Lishuang Yu

Determination of flavonoids in Houttuynia cordata Thunb. and Saururus chinensis (Lour.) Bail. by capillary electrophoresis with electrochemical detection

Four flavonoids (rutin, hyperoside, quercitrin and quercetin) in Houttuynia cordata Thunb. and Saururus chinensis (Lour.) Bail. were determined by capillary electrophoresis with wall-jet amperometric detection. The working electrode was a 500 microm diameter carbon disc electrode and the detection potential was +0.95 V (versus Ag/AgCl). Effects of several important factors, such as the running buffer and its corresponding pH and concentration, separation voltage, injection time were investigated to acquire the optimum conditions for separation of these four flavonoids. Baseline separation for the four flavonoids was obtained within 21 min in a 60 cm length capillary at a separation voltage of 15 kV with a 60 mmoL/L Na(2)B(4)O(7)-120 mmoL/L NaH(2)PO(4) buffer (pH 8.8) as running buffer. The relationship between peak currents and analyte concentrations was linear over about two orders of magnitude with detection limits (defined as S/N=3) ranging from 0.02 to 0.05 microg/mL for all analytes. This method was applied for the determination of the above four flavonoids in H. cordata Thunb. and S. chinensis (Lour.) Bail. with simple extraction procedures, and the assay results were satisfactory.

Field‐amplified on‐line sample stacking for simultaneous enantioseparation and determination of some β‐blockers using capillary electrophoresis

A capillary electrophoresis method using carboxymethyl‐β‐cyclodextrin as the chiral selector and mixture of methanol and ethanol as the organic additive was successfully developed for the simultaneous enantioseparation and determination of six β‐blockers, namely, carteolol, atenolol, sotalol, metoprolol, esmolol and propranolol in this paper. The most suitable running buffer for enantioseparation was found to be the solution of 20 mmol/L NaH2PO4–Na2HOP4 (pH 5.5) containing 1.5% w/v carboxymethyl‐β‐cyclodextrin, 5% v/v methanol and 5% v/v ethanol. Furthermore, field‐amplified sample injection as an on‐line sample stacking method was developed in order to increase the detection sensitivity. The experimental conditions for both simultaneous enantioseparation and the field‐amplified sample injection method had been investigated in detail. Under the optimum conditions, the detection limits (defined as S/N=3) of this method were 0.01, 0.05, 0.05, 0.05, 0.05 and 0.5 μg/mL for (±) carteolol, (±) atenolol, (±) sotalol, (±) metoprolol, (±) esmolol and (±) propranolol, respectively, which were much lower than those of the conventional methods. The enhancement factors were greatly improved by 25‐fold for the enantiomers of the β‐blockers except five‐fold for (±) propranolol. Eventually, the proposed method has been applied for the analysis of human serum sample.

Improved simultaneous enantioseparation of β-agonists in CE using β-CD and ionic liquids

In this study, approaches to improve chiral resolutions in simultaneous enantioseparation of β‐agonists by CE via a CD inclusion complexation modified with ionic liquids (ILs) are described. Different types of ILs, including tetraalkylammonium‐based ILs, alkylimidazolium‐based ILs and alkylpyridinium‐based ILs, were examined and compared for controlling the EOF in order to improve resolutions of β‐agonists enantiomers. In this regard, tetraalkylammonium‐based ILs were more effective because they could be used at much higher concentrations than other types of ILs. N‐octylpyridinium hexafluorophosphate gave poor resolutions of β‐agonists enantiomers. In addition, when different ILs were mixed to use, they would present particular properties of their own. Moreover, the presence of ILs was essential in the chiral separations of (±) salbutamol, (±) cimaterol and (±) formoterol, which were reportedly not enantioseparated by using the buffer electrolytes containing only β‐CD as a chiral selector.

Microemulsion electrokinetic chromatography coupling with field amplified sample injection and electroosmotic flow suppressant for analysis of some quinolizidine alkaloids

A new rapid and reproducible method using microemulsion electrokinetic chromatography (MEEKC) combining field amplified sample injection and electroosmotic flow suppressant for the analysis of five quinolizidine alkaloids is developed in this paper. For the separation of five quinolizidine alkaloids, a running buffer composed of 1.2% (v/v) 1-butanol, 0.6% (v/v) ethyl acetate and 98.2% (v/v) 1 mM Na(2)B(4)O(7)-2 mM NaH(2)PO(4) buffer solution containing 21 mM sodium cholate (SC) (pH 6.5) was developed. The resolution of the analytes was improved significantly by adding a divalent cation (e.g., Mg(2+)) to the running buffer as an electroosmotic flow modification. In order to analyze trace quinolizidine alkaloids in traditional Chinese herbal medicines, field amplified sample injection (FASI) was applied to increase the detection sensitivity. The detection limits (defined as S/N=3) for the analytes could be as low as 0.0001 microg/mL. This method was applied for the determination of quinolizidine alkaloids in real samples with simple extraction procedures, and the assay results were satisfactory.

Separation and determination of flavonoids using microemulsion EKC with electrochemical detection.

A selective and sensitive method of microemulsion EKC (MEEKC) with electrochemical detection (ED) was developed for separation and determination of 14 flavonoids. In order to obtain the better stability for the studied flavonoids, oil (ethyl acetate) with low interfacial surface tension was employed as organic solvent. A running buffer composed of 0.9% (w/v, 30 mM) SDS, 0.9% (w/v, 21 mM) sodium cholate (SC), 0.9% (w/v, 121 mM) butan‐1‐ol, 0.6% (w/v, 68 mM) ethyl acetate, and 98.2% v/v 10 mM Na2B4O7–20 mM H3BO3 buffer (pH 7.5) was applied for the separation of flavonoids. Under the optimum conditions, the relationship between peak currents and analyte concentrations was linear over about 1.3 and 1.7 orders of magnitude with detection limits (defined as S/N = 3) ranging from 0.02 to 0.5 μg/mL for all analytes. This method was applied for the determination of flavonoids in real samples with simple extraction procedures, and the assay results were satisfactory.

CE-ESI-MS coupled with dynamic pH junction online concentration for analysis of peptides in human urine samples.

In this article, an approach has been developed for the analysis of some small peptides with similar pI values by CE‐ESI‐MS based on the online concentration strategy of dynamic pH junction. The factors affected on the separation, detection and online enrichment, such as BGE, injection pressure, sheath flow liquid and separation voltage have been investigated in detail. Under the optimum conditions, i.e. using 0.5 mol/L formic acid (pH 2.15) as the BGE, preparing the sample in 50 mM ammonium acetate solution (pH 7.5), 50 mbar of injection pressure for 300 s, using 7.5 mM of acetic acid in methanol–water (80% v/v) solution as the sheath flow liquid and 20 kV as the separation voltage, four peptides with similar pI values, such as L‐Ala‐L‐Ala (pI=5.57), L‐Leu‐D‐Leu (pI=5.52), Gly‐D‐Phe (pI=5.52) and Gly‐Gly‐L‐Leu (pI=5.52) achieved baseline separation within 18.3 min with detection limits in the range of 0.2–2.0 nmol/L. RSDs of peak migration time and peak area were in the range of 1.45–3.57 and 4.93–6.32%, respectively. This method has been applied to the analysis of the four peptides in the spiked urine sample with satisfactory results.

Separation and detection of isoquinoline alkaloids using MEEKC coupled with field-amplified sample injection induced by ACN

New methods based on MEEKC coupling with field‐amplified sample injection (FASI) induced by ACN were proposed for five isoquinoline alkaloids (berberine, palmatine, jatrorrhizine, sinomenine and homoharringtonine) in no salt and high salt sample solution (HS). For the separation of five isoquinoline alkaloids, a running buffer composed of 18 mM sodium cholate, 2.4% v/v butan‐1‐ol, 0.6% v/v ethyl acetate, 10% v/v (or 30% v/v) methanol and 87.0% v/v (or 67% v/v) 5 mM Na2B4O7∼10 mM NaH2PO4 buffer (pH 7.5) was developed. In order to improve the sensitivity, FASI induced by ACN was applied to increase the detection sensitivity. The detection limit was found to be as low as 0.0002 μg/mL in no salt sample solution and 0.062 μg/mL in HS. The method has been applied for the analysis of human urine spiked with analytes, and the assay results were proved to be satisfactory, and also the determination of berberine in urine sample after oral administration berberine.

Affinity capillary electrophoresis coupling with partial filling technique and field‐amplified sample injection for enantioseparation and determination of DL‐tetrahydropalmatine

A novel, simple and sensitive method for the enantioseparation and determination of DL‐tetrahydropalmatine (DL‐THP) was developed using ACE in combination with partial filling technique and field‐amplified sample injection. A chiral selector, i.e. BSA, was used for the enantioseparation of DL‐THP in ACE. Effects of BSA concentration, pH and separation voltage on the effectiveness of the enantiomer separation were evaluated. In an optimal condition, D‐ and L‐THP were completely enantio‐separated in less than 9 min by partially filling an electrophoretic capillary with 50 μmol/L BSA (50 mbar, 100 s) and carrying out an electrophoresis with 20 mmol/L phosphate buffer (pH 7.4) at 15 kV. The sensitivity was further improved by making use of field‐amplified sample injection to lower the LOD (defined as S/N=3) down to 6 ng/mL. Real samples were also tested and promising results for the determination of DL‐THP enantiomers were obtained.

Solid‐phase extraction‐field‐amplified sample injection coupled with CE‐ESI‐MS for online pre‐concentration and quantitative analysis of brain‐gut peptides

In order to improve the sensitivity of CE‐ESI‐MS for the analysis of brain‐gut peptides, a solid‐phase extraction combined with field‐amplified sample injection was used for the pre‐concentration of the brain‐gut peptides. Compared with the conventional pressure injection method, the sensitivity in the detection of brain‐gut peptides was improved more than 100‐fold. The possible factors affecting sample stacking, such as the sample matrix, the composition and the length of the water column, the types and the volumes of eluent, have been investigated in detail. Under the optimum conditions, the detectable concentration of brain‐gut peptides was found to be as low as 0.02 μM. A linear response concentration for the detection was developed in the range of 0.08–25 μmol/L. A real sample of human cerebrospinal fluids, which was spiked with brain‐gut peptides, was also examined in order to evaluate the reliability of the proposed approach. The recovery of the method was in a range from 69.2 to 85.4%. The method was found to be reliable, accurate and potentially applicable for clinical drug analysis.

Label‐free aptamer‐based partial filling technique for enantioseparation and determination of dl‐tryptophan with micellar electrokinetic chromatography

In this study, a simple and reproducible method for enantioseparation and determination of dl‐tryptophan (dl‐Trp) was developed by using a partial filling technique in combination with MEKC. The corresponding l‐Trp specific DNA aptamer was used as a chiral selector. Sodium cholate was used to form the chiral micelles and to enhance the enantioseparation of the enantiomers. Effects of aptamer concentration, filling time, buffer composition, and separation voltage on the enantioseparation were evaluated. The Mg2+ and Na+ concentration in separation buffer was found to effectively affect the separation efficiency and reproducibility. Under the optimal conditions, d‐ and l‐Trp were completely enantioseparated in less than 9 min. This aptamer‐based partial‐filling approach has the potential to be extended to the separation of other enantiomers after the replacement of corresponding specific aptamers.