Systematic Approach to Links between Separations in MEKC and Reversed-Phase HPLC

Retention factors and partition coefficients in micellar electrokinetic chromatography (MEKC) and reversed-phase high-performance liquid chromatography (RP-HPLC) are compared for a series of alkylbenzenes and substituted phenols. In both techniques, separations are based on partitioning between an aqueous phase and an alkyl phase. In MEKC, this was an SDS (C12) micellar pseudostationary phase, and in RP-HPLC an ODS 2 (C18) stationary phase. A nonporous silica (Micra 1.5-μm NPS), which has a low carbon loading, was used rather than a standard porous silica to avoid excessive retention in HPLC and to allow identical mobile phase conditions to be used in both separation modes. The average ratio of analyte retention factors, k(MEKC):k(HPLC), was found to be equal to the ratio β(MEKC):β(HPLC), where β is the phase ratio. This implies that partition coefficients, P, are similar in both MEKC and HPLC, since P = k/β, and that the dominant contribution to stability within each alkyl phase arises from hydrophobic interactions which are common to both separation media. Since partition coefficients are similar in MEKC and HPLC under aqueous buffer conditions, information on retention in one technique may be transferred to the other, provided that the phase ratios are known. In MEKC and HPLC, linear correlations of log octanol-water partition coefficients, K(ow), vs log k for the test compounds were transformed, knowing the phase ratio, to give log P values as a function of log K(ow). This allows quantitative links between MEKC and HPLC to be extended to include octanol-water partitioning. The addition of acetonitrile as an organic modifier over the concentration range 0-20% (v/v) was found to have a greater effect on k in HPLC than in MEKC. This could be a result of a decrease in the MEKC phase ratio due to an increase in the critical micelle concentration.     

Determination of Aqueous Solubilities of a Series of 5-Ethyl-5-Alkylbarbituric Acids and Their Correlation with Log P and Melting Points

A rapid potentiometric method has been used to determine the aqueous solubilities of an homologues series of 5-ethyl-5-alkyl (n=0-9) barbituric acids. A plot of the logarithm of solubility against the number of carbon atoms in the side chain revealed two distinct linear regions with a definite break about the butyl compound. The solubilities of all the members of this series (except 5-ethyl barbituric acid) can be predicted from log P values and melting points by the semiempirical solubility equation of Yalkowsky.     

Electrokinetic chromatography using thermodynamically stable vesicles and mixed micelles formed from oppositely charged surfactants

The electrokinetic chromatography (EKC) of a novel mixed surfactant system consisting of oppositely charged surfactants, sodium dodecyl sulfate (SDS) and n-dodecyltrimethylammonium bromide (DTAB), was investigated. The chromatographic characteristics of large liposome-like spontaneous vesicles and rodlike mixed micelles formed from the mixture were explored and compared with those of SDS micelles. Separations of a series of n-alkylphenones showed that the spontaneous vesicles provided about a 2 times wider elution window than SDS micelles. Both vesicle and mixed micelle systems were found to provide larger methylene selectivity than SDS. The different elution order of a group of nitrotoluene geometric isomers with DTAB/SDS spontaneous vesicles and SDS micelles pseudostationary phases suggested the possibility of different separation mechanisms with these two systems. Comparisons of polar group selectivity, retention, and efficiency were made between vesicles, mixed micelles, and SDS micelles. The correlation between the logarithms of the retention factors (log k') and octanol-water partition coefficients (log P(ow)) for a group of 20 neutral compounds was also studied with DTAB/SDS vesicles. Spontaneous vesicles have great potential as a pseudostationary phase in electrokinetic chromatography.     

Measurement of drug lipophilicity and pKa using acoustics

Lipophilicity of chemicals and drug candidates is normally described in terms of octanol/water partitioning and log P. We investigated an alternate approach to lipophilicity determination using a mimic of an alkyl alcohol with compound partitioning quantified using acoustic sensing. A self-assembled monolayer composed of HSC(10)(CH(2)CH(2)O)(6)C(18) was formed on planar gold electrodes of a piezoelectric acoustic sensor. The system was challenged with compounds covering a 4-log range of log D values. As compounds partitioned in the interfacial layer, changes in sensor resonant frequency were found to correlate with compound partition coefficients (log P) and with distribution coefficients (log D). Linear concordance (R(2) = 0.933) was established between log(-dF/M(w)t) and log P and with log D in both water and biological buffers at variant pH (pH 5.2 to 7.8). In turn, drug pK(a) could be determined by profiling log D changes during pH titration. The lipophilicity/pH profile of a weakly basic drug (quinine; pK(a) = 7.95) was sigmoidal with respect to -dF/M(w) values, with a profile inverse to that of a weakly acidic drug (naproxen; pK(a) = 4.15).     

Evaluating "fast" micellar monolithic liquid chromatography for high-throughput quantitative structure-retention relationship screening

The recently introduced monolithic silica columns were tested for their use in micellar liquid chromatography. Micellar methods are utilized in high-throughput quantitative structure-retention relationships to estimate an indicator of the membrane permeability of drugs, namely, the octanol-water partition coefficient, log P. The monolithic column's ability to function at higher flow rates might be useful to speed up these chromatographic methods estimating the log P. Therefore, the elution behavior of diverse basic pharmaceutical substances was determined on a classical particle-based and a monolithic column, both with and without a micellar medium in the mobile phase. Utilizing among others principal component analysis, the extent to which these methods differ in retention characteristics was examined in the context of high-throughput determination of log P. Results indicate that combining monolithic columns with micellar media leads to faster log P and possibly even better permeability predictions.     

Solid-phase microextraction as a method for estimating the octanol-water partition coefficient

The determination of octanol-water partition coefficients (log K(ow)) is important for the prediction of the fate of organic pollutants in the environment. Traditionally, log K(ow) values are determined by shake-flask, estimated by, e.g., HPLC retention data, or calculated, e.g., from ClogP. In this paper, an alternative approach is reported that allows log K(ow) to be estimated from solid-phase microextraction (SPME) data. Previously reported attempts to correlate SPME data with log K(ow) are discussed. The results obtained in this work for six phenols, using an 85 μm polyacrylate-coated fiber, indicate that SPME is a viable method for estimating log K(ow) values <3.5.     

Scatchard-Hildebrand热力学模型在PA印刷层迁移预测中的应用

目的探讨Scatchard-Hildebrand热力学模型对塑料印刷层中有害物质迁移分配系数预测的适用性和准确性。方法以聚酰胺(PA)印刷层中5种迁移物为代表,选取相关文献中PA印刷层中有害物质向不同模拟液中迁移的实验测试结果数据,对分配系数实验值Fexp与模型预测计算值Fsim作对比分析,并用实验数据对Scatchard-Hildebrand统计热力学模型进行校正,研究了模型修正系数Ks(Ks=ln(Fsim/Fexp))与迁移物相对分子质量(M)及其醇-水分配系数(log P)以及模拟液极性(Ps)间的关系。结果 PA印刷层中有害物质迁移分配系数的预测模型修正系数Ks与M和Ps间呈现显著的线性关系,得到了修正后的分配系数预测模型。结论修正后的Scatchard-Hildebrand热力学模型对预测塑料PA印刷层中有害物质迁移分配系数具有一定的适用性。     

A comparison of gas-hexadecane and gas-apolane partition coefficients

Gas-apolane partition coefficients (L(87)) of 157 nonpolar and polar organic solutes, spanning a wide range of functional groups, dipolarities, and hydrogen-bonding capabilities, are measured by open tubular capillary gas chromatography at 40 °C. The experimental values compare well (R(2) = 0.999) with literature values of log L(87) from packed-column gas chromatography. The log L(87) values are also compared with gas-hexadecane partition coefficients (log L(16)) at 25 °C. A strong linear relationship exists between log L(87) and log L(16) for all solutes (R(2) = 0.994), as well as for chemically relevant subsets of the data. Therefore, unknown L(16) values at 25 °C can be predicted from the corresponding L(87) values, which can be measured on open-tubular or packed columns at higher temperatures, due to the low volatility of apolane. Predicted values of L(16) would be extremely useful, since log L(16) is often the major explanatory parameter in many linear solvation energy relationships.     

Chromatographic Hydrophobicity Index by Fast-Gradient RP-HPLC: A High-Throughput Alternative to log P/log D

A new chromatographic hydrophobicity index (CHI) is described which can be used as part of a protocol for high-throughput (50-100 compounds/day) physicochemical property profiling for rational drug design. The index is derived from retention times (t(R)) observed in a fast gradient reversed-phase HPLC method. The isocratic retention factors (log k') were measured for a series of 76 structurally unrelated compounds by using various concentrations of acetonitrile in the mobile phase. By plotting the log k' as a function of the acetonitrile concentration, the slope (S) and the intercept (log k'(w)) values were calculated. The previously validated index of hydrophobicity φ(0) was calculated as -log k'(w)/S. A good linear correlation was obtained between the gradient retention time values, t(R) and the isocratically determined φ(0) values for the 76 compounds. The constants of this linear correlation can be used to calculate CHI. For most compounds, CHI is between 0 and 100 and in this range it approximates to the percentage (by volume) of acetonitrile required to achieve an equal distribution of compound between the mobile and the stationary phases. CHI values can be measured using acidic, neutral, or slightly basic eluents. Values corresponding to the neutral form of molecules could be measured for 52 of the compounds and showed good correlation (r = 0.851) to the calculated octanol/water partition coefficient (c log P) values.     

Determination of oceanic carbon dioxide using a carbonate-selective electrode

Potentiometric properties of the PVC membrane-based electrodes prepared with molecular tweezer-type neutral carriers, 3,12-bis(TFAB)CA and deoxy-3,12-bis(TFAB)CA, and trifluoroacetyl-p-decylbenzene (TFADB) were measured in buffered electrolytes (0.1 M Tris-H2SO4, pH 8.6 and 8.0) and artificial seawater. It was observed that the deoxy-3,12-bis(TFAB)CA-based electrode provides greatly enhanced carbonate selectivity over chloride (log K(CO3(2-), Cl-)POT approximately -6) and other minor anions present in seawater. Thus, we explored the possibility of applying this new carbonate-selective electrode for direct determination of oceanic carbon dioxide. The total carbon dioxide (TCO2) level in surface Yellow Sea water was determined with the deoxy-3,12-bis(TFAB)CA-based electrode, Severinghaus-type CO2 gas sensor, and the traditional potentiometric titration methods. The results showed that the carbonate-selective electrode provides accurate oceanic TCO2 determination comparable to that obtainable with the other two methods. The analytical procedure based on a carbonate-selective electrode is clearly advantageous over other conventional methods: it does not require any sample pretreatment and extra reagents other than the standard calibration solutions, while providing the measured results directly and immediately.     

Potentiometric strip cell based on carbon nanotubes as transducer layer: toward low-cost decentralized measurements

In this study, we developed a potentiometric planar strip cell based on single-walled carbon nanotubes that aims to exploit the attributes of solid-contact ion-selective electrodes for decentralized measurements. That is, the ion-selective and reference electrodes have been simultaneously miniaturized onto a plastic planar substrate by screen-printing and drop-casting techniques, obtaining disposable strip cells with satisfactory performance characteristics (i.e., the sensitivity is 57.4 ± 1.3 mV/dec, the response time is ≤30 s within the linear range from log a(K+) = -5 to -2, and the limit of detection is -6.5), no need of maintenance during long dry storage, quick signal stabilization, and light insensitivity in short-term measurements. We also show how the new potentiometric strip cell makes it possible to perform decentralized and rapid determinations of ions in real samples, such as saliva or beverages.     

Determination of distribution coefficients of priority polycyclic aromatic hydrocarbons using solid-phase microextraction

The determination of distribution coefficients is important for prediction of the chemical pathways of organic compounds in the environment. Solid-phase microextraction (SPME) is a convenient and effective method to measure the distribution of chemicals in a two-phase system. In the present study, the SPME distribution coefficient (K(spme)) of 16 priority aromatic hydrocarbons (PAHs) was determined with 100-microm poly(dimethylsiloxane) (PDMS) and 85-microm polyacrylate (PA) fibers. The partition coefficients and LeBas molar volumes were used to describe the linearity of the log K(spme) values of PAHs. Also, the validation of the distribution coefficient was examined using different sample volumes. The extraction time was dependent on the types of PAHs, and 20 min to 60 h was needed to reach equilibrium. The determined log K(spme) values ranged from 3.02 to 5.69 and from 3.37 to 5.62 for 100-microm PDMS and 85-microm PA fibers, respectively. Higher K(spme) values of low-ring PAHs were observed using 85-microm PA fiber. Good linear relationships between log K(ow) and log K(spme) for PAHs from naphthalene to benzo[alpha]pyrene and from naphthalene to chrysene for 100-microm PDMS and 85-microm PA fibers, respectively, were obtained. The correlation coefficients were 0.969 and 0.967, respectively. The linear relationship between log K(spme) and the LeBas molar volume was only up to benz[alpha]anthracene for 85-microm PA fiber and up to chrysene for 100-microm PDMS fiber. Moreover, the effect of sample volume can be predicted using the partition coefficient theory and excellent agreement was obtained between the experimental and theoretical absorbed amounts of low-ring PAHs. This result shows that the determined log K(spme) is more accurate than the previous method for estimating analytes with log K(ow) < 6 as well as for predicting the partitioning behaviors between SPME fiber and water.     

High-throughput method for lipophilicity measurement

A high-throughput method has been developed for lipophilicity measurement. It measures the partition coefficient of a solute between a polymer phase and an aqueous phase (Ppw) in a 96-well format. The polymer is plasticized poly(vinyl chloride) (PVC), which is widely used as a material for clinical containers and ion-selective electrodes. The composition is 2:1 (w/w) dioctyl sebacate and PVC. With six repeats, log Ppw values of 15 solutes have been determined in one 96-well microplate in 4 h. A linear relationship between log Ppw and log Pow (octanol-water partition coefficient) values exists with a correlation coefficient of 0.979. The slope and intercept of the log Ppw vs log Pow plot are statistically indistinguishable from 1 and 0, respectively. Similar to the HPLC method, by using the correlation line as a calibration curve, the measured log Ppw values can be used to predict log Pow. This protocol is faster than the HPLC method. Moreover, it is straightforward to extend the protocol to the determination of the distribution coefficient and pKa of charged solutes. We show that the log Ppw of the neutral form of racemic econazole is 4.83(+/-0.06), for the cationic form (presumably as a dihydrogen phosphate ion pair) 1.68(+/-0.04), and the pKa is 6.15(+/-0.04). This method has great flexibility as well and is potentially fully automated.     

Dynamic permeation method to determine partition coefficients of highly hydrophobic chemicals between poly(dimethylsiloxane) and water

Measurement of partition coefficients between poly(dimethylsiloxane) (PDMS) and water (KPDMSw) becomes more and more difficult as the hydrophobicity of the compound increases. Experimental challenges include long extraction times, sorption to various surfaces and materials, and incomplete dissolution of the compound in the aqueous phase. In order to avoid these artifacts and to shorten experimental time, a dynamic permeation method was developed. According to steady-state diffusion theory, KPDMSw is inversely proportional to the permeation rate through the aqueous boundary layer (ABL) from the donor PDMS to the acceptor PDMS. A simple ABL permeation reactor can thus be applied to determine KPDMSw values of hydrophobic chemicals within a few days. The obtained values were in good agreement with those obtained using a conventional shaking method and the partition controlled delivery system. A good linear correlation was obtained between the logarithm of the 1-octanol/water partition coefficient (log Kow) from the literature and log KPDMSw over 6 orders of magnitude.     

Method for simultaneous determination of partition coefficients for cyclic volatile methylsiloxanes and dimethylsilanediol

Cyclic volatile methyl siloxanes (cVMS) such as octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6) may enter the environment through industrial activities and the use of various consumer products. Reliable air/water (K(AW)), 1-octanol/water (K(OW)), and octanol/air partition coefficients (K(OA)) for those compounds and their common degradation product, dimethylsilanediol, are critical for accurate prediction of the environmental fate, distribution, and transport of these materials. Challenges have been encountered in determining these properties for cVMS and their degradation products mainly due to the extremely low water solubility of the organosiloxanes, low volatility of their degradation products, and reactivity of those compounds in the water/1-octanol system that can lead to inconsistent and inaccurate partition coefficients. A novel direct method is presented for the simultaneous determination of K(AW), K(OW), and K(OA) of organic compounds and was applied to these organosilicon compounds. It was tested in a range of log K(AW) values from -6.8 to 3.1, log K(OW) values from -0.4 to 8.9, and log K(OA) values up to 7. The advantages of the new direct method include the improved accuracy, a shortened measurement time, simultaneous measurement of three partition coefficients of multiple compounds, self-consistency among resultant partition coefficients, and a wide range of applicability including materials that may be slowly reactive in the water/1-octanol system.     

Sol-gel derived potentiometric pH sensors

A potentiometric sol-gel derived (xerogel) pH sensor based on covalently attached amine groups is described. The sensor consists of a Ag/AgCl electrode coated with a hybrid aminosilane/alkylsilane xerogel film. Various combinations of aminosilanes and alkylsilanes are evaluated for their potentiometric response to pH. The optimal sensor design is composed of (aminoethylaminomethyl)phenethyltrimethoxysilane and methyltrimethoxysilane. This sensor exhibits near-Nerstian response (-55 mV.decade(-)(1)), responds rapidly (< or =3 s) to changes in pH, and has H(+) selectivity coefficients (log K(pot)(H)()+ (, )(j)()) of -13 and -11 for interfering j cations Na(+) and K(+), respectively. In vitro platelet adhesion tests indicate that the xerogel coatings are more blood compatible than conventional poly(vinyl chloride) and poly(urethane) ion-selective electrode coatings.     

The Behavior of Copper (II) Chelates of Aliphatic Aminocarboxylic Acids in Aqueous Solutions

Abstract

The effective overall stability constants (log Ke values) and molar combining ratios for nine aliphatic aminocarboxylic acids (C₂ to c₆) with copper (II) ion were calculated from spectrophotometric and potentiometric data at pH 3.0, 4.0 and 5.0 and 25°C. In addition, calculated overall stability constants (log Kn values), using Bjerrum's method of analyzing potentiometric data, showed that chelates of alpha- and beta-aminocarboxylic acids with copper (II) were more stable than those of gamma-and epsilon-aminocarboxylic acids. Using Job's method of continuous variations for analyzing absorbance data, it was observed that while the alpha- and beta-aminocarboxylic acids formed both bicoordinated (1:1) and tetracoordinated (2:1) chelates with copper (II) ion at pH 4:O and 5:O and the alpha-aminocarboxylic acids formed only a (1:1) chelate at pH 3.0, the gamma and epsilon-aminocarboxylic acids showed only bicoordinated (1:1) chelate formation at pH 5.0.

The complexation of epsilon-aminocarboxylic acid ligand to copper (II) ions appears to be through both the amino and the carboxylate groups rather than through carboxylate ion, alone as previously suggested by other workers. The implication of this finding is that the formation of chelate ring structures greater than eight members is a distinct possibility in the aliphatic aminocarboxylic acid series and further suggests the opportunity for more complex metallobinding in proteins as well.     

Thiomenthol derivatives as novel percutaneous absorption enhancers

Thiomenthol derivatives were synthesized and their promoting activity on the percutaneous absorption of ketoprofen from hydrogels was evaluated in rats. The apparent penetration rate (R_p) of ketoprofen absorbed from the hydrogel was estimated by the pharmacokinetic model derived under the assumption of a constant penetration rate through the skin after a lag time. As an index of promoting activity of thiomenthol derivatives, an enhancement factor (E_f) was defined as the ratio of the R_p value with enhancer to the value obtained with the control not containing enhancer. Skin irritancy evoked by these derivatives was investigated microscopically by using a cross-section of the excised skin onto which ketoprofen hydrogel was applied. Total irritation score (TIS) was estimated by summation of each irritation score in several parts of the skin. The physicochemical parameters of thiomenthol derivatives such as a partition coefficient (log P) and a steric energy were calculated and the quantitative relationships between these parameters and the E_f values or TIS values were investigated on the basis of multiple regression analysis. As a result, a parabolic relationship between log P and E_f was noted. A similar relationship was also observed in the case of TIS.     

Prediction of retention in micellar electrokinetic chromatography from solute structure. 1. Sodium dodecyl sulfate micelles.

Like other chromatographic techniques, retention factor, k, in micellar electrokinetic chromatography (MEKC) is directly related to solute partition coefficient and the chromatographic phase ratio as k = Kphi. Unlike conventional chromatography, however, the phase ratio and partition coefficient can be accurately determined in MEKC for a given micellar pseudostationary phase. This means that retention factor in MEKC can be predicted for solutes with known micelle-water partition coefficients without any prior experimentation. In this paper, the use of this simple relationship for prediction of retention behavior in MEKC is examined. The principle of additivity of functional group contribution to partitioning is used to calculate the micelle-water partition coefficient, Kmw, for SDS micellar pseudophase. The micellar substituent constants for 20 functional groups (training set) were determined. Using these substituent constants, the Kmw and retention factors for a group of 80 neutral solutes (test set) were predicted. The linear plot of predicted versus observed log k had an R2 = 0.97 and a slope equal to 1.01. It is shown that the retention times (thus chromatograms) in MEKC can be predicted from the calculated retention factors after only one initial experiment to measure teo and t(mc) under the experimental conditions.