Heteroconjugation-based capillary electrophoretic separation of phenolic compounds in acetonitrile and propylene carbonate

A mixture of methyl- and hydroxy-substituted phenols was separated by capillary electrophoresis in pure acetonitrile and propylene carbonate. Interactions between undissociated phenolic compounds and the background electrolytes were investigated. In the present work, benzyltriethylammonium chloride, tetrabutylammonium acetate, and two room temperature-molten salts, 1-butyl-3-methyl imidazolium trifluoroacetate and 1-butyl-3-methyl imidazolium heptafluorobutanoate, were used as background electrolytes. The formation of a negative complex between background electrolyte anion and neutral phenolic compound was observed and the formation constant calculated. The formation constants for anion-analyte complexes were approximately the same in propylene carbonate and in acetonitrile. In both solvents the formation constants were the highest for acetate and the lowest for trifluoroacetate. The separation of analytes was slightly influenced by the nature of the solvent: in acetonitrile the resolution between peaks was higher for 1,3-dihydroxyphenol and 1,3,5-trihydroxyphenol, in propylene carbonate 3-methylphenol and phenol were better separated. It was demonstrated that traces of water influence the mobilities of anion-phenol complexes in propylene carbonate.     

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.     

Rapid analysis of free fatty acid composition in Brassica rapa L. and Brassica napus L. extracts by surface-assisted laser desorption/ionization time-of-flight mass spectrometry

A rapid and simple technique was developed to identify free fatty acids in mixtures by the surface-assisted laser desorption/ionization time-of-flight mass spectrometry (SALDI-TOF MS) using powdered carbon aerogels as a “matrix”. A 3% solution of a neutral surfactant Triton X-100 was used as a fixing agent of carbon particles as well as a suppressor of background peaks. The extraction of free fatty acids from Brassica napus (rape) and Brassica rapa (turnip) seeds was performed by using both methanol-chloroform mixture and supercritical fluid extraction techniques. The quantitative analysis of the seed composition was carried out to demonstrate the effectiveness of the developed method. The detection limits were determined using a mixture of ten standard fatty acids and were found to be 3 ng of each acid per spot.     

Investigation of the surfactant type and concentration effect on the retention factors of glutathione and its analogues by micellar electrokinetic chromatography

In the present study, a micellar electrokinetic chromatographic method was used to determine the retention factors of hydrophilic monomeric and homodimeric forms of glutathione analogues. Ionic‐liquid‐based surfactant, 1‐tetradecyl‐3‐methylimidazolium chloride, as well as cetyltrimethylammonium bromide and phosphate buffer (pH 7.4) were employed in the experiments. Since the studied peptides possess a negative charge under physiological conditions, it is expected that the peptides interact with the oppositely charged 1‐tetradecyl‐3‐methylimidazolium chloride and cetyltrimethylammonium bromide micelles via hydrophobically assisted electrostatic forces. The dependence of the retention factor on the micellar concentration of 1‐tetradecyl‐3‐methylimidazolium chloride and cetyltrimethylammonium bromide is nonlinear and the obtained curves converge to a limiting value. The retention factor values of GSH analogues were in the range of 0.36–2.22 for glutathione analogues and –1.21 to 0.37 for glutathione when 1‐tetradecyl‐3‐methylimidazolium chloride was used. When cetyltrimethylammonium bromide was employed, the retention factor values were in the range of 0.27–2.17 for glutathione analogues and –1.22 to 0.06 for glutathione. If sodium dodecyl sulfate was used, the retention factor values of glutathione analogues with carnosine moiety were in the range of –1.54 to 0.38.     

Thermal marks as a signal processing aid for a portable capillary electropherograph

The interpretation of raw signals in capillary CE can be challenging if there are unknown peaks, or the signal is corrupt due to baseline fluctuations, EOF velocity drift, etc. Signal processing could be required before results can be interpreted. A suite of signal processing algorithms has been developed for CE data analysis, specifically for use in field experiments for the detection of nerve agents using portable CE instruments. Everything from baseline correction and electropherogram alignment to peak matching and identification is included in these programs. Baseline correction is achieved by interpolating a new baseline according to points found using all local extremes, by applying an appropriate outliers test. Irreproducible migration times are corrected by compensating for EOF drift, measured with the aid of thermal marks. Thermal marks are small disturbances in the capillary created by punctual heating that move with the velocity of EOF. Peaks in the sample electropherogram are identified using a fuzzy matching algorithm, by comparing peaks from the sample electropherogram to peaks from a reference electropherogram.     

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.     

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%.     

Nonaqueous CE using contactless conductivity detection and ionic liquids as BGEs in ACN

N,N'-Alkylmethylimidazolium cations have been separated in NACE when one of the N,N'-dialkylimidazolium salts (ionic liquids (ILs)) was used as an electrolyte additive to the organic solvent separation medium. The separated species were 1-methyl-, 1-ethyl-, 1-butyl-, 1-octyl-, 1-decyl-3-methylimidazolium and N-butyl-3-methylpyridinium cations and BGE composed of 1-ethyl-3-methylimidazolium ethylsulfate or 1-butyl-3-methylimidazolium trifluoroacetate [BMIm][FAcO] (A6; B2) diluted in ACN. It was demonstrated that contactless conductivity detection (CCD) may be applied to monitoring the separation process in nonaqueous separation media, allowing to use the UV light-absorbing imidazolium-based electrolyte additives. There could be marked three concentration regions of added ILs; at first ionic strength of BGE below 1-2 mM, and then the actual electrophoretic mobility of analytes rises from 0. At concentrations above 1-2 mM, the added IL facilitated separation. In concentration region of 1-20 mM, the actual electrophoretic mobility of analyzed imidazolium cations was increasing with decrease in separation medium ionic strength. At higher concentrations of BGE (above 30 mM), the conductivity of the separation media became too high for this detector. Some organic dyes were also successfully separated and detected by contactless conductivity detector in a 20 mM A6 separation electrolyte in ACN.     

Capillary zone electrophoresis on chemically bonded imidazolium based salts

In the present paper, fused-silica capillaries were chemically modified with an analogue of the imidazole-based ionic liquid and zwitterionic salt. The coated capillaries were examined for the behavior of the electroosmotic flow in both aqueous and non-aqueous electrolytes. The electroosmotic flow in the capillary coated with an ionic liquid analogue was found to be anodic (reversed) and dependent on the pH of the separation buffer. In the case of a zwitterionic capillary, the electroosmotic flow was cathodic and its velocity remained almost constant in the pH range of 4-7. The zeta-potentials of the modified surfaces were also calculated. The effectiveness of coating was investigated by comparing a separation of five inorganic ions and seven alkylphosphonic acids/monoesters in the modified and uncoated capillaries. All separations were successfully carried out in simple buffers and completed during a short analysis time. Finally, the run-to-run and day-to-day reproducibility of the coated capillaries in terms of the migration time of a neutral marker was determined.     

Simultaneous determination of fluoroquinolones,sulfonamides and tetracyclines in sewage sludge by pressurized liquid extraction and liquid chromatography electrospray ionization-mass spectrometry

A new scheme for the quantitative determination of traces of fluoroquinolones (FQs), tetracyclines (TCs) and sulfonamides (SAs) in sewage sludge was developed. The compounds were simultaneously extracted from sewage sludge by pressurized liquid extraction (PLE). A novel and effective method for PLE was developed. Solid-phase extraction was used for cleaning up the extracts. Identification and quantification of the compounds was done using high-performance liquid chromatography with electrospray ionization mass spectrometry in selected reaction monitoring mode. The best recovery of FQs and TCs was obtained by using hydrophilic–lipophilic balance cartridges, recoveries ranged 59% for norfloxacin to 82% for ofloxacin and 95% for doxycycline; for SAs strong cation-exchange cartridges were more efficient, recoveries were 96% for sulfamethoxazole and 43% for sulfadimethoxine. Limit of quantification ranged from 0.1 ng/g for SAs to 160 ng/g for tetracycline. Method precision for TCs was 5.06% and 1.12%, and for SAs 0.43% and 2.01%. FQs precision ranged from 0.77% to 1.89%.