Interactions of basic compounds with ionic liquids used as oils in microemulsion liquid chromatography.

Aqueous microemulsions (MEs), where an oil coexists with water in the presence of the anionic surfactant sodium dodecyl sulphate (SDS), have been proposed as a solution to decrease the amount of organic solvent in the mobile phase needed in reversed-phase liquid chromatography (RPLC). However, the oil phase of a typical ME is volatile, toxic and flammable, and although it is added in a small amount, it would be desirable to avoid it from an environmental perspective. This is the reason for the proposal of Peng et al. (J. Chromatogr. A 1499 (2017) 132‒139) to replace the oil in microemulsion liquid chromatography (MELC) by the apolar ionic liquid (IL) 1-hexyl-3-methylimidazolium hexafluorophosphate ([C6C1IM][PF6]), to analyse neutral phenolic acids at acidic pH. Based on this report, an MELC procedure is here proposed for ß-adrenoceptor antagonists, which are basic compounds where the dominant species is cationic. To verify the formation of MEs containing SDS and IL, and elucidate the interactions between the cationic basic compounds with the SDS anion, and the cation and anion in the IL, an extensive study was carried out with several methylimidazolium ILs containing the cations [C2C1IM]+, [C4C1IM]+, or [C6C1IM]+, combined with the anions Cl-, BF4-, or PF6-, using 1-butanol as co-surfactant. The behaviour was compared with that observed in classical MELC with octane, micellar liquid chromatography with SDS and 1-propanol, and RPLC with mobile phases containing an IL and acetonitrile.

Performance and modelling of retention in microemulsion liquid chromatography.

The capability of liquid chromatography with microemulsions (MEs) as mobile phases was studied for the analysis of four parabens (butylparaben, ethylparaben, methylparaben, and propylparaben) and seven ß-adrenoceptor antagonists (acebutolol, atenolol, carteolol, metoprolol, oxprenolol, propranolol, and timolol). MEs were formed by mixing aqueous solutions of the anionic surfactant sodium dodecyl sulphate, the alcohol 1-butanol that played the role of co-surfactant, and octane as oil. In order to guarantee the formation of stable MEs, a preliminary study was carried out to determine the appropriate ranges of concentrations of the three components. For this purpose, mixtures of variable composition were prepared, and the possible separation of two phases (formation of an emulsion) was visually detected. The advantage offered by the addition of octane to micellar mobile phases, inside the concentration range that allows the formation of stable MEs, was evaluated by comparing the retention behaviour, peak profile and resolution of mixtures of the probe compounds, in the presence and absence of octane. The final aim of this work was the proposal of a mathematical equation to model the retention behaviour in microemulsion liquid chromatography. The derived global model that considered the three factors (surfactant, alcohol and oil) allowed the prediction of retention times at diverse mobile phase compositions with satisfactory accuracy (in the 1.1‒2.5% range). The behaviour was compared with that found with mobile phases without octane. The model also yielded information about the retention mechanism and revealed that octane, when inserted inside the micelle, modifies the interaction between solutes and micelles.

Fatty acids-based microemulsion liquid chromatographic determination of multiple caffeoylquinic acid isomers and caffeic acid in honeysuckle sample.

A green and efficient microemulsion liquid chromatographic (MELC) method using fatty acid as co-surfactant and electrochemical detection was established and validated for the determination of four caffeoylquinic acid isomers and caffeic acid in honeysuckle samples. The influences of each individual component within the isocratic oil-in-water (O/W) microemulsion mobile phase were systematically investigated, such as the type and concentration of co-surfactant, concentration of sodium dodecyl sulphate (SDS), organic modifier addition, type and concentration of oil phase, pH and detection voltage. Results indicated that excellent resolution was achieved using 3.0% w/v of propionic acid, 0.5% w/v of ethyl acetate, 1.0% w/v of SDS, 5% w/v acetonitrile, 90.5% v/v of water and 25 mM sodium dihydrogen phosphate at pH = 3 as microemulsion mobile phase and 0.8 V as the optimal voltage value. Under the optimal condition, analytical performance of developed method was evaluated. The detection limits were below 17.3 ng/mL and intra-day and inter-day precisions by relative standard deviations (RSD%) were between 0.5% and 3.6%. Satisfactory recovery (in the range of 83.8-109.1%) with good repeatability lower than 4.7% (n = 3) was obtained. Therefore, the developed O/W MELC method was rapid, precise and accurate for simultaneous determination of neochlorogenic acid, chlorogenic acid, isochlorogenic acid A and isochlorogenic acid C in honeysuckle samples, with contents of 2.6, 28.7, 18.1 and 5.2 mg/g, respectively.

Modulation of retention and selectivity in oil-in-water microemulsion liquid chromatography: A review.

Microemulsions (MEs) are stable, isotropically clear solutions consisting of an oil and water stabilized by a surfactant and a co-surfactant. Oil-in-water microemuslion liquid chromatography (MELC) is a relatively new chromatographic mode, which uses an O/W ME as mobile phase. Retention, selectivity and efficiency can be modified by changing the concentration of the ME components and the ratio between the aqueous and oil phases. This work makes a critical survey on the information found in the literature about the mobile phase compositions that lead to the creation of successful O/W ME mobile phases, as well as the effect of pH for ionizable compounds and temperature. The viability of performing the analyses using isocratic and gradient elution is also considered. The complexity of the composition of a successful ME, and the fact that the different factors interact each other, may require many manipulations during method development to achieve an acceptable separation for complex mixtures. This is the reason of the proposal from several authors of a standard ME as starting point when developing a method for a new separation with no previous reports. Based on these initial conditions, the interest of several authors in applying computer-assisted approaches to optimize the composition of ME mobile phases, and reduce significantly the time and reagent consumption for method development, is described. Some practical tips are given to prepare stable ME mobile phases that yield reproducible results.

Determination of quantitative retention-activity relationships between pharmacokinetic parameters and biological effectiveness fingerprints of Salvia miltiorrhiza constituents using biopartitioning and microemulsion high-performance liquid chromatography.

In this study, we analyzed danshen (Salvia miltiorrhiza) constituents using biopartitioning and microemulsion high-performance liquid chromatography (MELC). The quantitative retention-activity relationships (QRARs) of the constituents were established to model their pharmacokinetic (PK) parameters and chromatographic retention data, and generate their biological effectiveness fingerprints. A high-performance liquid chromatography (HPLC) method was established to determine the abundance of the extracted danshen constituents, such as sodium danshensu, rosmarinic acid, salvianolic acid B, protocatechuic aldehyde, cryptotanshinone, and tanshinone IIA. And another HPLC protocol was established to determine the abundance of those constituents in rat plasma samples. An experimental model was built in Sprague Dawley (SD) rats, and calculated the corresponding PK parameterst with 3P97 software package. Thirty-five model drugs were selected to test the PK parameter prediction capacities of the various MELC systems and to optimize the chromatographic protocols. QRARs and generated PK fingerprints were established. The test included water/oil-soluble danshen constituents and the prediction capacity of the regression model was validated. The results showed that the model had good predictability.

Analysis of phenolic acids by ionic liquid-in-water microemulsion liquid chromatography coupled with ultraviolet and electrochemical detector.

An environmentally friendly ionic liquid-in-water (IL/W) microemulsion was established and applied as mobile phase in microemulsion liquid chromatography (MELC) with ultraviolet (UV) detection or electrochemical detector (ECD) for analysis of phenolic compounds in real samples. The optimal condition of the method was using the best composition of microemulsion (0.2% w/v [HMIM]PF6, 1.0% w/v SDS, 3.0% w/v n-butanol, 95.8% v/v water, pH 2.5) with UV detection. The validation results indicated that the method provided high degree of sensitivity, precision and accuracy with the low limit of detections ranged from 17.9-238ng/mL, satisfactory mean recovery values in the range of 80.1-105% and good linearity (r2>0.9994). Additionally, this method exhibited high selectivity and resolution for the analytes and was more eco-friendly compared with traditional MELC method. Consequently, the established IL/W MELC method was successfully applied to simultaneously separate and determine target compounds in Danshen sample and its preparation.

Simultaneous Determination of 4 Flavonoids from Poacynum hendersonii Leaf Extract in Rat Plasma Sam-ple with Intragastric Administration by Microemulsion Liquid Chromatography / 中国药房

OBJECTIVE:To establish the method for the simultaneous determination of 4 flavonoids in rat plasma sample, such as quercetin-3-O-sophoroside,isoquercitrin,hyperin,rutin. METHODS:8 SD rats were selected and given Poacynum hender-sonii leaf extract suspension intragastrically 0.1 g/kg(calculated by extract). The blood sample 1 mL was collected from orbit 1 h after medication. The contents of 4 flavonoids were determined by microemulsion liquid chromatography(MELC)after centrifuga-tion. The separation was performed on Zorbax SB-C18 column with mobile phase consisted of microemulsion [polyoxyethylene lau-ryl ether-n-butanol-ethyl acetate-triethylamine-water (volume ratio was 2.5:3.0:1.7:0.3:92.5,pH=5.0)] at the flow rate of 0.8 mL/min. The column temperature was set at 30 ℃,and detection wavelength was 360 nm. Sample size was 10 μL,and internal standard was bergenin. It was compared with HPLC method using organic solvent as mobile phase(mobile phase consisted of 0.2%phosphoric acid solution-acetonitrile,gradient elution,other condition same as MELC method). RESULTS:In rats plasma,the lin-ear range of quercetin-3-O-sophoroside,isoquercitrin,hyperin,rutin were 10-1000 ng/mL(r≥0.9980). The limits of quantitation was 10 ng/mL(S/N=10). RSDs of precision,stability and reproducibility tests were all below 7.5%(n=5). Average sample recov-eries were between 97.4%-98.5% and RSDs were less than 5.3%(n=5). Extraction recoveries were between 88.7%-100.3%(RSD≤6.14%,n=5). The methodological evaluation results of MELC and HPLC method were all in line with the regulations of pharmacopeia,and the results of blood concentration betweenthem were similar. Compared with HPLC method, MELCmethod could shorten detection time(10 min vs. 40 min)andreduce the amount of organic solvent (0.38 mL vs. 28 mL).CONCLUSIONS:Established MELC method is rapid,simpleand green,and can be used for 4 flavonoids from P. henderso-nii leaf in rat plasma sample.

Analysis of isoquinoline alkaloids using chitosan-assisted liquid-solid extraction followed by microemulsion liquid chromatography employing a sub-2-micron particle stationary phase.

A simple, efficient, and green chitosan-assisted liquid-solid extraction method was developed for the sample preparation of isoquinoline derivative alkaloids followed by microemulsion LC. The optimized mobile phase consisted of 0.8% w/v of ethyl acetate, 1.0% w/v of SDS, 8.0% w/v of n-butanol, 0.1% v/v acetic acid, and 10% v/v ACN. Compared to pharmacopoeia method and organic solvent extraction, this new approach avoided the use of volatile organic solvents, replacing them with relatively small amounts of chitosan. Under the optimum conditions, good linearity (r2 > 0.9980) for all calibration curves and low detection limits between 0.05 and 0.10 µg/mL were achieved. The presented procedure was successfully applied to determine alkaloids in Rhizoma coptidis with satisfactory recoveries (81.3-106.4%).

Determination of the lipophilicity of Salvia miltiorrhiza Radix et Rhizoma (danshen root) ingredients by microemulsion liquid chromatography: optimization using cluster analysis and a linear solvation energy relationship-based method.

We evaluated 26 microemulsion liquid chromatography (MELC) systems for their potential as high-throughput screening platforms capable of modeling the partitioning behaviors of drug compounds in an n-octanol-water system, and for predicting the lipophilicity of those compounds (i.e. logP values). The MELC systems were compared by cluster analysis and a linear solvation energy relationship (LSER)-based method, and the optimal system was identified by comparing their Euclidean distances with the LSER coefficients. The most effective MELC system had a mobile phase consisting of 6.0% (w/w) Brij35 (a detergent), 6.6% (w/w) butanol, 0.8% (w/w) cyclohexane, 86.6% (w/w) buffer solution and 8 mm cetyltrimethyl ammonium bromide. The reliability of the established platform was confirmed by the agreement between the experimental data and the predicted values. The logP values of the ingredients of danshen root (Salvia miltiorrhiza Radix et Rhizoma) were then predicted. Copyright © 2015 John Wiley & Sons, Ltd.

Selective microemulsion liquid chromatography analysis of dopamine receptor antagonist LE300 and its N-methyl metabolite in mouse sera by using a monolithic silica column.

A highly selective, sensitive, and rapid microemulsion liquid chromatography (MELC) method was developed and validated for the simultaneous determination of a novel type of dopamine receptor antagonist LE300 and its N-methyl metabolite in mouse sera. LE300, its N-methyl metabolite, and pindolol (an internal standard) were detected using excitation and emission wavelengths of 275 and 340 nm, respectively. HPLC analysis by using a monolithic column was performed by directly injecting the sample after appropriate dilution with the microemulsion mobile phase. The chromatographic behaviour of these compounds was studied to demonstrate their chromatographic efficiency, retention, and peak symmetry. The MELC method was validated for its specificity, linearity, accuracy, precision, robustness and stability. An experimental design was used during validation to evaluate method robustness. The calibration curves in serum showed excellent linearity (r=0.997) over concentrations ranging from 10 to 400 ngmL(-1) for LE300 and 15 to 500 ngmL(-1) for its N-methyl metabolite. The mean relative standard deviation (RSD) of the results of inter- and intra-day precision and accuracy of LE300 and its N-methyl metabolite were ≤5%. The overall recoveries of LE300 and its N-methyl metabolite from mouse sera were in the range 97.9-101.5% with %RSD ranging from 0.98% to 3.63%, which were in line with ICH guidelines. The assay was successfully applied in a pharmacokinetic study.

Multiobjective optimization strategy based on desirability functions used for the microemulsion liquid chromatographic separation and quantification of norfloxacin and tinidazole in plasma and formulations.

The aim of the present study was to optimize a microemulsion liquid chromatography method for the simultaneous determination of norfloxacin and tinidazole binary mixture using a chemometric protocol. Optimization experiments were conducted through a process of screening and optimization. A 2(7-4) fractional factorial design was used as screening design. While the location of optimum conditions was established by applying Derringer's desirability function. The optimal mobile phase composition was predicted to be: 3.5% w/v SDS, 10.03% v/v 1-propanol, 0.5% v/v 1-octanol, and 0.3% triethylamine in 0.02 M phosphoric acid at pH 6.5. The mobile phase was delivered isocratically at a flow rate of 1 mL/min with UV detection at 290 nm. Tinidazole and norfloxacin were eluted with retention times of 1.8 and 5.8 min, respectively. The calibration plots displayed good linear relationships in the concentration ranges of 0.5-50 and 0.75-75 µg/mL for norfloxacin and tinidazole, respectively. The method was successfully applied for determination of both drugs in pharmaceutical dosage forms and real human plasma. Where the accuracy was proved by the low values of % error and high values of recovery, also the relative standard deviation for the results did not exceed 1.5%, proving the precision of the method.

Determination of the lipophilicity (log P o/w) of organic compounds by microemulsion liquid chromatography.

Four microemulsion liquid chromatography (MELC) systems and one micellar liquid chromatography (MLC) system have been evaluated as potential high-throughput screening platforms capable of modeling the partitioning behaviors of drug compounds in an n-octanol/water system and predicting their lipophilicity (i.e., log P values). The microemulsion mobile phases is consisted of sodium dodecyl sulfate (SDS), butanol, octane, heptanes, octanol and water. A linear solvation energy relationship (LSER)-based method was used to compare the MELC and MLC systems, as well as several other biochemical systems, and to identify the optimal system by comparing their Euclidean distances with the LSER coefficients. The most effective MELC system had a mobile phase consisting of 3.0% (w/w) SDS, 6.0% (w/w) butanol, 0.8% (w/w) octanol, and 90.2% (w/w) water (pH 6.4). The results showed that it gave superior results to the other chromatographic systems in terms of its ability to predict the log P values of drug compounds.

Simultaneous Determination of Five Major Lignans from Schisandrae Sphenantherae Fructus and Schisandrae Chinensis Fructus by Microemulsion Liquid Chromatography / 中国中医药信息杂志

Objective To develop a stable and sensitive microemulsion liquid chromatography method by using oil-in-water microemulsion mobile phase;To simultaneously determine five major active lignans, namely, schizandrin, schisandrol B, schisantherin A, deoxyschizandrin, and schisandrin B, in Schisandrae Sphenantherae Fructus and Schisandrae Chinensis Fructus. Methods The quantitative determination was conducted by using Kromasil C18 column (250 mm × 4.6 mm, 5μm). The influence of the composition of the microemulsion system on separation effect and retention time was studied. The method was optimized to sodium dodecyl sulfonate 3.3%(SDS, W/V), n-octanol 1.2% (V/V) and n-butanol 6.6% (V/V) in 20 mmol/L phosphate buffer (adjusted to pH 7.0 with orthophosphoric acid and sodium hydroxide). The flow rate was set at 0.7 mL/min. The compound peaks were detected by UV at 254 nm. The column temperature was 25 ℃. Results Under the optimized conditions, baseline separation of 5 lignans was achieved in 20 min, and the separation was satisfying. Schizandrin, schisandrol B, schisantherin A, deoxyschizandrin, and schisandrin B were in linear relation within the ranges of 2.0-155.0, 8.3-166.7, 8.8-175.8, 8.3-166.7, 8.9-177.1μg/mL (r>0.999). The limit of detection ranged from 0.06μg/mL to 0.18μg/mL, and the limit of quantitation ranged from 0.17μg/mL to 0.55μg/mL. The average recovery values were between 97.22%and 100.8%. Conclusion This method is simple, accurate and reliable, and can be successfully applied to simultaneous determination of five major active lignans in 13 batches of Schisandrae Sphenantherae Fructus and Schisandrae Chinensis Fructus from different producing areas in China.

Optimization of liquid chromatographic method for the separation of nine hydrophilic and hydrophobic components in Salviae miltiorrhizae Radix et Rhizoma (Danshen) using microemulsion as eluent.

In this study, we have proposed and developed a novel, environmental-friendly and simple method for separation of nine hydrophilic and hydrophobic components in Danshen using microemulsion liquid chromatography. The proposed method was optimized via the preliminary screening experiment and the experimental design. The following factors were investigated in preliminary screening experiment: pH of mobile phase, column type, the nature of surfactant, the nature of oil phase and additives. In order to simultaneously optimize resolution and analysis time, the chromatographic optimization function (COF) was adopted to evaluate chromatograms. The central composite design (CCD) was used to create the matrix of experiments for mapping the chromatographic response surface. Finally, the COF values were fitted into a second order polynomial model and the response surface methodology (RSM) was employed to find the optimal eluent constituents. The reliability of the established model was confirmed by the good agreement obtained between experimental data and predictive values. Based on the results from the preliminary screening experiment and the CCD optimization, the optimal mobile phase was identified as a solution consisting of 6.68% (w/w) polyoxyethylene lauryl ether (Brij35), 0.84% (w/w) cyclohexane, 6.92% (w/w) n-butanol, 85.56% (w/w) phosphate buffer (pH 6.60) and 8mM cetyltrimethyl ammonium bromide (CTAB).

Simultaneous separation and determination of four phenylethanoid glycosides in rat plasma sample after oral administration of Cistanche salsa extract by microemulsion liquid chromatography.

A simple, rapid and specific method was developed to separate as well as to determine the four phenylethanoid glycosides (PhGs) (echinacoside, tubuloside B, acteoside and isoacteoside) in rat plasma after oral administration of Cistanche salsa extract by reversed phase high performance liquid chromatography using a microemulsion as the mobile phase. The separations were performed on a Zorbax Extend-C18 column at 25°C. Photodiode-array detector was conducted at 322nm and with a flow rate of 0.8mLmin(-1). The optimized microemulsion mobile phase consisted of 0.3% triethylamine in 20mM phosphoric acid at pH 6.0, 0.8% (v/v) ethyl acetate as oil phase, 1.5% (v/v) Genapol X-080 as surfactant, 2.5% (v/v) n-propanol as co-surfactant. Under the optimal conditions, the calibration curve for four PhGs was linear in the range of 10-1000ngmL(-1) with the correlation coefficients greater than 0.9994. The intra-day and inter-day precision (RSD) were below 8.64% and the limits of detection (LOD) for the four PhGs were 0.4-1.3ngmL(-1) (S/N=3). The microemulsion liquid chromatography (MELC) method was successfully applied to separate and determine the four PhGs in rat plasma after oral administration of C. salsa extract.

Retention Behavior of Acid Drugs in Microemulsion Liquid Chromatography / 分析化学

The retention mechanism of acid drugs such as naproxen, ibuprofen, diclofenac, phenylalanine and 2,4-dichlorophenoxy acetic acid in microemulsion liquid chromatography has been discussed.The reten tion modeling between capacity factors (k') of acid drugs in microemulsion liquid chromatography and concen tration of surfactant, cosurfactant and lipophilic organic solvent and pH value was set.The results showed the effect of change of concentration of surfactant, cosurfactant and lipophilic organic solvent and pH value on capacity factors of acid drugs in microemulsion liquid chromatography was fully consistent with the theoretical modeling.The retention modeling can better reflect the effect of composition of microemulsion solution and pH value on retention behavior of acid drugs in microemulsion liquid chromatography.

Simultaneous Determination of Four Flavonoids in Extract of Hawthorn Leaves by Microemulsion Liquid Chromatography / 分析化学

A novel system was developed for the rapid determination of vitexin, vitexin-2″-O-rhamnoside, rutin, and hyperoside in the extract of hawthorn(Crataegus pinnatifida Bge.) leaves with microemulsion liquid chromatography(MELC). The effect on the chromatography of varying the operating paramers was studied. The optimized MELC system with microemulsion was consisting of 1.0%(w/w) brij35-1.1%(w/w) n-butanol-0.1%(w/w) n-octanol-0.3%(v/v) triethylamine, the pH was adjusted to 2.5 with phosphoric acid. The type and concentration of surfactant, types of oil phase, the pH and triethylamine as the organic additive in microemulsion played an important role for separation of four flavonoids. MELC analysis was performed on a Venusil ASB C_(18) analytical column (150 mm× 4.6 mm, i.d., 5 μm). The flow rate was set at 0.8 mL/min and the eluent was detected at 340 nm for the four flavonoids. The calibration curves of the four flavonoids are linear(r>0.9995) over the concentration range of 0.95-140.0 mg/L. The mean recoveries are 98.6% to 101.6%. The results indicate that the optimized method was successfully applied to the analysis of four important flavonoids in the extract of hawthorn leaves.