Merike Vaher

Ionic liquids as electrolytes for nonaqueous capillary electrophoresis

Acetonitrile is a well‐suited medium for nonaqueous capillary electroseparations and enables extending the range of applications of capillary electrophoresis (CE) techniques to more hydrophobic species. In this study, the dialkylimidazolium‐based low temperature melting organic salts know as “ionic liquids” (ILs) are used as electrolytes. At room temperature these liquids are miscible with acetonitrile which makes it easy to use them for adjustment of analyte mobility and separation. The anionic part as well as the concentration of an IL influence the general electrophoretic mobility of the buffer system. The separation of different analytes is achieved because they become charged in the presence of ILs in separation media. There is also a possibility for a complex formation between the solute and the electrolyte which alters the mobility of the solute. A selected application of separations of phenols and aromatic acids will be discussed.

Application of 1-alkyl-3-methylimidazolium-based ionic liquids in non-aqueous capillary electrophoresis

In many cases salts, which are liquid at room temperature show a better solubility in organic solvents, and can be used in nonaqueous capillary zone electrophoresis as ionic additives. In this study 1-alkyl-3-methylimidasolium-based ionic liquids were used as additives in separation media to assess the interactions between the analytes and the ionic additive present and to find an influence of the type and concentration of the ionic additive, also the nature of the nonaqueous medium employed. Different organic solvents (acetonitrile and methanol) contribute differently to the conversion of analytes into a charged form. Complexes with either an anionic or a cationic part of the ionic liquid additive were formed. This was the case for electrophoresis separation of Brønsted acids and polyphenolic compounds.

Separation of polyphenolic compounds extracted from plant matrices using capillary electrophoresis.

Phenolic compounds constitute a large group of secondary plant products whose chemical structure may range from quite simple compounds to highly polymerized substances. The polyphenols content have been investigated in the alcoholic extract of the fruits of three different plants: sweet gale, sea buckthorn, hiprose. The trans-resveratrol content we have studied in roots, stems, leaves and flowers of Japanese knotweed grown in Estonia. Plant material was pre-treated in two different ways: by infusing with methanol and by supercritical fluid extraction with carbon dioxide modified with different alcohols. The relationship between variables (pressure, temperature, modifier amount) and yields are examined. The capillary zone electrophoresis methods were developed for the separation of polyphenolic anti-oxidative compounds. Using both water based borate buffer and acetonitrile based non-aqueous media it was possible to get reliable separation of several polyphenolic compounds. Based on that there has been identified such as flavone, trans-resveratrol, catechin, chlorogenic acid, quercetin and myricetin in plant extracts. Changes in the relative concentrations of trans-resveratrol in different parts of the knotweed have been established.

Capillary electrophoretic analysis of neutral carbohydrates using ionic liquids as background electrolytes

In this study, ionic liquids (ILs) as BGE additives were applied for the analysis of neutral carbohydrates in CE. The ILs served primarily as chromophores for indirect UV detection. The influence of imidazolium‐based ionic liquids on the separation, detection limits and mobility of underivatized neutral carbohydrates was investigated. BGEs consisting of 10–50 mM of ILs at pH 12.4 without other additives provided fast separation of neutral sugars. This method was used to determine sucrose, glucose and fructose in certain vegetable juices.

Evaluation of antioxidative capability of the tomato (Solanum lycopersicum) skin constituents by capillary electrophoresis and high-performance liquid chromatography.

In the current study, phenolic compounds of the tomato (Solanum lycopersicum) skin extract were separated and their composition was determined by capillary electrophoresis and tandem high‐performance liquid chromatography diode array detection‐mass spectrometry (HPLC‐DAD‐MS/MS). Both the total phenolic and flavonoid contents of the extract were determined. The antioxidative capability of the extract was measured using a stable free radical 2,2‐diphenyl‐1‐picrylhydrazyl assay. The monitoring of the radical scavenging capability of specific phenolic compounds was carried out both by capillary electrophoresis and HPLC‐MS/MS.

A portable capillary electropherograph equipped with a cross-sampler and a contactless-conductivity detector for the detection of the degradation products of chemical warfare agents in soil extracts

A fully portable CE device equipped with a capacitively coupled contactless‐conductivity detector and a cross‐injection device is put to the test in laboratory conditions. The portable device is capable of working on batteries for at least 4 h. After that, its performance is strongly affected by the drop in the high‐voltage output and analysis may be interrupted if its length exceeds a reasonable time. The concentration of the BGE affects both ionic strength and conductivity. Choosing an optimal concentration of BGE is therefore about finding a good compromise between selectivity and sensitivity. All experiments were performed using a mixture of histidine and MES with a concentration of 15 mM as BGE. The performance of the cross‐injection device is optimized by the use of internal standards. Satisfactory reproducibility is gained as the RSD of peak areas is reduced to 8% or less. LODs for different phosphonic acids are in the range of 2.5–9.7 μM. For the analysis of adsorption of phosphonic acids in sand and loamy soil samples, calibration curves are constructed. Linearity in a measured concentration range of 10–100 μM is excellent, as the squares of correlation constants are ∼1. The concentration analysis of phosphonic acids in soil extracts demonstrates that their adsorption curves in sand and loamy soil follow different adsorption isotherms.

Analysis of the stable free radical scavenging capability of artificial polyphenol mixtures and plant extracts by capillary electrophoresis and liquid chromatography–diode array detection–tandem mass spectrometry

The oxidation process of phenolic compounds of an artificial mixture consisting of six polyphenols and the extract of eggplant (Solanum melongena) skin was monitored by using capillary zone electrophoresis and liquid chromatography-diode array detection-tandem mass spectrometry. The methods developed enabled simultaneous evaluation of the antioxidative capability of each compound. The above oxidation process was carried out using two radicals, viz. the 2,2-diphenyl-1-picrylhydrazyl and hydroxyl radicals generated via the Fenton reaction. The radical scavenging effects of artificial and natural polyphenol mixtures were compared.

Application of capillary zone electrophoresis to the separation and characterization of poly(amidoamine) dendrimers with an ethylenediamine core

Generations 0 through 5 of ethylenediamine-core poly(amidoamine) dendrimers were synthesized and capillary zone electrophoresis has been applied to the separation of different generations of synthesized dendrimers and for the characterization of individual generations.

Monitoring of the electroosmotic flow of ionic liquid solutions in non-aqueous media using thermal marks

The possibility of applying a new method employing thermal marks to measuring the rate of the electroosmotic flow (EOF) in non-aqueous capillary electrophoresis (NACE) was investigated. The thermal marks were monitored by using a contactless conductivity detection. During one experiment and in between the series of experiments the reproducibility of the method was excellent. The EOF rate was measured 4-7 times during one experiment, the precision of measurement being around 0.5%. In this study, the influence of 1-butyl-3-methyl-imidazolium salts in organic solvents on the rate of the EOF was investigated. Various organic solvents were mixed with an ionic liquid of various concentrations and the EOF rate was measured using thermal marks. The accuracy of the method was compared with that of the neutral marker one. Five benzoic acid derivatives were separated while the EOF was monitored. The relative standard deviations of the corrected effective mobilities of the above analytes were in the range of 1.0-6.1%.

Fraction collection in capillary electrophoresis for various stand-alone mass spectrometers.

A procedure for collecting fractions during capillary electrophoresis for their analysis using various stand-alone instruments is described. The results of a systematic study of the optimization and application of capillary electrophoresis (CE) in conjunction with a reverse-phase high-performance liquid chromatography electrospray ionization quadrupole time of flight-tandem mass spectrometry (RP-HPLC-ESI-Q-TOF-MS/MS) and inductively-coupled mass spectrometry (ICP-MS) to the analysis of the seed extract of the Japanese Pagoda Tree (Sophora japonica) are presented. The off-line coupling of CE to the matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF-MS) for the proteins mixture was applied. The cathode end of the capillary was placed inside a stainless steel needle using a coaxial liquid-sheath-flow configuration. The optimization of experimental parameters resulted in an efficient methodology for MS analysis of fractions. Several components contained in the extract of S. japonica were identified, some not previously known. It was demonstrated that low sensitivity, which is a real problem in off-line CE-MS analysis, could be tolerated because of a more flexible optimization of the CE separation conditions and the choice of independent stand-alone instruments for analysis of separated fractions. The estimated limit of detection for CE-RP-HPLC-ESI-Q-TOF-MS was 50 microM of polyphenols and for CE-ICP-MS, 1-100 microg/l.