solute or pair of solutes and, after their validation, the global separation was accomplished by means of Derringer's desirability functions. For neutral compounds a 37:12:8:43 (v/v/v/v) percentage mixture of ACN:MeOH:THF:H₂O with the C18 column and for pesticides a 15:15:70 (v/v/v) ACN:THF:H₂O mixture with the C8 column provide excellent resolution of all peaks.     

Critical assessment of three high performance liquid chromatography analytical methods for food carotenoid quantification

Three sets of extraction/saponification/HPLC conditions for food carotenoid quantification were technically and economically compared. Samples were analysed for carotenoids α-carotene, β-carotene, β-cryptoxanthin, lutein, lycopene, and zeaxanthin. All methods demonstrated good performance in the analysis of a composite food standard reference material for the analytes they are applicable to. Methods using two serial connected C₁₈ columns and a mobile phase based on acetonitrile, achieved a better carotenoid separation than the method using a mobile phase based on methanol and one C₁₈-column. Carotenoids from leafy green vegetable matrices appeared to be better extracted with a mixture of methanol and tetrahydrofuran than with tetrahydrofuran alone. Costs of carotenoid determination in foods were lower for the method with mobile phase based on methanol. However for some food matrices and in the case of E–Z isomer separations, this was not technically satisfactory. Food extraction with methanol and tetrahydrofuran with direct evaporation of these solvents, and saponification (when needed) using pyrogallol as antioxidant, combined with a HPLC system using a slight gradient mobile phase based on acetonitrile and a stationary phase composed by two serial connected C₁₈ columns was the most technically and economically favourable method.     

Enhanced‐fluidity liquid chromatography using mixed‐mode hydrophilic interaction liquid chromatography/strong cation‐exchange retention mechanisms

The potential of enhanced‐fluidity liquid chromatography, a subcritical chromatography technique, in mixed‐mode hydrophilic interaction/strong cation‐exchange separations is explored, using amino acids as analytes. The enhanced‐fluidity liquid mobile phases were prepared by adding liquefied CO₂ to methanol/water mixtures, which increases the diffusivity and decreases the viscosity of the mixture. The addition of CO₂ to methanol/water mixtures resulted in increased retention of the more polar amino acids. The “optimized” chromatographic performance (achieving baseline resolution of all amino acids in the shortest amount of time) of these methanol/water/CO₂ mixtures was compared to traditional acetonitrile/water and methanol/water liquid chromatography mobile phases. Methanol/water/CO₂ mixtures offered higher efficiencies and resolution of the ten amino acids relative to the methanol/water mobile phase, and decreased the required isocratic separation time by a factor of two relative to the acetonitrile/water mobile phase. Large differences in selectivity were also observed between the enhanced‐fluidity and traditional liquid mobile phases. A retention mechanism study was completed, that revealed the enhanced‐fluidity mobile phase separation was governed by a mixed‐mode retention mechanism of hydrophilic interaction/strong cation‐exchange. On the other hand, separations with acetonitrile/water and methanol/water mobile phases were strongly governed by only one retention mechanism, either hydrophilic interaction or strong cation exchange, respectively.     

Separation of water-soluble vitamins on silica modified with L-cysteine–stabilized gold nanoparticles using HPLC

Silica functionalized with 3-aminopropyltriethoxysilane and L-cysteine–stabilized gold nanoparticles has been obtained (SiO₂–NH₂–Au–L-cysteine). The influence of pH and the content of acetonitrile in the mobile phase on retention and separation selectivity of eight vitamins has been studied. The chromatographic conditions that enable the separation of C, B3, B12, B5 and B1, B2, B6, B10 mixtures of vitamins have been proposed. It has been found that it is possible to separate a mixture of vitamins C, B3, B12, and B5 in 8 min, and a mixture of vitamins B1, B2, B6, B10 in 12 min on the synthesized sorbent SiO₂–NH₂–Au–L-cysteine in isocratic elution mode.     

Combined Effect of Temperature and Ternary Mobile Phase Composition on the Retention in Ternary Isocratic and Gradient Elution RP-LC under Isothermal Conditions. Application to the Retention Prediction of Four Macrolide Antibiotics

Retention models considering simultaneously ternary mobile phase organic contents and column temperature (T) were developed by a direct combination of equations expressing separately the dependence of the retention upon each of these factors. Thus, a combination of a linear dependence of the logarithm of the solute retention factor, ln k (T), against 1/T, i.e. ln k (T) = a + b/T, and of either a linear or a quadratic dependence of ln k upon the compositions of a binary mobile phase, gave two different expressions for the logarithm of the solute retention factor in terms of both temperature and organic contents in the ternary mobile phase. The effectiveness of the above models was tested in the prediction of isothermal retention of a mixture of four macrolide antibiotics under ternary isocratic and gradient elution conditions using mobile phases modified by methanol and acetonitrile. The limiting case of using ternary eluents with constant ratio of the concentrations of the two organic modifiers was also tested.     

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.