Commercial C18 Column Research Articles

Preparation of hollow porous molecularly imprinted polymers and their applications to solid-phase extraction of triazines in soil samples

Porous polymers have aroused increasing interest due to their controllable hole(s) structure in favor of mass transfer. In this work, three types of porous molecularly imprinted polymers (MIPs) using atrazine as template, namely single-hole hollow molecularly imprinted polymers (s-HMIPs), multihole hollow molecularly imprinted polymers (m-HMIPs) and porous solid molecularly imprinted polymers (ps-MIPs), were prepared, and applied as solid-phase extraction (SPE) sorbent for selective preconcentration and specific recognition of triazines in soil samples. For porous MIPs, most of the binding cavities were located in the proximity of the surface, which remarkably facilitated template removal and mass transfer. The resultant atrazine imprinted porous polymers of s-HMIPs and m-HMIPs exhibited more than triple imprinting capacity for triazines compared to the ps-MIPs, while they were all greatly superior to that of the non-imprinted polymers (NIPs). Accordingly, the s-HMIPs employed as SPE sorbent presented much higher extraction efficiency for several triazines based on the large specific surface area and high adsorption capacity in comparison with the commercial C18 column. The validated method was also successfully applied to soil sample analysis, and satisfactory recoveries were attained, such as 73.5–102.0% with the precision of 1.17–4.24% for five triazines at 10 μg L−1. The s-HMIPs-SPE proves to be a highly-effective cleanup and enrichment method for simultaneous separation and sensitive determination of triazines in complicated samples.

Determination of 16 polycyclic aromatic hydrocarbons in environmental water samples by solid-phase extraction using multi-walled carbon nanotubes as adsorbent coupled with gas chromatography–mass spectrometry

A solid-phase extraction (SPE) using multi-walled carbon nanotubes (MWCNTs) as adsorbent coupled with gas chromatography–mass spectrometry (GC–MS) method was developed for the determination of 16 polycyclic aromatic hydrocarbons (PAHs) in environmental water samples. Several condition parameters, such as extraction adsorbents, elution solvents and volumes, and sample loading flow rate and volume were optimized to obtain high SPE recoveries and extraction efficiency. 150 mg MWCNTs as sorbent presented high extraction efficiency of 16 PAHs due to the large specific surface area and high adsorption capacity of MWCNTs compared with the commercial C18 column (250 mg/2 mL). The calibration curves of 16 PAHs extracted were linear in the range of 20–5000 ng L −1, with the correlation coefficients ( r 2) between 0.9848 and 0.9991. The method attained good precisions (relative standard deviation, RSD) from 1.2% to 12.1% for standard PAHs aqueous solutions; method recoveries ranged in 76.0–125.5%, 74.5–127.0%, and 70.0–122.0% for real spiked samples from river water, tap water and seawater, respectively. Limits of detection (LODs, S/N = 3) of the method were determined from 2.0 to 8.5 ng L −1. The optimized method was successfully applied to the determination of 16 PAHs in real environmental water samples.

A study of the effects of column porosity on gradient separations of proteins

The type of the stationary phase for reversed-phase liquid chromatography significantly affects the sample elution. Hydrodynamic properties, efficiency and gradient elution of proteins were investigated on five commercial C18 columns with wide-pore totally porous particles, with superficially porous layer particles, non-porous particles and a silica-based monolithic bed. The efficiency in the terms of reduced plate height is higher for low-molecular ethylbenzene than for proteins, but depends on the character of the pores in the individual columns tested. The superficially porous Poroshell and the non-porous Micra columns provide the best efficiency for proteins at high mobile phase flow rates, probably because of similar pore architecture in the stationary phase. The Zorbax column with similar pore architecture as the Poroshell active layer, i.e. narrow pore distribution of wider pores shows better efficiency than the packed column with narrow pores and broad pore distribution. The monolithic column shows lower efficiency for proteins at high flow rates, but it performs better than the broad-pore distribution totally porous particulate columns. Different pore architecture affects also the retention and selectivity for proteins on the individual columns. The retention times on all columns can be predicted using the model for reversed-phase gradient elution developed originally for low-molecular compounds. Consideration of the limited pore volume accessible to the biopolymers has negligible effect on the prediction of retention on the columns packed with non-porous or superficially porous particles, but improves the accuracy of the predicted data for the totally porous columns with broad pore distribution.

Electron-impact and glow-discharge ionization LC–MS analysis of green tea tincture

A liquid chromatography-particle-beam mass spectrometer (LC-PB/MS) with interchangeable electron-impact (EI) and glow-discharge (GD) ion sources was evaluated for future application in analysis of botanical extracts. In this work a green tea tincture was characterized for a series of catechin components (catechin, epicatechin, epigallocatechin, and epigallocatechin gallate (EGCG)) and caffeine. Special emphasis was given to EGCG and caffeine, because they are important in determining the possible health effects of the green tea. The effects of instrument operating conditions were evaluated for the EI and GD ionization sources to determine their effect on analyte intensities and fragmentation patterns. These studies furnished information about the effects of these conditions in determining possible ionization pathways in the two ion sources. The mass spectra of these compounds obtained with the GD ion source are EI-like in appearance, with clearly identified molecular ions and fragmentation patterns that are easily rationalized. The absolute limits of detection for EGCG and caffeine were, respectively, 11 ng and 0.77 ng for the EI source and 3.2 ng and 0.61 ng for the GD source. The PB/EIMS and PB/GDMS combinations can be operated in a flow-injection mode, wherein the analyte is injected directly into the mobile phase, or coupled to high-performance liquid chromatography (HPLC), enabling LC-MS analysis of complex mixtures. A reversed-phase chromatographic separation of the green tea tincture was performed on a commercial C18 column using a gradient of water (containing 0.1% TFA) and ACN. Quantification of EGCG and caffeine was performed by the standard addition method. The amounts of EGCG and caffeine in the tested green tea tincture were each approximately 14 mg mL-1.

Comparison of column properties in reversed-phase chromatography: monolithic, cholesterolic and mixed bonded stationary phases

A commercial Chromolith C18 column and two new stationary phases with mixed ligands bonded on the Kromasil silica gel support, SG-MIX and SG-Chol, were characterized using simple tests based on the retention of non-polar, basic and acidic compounds. Polar and methylene selectivity tests in acetonitrile–water and methanol–water mobile phases revealed lower hydrophobicities of the SG-MIX and SG-Chol columns in comparison to the Chromolith column. The columns were further characterized using new test criteria – gradient oligomer capacity and isomeric selectivity and peak symmetry of naphthalene di-sulphonic acids in aqueous mobile phases. The cholesterolic column shows greater gradient oligomer selectivity for the separation of oligoethylene glycol samples than the SG-MIX and the Chromolith columns. Increased retention and peak tailing, but decreased isomeric selectivity for naphthalene-di-sulphonic acids was observed with the SG-MIX column, because of interactions with various polar bonded groups.

Retention characteristics and selected applications of cyclic siloxane-based octadecylsilyl bonded phases in reversed-phase high-performance liquid chromatography

Hexamethylcyclotrisiloxane- and octaphenylcyclotetrasiloxane-based octadecyl bonded phases are prepared and evaluated for liquid chromatographic separation of various compounds. As revealed by the retention data of the selected solutes, the hexamethylcyclotrisiloxane–octadecyl phase exhibits better chromatographic performance than octaphenylcyclotetrasiloxane–octadecyl phase presumably due to greater accessibility of the bonded alkyl groups on the siloxane skeleton for interactions. The peak asymmetry values showed that solute retention on the hexamethylcyclotrisiloxane–octadecyl phase is essentially controlled by dispersive interactions. The retention characteristics of the cyclicsiloxane-based phases were compared to those obtained on a commercial C18 column and an octadecyl phase synthesized through a silanization–hydrosilation addition process. While the two phases gave baseline separation of polycyclic aromatic hydrocarbons and barbiturates, the hexamethylcyclotrisiloxane–octadecyl phase allowed fast and efficient separation of a four-component standard mixture of tricyclic antidepressants.

Determination of column selectivity toward polycyclic aromatic hydrocarbons

AbstractAn empirical test is described for the evaluation of column selectivity in reversed‐phase liquid chromatography. Using a test mixture of three polycyclic aromatic hydrocarbons (PAH), overall column selectivity toward PAH was assessed for over 20 different commercial C18 columns. Retention behavior was correlated to phase type (i.e., monomeric and polymeric surface modification chemistry) for custom synthesized phases. A classification scheme is proposed in which commercial C18 columns are grouped into three classes based on retention behavior: monomeric‐like, polymeric‐like, and intermediate phase selectivity toward PAH. Correlation of retention behavior of the test mixture with the separation of PAH mixtures and with more general column properties (e.g., phase thickness) is discussed.

Custom made C18 columns for the hydrophobic chromatography of cephalosporin C derivatives.

"Hydrophobic chromatography", which is a variation of reverse phase chromatography, is applicable to the analysis of cephalosporin C derivatives, especially in fermentation broths. Unfortunately, there are no commercial C18 columns which are entirely suitable for this class of compounds. For this reason C18 columns were prepared by an in-situ bonding technique and were optimally designed for cephalosporin C derivatives. Mono-, di- and trifunctional octadecyl bonding agents were used with 10 microns silica of both 60 A and 100 A pore diameter. The best results were obtained with the difunctional agent, methyloctadecyldichlorosilane, and 100 A silica. "Endcapping" of residual silanol groups with a trimethylsilylation agent was optional, since good results were obtained with both a plain C18 column and one that was "endcapped".