Admicelles in open-tube capillaries for chromatography and electrochromatography

Chiral metal-organic frameworks grown in situ for monolithic capillary electrochromatographic enantioseparation

Chiral metal-organic frameworks (CMOFs) with chiral selectivity are one of the high-quality stationary phases for capillary electrochromatography (CEC). However, there is a problem of unsatisfactory enantioseparation performance of capillary columns due to insufficient loading. In this work, a lamellar CMOF (Cu-TC) was grown in situ on the surface of the monolith in a capillary monolithic column to obtain a Cu-TC@monolithic column. The CEC system constructed based on the Cu-TC@monolithic column shows a satisfactory chiral separation performance. Compared with the Cu-TC-based coated column (Cu-TC@coated column), the enantioseparation performance of the CEC system based on the Cu-TC@monolithic column was greatly improved, and the resolutions (Rs) of the model analytes were increased by 80-500%. In addition, the effects of experimental conditions such as the number of cycles of Cu-TC in situ growth, buffer concentration, buffer pH, organic solvent addition and applied voltage on the performance of CEC were also investigated. Finally, the chiral selection mechanism of the stationary phase was explored by selective adsorption experiments. The present work provides a new idea for the development of capillary stationary phases, which has great potential considering the diversity of CMOFs.

Dynamic in situ growth of bonded-phase silica nanospheres on silica capillary inner walls for open-tubular liquid chromatography

Silica nanospheres (SNS) were grown on the inner walls of silica capillaries through a dynamic in situ nucleation process to prepare a highly porous and large accessible surface area substrate. The SNS were then functionalized with octadecyl (C18), 3-aminopropyltriethoxysilane (APTES), beta-cyclodextrin (β-CD), and amino groups to develop robust and efficient chromatographic stationary phases. The modified silica capillaries were exploited for open-tubular liquid chromatography (OT-LC) and open-tubular capillary electrochromatography (OT-CEC) applications. The prepared stationary phases were compared to conventional capillaries in terms of separation performance. The synthesis process was optimized, and the bonded-phase stationary phases were characterized by the electron microscopy technique. The effects of different solvents, additives, and functional groups on the geometry and chromatographic resolving power of the SNS were envisaged. The capillaries modified with octadecyl groups were evaluated for the separation of non-steroidal anti-inflammatory drugs, phenones, alkenylbenzenes, and enantiomers of chlorophenoxy herbicides. As an application instance, an SNS-C18-coated capillary was utilized for the separation of alkenylbenzenes from clove extract and protein digest medium, through OT-LC and OT-CEC techniques, respectively. The β-CD functionalized capillary was applied for the OT-CEC separation of a dichlorprop racemic mixture.

In-line sample concentration in capillary electrophoresis by cyclodextrin to admicelle microextraction

Cyclodextrins (CDs) as a pseudophase in pseudophase-to-pseudophase microextraction (P²ME) in capillary zone electrophoresis (CZE) are proposed. In this P²ME mode called CD to admicelle ME, a long plug of dilute analyte solution prepared in cetyltrimethylammonium bromide (CTAB) at the critical micellar concentration was injected into the capillary. This formed CTAB admicelles at the interface between the solution and the negatively charged capillary surface, where the analytes were trapped. The injection of CD solution released the admicelles and the analytes from the capillary surface due to the formation of stable CD/CTAB inclusion complexes. The analytes are concentrated at the CD front during injection and voltage separation. Various neutral CDs were found to be effective for CD to admicelle ME. To implement this in-line sample concentration technique in CZE, CD concentration, sample injection time, and sample:CD solution injection ratio were optimized. The optimized conditions for five model anionic analytes, namely, 4-bromophenol, sulindac, sulfamethizole, 4-vinylbenzoic acid, and succinylsulfathiazole, were 20 mM α-CD in 20 mM sodium tetraborate (pH 9.2) solution, sample injection time of 370 s, and CD:sample injection ratio of 1:2. The sensitivity enhancement factors (SEFs) were between 112 and 168. The SEFs of sulindac and sulfamethizole in particular were similar to previously published off-line microextraction techniques, which are typically time-consuming. The calculated values of LOQ, intra-/inter-day (n = 6/n = 10, 3 days) repeatability, and linearity (R²) of CD to admicelle ME were 0.0125-0.05 µg/mL, 1.5-4.6%, 1.8-4.8%, and ≥0.999, respectively. Finally, the potential of CD to admicelle ME to the analysis of artificial urine samples was demonstrated.

Chiral separation using cyclodextrins as mobile phase additives in open-tubular liquid chromatography with a pseudophase coating

The chiral separation of various analytes (dichlorprop, mecoprop, ibuprofen, and ketoprofen) was demonstrated with different cyclodextrins as mobile phase additives in open‐tubular liquid chromatography using a stationary pseudophase semipermanent coating. The stable coating was prepared by a successive multiple ionic layer approach using poly(diallyldimethylammonium chloride), polystyrene sulfonate, and didodecyldimethyl ammonium bromide. Increasing concentrations (0–0.2 mM) of various native and derivatized cyclodextrins in 25 mM sodium tetraborate (pH 9.2) were investigated. Chiral separation was achieved for the four test analytes using 0.05–0.1 mM β‐cyclodextrin (resolution between 1.11 and 1.34), γ‐cyclodextrin (resolution between 0.78 and 1.27), carboxymethyl‐β‐cyclodextrin (resolution between 1.64 and 2.59), and 2‐hydroxypropyl‐β‐cyclodextrin (resolution between 0.71 and 1.76) with the highest resolutions obtained with 0.1 mM carboxymethyl‐β‐cyclodextrin. %RSD values were <10%. This is the first demonstration of chiral open‐tubular liquid chromatography using achiral chromatographic coatings and cyclodextrins as mobile phase additives.

Pseudophase microextraction for in-line sample concentration in micellar electrokinetic chromatography

Pseudophase microextraction (PPME) as a simple in-line sample concentration technique in micellar electrokinetic chromatography (MEKC) is presented. In contrast to popular electric-field driven stacking techniques in MEKC such as sweeping, PPME is pressure-driven. The technique afforded up to 403-2968x improvements in peak heights for fenoprop, dichlorprop, 1- and 2-naphthol compared to typical injection. Under the same MEKC conditions, the improvements in PPME were up to 23-59x better compared to sweeping. Briefly in PPME, the entire capillary was loaded (up to 20 capillary volumes) with the sample prepared in a dilute solution of cetyltrimethylammonium bromide ([CTAB] > critical surface aggregation concentration). The CTAB formed aggregates at the inner capillary walls and these aggregates acted as a stationary chromatographic pseudophase. After clean-up via flushing the capillary with purified water, the MEKC background solution (BGS) with sodium dodecyl sulfate was then introduced by pressure from the outlet end to elute the retained analytes. The analytes concentrate at front of the BGS and the front was moved to the inlet end of the capillary prior to MEKC. Optimization strategies and current limitations in PPME-MEKC are described. The linear ranges using a 4 capillary volume sample load obtained for fenoprop, dichlorprop, 1- and 2-naphthol were between 1 and 160 ng/mL (r²s ≥ 0.996), LOQs = 1-2.5 ng/mL and repeatability %RSDs (n = 6) were ≤5% (intra-day) and ≤7% (inter-day) (using low analyte concentrations 1-5x LOQ). PPME-MEKC with simple dilution of fortified real samples (no off-line sample concentration) was also able to detect low levels of dichlorprop (10 ng/mL, limit set in Australia) and 1- and 2-naphthol (7.5-15 ng/mL) in a drinking water and natural water sample, respectively (% recovery = 84-108%). The concept of PPME may find use in other modes of capillary electrophoresis and other nano-microscale separations.

Electroosmotic flow assisted pseudophase to pseudophase microextraction for stacking in capillary zone electrophoresis

A stacking technique is proposed to improve the poor detection sensitivity of capillary zone electrophoresis (CZE) with UV detection. A long injection (e.g., 12.4 cm plug) of model anionic analytes prepared in a dilute solution of hexadecyltrimethylammonium bromide (CTAB) was enriched 26-34-x (compared to a typical or 2.1 mm sample injection) via the injection of a micellar solution of sodium dodecyl sulfate (SDS) prior to CZE separation. During sample injection, the CTAB formed a stationary pseudophase coating, which trapped the analytes at the inner walls of a fused silica capillary. The SDS micelles then released the CTAB admicelles via the formation of solution CTAB-SDS catanionic micelles during SDS plug injection and voltage application. As the SDS micelles moved through the sample zone, the formation of the catanionic micelles then released and accumulated the analytes at the front of the injected SDS zone. The stacking technique is called electroosmotic flow (EOF) assisted pseudophase to pseudophase microextraction because the EOF was essential for the formation of CTAB-SDS catanionic micelles for microextraction. Also, the CTAB and SDS aggregates are both pseudophases, which were used to retain and release the analytes from the capillary wall, respectively.

Micelle to cyclodextrin stacking in open-tubular liquid chromatography using capillaries coated with surfactant admicelles

In-line sample concentration by micelle to cyclodextrin stacking (MCDS) in open-tubular liquid chromatography (OT-LC) with UV detection is described. OT-LC of two sets of analytes (small-molecule drugs and neutral alkenylbenzenes) was by the use of a fused-silica capillary that was coated with admicelles of didodecyldimethyl ammonium bromide (DDAB). These admicelles acted as a stationary chromatographic pseudophase. The mobile phase was 25 mM sodium tetraborate in 10% methanol, pH 9.2. MCDS was by long pressure injection of samples prepared in 10 mM hexadecyltrimethyl ammonium bromide (CTAB) in 25 mM sodium tetraborate, pH 9.2 (buffer), followed by injection of 50 mM α-CD in buffer (CD solution). Stacking was by application of voltage at -20 kV prior to pressure-driven OT-LC. The factors that influenced MCDS such as type and concentration of CD, concentration of CTAB in the sample, injection time ratio of the sample and the CD solution and stacking time were studied. Under optimised conditions, sensitivity enhancement factors (SEFs) were between 19 and 23, linear ranges were between 0.5 and 10 µg/mL with r² > 0.99 and inter-day/intra-day repeatability in retention time and peak area were ≤5.6% for the model small-molecule drugs. Application to real samples was by the determination of potentially toxic alkenylbenzenes (SEFs = 10 to 12) in basil-leaf and whole-clove extracts. The assay results were comparable to those obtained from an in-house high-performance liquid chromatography-UV method.

A stationary pseudophase semi-permanent coating for open-tubular capillary liquid chromatography and electrochromatography

We describe the chromatographic and electrochromatographic separation of small neutral and charged analytes using a fused silica capillary with a stationary pseudophase semi-permanent coating of didodecyldimethyl ammonium bromide (DDAB) aggregates. The coating was prepared by flushing the capillary with a DDAB solution that was rinsed out with the mobile phase. Our studies (i.e., electroosmotic flow measurements by capillary electrophoresis, chromatographic retention of a neutral probe and atomic force microscopy) suggested the formation of DDAB patchy admicelle, complete admicelle, or larger aggregates at the solid surface - liquid interface inside the capillary, depending on the concentration of DDAB used in coating the capillary. The analytical figures of merit for open tubular liquid chromatography (OT-LC, pressure driven) and open tubular capillary electrochromatography (OT-CEC, voltage driven) using a capillary coated with 0.5 mM DDAB and mobile phase/background solution of 25 mM borate buffer at pH 9.5 with 10% MeOH were the following: LOD = 3.0-5.0 µg/mL (OT-LC) and 2.5-5.0 µg/mL (OT-CEC); linearity R2 > 0.99 (peak area (OT-LC) and corrected peak area (OT-CEC)), intraday and interday repeatability%RSD < 5% (n = 12) for retention/migration time, peak area (OT-LC) and corrected peak area (OT-CEC). The reversed-phase and anion-exchange property of the stationary pseudophase was studied by the addition of organic solvents and sodium chloride to the mobile phase, respectively. We also demonstrate the increase in the ks of the tested analytes by implementing successive multiple ionic layer (SMIL) coating strategies with DDAB in combination with a cationic and/or anionic polyelectrolyte. The use of a stationary pseudophase coating is potentially an easy alternative way to conduct open-tubular liquid chromatography and electrochromatography.

A comparison of 2 micron inner diameter open tubular column liquid chromatography with pressure-driven isocratic, slip-flow, and electrochromatographic modes of operation: a theoretical study

Modifications to the flow profile used in open tube capillary liquid chromatography (OT-CLC) include using slip-flow walls and using electroosmosis as a fluid pump as practiced in electrochromatography. These modifications are implemented experimentally by changing the capillary surface and solvent conditions which results in the change of boundary conditions at the capillary wall. In this paper we employ a theory-based study and compare the zone broadening of simple solutes using parabolic flow from a liquid pump, slip-flow from a highly hydrophobic inner surface with water eluent, and electroosmosis for the conditions of pure water and dilute salt utilizing 2 µm inner diameter OT capillaries. In general, the two types of flow other than parabolic exhibit thin zones in the early part of the chromatogram, consistent with previous studies of slip-flow and electroosmotic flow used in electrochromatography. Electrochromatography is shown to yield higher efficiency and less zone broadening than parabolic and slip-flow conditions used in this study. Nonetheless, it is found that the zone standard deviations are shown to be similar for these flow profiles as is the number of plates for these different flow profiles under the conditions utilized here. It is revealed that these modifications do not warrant the effort to maintain the special solvent conditions when compared to gradient elution OT-CLC, which gives a nearly constant peak width throughout the chromatogram, is easiest to implement, and is the method of choice for complex analysis.

Solvent-free mixed micellar mobile phases: An advanced green chemistry approach for reversed-phase HPLC determination of some antihypertensive drugs

Minimizing the amount of organic solvents without loss in chromatographic performance has been an important step toward greening analytical methodologies. Mobile-phase composition is the key for maintaining separation efficiency in liquid chromatography while decreasing the procedure hazardousness. If sodium dodecyl sulfate is mixed with Brij-35 in the mobile phase, they could be used as a green alternative for using organic modifiers. In this research, the effect of changing the relative amounts of both surfactants was studied on the chromatographic performance and separation efficiency of ten antihypertensive drugs belonging to different categories. The use of surfactants has many advantages including low cost and toxicity, safe environmental disposal, unique selectivity besides high solubilization capabilities. The optimum separation was maintained using a mobile phase (0.01 M Brij-35, 0.08 M sodium dodecyl sulfate and 0.01 M sodium dihydrogen phosphate/pH 5) on reversed-phase C18 core-shell column at flow rate 1.5 mL/min and temperature 30°C. The method was successfully applied for the determination of the drugs in various marketed dosage forms. International Conference of Harmonization guidelines were followed to validate the developed method. Additionally, the method was verified on the Green Analytical Procedure Index in regards to the greenness and found to be an excellent green alternative method.

Open-tubular admicellar electrochromatography of charged analytes

Fundamental studies on the separation of cationic and anionic analytes in open-tubular admicellar electrochromatography (OT-AMEC) using cetyltrimethylammonium bromide (CTAB) and fused silica capillaries are presented. OT-AMEC was compared with open-tubular admicellar liquid chromatography (OT-AMLC) by running the two methods using the same mobile phases. The mobile phases were buffered at pH ≥ 6 and contained a low concentration (above the critical surface aggregation concentration and below the critical micelle concentration) of CTAB. The stationary pseudophase of CTAB admicelles were formed at the solid surface and liquid interface inside the capillary by simply conditioning the capillary with the mobile phase. Separations were performed in a 30 cm (21.5 cm to UV detector) long and 50 μm inner diameter capillary, using low pressure (50 mbar) in OT-AMLC and high voltage (15 kV at negative polarity) in OT-AMEC. The appropriate equations for the experimental estimation of retention factor (k) values of analytes were discussed. For anionic analytes, k in OT-AMEC were carefully determined by considering the observed interaction between CTAB monomers and tested analytes. The calculated k for each analyte was found similar in OT-AMLC and OT-AMEC, although the mechanism of retention was not entirely different due to the contribution of electrophoresis in OT-AMEC. Studies on the addition of a typical (i.e., acetonitrile) and atypical modifier (i.e., nonyl-β-glucoside) into the mobile phase, and sample focusing with >10x improvement in peak height under isocratic conditions were also conducted.

Recent advancements in open-tubular liquid chromatography and capillary electrochromatography during 2014-2018

This review critically discusses the developments on open-tubular liquid chromatography (OT-LC) and open-tubular capillary electrochromatography (OT-CEC) during 2014-2018. An appropriate Scopus search revealed 5 reviews, 4 theoretical papers on open-tubular format chromatography, 29 OT-LC articles, 68 OT-CEC articles and 4 OT-LC/OT-CEC articles, indicating a sustained interest in these areas. The open-tubular format typically uses a capillary column with inner walls that are coated with an ample layer or coating of solid stationary phase material. The ratio between the capillary internal diameter and coating thickness (CID/CT) is ideally ≤ 100 for appropriate chromatographic retention. We, therefore, approximated the CID/CT ratios and found that 22 OT-LC papers have CID/CT ratios ≤100. The other 7 OT-LC papers have CID/CT ratio >100 but have clearly demonstrated chromatographic retention. These 29 papers utilised reversed phase or ion exchange mechanisms using known or innovative solid stationary phase materials (e.g. metal organic frameworks), stationary pseudophases from ionic surfactants or porous supports. On the other hand, we found that 68 OT-CEC papers, 7 OT-LC papers and 4 OT-LC & OT-CEC papers have CID/CT ratios >100. Notably, 44 papers (42 OT-CEC and 2 OT-LC & OT-CEC) did not report the retention factor and/or effective electrophoretic mobility of analytes. Considering all covered papers, the most popular activity was on the development of new chromatographic materials as coatings. However, we encourage OT-CEC researchers to not only characterise changes in the electroosmotic flow but also verify the interaction of the analytes with the coating. In addition, the articles reported were largely driven by stationary phase or support development and not by practical applications.