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1.
J Chromatogr A ; 1602: 397-408, 2019 Sep 27.
Artigo em Inglês | MEDLINE | ID: mdl-31285058

RESUMO

In reversed-phase liquid chromatography, the performance for basic compounds is affected by the interaction of the protonated (cationic) species with the anionic free silanols on the alkyl-bonded stationary phases. Using aqueous-organic mobile phases in the absence of additives, the retention may be too high, and the peaks be broad and asymmetric. The performance is improved by addition to the mobile phase of ionic liquids, from which 1-hexyl-3-methylimidazolium chloride ([C6MIm][Cl]) has especially good characteristics. A recent report has also revealed that the use of the phosphate system as buffer, at varying concentration and pH, may have a significant role in the chromatographic performance of basic compounds, with effects on both retention and peak shape. In this work, this study has been extended to other three buffer systems (acetate, citrate, and formate), at increasing concentrations and pH 3 and 7, in the presence and absence of [C6MIm][Cl]. The results have been compared with those obtained with the phosphate system. The retention increases by addition of larger concentration of all buffers, in both absence and presence of [C6MIm][Cl]. Without additive, peak performance is also enhanced significantly. This effect is minimal in the presence of [C6MIm][Cl], which yields highly symmetrical peaks at all buffer concentrations, due to an effective blocking of the silanol activity.


Assuntos
Boratos/química , Cromatografia de Fase Reversa/métodos , Imidazóis/química , Acetonitrilos/química , Antagonistas Adrenérgicos beta/análise , Tampões (Química) , Cromatografia Líquida de Alta Pressão/métodos , Concentração de Íons de Hidrogênio , Líquidos Iônicos/química , Solventes , Água/química
2.
J Chromatogr A ; 1592: 91-100, 2019 May 10.
Artigo em Inglês | MEDLINE | ID: mdl-30704775

RESUMO

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.


Assuntos
Cromatografia Líquida , Óleos/química , Água/química , Cromatografia Líquida/normas , Tensoativos/química
3.
J Chromatogr A ; 1559: 118-127, 2018 Jul 20.
Artigo em Inglês | MEDLINE | ID: mdl-29203112

RESUMO

The reversed-phase chromatographic behaviour of six tricyclic antidepressants (amitryptiline, clomipramine, doxepin, imipramine, nortryptiline and maprotiline) was examined in this work with acetonitrile-water mobile phases, in the absence and presence of the ionic liquids 1-hexyl-3-methylimidazolium chloride and 1-hexyl-3-methylimidazolium tetrafluoroborate, which have interesting features for the separation of basic compounds, in terms of peak shape combined with reduced retention. Tricyclic antidepressants are low polarity drugs that strongly associate to the alkyl chains of conventional stationary phases. They are also positively charged in the usual working pH range (2-8) in reversed-phase liquid chromatography, due to their strong basic character. In consequence, they may interact with the residual ionised silanols present in conventional silica-based stationary phases, which is translated in stronger retention, and tailed and broad peaks. A simple chromatographic procedure for the control of tricyclic antidepressants in pharmaceutical formulations was developed using a C8 column and a mobile phase containing 30% acetonitrile/10 mM 1-hexyl-3-methylimidazolium chloride at pH 3, with UV detection. Intra- and inter-day precisions were usually below +1.0%, and intra- and inter-day bias (trueness) ranged between ‒2.1% and +2.4%, and between ‒3.0% and +2.3%, respectively. Sample preparation was simple and only required solubilisation and filtration previous to injection.


Assuntos
Antidepressivos Tricíclicos/química , Boratos/química , Imidazóis/química , Líquidos Iônicos/química , Amitriptilina/análise , Amitriptilina/química , Antidepressivos Tricíclicos/análise , Cromatografia de Fase Reversa/métodos , Doxepina/análise , Doxepina/química , Composição de Medicamentos , Limite de Detecção , Nortriptilina/análise , Nortriptilina/química , Espectrofotometria Ultravioleta
4.
J Chromatogr A ; 1559: 2-16, 2018 Jul 20.
Artigo em Inglês | MEDLINE | ID: mdl-28958758

RESUMO

The molten organic salts with melting point below 100°C, commonly called ionic liquids (ILs) have found numerous uses in separation sciences due to their exceptional properties as non molecular solvents, namely, a negligible vapor pressure, a high thermal stability, and unique solvating properties due to polarity and their ionic character of molten salts. Other properties, such as viscosity, boiling point, water solubility, and electrochemical window, are adjustable playing with which anion is associated with which cation. This review focuses on recent development of the uses of ILs in separation techniques actualizing our 2008 article (same authors, J. Chromatogr. A, 1184 (2008) 6-18) focusing on alkyl methylimidazolium salts. These developments include the use of ILs in nuclear waste reprocessing, highly thermally stable ILs that allowed for the introduction of polar gas chromatography capillary columns able to work at temperature never seen before (passing 300°C), the use of ILs in liquid chromatography and capillary electrophoresis, and the introduction of tailor-made ILs for mass spectrometry detection of trace anions at the few femtogram level. The recently introduced deep eutectic solvents are not exactly ILs, they are related enough so that their properties and uses in countercurrent chromatography are presented.


Assuntos
Cromatografia Gasosa/métodos , Cromatografia Líquida/métodos , Eletroforese Capilar/métodos , Líquidos Iônicos/química , Distribuição Contracorrente , Gases/química , Solventes/química , Espectrometria de Massas por Ionização por Electrospray , Viscosidade
5.
J Chromatogr A ; 1559: 112-117, 2018 Jul 20.
Artigo em Inglês | MEDLINE | ID: mdl-28602502

RESUMO

The presence of anionic free silanols in the silica-based stationary phases gives rise to broad and asymmetrical peaks when cationic basic compounds are chromatographed using hydro-organic mobile phases. The addition to the mobile phase of a reagent with ionic character prevents the access of analytes to the free silanols, improving the peak shape. The silanol activity can be affected by the buffer concentration and mobile phase pH, factors that are not always considered sufficiently in the literature. In this work, the chromatographic behaviour of three basic ß-adrenoceptor antagonists (acebutolol, nadolol and timolol), using mobile phases containing acetonitrile, was examined at different phosphate buffer concentrations (5-50mM) and mobile phase pH (2-8), in the absence and presence of three imidazolium-based ionic liquids (1-ethyl-, 1-butyl- and 1-hexyl-3-methylimidazolium chloride). All factors were evaluated through both the retention and peak shape. The imidazolium cations can block the access of cationic analytes through electrostatic interaction with the anionic silanols, or association with the alkyl chains bound to the stationary phase. In previous reports, the protection mechanism was demonstrated to be directly related to the cation size. The studies in this work reveal that the effectiveness of the mobile phase additive as silanol blocker also depends on the concentration of the buffer anion and the protonation degree of the silanols on the stationary phase. Increasing amounts of phosphate at low pH give rise to increasing retention times. Also, the peak shape is improved, which indicates the influence of phosphate on blocking the activity of free silanols. However, the benefits obtained by the combined effect of buffering the mobile phase at low pH and the use of a bulky additive are lost at pH>6.


Assuntos
Líquidos Iônicos/química , Fosfatos/química , Silanos/química , Tampões (Química) , Cromatografia de Fase Reversa , Concentração de Íons de Hidrogênio , Eletricidade Estática
6.
J Chromatogr A ; 1465: 98-106, 2016 Sep 23.
Artigo em Inglês | MEDLINE | ID: mdl-27586325

RESUMO

In reversed-phase liquid chromatography, cationic basic compounds yield broad and asymmetrical peaks, as a result of their ionic interaction with the anionic free silanol groups present in the silica-based stationary phases (commonly derivatised with C18 groups). A simple way to improve the peak shape is the addition to the hydro-organic mobile phase of a reagent (usually called additive) with cationic character. This associates with the stationary phase to prevent the access of analytes to the free silanol groups. Cationic additives may interact electrostatically with the anionic silanols. The hydrophobic region of the additive may also associate with the alkyl chains bound to the stationary phase, with the positive charge oriented towards the mobile phase. The access to the silanol groups is thus blocked, but in turn, the stationary phase is positively charged and will repel the protonated basic compounds, which unless their polarity is sufficiently low, will elute at very short times. In this work, a comparative study of the performance of a group of amines (butylamine, pentylamine, hexylamine, cyclopentylamine, cycloheptylamine, N,N-dimethyloctylamine and tributylmethylammonium chloride), as modifiers of the chromatographic behaviour of basic compounds, is carried out. The behaviour is compared with that obtained with the ionic liquids 1-butyl-3-methylimidazolium chloride and 1-hexyl-3-methylimidazolium chloride, used as additives. The study revealed that the performance of the cationic additives to block the silanol activity is mainly explained by the additive size and its ability to be adsorbed onto the stationary phase.


Assuntos
Aminas/química , Cromatografia de Fase Reversa , Silanos/química , Antagonistas Adrenérgicos beta/química , Antagonistas Adrenérgicos beta/isolamento & purificação , Boratos/química , Interações Hidrofóbicas e Hidrofílicas , Imidazóis/química , Líquidos Iônicos/química
7.
Anal Chim Acta ; 883: 1-21, 2015 Jul 09.
Artigo em Inglês | MEDLINE | ID: mdl-26088771

RESUMO

The popularity of ionic liquids (ILs) has grown during the last decades in several analytical separation techniques. Consequently, the number of reports devoted to the applications of ILs is still increasing. This review is focused on the use of ILs (mainly imidazolium-based associated to chloride and tetrafluoroborate) as mobile phase additives in high-performance liquid chromatography (HPLC). In this approach, ILs just function as salts, but keep several kinds of intermolecular interactions, which are useful for chromatographic separations. Both cation and anion can be adsorbed on the stationary phase, creating a bilayer. This gives rise to hydrophobic, electrostatic and other specific interactions with the stationary phase and solutes, which modify the retention behaviour and peak shape. This review updates the advances in this field, with emphasis on topics not always deeply considered in the literature, such as the mechanisms of retention, the estimation of the suppressing potency of silanols, modelling and optimisation of the chromatographic performance, and the comparison with other additives traditionally used to avoid the silanol problem.

8.
J Chromatogr A ; 1380: 96-103, 2015 Feb 06.
Artigo em Inglês | MEDLINE | ID: mdl-25591403

RESUMO

In reversed-phase liquid chromatography in the absence of additives, cationic basic compounds give rise to broad and asymmetrical peaks as a result of ionic interactions with residual free silanols on silica-based stationary phases. Ionic liquids (ILs), added to the mobile phase, have been suggested as alternatives to amines to block the activity of silanols. However, the dual character of ILs should be considered: both cation and anion may be adsorbed on the stationary phase, thereby creating a double asymmetrical layer positively or negatively charged, depending on the relative adsorption of both ions. In this work, a study of the performance of six imidazolium-based ILs (the chlorides and tetrafluoroborates of 1-ethyl-, 1-butyl- and 1-hexyl-3-methylimidazolium) as modifiers of the chromatographic behaviour of a group of 10 ß-blockers is performed, and compared with triethylamine and dimethyloctylamine. In order to gain more insight in the behaviour of ILs in RPLC, the changes in the nature of the chromatographic system, at increasing concentration of the additives, were followed based on retention and peak shape modelling. The multiple interactions that amines and ILs experience inside the chromatographic system suggest that the suppressing potency should be measured based on the shape of chromatographic peaks and not on the changes in retention. The ILs 1-hexyl-3-methyl-imidazolium chloride and tetrafluoroborate offered the most interesting features for the separation of the basic drugs.


Assuntos
Cromatografia Líquida de Alta Pressão/métodos , Cromatografia de Fase Reversa/métodos , Imidazóis/química , Líquidos Iônicos/química , Antagonistas de Receptores Adrenérgicos beta 1/isolamento & purificação , Aminas/química , Boratos/química , Cloretos/química
9.
J Sep Sci ; 38(4): 550-5, 2015 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-25476819

RESUMO

Micellar liquid chromatography makes use of aqueous solutions or aqueous-organic solutions containing a surfactant, at a concentration above its critical micelle concentration. In the mobile phase, the surfactant monomers aggregate to form micelles, whereas on the surface of the nonpolar alkyl-bonded stationary phases they are significantly adsorbed. If the mobile phase contains a high concentration of organic solvent, micelles break down, and the amount of surfactant adsorbed on the stationary phase is reduced, giving rise to another chromatographic mode named high submicellar liquid chromatography. The presence of a thinner coating of surfactant enhances the selectivity and peak shape, especially for basic compounds. However, the risk of full desorption of surfactant is the main limitation in the high submicellar mode. This study examines the adsorption of the anionic surfactant sodium dodecyl sulfate under micellar and high submicellar conditions on a C18 column, applying two methods. One of them uses a refractive index detector to obtain direct measurements of the adsorbed amount of sodium dodecyl sulfate, whereas the second method is based on the retention and peak shape for a set of cationic basic compounds that indirectly reveal the presence of adsorbed monomers of surfactant on the stationary phase.

10.
J Chromatogr A ; 1353: 2-9, 2014 Aug 01.
Artigo em Inglês | MEDLINE | ID: mdl-24947888

RESUMO

Method validation is being applied in the reported analytical methods for decades. Even before this protocol was defined, authors already somehow validated their methods without full awareness. They wished to assure the quality of their work. Validation is an applied approach to verify that a method is suitable and rugged enough to function as a quality control tool in different locations and times. The performance parameters and statistical protocols followed throughout a validation study vary with the source of guidelines. Before single laboratory validation, an analytical method should be fully developed and optimized. The purpose of the validation is to confirm performance parameters that are determined during method development, and it should provide information on how the method will perform under routine use. An unstable method may require re-validation. Further method development and optimization will be needed if validation results do not meet the accepted performance standards. When possible, the validation protocol should also be conducted as a collaborative study by multiple laboratories, on different instruments, reagents, and standards. At this point, it would be interesting to know how people are validating their methods. Are they evaluating all defined validation parameters? Are they indicating the followed guidelines? Is re-validation really currently used? Is validation performed by a single laboratory, or is it a collaborative work by several laboratories? Is it an evolving discipline? In this survey, we will try to answer these questions focused to the field of liquid chromatography.


Assuntos
Cromatografia Líquida/métodos , Estudos de Validação como Assunto , Coleta de Dados , Humanos , Controle de Qualidade
11.
J Chromatogr A ; 1344: 31-41, 2014 May 30.
Artigo em Inglês | MEDLINE | ID: mdl-24767834

RESUMO

In micellar liquid chromatography (MLC), chromatographic peaks are more evenly distributed compared to conventional reversed-phase liquid chromatography (RPLC). This is the reason that most procedures are implemented using isocratic elution. However, gradient elution may be still useful in MLC to analyse mixtures of compounds within a wide range of polarities, decreasing the analysis time. Also, it benefits the determination of moderately to low polar compounds in physiological fluids performing direct injection: an initial micellar eluent with a low organic solvent content, or a pure micellar (without surfactant) solution, will provide better protection of the column against the proteins in the physiological fluid, and once the proteins are swept away, the elution strength can be increased using a positive linear gradient of organic solvent to reduce the analysis time. This work aims to encourage analysts to implement gradients of organic solvent in MLC, which is rather simple and allows rapid analytical procedures without pre-treatment or the need of re-equilibration. The implementation of gradient elution is illustrated through the separation of eight basic compounds (ß-blockers) in urine samples directly injected into the chromatograph, the most hydrophobic showing large retention in both conventional RPLC and MLC. The use of the DryLab(®) software to optimise gradients of organic solvent with eluents containing a fixed amount of surfactant above the critical micellar concentration is shown to provide satisfactory predictions, and can facilitate greatly the implementation of gradient protocols.


Assuntos
Antagonistas Adrenérgicos beta/urina , Solventes/química , Cromatografia de Fase Reversa/métodos , Humanos , Indicadores e Reagentes/química , Micelas , Software , Tensoativos/química
12.
J Chromatogr A ; 1344: 76-82, 2014 May 30.
Artigo em Inglês | MEDLINE | ID: mdl-24767835

RESUMO

Column selection in reversed-phase liquid chromatography (RPLC) can become a challenge if the target compounds interact with the silica-based packing. One of such interactions is the attraction of cationic solutes to the free silanols in silica-based columns, which is a slow sorption-desorption interaction process that gives rise to tailed and broad peaks. The effect of silanols is minimised by the addition of a competing agent in the mobile phase, such as the anionic surfactant sodium dodecyl sulphate (SDS). In micellar-organic RPLC, the adsorption of an approximately fixed amount of SDS monomers gives rise to a stable modified stationary phase, with properties remarkably different from those of the underlying bonded phase. The chromatographic behaviour (in terms of selectivity, analysis time and peak shape) of eight C18 columns in the analysis of weakly acidic phenols and basic ß-blockers was examined with hydro-organic and micellar-organic mobile phases. The behaviour of the columns differed significantly when the cationic basic drugs were eluted with hydro-organic mobile phases. With micellar-organic mobile phases, the adsorption of surfactant, instead of making the columns similar, gave rise to a greater diversity of behaviours (especially in terms of selectivity and analysis time), for both groups of phenols and ß-blockers, which should be explained by the residual effect of the underlying bonded stationary phase and the different amount of surfactant covering the packing. Therefore, the implementation of a micellar-organic procedure in RPLC will depend significantly on the selected type of C18 column.


Assuntos
Cromatografia de Fase Reversa/métodos , Acetonitrilos/química , Antagonistas Adrenérgicos beta/química , Adsorção , Cromatografia de Fase Reversa/instrumentação , Micelas , Fenóis/química , Silanos/química , Dodecilsulfato de Sódio/química , Tensoativos/química , Água/química
13.
J Chromatogr A ; 1314: 142-53, 2013 Nov 01.
Artigo em Inglês | MEDLINE | ID: mdl-24055228

RESUMO

Peak profiles in chromatography are characterised by their height, position, width and asymmetry; the two latter depend on the values of the left and right peak half-widths. Simple correlations have been found between the peak half-widths and the retention times. The representation of such correlations has been called half-width plots. For isocratic elution, the plots are parabolic, although often, the parabolas can be approximated to straight-lines. The plots can be obtained with the half-widths/retention time data for a set of solutes experiencing the same kinetics, eluted with a mobile phase at fixed or varying composition. When the analysed solutes experience different resistance to mass transfer, the plots will be solute dependent, and should be obtained with the data for each solute eluted with mobile phases at varying composition. The half-width plots approach is a simple tool that facilitates the prediction of peak shape (width and asymmetry) with optimisation purposes, reveal the interaction kinetics of solutes in different columns, and characterise chromatographic columns. This work shows half-width plots for different situations in isocratic elution, including the use of different flows, the effect of temperature, the modification of the stationary phase surface by an additive, the existence of specific interactions within the column, and the comparison of columns. The adaptation to gradient elution is also described. Previous knowledge on half-width plots is structured and analysed, to which new results are added.


Assuntos
Cromatografia Líquida/instrumentação , Cinética , Modelos Teóricos
14.
J Chromatogr A ; 1281: 94-105, 2013 Mar 15.
Artigo em Inglês | MEDLINE | ID: mdl-23398999

RESUMO

Although there is a great deal of stationary phases having different selectivities (even practically orthogonal selectivities), these very rarely are taken as a factor to be optimized during method development. The chromatographer selects the stationary phase usually in a trial-and-error fashion (or based on the solute nature and expected interactions), and then optimizes continuous factors as the mobile phase composition, pH, temperature and flow-rate. However, the optimization of the stationary phase nature and column length (which are discrete factors) may be interesting. In this regard, the optimization of the coupling of individual columns may yield separations that are not possible with a single column, based on the combined selectivities and the potential increase in plate count. The idea is highly attractive, but there are only few reports in the literature using this approach. The theory behind the use of serially coupled columns is indeed rather simple, but its implementation may be troublesome. The most problematic factor is the connection of the serial columns, which ideally should not modify the result of the sum of behaviors of the columns. A proper serial connection of columns needs a zero-dead volume (ZDV) union and a system to link tightly the columns to each other. In this work, two different approaches to solve this problem are compared using isocratic elution, one consisting in the use of holders, which are screwed to maintain attached the columns linked by ZDV junctions, and the other using ZDV fingertight column couplers, which are screwed directly to the columns maintaining them tightly attached without the need of column holders. The advantages and problems associated to these approaches are described, and information on the accuracy in the prediction of retention times, peak widths and asymmetries are given for the combined columns. Guidelines to prepare software to make reliable predictions are also presented. A set of 15 sulphonamides were used to probe the systems.


Assuntos
Cromatografia Líquida/instrumentação , Cromatografia Líquida/métodos , Modelos Teóricos , Software , Reprodutibilidade dos Testes , Sulfonamidas/isolamento & purificação
15.
J Chromatogr A ; 1258: 168-74, 2012 Oct 05.
Artigo em Inglês | MEDLINE | ID: mdl-22944385

RESUMO

Ionic liquids are dual modifiers composed by a large anion and a large cation, which interact with both the hydrophobic alkyl-bonded phase and the anionic residual silanols in C18 columns. The deactivation of the silanol groups has important implications on the chromatographic analysis of basic drugs, being the improvement of peak profiles and shorter retention times the most noticeable features. However, other characteristics as selectivity or resolution are not usually considered, or are only examined for selected chromatographic conditions. In this work, the effect of the addition of the ionic liquid 1-hexyl-3-methyl imidazolium tetrafluoroborate to acetonitrile-water mixtures in a wide range of concentrations, using three C18 columns (Zorbax SB-C18, Nucleosil and Spherisorb) was studied for the separation of a group of ß-adrenolytic drugs. The extremely poor chromatographic performance was highly improved in the presence of the ionic liquid, giving rise to shorter analysis times, smaller consumption of acetonitrile, significantly narrower and more symmetric peaks, and high resolution. The improvement was especially noteworthy for the Spherisorb column, which yielded the poorest results in the absence of additive.


Assuntos
Boratos/química , Cromatografia de Fase Reversa/instrumentação , Cromatografia de Fase Reversa/métodos , Imidazóis/química , Preparações Farmacêuticas/isolamento & purificação , Concentração de Íons de Hidrogênio , Preparações Farmacêuticas/análise
16.
J Chromatogr A ; 1232: 166-75, 2012 Apr 06.
Artigo em Inglês | MEDLINE | ID: mdl-22196243

RESUMO

Residual silanols on C18 columns yield undesirable slow-kinetics ion-exchange interactions with positively charged basic compounds that result in asymmetrical peaks, low efficiencies and long retention times. The purity of the silica employed as supporting material, and the technique used to form the bonded phase, which varies with the brand and manufacturer, give rise to different amounts of residual silanols in the packings, and consequently, different chromatographic performance. One of the most efficient and widespread strategies to reduce or even eliminate the different performance among columns is the addition of a reagent to the mobile phase to block the silanol sites. However, the intrinsic nature of both stationary phase and additive leads to particular results. In this work, a group of basic compounds were analysed using six C18 stationary phases (Zorbax SB-C18, X-Terra MS C18, Kromasil, Lichrospher, Nucleosil, and Spherisorb) and acetonitrile-water mixtures. Two ionic liquids (ILs), 1-butyl- and 1-hexyl-3-methyl-imidazolium tetrafluoroborates, were added to the mobile phases to evaluate their silanol suppressing potency, based on the decreased retention of the basic compounds when the silanols are blocked (described by the Horváth equation), and the improvement in peak profile (described by the plots of the peak half-widths at diverse retention times). The suppressing potency based on the retention can be misleading when the adsorption of the IL anion is not negligible, since the anion attracts the cationic basic compounds increasing the retention. However, the accessibility of basic compounds to the silanols is prevented by both IL cation and anion, improving the peak profiles for all stationary phases. This was especially remarkable for Spherisorb, which in the absence of additive yielded by far the worst performance. 1-Hexyl-3-methyl-imidazolium tetrafluoroborate was the best additive in terms of retention and peak profile (width and asymmetry).


Assuntos
Cromatografia Líquida/instrumentação , Líquidos Iônicos/química , Silanos/química , Acetonitrilos/química , Antagonistas Adrenérgicos beta/química , Antagonistas Adrenérgicos beta/isolamento & purificação , Cromatografia Líquida/métodos , Imidazóis/química
17.
J Chromatogr A ; 1218(36): 6044-52, 2011 Sep 09.
Artigo em Inglês | MEDLINE | ID: mdl-21269630

RESUMO

Countercurrent chromatography (CCC) is a liquid chromatography technique in which the stationary phase is also a liquid. The main chemical process involved in solute separation is partitioning between the two immiscible liquid phases: the mobile phase and the support-free liquid stationary phase. The octanol-water partition coefficients (P(o/w)) is the accepted parameter measuring the hydrophobicity of molecules. It is considered to estimate active principle partitioning over a biomembrane. It was related to the substance biological activity. CCC is able to work with an octanol stationary phase and an aqueous mobile phase. In this configuration, CCC is a useful and easy alternative to measure directly the P(o/w) of the molecules compared to other methods including the classical and tedious shake-flask method. Three ketones are used as model compounds to illustrate the CCC protocol of P(o/w) measurement. The focus of this work is put on ionisable molecules whose apparent P(o/w) is completely changed by ionization. ß-Blockers, diuretics and sulfonamides are compound classes that were studied. Some of the experimentally determined P(o/w) coefficients of the molecular forms disagreed with calculated and experimental values available in the literature. The P(o/w) coefficients of the ionic forms and the acidity constants were also calculated using a theoretical model. Relationships between biological properties and hydrophobicity are also discussed.


Assuntos
Antagonistas Adrenérgicos beta/química , Distribuição Contracorrente/métodos , Diuréticos/química , Preparações Farmacêuticas/química , Interações Hidrofóbicas e Hidrofílicas
18.
J Chromatogr A ; 1218(3): 398-407, 2011 Jan 21.
Artigo em Inglês | MEDLINE | ID: mdl-21176907

RESUMO

The cationic nature of basic drugs gives rise to broad asymmetrical chromatographic peaks with conventional C18 columns and hydro-organic mixtures, due to the ionic interaction of the positively charged solutes with the free silanol groups on the alkyl-bonded reversed-phase packing. Ionic liquids (ILs) have recently attracted some attention to reduce this undesirable silanol activity. ILs are dual modifiers (with a cationic and anionic character), which means that both cation and anion can be adsorbed on the stationary phase, giving rise to interesting interactions with the anionic free silanols and the cationic basic drugs. A comparative study of the performance of four imidazolium-based ILs as modifiers of the chromatographic behaviour of a group of ß-blockers is shown. The ILs differed in the adsorption capability of the cation and anion on C18 columns. Mobile phases without additive and containing a cationic (triethylamine, TEA) or anionic (sodium dodecyl sulphate, SDS) additive were used as references for the interpretation of the behaviours. The changes in the nature of the chromatographic system, at increasing concentration of the additives, were followed based on the changes in retention and peak shape of the ß-blockers. The silanol suppressing potency of the additives, and the association constants between the solutes and modified stationary phase or additive in the mobile phase, were estimated. The study revealed that SDS and the ionic liquid 1-hexyl-3-methylimidazolium tetrafluoroborate are the best enhancers of chromatographic peak shape among those studied.


Assuntos
Cromatografia de Fase Reversa/métodos , Líquidos Iônicos/química , Modelos Químicos , Preparações Farmacêuticas/isolamento & purificação , Boratos/química , Etilaminas/química , Concentração de Íons de Hidrogênio , Imidazóis/química , Modelos Lineares , Silanos/química , Dodecilsulfato de Sódio/química
19.
J Chromatogr A ; 1217(45): 7090-9, 2010 Nov 05.
Artigo em Inglês | MEDLINE | ID: mdl-20934180

RESUMO

Organic solvents are traditionally added to micellar mobile phases to achieve adequate retention times and peak profiles, in a chromatographic mode which has been called micellar liquid chromatography (MLC). The organic solvent content is limited to preserve the formation of micelles. However, at increasing organic solvent contents, the transition to a situation where micelles do not exist is gradual. Also, there is no reason to neglect the potentiality of mobile phases containing only surfactant monomers instead of micelles (high submicellar chromatography, HSC). This is demonstrated here for the analysis of ß-blockers. The performance of four organic solvents (methanol, ethanol, 1-propanol, and acetonitrile) was compared in mobile phases containing the anionic surfactant sodium dodecyl sulphate in the MLC and HSC modes. The association of the organic solvent molecules with micelles gives rise to a significant loss in the elution strength of the organic solvent; whereas upon disruption of micelles, it tends to that observed in the hydro-organic mode. The elution behaviour of the ß-blockers was modelled to predict the retention times. This allowed the detailed exploration of the selectivity and resolution of the chromatographic systems in relatively wide ranges of concentration of surfactant and organic solvent. The best performance in terms of resolution and analysis time was achieved using HSC with acetonitrile, being able to base-line resolve a mixture of eight ß-blockers. Ethanol also provided a good separation performance, significantly improved with respect to methanol and 1-propanol. In contrast, the hydro-organic mode using acetonitrile or any of the short-chain alcohols could not succeed with the separation of the ß-blockers, owing to the poorer selectivity and wider peaks.


Assuntos
Acetonitrilos/química , Antagonistas Adrenérgicos beta/isolamento & purificação , Álcoois/química , Cromatografia de Fase Reversa/métodos , Tensoativos/química , Álcoois/isolamento & purificação , Algoritmos , Interpretação Estatística de Dados , Micelas , Modelos Químicos , Dodecilsulfato de Sódio/química
20.
J Chromatogr A ; 1217(45): 7082-9, 2010 Nov 05.
Artigo em Inglês | MEDLINE | ID: mdl-20932525

RESUMO

The behaviour of ß-blockers in a reversed-phase liquid chromatographic (RPLC) column with mobile phases containing a short-chain alcohol (methanol, ethanol or 1-propanol), with and without the surfactant sodium dodecyl sulphate (SDS), was explored. Two surfactant-mediated RPLC modes were studied, where the mobile phases contained either micelles or only surfactant monomers at high concentration. Acetonitrile was also considered for comparison purposes. A correlation was found between the effects of the organic solvent on micelle formation (monitored by the drop weight procedure) and on the nature of the chromatographic system (as revealed by the retention, elution strength and peak shape of ß-blockers). When SDS is added to the mobile phase, the free surfactant monomers bind the C18 bonded chains on the stationary phase, forming an anionic layer, which attracts strongly the cationic ß-blockers. The retention is modified as a consequence of the solving power of the organic solvent, micelles and surfactant monomers. The molecules of organic solvent bind the micelles, modify their shape, and may avoid their formation. They also bind the monomers of surfactant, desorbing them from the stationary phase, which affects the retention. The remaining surfactant covers the free silanols on the siliceous support, avoiding the interaction with the cationic solutes. The retention of ß-blockers results from a combination of electrostatic and hydrophobic interactions, the latter being weaker compared to the hydro-organic system. The peak efficiencies and asymmetries are excellent tools to probe the surfactant layer on the stationary phase in an SDS/organic solvent system. The peaks will be nearly symmetrical wherever enough surfactant coats the stationary phase (up to 60% methanol, 40% ethanol, 35% 1-propanol, and 50% acetonitrile).


Assuntos
Antagonistas Adrenérgicos beta/química , Álcoois/química , Cromatografia de Fase Reversa/métodos , Dodecilsulfato de Sódio/química , Tensoativos/química , 1-Propanol , Acetonitrilos/química , Interpretação Estatística de Dados , Etanol , Metanol , Micelas
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