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

Open tubular liquid chromatography: Recent advances and future trends

Nano-liquid chromatography (nanoLC) is gaining significant attention as a primary analytical technique across various scientific domains. Unlike conventional high-performance LC, nanoLC utilizes columns with inner diameters (i.ds.) usually ranging from 10 to 150 μm and operates at mobile phase flow rates between 10 and 1000 nl/min, offering improved chromatographic performance and detectability. Currently, most exploration of nanoLC has focused on particle-packed columns. Although open tubular LC (OTLC) can provide superior performance, optimized OTLC columns require very narrow i.ds. (< 10 μm) and demand challenging instrumentation. At the moment, these challenges have limited the success of OTLC. Nevertheless, remarkable progress has been made in developing and utilizing OTLC systems featuring narrow columns (< 2 μm). Additionally, significant efforts have been made to explore larger columns (10-75 μm i.d), demonstrating practical applicability in many situations. Due to their perceived advantages, interest in OTLC has resurged in the last two decades. This review provides an updated outlook on the latest developments in OTLC, focusing on instrumental challenges, achievements, and advancements in column technology. Moreover, it outlines selected applications that illustrate the potential of OTLC for performing targeted and untargeted studies.

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.

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.