Monolithic Column with Zwitterionic Stationary Phase for Capillary Electrochromatography

Abstract

A capillary electrochromatography (CEC) monolithic column with zwitterionic stationary phases was prepared by in situ polymerization of butyl methacrylate, ethylene dimethacrylate, methacrylic acid, and 2-(dimethyl amino) ethyl methacrylate in the presence of porogens. The stationary phases have zwitterionic functional groups, that is, both tertiary amine and acrylic acid groups, so the ionization of those groups on the zwitterionic stationary phase was affected by the pH values of the mobile phase, and further affects the strength and direction of the electroosmotic flow (EOF). Separations of alkylbenzenes and polycylic aromatic hydrocarbons based on the hydrophobic mechanism were obtained. Separation of various types of polar compounds, including phenols, anilines, and peptides, on the prepared column were performed under CEC mode with anodic and cathodic EOF, and different separation selectivities of those polar analytes were observed on the monolithic capillary column by using mobile phases with different pH values.Ionizable groups on the surface of the stationary phase are necessary to generate substantial electoosmotic flow (EOF) for capillary electrochromatography (CEC). 1 Strictly speaking, most stationary phases used in CEC are ion-exchangers. Silica-based packing materials, which have been most widely used in CEC and can be regarded as weak cation exchangers, generate cathodic EOF due to the ionization of the residual silanol groups on the surface of packings. Recently, strong ion exchangers and so-called mixed-mode packing materials consisting of ionic groups such as sulfonic acids or quaternary amines and hydrocarbon chains have attracted much attention in CEC, because these packings ensure stable EOF over an extended pH range. 2 The direction of the EOF depends on the charges on the surface of the stationary phases, so stationary phases with positively charged functional groups, such as amino groups or ammonium groups, generate an EOF from cathode to anode, whereas stationary phases carrying negatively charged groups, such as sulfonic acid and carboxylic acid, generate cathodic EOF. To date, the vast majority of reports on CEC concern EOF with one direction, that is, cathodic or anodic EOF, for a CEC column; however, zwitterionic stationary phases, which can make it possible to generate EOF with different directions for one column, are seldom investigated in CEC. In fact, there are many reports concerning chromatographic separation with zwitterionically modified materials, although it is still one of the new liquid chromatography separation modes studied in recent years. [3][4][5][6] Hu et al. 7 established the zwitterionic functionality via dynamically adsorbing the ODS (octadecyl silica) column with a sulfobetaine-type zwitterionic surfactant by the interaction between the C 18 groups on the surface and the hydrophobic tail of the surfactant. To overcome the drawback of inferior stability due to loss of functional moieties from the dynamically attached layer on the surface of stationary phases, ...

Hybrid organic-inorganic phenyl monolithic column for capillary electrochromatography

“…Up till now, both organic polymer [3][4][5] and silica-based [6][7][8][9][10] monoliths have been developed. Polymeric monolithic stationary phases have the advantage of simple polymerization procedure and easy tuning of porosity and surface chemistry, but they suffer from shrinking or swelling when exposed to different organic mobile phases, leading to lack of mechanical stability.…”

Fases estacionárias monolíticas para separações cromatográficas

“…Outra vantagem do uso de monolitos poliméricos em ECC é a boa estabilidade química, entre pH 2 e 12, visto que, freqüentemente, se realizam separações em condições de pH fora da faixa de estabilidade da sílica (pH de 2 a 8), para enriquecimento do fluxo eletroosmótico 1,3,27 . Até o momento três tipos diferentes de polímeros orgânicos foram preparados na forma de suportes cromatográficos monolíticos em CLAE e ECC: poliacrilamida 21,28 , poliestireno-divinilbenzeno 11,24,[29][30][31] e polimetacrilato 25,[32][33][34][35][36] . Os polimetacrilatos são os mais empregados, devido à ampla variedade de monômeros funcionais e a fácil derivatização após a polimerização.…”

“…A polimerização é iniciada por aquecimento, em temperaturas que variam de 50 a 80 °C 25,31,[32][33][34][35][36] , ou por luz UV [37][38][39] . A principal vantagem da iniciação por luz UV é a polimerização local do monolito.…”

“…Em todas as aplicações, as eficiências em ECC foram superiores às obtidas em µCLAE. Diversas moléculas já foram analisadas no modo de fase reversa da eletrocromatografia capilar, a citar: hidrocarbonetos aromáticos policí-clicos 18,35,36,55,56 , compostos básicos 33,35,36,63 e ácidos 33,36,55,62 , fármacos 28 , pesticidas 41 , entre outros. Estes compostos são freqüentemente separados com rapidez, devido à presença de moléculas na FM que fornecem altas velocidades de fluxo eletroosmótico e às altas eficiências cromatográficas, provenientes do perfil planar do FEO.…”

“…Desta forma, evitaram-se as interações eletrostáticas indesejáveis entre os polipeptídeos e os grupos amino, obtendo boas separações e picos essencialmente simétricos. De maneira similar, Fu et al 36 analisaram bi-e tripeptídeos em dois valores de pH (2 e 8) empregando uma coluna monolítica à base de polimetacrilatos com grupos carregados positiva ou negativamente, dependendo do pH da fase móvel. Em pH 2,0 a eluição dos seis peptídeos analisados foi aproximadamente três vezes mais rápida que em pH 8,0, além de apresentar melhores simetrias para os picos das macromoléculas, devido à inexistência de interações eletrostática entre as macromoléculas e os grupos iônicos da superfície do monolito.…”