Effect of Ionic Liquids as Mobile-Phase Additives on Chromatographic Parameters of Neuroleptic Drugs in Reversed-Phase High-Performance Liquid Chromatography

Abstract

Ionogenic basic compounds belonging to phenothiazine derivatives were analyzed in the reversed-phase system and were modified with the addition of three ionic liquids: 1-ethyl-3-methyl-imidazolium hexafluorophospate (EMIM PF₆), 1-butyl-3-methyl-imidazolium hexafluorophosphate (BMIM PF₆), and 1-butyl-3-methyl-imidazolium chloride (BMIM Cl). The effects of the concentration and the type of ionic liquid on the analytes' retention, peak symmetry, and efficiency were examined. The following trends increase analytes' retention factor and improve system efficiency: BMIM PF₆ > EMIM PF6 > BMIM Cl. With its asymmetric cation enlarged with hydrophobic substituents and a chaotropic anion, BMIM PF₆ appeared to be the most advantageous one. The isotherm of adsorption of this reagent presents a typical Langmuir course. By the application of high-performance liquid chromatography, lipophilicity parameters were established (lnk_w, S, ? ₀) for the investigated compounds. Chromatographic systems modified with ionic liquids were compared to buffered organic–aqueous mobile phase and eluent containing chaotropic salt additive.     

Comparison of chaotropic salt and ionic liquid as mobile phase additives in reversed-phase high-performance liquid chromatography of biogenic amines

Highly hydrophilic compounds belonging to biogenic amines were analysed in the reversed-phase system, modified with the addition of ionic liquids: 1-ethyl-3-methyl-imidazolium hexafluorophosphate (EMIM PF(6)) and chaotropic salt NaPF(6) on Discovery HS C18 column at acidic conditions. The effect of the additives concentration and the presence of organic solvent on the analytes' chromatographic parameters such as retention factor, tailing factor and theoretical plates number were all determined. On the basis of k versus ionic liquid concentration in aqueous-organic mobile phase with constant amount of phosphate buffer, retention mechanism was studied. It was established that the presence of organic solvent with low dielectric constant and ionic liquid with both chaotropic ions allows achieving typical Langmuir shape of this relationship. Investigated mobile phase additives are comparable according to efficiency and selectivity towards biogenic amines analysis. However, the sensitivity was found to be better for the eluent system modified with chaotropic salt.     

Influence of inorganic mobile phase additives on the retention and separation efficiency of selected amino acids in thin-layer chromatography on cellulose layers

Selected amino acid standards are investigated on cellulose layers using organic-aqueous eluent systems modified with neutral and chaotropic salts: chlorides, iodides, nitrates, thiocyanates, perchlorates, and hexafluorophosphates at low concentrations ranging from 10 up to 80mM in whole mobile phase. The effect of salts used as the mobile phase modifier is analyzed by the comparison of densitograms, peak symmetry coefficient, and theoretical plate number. The efficiency of chromatographic systems modified with inorganic salts additives depends primarily on the kind of salt and organic solvent in the mobile phase. The best efficiency is obtained through the addition of ammonium thiocyanate to the mobile phase containing acetonitrile as an organic modifier.     

Usefulness of the chaotropic effect in sample preparation for chromatographic analysis of acidic xenobiotics in human plasma

In this study a new RP‐HPLC with photo‐diode array detection method for the determination of ibuprofen ((RS)‐2‐(4‐isobutylphenyl)propionic acid) in human plasma samples was developed. Samples were prepared by SPE and analyzed by an isocratic elution mode over a C₁₈ column using 80% methanol. A novel sample pretreatment method, based on the addition of ionic liquids possessing chaotropic ions to small human plasma sample (100 μL), was elaborated. 1‐Butyl‐3‐methylimidazolium chloride and 1‐butyl‐3‐methylimidazolium tetrafluoroborate (BMIM BF₄) were tested from the point of view of extraction yield. Quantification was based on calibration curve applying diclofenac as the internal standard. Owing to dilution of plasma sample by 2 mM aqueous solution of BMIM BF₄ before SPE, appropriate sample purification and extraction yields higher than 95% with precision lower than 2% can be achieved. Linear coefficients of correlation (r²) were >0.99 in the range of 0.3–5 μg/mL ibuprofen concentration in plasma. The limit of quantification was 65 ng/mL and the detection limit for ibuprofen was 19.5 ng/mL.     

Identification of ionic liquid components by RP‐HPLC with diode array detector using chaotropic effect and perturbation technique

Sodium hexafluorophosphate, perchlorate and tetrafluoroborate were applied as the ion‐pair reagents in reversed‐phase chromatography of several imidazolium‐based ionic liquids. The optimization of the retention was performed by changing the kind of organic modifier (methanol, acetonitrile), concentration and the kind of the ion‐pair reagents in the mobile phase and the column kind (Zorbax SB‐C18, Zorbax SB‐Phenyl, Zorbax SB‐CN, Zorbax SB‐NH₂ and Supelcosil LC‐F). The selectivity of the proposed chromatographic systems according to the cation kind was compared on the basis of the resolution of ionic liquid mixture. The perturbation method was applied to identify the anion kind. The formation of ion‐associated complexes between promethazine as counter‐cation and chaotropic anions controlling their retention was confirmed.     

A New Anionic Micellar Mobile Phase System for the Normal-Phase Thin-Layer Chromatography of Amino Compounds: Simultaneous Separation of Aminophenol Isomers

The chromatography of some amino compounds was performed on high-performance thin-layer plates (silica-gel 60 F₂₅₄, catalog no. 1.05554, Merck, Germany) using aqueous and alcoholic (MeOH) and aqueous–organic solutions of cationic, anionic, and nonionic surfactants. The results obtained with 0.01 M aqueous sodium dodecyl sulfate (SDS) have been compared with distilled water (i.e., zero SDS) as the eluent. The effects of the surfactant concentration, the nature of the alcohol, and the presence of inorganic salts (NaCl, LiCl, CaCl₂, and CoCl₂) in the mobile phase were examined in order to understand the mobility pattern of amino compounds. Among the added salts, CaCl₂ was found to be the most effective for facilitating an analytically important separation of coexisting ortho-, meta-, and para-aminophenols. The TLC system consisting of a precoated high-performance silica layer as the stationary phase and 0.01 M methanolic SDS plus 0.1 M CaCl₂ in a 3 : 7 ratio as the mobile phase was identified as the most suitable system for the separation of o-, m-, and p-APHS from their mixtures. The influence of various impurities, such as amines, phenols, and inorganic cations, on the mobility and the separation of coexisting aminophenol isomers has also been examined. The lower limits of detection of aminophenols were determined on HPTLC plates using I₂ vapors as the detection reagent.     

Chromatographic determination of solubilities in superheated water

Superheated water (SHW) is an effective solvent for the extraction of a variety of environmental pollutants, but knowledge of the solubilities in water at elevated temperatures necessary to maximise the efficiency of the process is often lacking. Ambient temperature aqueous solubilities have been measured by reverse‐phase HPLC from correlations with retention factors, k, but for poorly soluble organics the eluent must contain a proportion of organic modifier followed by extrapolation to pure water. The use of SHW as mobile phase allows direct determination of aqueous solubility from measurement of k on a modified HPLC system in which the eluent is cooled before detection to improve baseline stability. Alumina‐bonded octadecylsilane columns were found to be more stable in SHW chromatography than their silica‐bonded counterparts. To validate the procedure, measurements of k were made between 100 and 200°C for toluene and correlated with literature solubilities; the solubilities at 170°C of a number of related aromatics were then determined from their k‐values.     

Effect of analyte lipophilicity on the resolution of α‐ and β‐amino acids on liquid chromatographic ligand exchange chiral stationary phases

Separation of the two enantiomers of racemic α‐ and β‐amino acids on two ligand exchange chiral stationary phases (CSPs) prepared previously by covalently bonding sodium N‐((S)‐1‐hydroxymethy‐3‐methylbutyl)‐N‐undecylaminoacetate or sodium N‐((R)‐2‐hydroxy‐1‐phenylethyl)‐N‐undecylaminoacetate on silica gel was studied with variation of the organic modifier (methanol) concentration in the aqueous mobile phase. In particular, the variation of retention factors with changing organic modifier concentration in the aqueous mobile phase was found to be strongly dependent on both the analyte lipophilicity and the stationary phase lipophilicity. In general, the retention factors of relatively lipophilic analytes on relatively lipophilic CSPs tend to increase with increasing organic modifier concentration in the aqueous mobile phases while those of less lipophilic or hydrophilic analytes tend to increase. However, only highly lipophilic analytes show decreasing retention factors with increasing organic modifier concentration in the aqueous mobile phase on less lipophilic CSPs. The contrasting retention behaviors on the two CSPs were rationalized by the balance of the two competing interactions, viz. hydrophilic interaction of analytes with polar aqueous mobile phase and the lipophilic interaction of analytes with the stationary phase.     

Modeling the retention mechanism for high‐performance liquid chromatography with a chiral ligand mobile phase and enantioseparation of mandelic acid derivatives

The chromatographic retention mechanism describing relationship between retention factor and concentration of Cu²⁺(l ‐phenylalanine)₂ using chiral ligand mobile phase was investigated and eight mandelic acid derivatives were enantioseparated by chiral ligand exchange chromatography. The relationship between retention factor and concentration of the Cu²⁺(l ‐phenylalanine)₂ complex was proven to be in conformity with chromatographic retention mechanism in which chiral discrimination occurred both in mobile and stationary phase. Different copper(II) salts, chiral ligands, organic modifier, pH of aqueous phase, and conventional temperature on retention behavior were optimized. Eight racemates were successfully enantioseparated on a common reversed‐phase column with an optimized mobile phase composed of 6 mmol/L of l ‐phenylalanine or N,N‐dimethyl‐l ‐phenylalanine and 3 mmol/Lof copper(II) acetate or copper(II) sulfate aqueous solution and methanol.     

Application of chaotropic effect in reversed-phase liquid chromatography of structurally related phenothiazine and thioxanthene derivatives

Chromatographic behavior of two main classes of neuroleptics, derivatives of phenothiazine and thioxanthene in RP systems modified by anionic additives: sodium perchlorate and sodium hexafluorophosphate possessing chaotropic properties, was examined. Influence of the method of pH lowering (by addition of acids: trifluoroacetic or perchloric or by adding the appropriate concentration of phosphate buffer) and the kind of organic modifier in the mobile phase (methanol, acetonitrile) were estimated. Stability of complexes created between protonated drugs and anions of added salts was evaluated by comparison of their desolvation parameters (K), limiting retention factors for unsolved molecules calculated on the basis of chromatographic data. Experimentally obtained parameters were used in QSAR studies. It appeared that chosen parameters reflect not only physico-chemical properties of analytes but also contain information about the strength of their antipsychotic activity. Multidimensional cluster analysis has been performed. On the basis of the results obtained, it could be concluded that chaotropic systems can generate useful parameters for further QSAR studies.     

Method development for separating organic carbonates by ion‐moderated high‐performance liquid chromatography

An ion‐moderated partition high‐performance liquid chromatography method was developed for the separation and identification of common organic carbonates. The separation of organic carbonates was achieved on an ion exclusion column with an exchangeable hydrogen ion. An isocratic, aqueous mobile phase was used for elution and detection was performed with a refractive index detector. The developed method was validated for specificity, linearity, limits of detection and quantification, precision and accuracy. All calibration curves showed excellent linear regression (R² > 0.9990) within the testing range. The limits of detection were 3.8–30.8 ppm for the analyzed carbonates. Improvements in the peak resolution of the chromatograms were achieved by decreasing the column temperature. Addition of the organic modifier, acetonitrile, to the eluent was found to have insignificant effects on the peak resolution. The developed method was demonstrated for analyzing organic carbonate components in the electrolyte system of a commercial lithium ion battery.     

Testing the capability of a polynomial‐modified gaussian model in the description and simulation of chromatographic peaks of amlodipine and its impurity in ion‐interaction chromatography

In this paper, the capability of a polynomial‐modified Gaussian model to relate the peak shape of basic analytes, amlodipine, and its impurity A, with the change of chromatographic conditions was tested. For the accurate simulation of real chromatographic peaks the authors proposed the three‐step procedure based on indirect modeling of peak width at 10% of peak height (W_0.1), individual values of left‐half width (A) and right‐half width (B), number of theoretical plates (N), and tailing factor (Tf). The values of retention factors corresponding to the peak beginning (k_B), peak apex (k_A), peak ending (k_E), and peak heights (H₀) of the analytes were directly modeled. Then, the investigated experimental domain was divided to acquire a grid of appropriate density, which allowed the subsequent calculation of W_0.1, A, B, N, and Tf. On the basis of the predicted results for Tf and N, as well as the defined criteria for the simulation the following conditions were selected: 33% acetonitrile/67% aqueous phase (55 mM perchloric acid, pH 2.2) at 40°C column temperature. Perfect agreement between predicted and experimental values was obtained confirming the ability of polynomial modified Gaussian model and three‐step procedure to successfully simulate the real chromatograms in ion‐interaction chromatography.     

Preparation and Structures of 2‐Substituted 5‐Benzyl‐3‐methylimidazolidin‐4‐one‐Derived Iminium Salts,Reactive Intermediates in Organocatalytic Transformations Involving α,β‐Unsaturated Aldehydes

Preparations of the title compounds, 5 – 7 (Scheme 1 and Table 1), of their ammonium salts, 9 – 11 (Scheme 2 and Table 2), and of the corresponding cinnamaldehyde‐derived iminium salts 12 – 14 (Scheme 3 and Table 3) are reported. The X‐ray crystal structures of 15 cinnamyliminium PF₆ salts have been determined (Table 4). Selected ¹H‐NMR data (Table 5) of the ammonium and iminium salts are discussed, and structures in solution are compared with those in the solid state.     

Enantioseparation of chiral sulfonates by liquid chromatography and subcritical fluid chromatography

Tert‐butylcarbamoyl‐quinine and ‐quinidine weak anion‐exchange chiral stationary phases (Chiralpak® QN‐AX and QD‐AX) have been applied for the separation of sodium β‐ketosulfonates, such as sodium chalconesulfonates and derivatives thereof. The influence of type and amount of co‐ and counterions on retention and enantioresolution was investigated using polar organic mobile phases. Both columns exhibited remarkable enantiodiscrimination properties for the investigated test solutes, in which the quinidine‐based column showed better enantioselectivity and slightly stronger retention for all analytes compared to the quinine‐derived chiral stationary phase. With an optimized mobile phase (MeOH, 50 mM HOAc, 25 mM NH₃), 12 of 13 chiral sulfonates could be baseline separated within 8 min using the quinidine‐derivatized column. Furthermore, subcritical fluid chromatography (SubFC) mode with a CO₂‐based mobile phase using a buffered methanolic modifier was compared to HPLC. Generally, SubFC exhibited slightly inferior enantioselectivities and lower elution power but also provided unique baseline resolution for one compound.     

Determination of rimantadine in human urine by HPLC using a monolithic stationary phase and on‐line post‐column derivatization

In the present study, we propose the first HPLC method coupled to postcolumn derivatization for the determination of rimantadine in human urine samples. The analyte and amantadine (internal standard) were isocratically separated using an RP monolithic stationary phase (100 × 4.6 mm id) with a mobile phase consisting of CH₃OH/phosphate buffer (25 mmol/L, pH 3.0) at a volume ratio of 50:50. Postcolumn derivatization involved on‐line reaction with o‐phthalaldehyde (20 mmol/L) and N‐acetyl‐cysteine (5 mmol/L) at alkaline medium (100 mmol/L borate pH 11.0). Spectrofluorimetric detection at λ_ex/λ_em = 340/455 nm enabled the selective and sensitive determination of rimantadine in urine samples at a range of 50–500 ng/mL with an LOD of 5 ng/mL. Human urine samples were analyzed successfully after SPE using hydrophilic‐lipophilic balanced RP cartridges (30 mg/mL, Oasis HLB). Recoveries ranged between 89.7 and 102.7%.     

Investigations of mobile phase contributions to enantioselective anion- and zwitterion-exchange modes on quinine-based zwitterionic chiral stationary phases

Novel chiral stationary phases (CSPs) based on zwitterionic Cinchona alkaloid-type low-molecular mass chiral selectors (SOs), as they have been reported recently, were investigated in HPLC towards effects on their chromatographic behavior by mobile phase composition. Mobile phase characteristics like acid-to-base ratio and type of acidic and basic additives as well as effect of type of bulk solvents in nonaqueous polar organic and aqueous reversed-phase (RP) eluent systems were varied in order to illustrate the variability and applicability of zwitterionic CSPs with regard to mobile phase aspects. Chiral SOs of the five zwitterionic CSPs investigated herein contained weak and strong cation-exchange (WCX, SCX) sites at C9- and C6′-positions of the Cinchona alkaloid scaffold which itself accommodated the weak anion-exchange (WAX) site. The study focused on zwitterion-exchange (ZX) operational mode and chiral amino acids as target analytes. Besides, also the anion-exchange (AX) mode for chiral N-blocked amino acid analytes was considered, because of the intramolecular counterion (IMCI) property available in AX mode. Overall, most general and successful conditions in ZX mode were found to be weakly acidic methanolic mobile phases. In aqueous eluents RP contributions to retention came into play but only at low organic modifier content because of the highly polar character of zwitterionic analytes. At higher acetonitrile content, HILIC-related retention phenomena were observed. When using weakly basic eluent system in AX mode remarkably fast enantiomer separations involving exclusion phenomena were possible with one enantiomer eluting before and the other after void volume.     

Occurrence and behavior of system peaks in RP HPLC with solely aqueous mobile phases

System peaks are important but often also disturbing phenomena occurring in separation systems. Behavior of system peaks was studied in reversed phase high performance liquid chromatography (RP HPLC) systems consisting of an RP Amide C16 column and aqueous solutions of organic acids with alkaline metal hydroxides as mobile phases. Binary mobile phases, composed of benzoic acid and lithium hydroxide (LiOH) or cesium hydroxide (CsOH), yielded two system peaks. The first peak was stationary and the second one moved with dilution of the mobile phase or with changes of the alkaline metal hydroxide concentration. The latter changes affected dissociation of the benzoic acid present in the mobile phase and thereby its retention. The presumption that the first system peak is not influenced by the type of alkaline metal cation and that it is related to the non‐adsorbed component of the mobile phase was confirmed by a cyclic procedure. Three‐component mobile phases composed of benzoic acid, tropic acid, and a hydroxide gave rise to three system peaks as expected. The first peak was again stationary and the two others shifted depending on the concentration variation of both acids. Resonance causing a zigzag peak, well described in capillary zone electrophoresis (CZE), was observed if 1‐pentanol was injected into a chromatographic system with one‐component mobile phase.     

Separation of chlorogenic acid and concentration of trace caffeic acid from natural products by pH‐zone‐refining countercurrent chromatography

Chlorogenic acid and caffeic acid were selected as test samples for separation by the pH‐zone‐refining countercurrent chromatography (CCC). The separation of these test samples was performed with a two‐phase solvent system composed of methyl‐tert‐butyl‐ether/acetonitrile/water at a volume ratio of 4:1:5 v/v/v where trifluoroacetic acid (TFA; 8 mM) was added to the organic stationary phase as a retainer and NH₄OH (10 mM) to the aqueous mobile phase as an eluter. Chlorogenic acid was successfully separated from Flaveria bidentis (L.) Kuntze (F. bidentis) and Lonicerae Flos by pH‐zone‐refining CCC, a slightly polar two‐phase solvent system composed of methyl‐tert‐butyl‐ether/acetonitrile/n‐butanol/water at a volume ratio of 4:1:1:5 v/v/v/v was selected where TFA (3 mM) was added to the organic stationary phase as a retainer and NH₄OH (3 mM) to the aqueous mobile phase as an eluter. A 16.2 mg amount of chlorogenic acid with the purity of 92% from 1.4 g of F. bidentis, and 134 mg of chlorogenic acid at the purity of 99% from 1.3 g of crude extract of Lonicerae Flos have been obtained. These results suggest that pH‐zone‐refining CCC is suitable for the isolation of the chlorogenic acid from the crude extracts of F. bidentis and Lonicerae Flos.     

Micellar liquid chromatography of polycyclic aromatic hydrocarbons: Alkylpyridinium chloride as mobile phase modifier and selective fluorescence quencher

The dual role of alkylpyridinium chlorides, cetylpyridinium chloride (CPC) and dodecylpyridinium chloride (DDPC), as micellar mobile phase modifiers and selective fluorescence quenching agents of polycyclic aromatic hydrocarbons (PAHs), in micellar liquid chromatographic separation of PAHs is reported. The replacement of 0.037 M cetyltrimethylammonium chloride in the aqueous mobile phase with CPC/DDPC quencher greatly simplifies the observed fluorescence‐detected chromatograms, facilitating PAH identification. The resulting chromatograms are similar to those obtained from the conventional approach – pyridinium chloride quencher in an acetonitrile mobile phase. To quantify the quenching, the (F₀ /F – 1) values from the Stern‐Volmer equation are calculated from the chromatograms and compared. The feasibility of using CPC or DDPC as a dual reagent under isocratic and gradient conditions is shown.     

Extraction chromatography of common anions in liquid-liquid anion exchange systems. Part. III. Monobasic aliphatic organic acids and their sodium salts as eluants

Summary The chromatographic behaviour of anions on paper strips treated with tri-n-octylamine (TOA) salts or Aliquat 336 and developed with aqueous solutions of acetic, formic, monochloroacetic or trifluoroacetic acids and their sodium salts was investigated. Liquid-liquid extraction of organic acids by 0.1 M solution of TOA in benzene as well as the anion exchange between benzene solutions of TOA salts or Aliquat 336 (in acetate form) in benzene and aqueous solutions of sodium halides was also studied. It was found that extraction increases in the following order of the acids: CH₃COOOH<HCOOH<CH₂CICOOH<CF₃COOH; the relative affinity of organic anions to the quaternary alkyl-ammonium cation also increase in a similar order. The extraction of acid in excess over the amount necessary to neutralize the amine was observed for all four acids. The R_F values of anions investigated depend markedly on the type of organic acids or their salts and their concentration in the mobile phase. Halide ions are more strongly retained on paper treated with Aliquat 336 as compared with TOA salts. The chromatographic systems investigated offer many possibilities to separate various anion mixtures.Parts and II: refs. [1, 2].