Solvent exchange using hollow fiber prior to separation and determination of some antioxidants by high performance liquid chromatography

This study presents a simple and rapid solvent exchange procedure using a hollow fiber. Antioxidants (Irganox 1010, Irganox 1076 and Irgafos 168) and solvents such as tetrahydrofuran (THF), carbon tetrachloride and toluene were selected as model compounds and sample solvents, respectively. After injection of the sample solution into the hollow fiber and solvent evaporation, the precipitated analytes in lumen and pores of the fiber were washed with methanol (the mobile phase for separation and determination by HPLC-diode array detection) and good chromatographic peaks were obtained. The effect of different parameters such as fiber length, volumes of sample and washing solvents were investigated and the optimum conditions were selected. The repeatability of the method was tested and it was found that the relative standard deviation (R.S.D.) was less than 10% for all analytes. Also enrichment factors of 3.03, 2.21 and 1.19 times were obtained for Irganox 1010, Irganox 1076 and Irgafos 168, respectively, when 200 μL sample and 50 μL methanol (washing solvent) were used.     

Solvent-bar microextraction of herbicides combined with non-aqueous field-amplified sample injection capillary electrophoresis

Solvent-bar microextraction (SBME) based on two-phase (water-to-organic) extraction was for the first time used as the sample pretreatment method for the non-aqueous capillary electrophoresis (NACE) of herbicides of environmental concern. Due to the compatibility of the extractant organic solvent and the NACE separation system, the extract could be introduced directly to the CE system after SBME. Through investigations of the effect of sample pH, extraction time, agitation speed and salt addition on extraction efficiency, the most suitable extraction conditions were determined: sample solution at a pH of 1, without added salt, and stirring at 700 revolutions per minute for 30 min. SBME as applied here was also compared with single-drop microextraction and hollow fiber-protected liquid-phase microextraction. SBME showed the highest extraction efficiency. In addition, field-amplified sample injection with pre-introduced organic solvent plug removal using the electroosmotic flow as a pump (FAEP) was used to enhance the sensitivity further in NACE. Based on studies of the effect of different organic solvents, different lengths of the organic plugs and different volumes of sample injection on stacking efficiency under the most suitable separation conditions, methanol was found to be the most efficient solvent for on-line preconcentration. Combined with SBME, FAEP-NACE achieved limits of detection of between 0.08 ng/mL and 0.14 ng/mL for the studied analytes. This preconcentration approach for NACE was demonstrated to be amenable to aqueous environmental samples by applying it to spiked river water.     

Solvent-bar microextraction—Using a silica monolith as the extractant phase holder

In this paper, a novel liquid-phase microextraction (LPME) approach, based on solvent-bar microextraction (SBME), was developed in which a silica monolith was used as the extractant solvent holder. Owing to the porous nature of the monolith, the extractant solvent could be easily held in the material; when the monolith containing the extractant solvent was exposed to the sample solution, analytes could directly diffuse from the sample solution into the extractant solvent. Polycyclic aromatic hydrocarbons (PAHs) were used as model analytes to evaluate the procedure. Through the investigation of the effect of agitation speed, extraction time, length of the monolith (that determined the volume of organic extractant solvent) and salt concentration on extraction efficiency, the following optimal extraction conditions were obtained: stirring at 1000 rpm for 30 min without salt addition using a 4-mm silica monolith. The limits of detection ranged from 3.9 pg/mL to 28.8 pg/mL, with relative standard deviations of between 8.16% and 10.5% on the same silica monolith. The linearity was 0.05–200 ng/mL for fluoranthene and pyrene, and 0.5–200 ng/mL for chrysene and benzo[b]fluoranthene, with acceptable correlation coefficient. When this method was applied for the spiked real river sample, the relative recoveries ranged from 87.1% to 100.7% for the tested PAHs. This method was also compared to polymeric hollow fiber-based SBME and hollow fiber-protected LPME and found to provide better results. Additionally, compared with the polymeric hollow fiber, the silica monolith possesses good resistance to extreme conditions, such as high temperature and pH, and is more compatible with various organic solvents. This is the first report of an application of a monolithic material for LPME, and as a solvent holder for SBME. It extends the scope of applications of such materials, to analytical chemistry, specifically to sample preparation.     

Investigation of polar organic solvents compatible with Corona Charged Aerosol Detection and their use for the determination of sugars by hydrophilic interaction liquid chromatography

A range of organic solvents (ethanol, isopropanol and acetone) has been investigated as alternatives to acetonitrile and methanol when used in conjunction with Corona Charged Aerosol Detection (Corona CAD). These solvents have been evaluated with regard to their effect on the response of the Corona CAD. Three dimensional response surfaces were constructed using raw data showing the relationship between detector response, analyte concentration and percentage of organic solvent in the mobile phase, using sucralose or quinine as the test analyte. The detector response was non-linear in terms of analyte concentration for all solvents tested. However, detector response varied in an approximately linear manner with percentage of organic solvent over the range 0–40% for ethanol or isopropanol and 0–80% for acetone and methanol. The chromatographic performance of the various solvents when used as aqueous–organic mobile phases was evaluated for isocratic and gradient separations of sugars and sugar alcohols by hydrophilic interaction liquid chromatography (HILIC) using an Asahipak NH2P-504E column coupled with Corona CAD detection. It was found that whilst acetonitrile provided the highest column efficiencies and lowest detection limits of the solvents studied, acetone also performed well and could be used to resolve the same number of analytes as was possible with acetonitrile. Typical efficiencies and detection limits of 5330 plates m⁻¹ and 1.25 μg mL⁻¹, respectively, were achieved when acetone was used as the organic modifier. Acetone was utilised successfully as an organic modifier in the HILIC separation of carbohydrates in a beer sample and also for a partially digested dextran sample.     

Development of a solid phase extraction procedure for HPLC-DAD determination of several angiotensin II receptor antagonists in human urine using mixture design

The optimisation of a solid phase extraction procedure involves several variables whose influence has been widely studied. However, in most cases, only process variables are taken into account. In this work, the influence of those process variables together with the fact of using mixtures of solvents during the elution step of the solid phase extraction of four angiotensin II receptor antagonist drugs has been studied. Since the influence on the extraction efficiency of several process variables were simultaneously tested, a D-optimal design was constructed. The composition of the elution solvent (a mixture of methanol, acetonitrile, ethanol and acetone at different proportions from 0 to 100% each solvent), the percentage and pH of the buffer solution added to the urine samples at the beginning of the extraction procedure; the percentage of the organic component and the volume of the washing solution, the drying time and the volume of the elution solvent were the studied variables. The chromatographic separation was carried out by gradient elution mode with 0.026% trifluoroacetic acid (TFA) in the organic phase and 0.031% TFA in the aqueous phase using an Atlantis dC18, 100 mm × 3.9 mm I.D. chromatographic column at a flow rate of 1 mL/min and a column temperature of 35 ± 0.2 °C. For detection a diode array detector set at 232 nm was used. The extraction procedure for spiked human urine samples was developed using C8 cartridges, phosphate buffer pH 6.8 as conditioning agent, a drying step of 10 min, a washing step with methanol-phosphate buffer (20:80, v/v) and methanol as eluent. Recovery percentages obtained: 84% for eprosartan, 74% for telmisartan, 74% for irbesartan and 89% for valsartan allow the determination of these drugs concentration levels in urine.     

Effects of the operation parameters on Hydrophilic Interaction Liquid Chromatography separation of phenolic acids on zwitterionic monolithic capillary columns

Strongly polar phenolic acids are weakly retained and often poorly separated in reversed-phase (RP) liquid chromatography. We prepared zwitterionic polymethacrylate monolithic columns for micro-HPLC by in situ co-polymerization in fused-silica capillaries. The capillary monolithic columns prepared under optimized polymerization conditions show some similarities with the conventional particulate commercial ZIC-HILIC silica-based columns, however have higher retention and better separation selectivity under reversed-phase conditions, so that they can be employed for dual-mode HILIC-RP separations of phenolic acids on a single column. The capillary polymethacrylate monolithic sulfobetaine columns show excellent thermal stability and improved performance at temperatures 60–80 °C. The effects of the operation conditions on separation were investigated, including the type and the concentration of the organic solvent in the aqueous-organic mobile phase (acetonitrile and methanol), the ionic strength of the acetate buffer and temperature. While the retention in the RP mode decreases at higher temperatures in mobile phases with relatively low concentrations of acetonitrile, it is almost independent of temperature at HILIC conditions in highly organic mobile phases. The best separation efficiency can be achieved using relatively high acetate buffer ionic strength (20–30 mmol L⁻¹) and gradient elution with alternately increasing (HILIC mode) and decreasing (RP mode) concentration of aqueous buffer in aqueous acetonitrile. Applications of the monolithic sulfobetaine capillary columns in alternating HILIC-RP modes are demonstrated on the analysis of phenolic acids in a beer sample.     

Low-density magnetofluid dispersive liquid–liquid microextraction for the fast determination of organochlorine pesticides in water samples by GC-ECD

A new micro-extraction technique named low-density magnetofluid dispersive liquid–liquid microextraction (LMF-DMMLE) has been developed, which permits a wider range of solvents and can be combined with various detection methods. Comparing with the existing low density solvents micro-extraction methods, no special devices and complicated operations were required during the whole extraction process. Dispersion of the low-density magnetofluid into the aqueous sample is achieved by using vortex mixing, so disperser solvent was unnecessary. The extraction solvent was collected conveniently with an external magnetic field placed outside the extraction container after dispersing. Then, the magnetic nanoparticles were easily removed by adding precipitation reagent under the magnetic field. In order to evaluate the validity of this method, ten organochlorine pesticides (OCPs) were chosen as the analytes. Parameters influencing the extraction efficiency such as extraction solvents, volume of extraction solvents, extraction time, and ionic strength were investigated and optimized. Under the optimized conditions, this method showed high extraction efficiency with low limits of detection of 1.8–8.4 ng L⁻¹, good linearity in the range of 0.05–10.00 μg L⁻¹ and the precisions were in the range of 1.3–9.6% (RSD, n = 5). Finally, this method was successfully applied in the determination of OCPs in real water samples.     

On-line sequential injection dispersive liquid-liquid microextraction system for flame atomic absorption spectrometric determination of copper and lead in water samples

A simple, sensitive and powerful on-line sequential injection (SI) dispersive liquid-liquid microextraction (DLLME) system was developed as an alternative approach for on-line metal preconcentration and separation, using extraction solvent at microlitre volume. The potentials of this novel schema, coupled to flame atomic absorption spectrometry (FAAS), were demonstrated for trace copper and lead determination in water samples. The stream of methanol (disperser solvent) containing 2.0% (v/v) xylene (extraction solvent) and 0.3% (m/v) ammonium diethyldithiophosphate (chelating agent) was merged on-line with the stream of sample (aqueous phase), resulting a cloudy mixture, which was consisted of fine droplets of the extraction solvent dispersed entirely into the aqueous phase. By this continuous process, metal chelating complexes were formed and extracted into the fine droplets of the extraction solvent. The hydrophobic droplets of organic phase were retained into a microcolumn packed with PTFE-turnings. A portion of 300 μL isobutylmethylketone was used for quantitative elution of the analytes, which transported directly to the nebulizer of FAAS. All the critical parameters of the system such as type of extraction solvent, flow-rate of disperser and sample, extraction time as well as the chemical parameters were studied. Under the optimum conditions the enhancement factor for copper and lead was 560 and 265, respectively. For copper, the detection limit and the precision (R.S.D.) were 0.04 μg L⁻¹ and 2.1% at 2.0 μg L⁻¹ Cu(II), respectively, while for lead were 0.54 μg L⁻¹ and 1.9% at 30.0 μg L⁻¹ Pb(II), respectively. The developed method was evaluated by analyzing certified reference material and applied successfully to the analysis of environmental water samples.     

Three-phase system in solvent bar microextraction: An approach for the sample preparation of ionizable organic compounds prior to liquid chromatography

In this article, a simple new solvent microextraction technique is described for the extraction of ionizable organic compounds. This involves performing simultaneous forward- and back-extraction across an organic film immobilized in the pores of a porous polypropylene hollow fiber. Four chlorophenoxyacetic acid herbicides were chosen as model compounds. The target compounds are extracted from the stirred acidic aqueous sample (adjusted to 0.5 M HCl; donor phase) through a thin film of an organic solvent residing in the pores of a polypropylene hollow fiber; they are then finally extracted into another alkaline aqueous phase (1 M NaOH; acceptor phase). Both ends of the fiber are pressure-sealed. The acceptor phase was analyzed by liquid chromatography (LC). This method gave good enrichment (by a factor of 438-553) of the analytes in 40 min extraction time with reasonably good reproducibility. The analytical potential of the method was demonstrated by applying the method to spiked river water sample.     

Development of a new microextraction method based on a dynamic single drop in a narrow-bore tube: application in extraction and preconcentration of some organic pollutants in well water and grape juice samples

A novel sample preparation technique, the microextraction method based on a dynamic single drop in a narrow-bore tube, coupled with gas chromatography-flame ionization detection (GC-FID) is presented in this paper. The most important features of this method are simplicity and high enrichment factors. In this method, a microdrop of an extraction solvent assisted by an air bubble was repeatedly passed through a narrow-bore closed end tube containing aqueous sample. It has been successfully used for the analysis of some pesticides as model analytes in aqueous samples. Parameters affecting the method's performance such as selection of extraction solvent type and volume, number of extractions, volume of aqueous sample (tube length), and salt effect were studied and optimized. Under the optimal conditions, the enrichment factors (EFs) for triazole pesticides were in the range of 141-214 and the limits of detection (LODs) were between 2 and 112 μg L⁻¹. The relative standard deviations (C = 1000 μg L⁻¹, n = 6) were obtained in the range of 2.9-4.5%. The recoveries obtained for the spiked well water and grape juice samples were between 71 and 106%. Low cost, relatively short sample preparation time and less solvent consumption are other advantages of the proposed method.     

On-line coupling of counter-current chromatography and macroporous resin chromatography for continuous isolation of arctiin from the fruit of Arctium lappa L.

In this work, we have developed a novel hybrid two-dimensional counter-current chromatography and liquid chromatography (2D CCC × LC) system for the continuous purification of arctiin from crude extract of Arctium lappa. The first dimensional CCC column has been designed to fractionalize crude complex extract into pure arctiin effluent using a one-component organic/salt-containing system, and the second dimensional LC column has been packed with macroporous resin for on-line adsorption, desalination and desorption of arctiin which was effluent purified from the first CCC dimension. Thus, the crude arctiin mixture has been purified efficiently and conveniently by on-line CCC × LC in spite of the use of a salt-containing solvent system in CCC separation. As a result, high purity (more than 97%) of arctiin has been isolated by repeated injections both using the ethyl acetate–8% sodium chloride aqueous solution and butanol–1% sodium chloride aqueous solution. By contrast with the traditional CCC processes using multi-component organic/aqueous solvent systems, the present on-line CCC × LC process only used a one-component organic solvent and thus the solvent is easier to recover and regenerate. All of used solvents such as ethyl acetate, n-butanol and NaCl aqueous solution are low toxicity and environment-friendly. Moreover, the lower phase of salt-containing aqueous solution used as mobile phase, only contained minor organic solvent, which will save much organic solvent in continuous separation. In summary, our results indicated that the on-line hybrid 2D CCC × LC system using one-component organic/salt-containing aqueous solution is very promising and powerful tool for high-throughput purification of arctiin from fruits of A. lappa.     

A new concept of hollow fiber liquid–liquid–liquid microextraction compatible with gas chromatography based on two immiscible organic solvents

A new concept of liquid–liquid–liquid microextraction (LLLME) was introduced based on applying two immiscible organic solvents in lumen and wall pores of hollow fiber (HF). With this methodology, analytes of interest can be extracted from aqueous sample, into a thin layer of organic solvent (dodecane) sustained in the pores of a porous hollow fiber, and further into a μL volume of organic acceptor (acetonitrile or methanol) located inside the lumen of the hollow fiber. Some chlorophenols (CPs) were selected as model compounds for developing and evaluating of the method performance. The analysis was performed by gas chromatography–electron capture detection (GC–ECD) without derivatization. The factors affecting the HF-LLLME of target compounds were investigated and the optimal extraction conditions were established. Under the optimum conditions, preconcentration factors in a range of 208–895 were obtained. The performance of the proposed method was studied in terms of linear dynamic ranges (LDRs from 0.02 to 100 ng mL⁻¹), linearity (R² ≥ 0.995), precision (RSD % ≤ 8.1) and limits of detection (LODs in the range of 0.006–0.2 ng mL⁻¹). In addition to preconcentration, HF-LLLME also served as a technique for sample clean-up.     

Temperature-controlled headspace liquid-phase microextraction device using volatile solvents

A novel temperature-controlled headspace liquid-phase microextraction (TC-HS-LPME) device was established in which volatile solvents could be used as extractant. In this device, a PTFE vial cap with a cylindrical cavity was used as the holder of the extraction solvent. Up to 40 μl of extraction solvent could be suspended in the cavity over the headspace of aqueous sample in the vial. A cooling system based on thermoelectric cooler (TEC) was used to lower the temperature of extractant in PTFE vial cap to reduce the loss of volatile solvent during extraction process and increase the extraction efficiency. The selection of solvents for HS-LPME was then extended to volatile solvents, such as dichloromethane, ethyl acetate and acetone. The use of volatile extraction solvents instead of semi-volatile solvent reduced the interference of the large solvent peak to the analytes peaks, and enhanced the compatibility of HS-LPME with gas chromatograph (GC). Moreover, the use of larger volume of extractant solvent increases the extraction capacity and the injection volume of GC after extraction, thus improving detection limits. Several critical parameters of this technique were investigated by using chlorobenzenes (CBs) as the model analytes. High enrichment factors (498–915), low limits of detection (0.004–0.008 μg/L) and precision (3.93–5.27%) were obtained by using TC-HS-LPME/GC-FID. Relative recoveries for real samples were more than 83%.     

Novel application of modified multiwalled carbon nanotubes as a solid phase extraction adsorbent for the determination of polyhalogenated organic pollutants in aqueous solution

This paper describes a novel application of pristine and chemically modified multiwalled carbon nanotubes (MWCNTs) as the packing materials for the determination of different polyhalogenated organic pollutants, pentachlorophenol, 2,4,5-trichlorophenol, 3,3′,4,4′-tetrachlorobiphenyl and 2,2′,5,5′-tetrabromobiphenyl, from aqueous solution based on solid phase extraction. The modified MWCNTs were characterized using different techniques and the results revealed the successful modification of the MWCNTs with octadecyl amine and poly(ethylene glycol), separately. Factors that maybe influence the preconcentration efficiency, such as sample flow rate, adsorbent mass, sample pH and sample volume, were studied. Desorption of the target analytes was studied using different solvents and the results showed that acetone was the best solvent for all the analytes compared with methanol and hexane. All the results indicated that the proposed method could be used for the simultaneous determination of different pollutants in environmental water samples at trace levels.     

Separation of antidepressants by capillary electrophoresis with in-line solid-phase extraction using a novel monolithic adsorbent

The separation of three selective serotonin reuptake inhibitors (SSRIs) by capillary electrophoresis (CE) with fully integrated solid-phase extraction (SPE) is described. Polymeric monolithic SPE modules were prepared in situ within a fused silica capillary from either butyl methacrylate-co-ethylene dimethacrylate or 3-sulfopropyl methacrylate-co-butyl methacrylate-co-ethylene dimethacrylate. Using a 1 cm SPE module placed at the inlet of the capillary, a mixture of sertraline, fluoxetine and fluvoxamine was extracted from aqueous solution by applying a simple pressure rinse. Under pressure-driven conditions, efficient elution was possible from both SPE materials investigated using 50 mM phosphate buffer, pH 3.5 in acetonitrile (20/80, v/v). Two different strategies were investigated for the efficient elution and subsequent CE separation. Injection of an aqueous sample plug directly into the non-aqueous elution/separation buffer was found to be unsuitable with poor elution profiles observed in the electrodriven mode. Alternatively, a sample plug equivalent to several capillary volumes could be injected by pressure followed by filling the capillary with the non-aqueous elution/separation buffer from the outlet end using a combination of pressure and electrodriven flow. Using a neutral monolith, efficient elution/separation was not possible due to an unstable electroosmotic flow (EOF), however, by adding the ionisable monomer, 3-sulfopropyl methacrylate to the SPE module to increase and stabilise the EOF, it was possible to achieve efficient elution from the SPE module, followed by baseline separation by CE using a 200 mM acetate buffer, pH 3.5 in acetonitrile (10/90, v/v). With enrichment factors of over 500 achieved for each of the analytes this demonstrates the potential of in-line SPE-CE for the sensitive analysis of these drugs.     

Determination of pharmaceutically related compounds by suppressed ion chromatography: I. Effects of organic solvent on suppressor performance

This overall study aims to investigate gradient elution ion-exchange chromatography of pharmaceutically relevant compounds using universal nebulisation detectors, such as evaporative light scattering detection (ELSD). Addition of organic solvents to the eluent is necessary to minimise hydrophobic adsorption on the polymeric stationary phase and improve solubility of analytes. It is also necessary to de-salt the eluent prior to detection, and in this work, ion chromatography suppressors were used for this step. Such suppressors have been designed for aqueous eluents, so the purpose of the present study was to investigate the effects of methanol and acetonitrile on suppressor performance. Chemical and electrolytic suppressors were evaluated for baseline drift, noise and efficiency of suppression using aqueous/organic eluents containing up to 40% (v/v) methanol or acetonitrile. Chemical suppression of aqueous/organic eluents showed minimal noise levels, uniform low baseline and low gradient drift. Electrolytic suppression gave good performance, but with higher baseline conductivity levels and baseline drift than chemical suppression. The elevated baseline was found not to be caused by incomplete suppression of the eluent, but was attributed to chemical reactions involving the organic solvents and facilitated by high electric currents and heat generation. It was demonstrated that suppressed ion-exchange separation using a complex KOH elution profile could be coupled with ELSD, with the suppressor effectively de-salting the eluent, producing a stable baseline. Finally, complementary separation selectivity was demonstrated using a set of pharmaceutically related organic acids separated by reversed-phase and ion-exchange methods.     

Determination of organochlorine pesticides in water samples by dispersive liquid-liquid microextraction coupled to gas chromatography-mass spectrometry

A rapid and simple dispersive liquid-liquid microextraction (DLLME) has been developed to preconcentrate eighteen organochlorine pesticides (OCPs) from water samples prior to analysis by gas chromatography-mass spectrometry (GC-MS). The studied variables were extraction solvent type and volume, disperser solvent type and volume, aqueous sample volume and temperature. The optimum experimental conditions of the proposed DLLME method were: a mixture of 10 μL tetrachloroethylene (extraction solvent) and 1 mL acetone (disperser solvent) exposed for 30 s to 10 mL of the aqueous sample at room temperature (20 °C). Centrifugation of cloudy solution was carried out at 2300 rpm for 3 min to allow phases separation. Finally, 2 μL of extractant was recovered and injected into the GC-MS instrument. Under the optimum conditions, the enrichment factors ranged between 46 and 316. The calculated calibration curves gave a high-level linearity for all target analytes with correlation coefficients ranging between 0.9967 and 0.9999. The repeatability of the proposed method, expressed as relative standard deviation, varied between 5% and 15% (n = 8), and the detection limits were in the range of 1-25 ng L⁻¹. The LOD values obtained are able to detect these OCPs in aqueous matrices as required by EPA methods 525.2 and 625. Analysis of spiked real water samples revealed that the matrix had no effect on extraction for river, surface and tap waters; however, urban wastewater sample shown a little effect for five out of eighteen analytes.     

Air-assisted liquid–liquid microextraction method as a novel microextraction technique; Application in extraction and preconcentration of phthalate esters in aqueous sample followed by gas chromatography–flame ionization detection

A novel microextraction technique, air-assisted liquid–liquid microextraction (AALLME), which is a new version of dispersive liquid–liquid microextraction (DLLME) method has been developed for extraction and preconcentration of phthalate esters, dimethyl phthalate (DMP), diethyl phthalate (DEP), di-iso-butyl phthalate (DIBP), di-n-butyl phthalate (DNBP), and di-2-ethylhexyl phthalate (DEHP), from aqueous samples prior to gas chromatography–flame ionization detection (GC–FID) analysis. In this method, much less volume of an organic solvent is used as extraction solvent in the absence of a disperser solvent. Fine organic droplets were formed by sucking and injecting of the mixture of aqueous sample solution and extraction solvent with a syringe for several times in a conical test tube. After extraction, phase separation was performed by centrifugation and the enriched analytes in the sedimented phase were determined by GC–FID. Under the optimum extraction conditions, the method showed low limits of detection and quantification between 0.12–1.15 and 0.85–4 ng mL⁻¹, respectively. Enrichment factors (EFs) and extraction recoveries (ERs) were in the ranges of 889–1022 and 89–102%, respectively. The relative standard deviations (RSDs) for the extraction of 100 ng mL⁻¹ and 500 ng mL⁻¹ of each phthalate ester were less than 4% for intra-day (n = 6) and inter-days (n = 4) precision. Finally some aqueous samples were successfully analyzed using the proposed method and three analytes, DIBP, DNBP and DEHP, were determined in them at ng mL⁻¹ level.     

A vacuum assisted dynamic evaporation interface for two-dimensional normal phase/reverse phase liquid chromatography

A vacuum assisted dynamic solvent evaporation interface for coupling of two-dimensional normal phase/reverse phase liquid chromatography was developed and evaluated. A normal-phase liquid chromatographic (NPLC) column of a 250 mm × 4.6 mm I.D. 5 μm CN phase was used as the first dimension, and a reversed-phase liquid chromatographic (RPLC) column of 250 mm × 4.6 mm I.D. 5 μm C₁₈ phase was used as the second dimension. The eluent from the first dimension flowed into a fraction loop, and the solvent in the eluent was dynamically evaporated and removed by vacuum as it was entering the fraction loop of the interface. The non-evaporable analytes was retained and enriched in about 5–25 μL solution within the loop. Up to 1 mL/min of mobile phase from the first dimension can be evaporated and removed dynamically by the interface. The mobile phase from the second dimension then entered the loop, and dissolved the concentrated analytes retained inside the loop, and carried them onto the second dimension column for further separation. The operation conditions of the two dimensions were independent from each other, and both dimensions were operated at their optimal chromatographic conditions. We evaluated the interface by controlling the loop temperature in a water bath at normal temperature, and investigated the sample losses by using standard samples with different boiling points. It was found that the sample loss due to evaporation in the interface was negligible for non-volatile samples or for components with boiling point above 340 °C. The interface realizes fast solvent removal of mL volume of fraction and concentration of the fraction into tenth of μL volume, and injection of the concentrated fraction on the secondary column. The chromatographic performance of the two-dimensional LC system was enhanced without compromise of separation efficiency and selectivity on each dimension.     

Homogeneous ionic liquid microextraction of the active constituents from fruits of Schisandra chinensis and Schisandra sphenanthera

Homogeneous ionic liquid microextraction (HILME) was developed for the extraction of schizandrin, schisantherin A and deoxyschizandrin from Schisandra chinensis and Schisandra sphenanthera. 1-Butyl-3-methylimidazolium tetrafluoroborate ([C₄MIM][BF₄]) aqueous solution was used as extraction solvent, and ammonium hexafluorophosphate ([NH₄][PF₆]) was used as ion-pairing agent. 1-Butyl-3-methylimidazolium hexafluorophosphate ([C₄MIM][PF₆]), which is barely soluble in water, was formed in situ, and was used as sample solution. High-performance liquid chromatography (HPLC) was employed for separation and determination of the analytes. The calibration curve showed good linear relationship (r > 0.9998). The recoveries were between 69.71% and 88.33% with RSDs lower than 4.86%. External standard method was adopted in the proposed method, and internal standard method was applied for the evaluation of the proposed method. The two methods were compared and the results indicated that the proposed method was acceptable and simple. The HILME is free of volatile organic solvents, and represents lower expenditures of sample, extraction time and solvent, compared with ultrasonic and Soxhlet extraction. There was no obvious difference in the extraction yields of active constitutions obtained by the three extraction methods.