Porous polymer monoliths functionalized through copolymerization of a C60 fullerene-containing methacrylate monomer for highly efficient separations of small molecules.

Monolithic poly(glycidyl methacrylate-co-ethylene dimethacrylate) and poly(butyl methacrylate-co-ethylene dimethacrylate) capillary columns, which incorporate the new monomer [6,6]-phenyl-C(61)-butyric acid 2-hydroxyethyl methacrylate ester, have been prepared and their chromatographic performance have been tested for the separation of small molecules in the reversed phase. While addition of the C60-fullerene monomer to the glycidyl methacrylate-based monolith enhanced column efficiency 18-fold, to 85,000 plates/m at a linear velocity of 0.46 mm/s and a retention factor of 2.6, when compared to the parent monolith, the use of butyl methacrylate together with the carbon nanostructured monomer afforded monolithic columns with an efficiency for benzene exceeding 110,000 plates/m at a linear velocity of 0.32 mm/s and a retention factor of 4.2. This high efficiency is unprecedented for separations using porous polymer monoliths operating in an isocratic mode. Optimization of the chromatographic parameters affords near baseline separation of 6 alkylbenzenes in 3 min with an efficiency of 64,000 plates/m. The presence of 1 wt % or more of water in the polymerization mixture has a large effect on both the formation and reproducibility of the monoliths. Other factors such as nitrogen exposure, polymerization conditions, capillary filling method, and sonication parameters were all found to be important in producing highly efficient and reproducible monoliths.

Remotely detected NMR for the characterization of flow and fast chromatographic separations using organic polymer monoliths.

An application of remotely detected magnetic resonance imaging is demonstrated for the characterization of flow and the detection of fast, small molecule separations within hypercrosslinked polymer monoliths. The hyper-cross-linked monoliths exhibited excellent ruggedness, with a transit time relative standard deviation of less than 2.1%, even after more than 300 column volumes were pumped through at high pressure and flow. Magnetic resonance imaging enabled high-resolution intensity and velocity-encoded images of mobile phase flow through the monolith. The images confirm that the presence of a polymer monolith within the capillary disrupts the parabolic laminar flow profile that is characteristic of mobile phase flow within an open tube. As a result, the mobile phase and analytes are equally distributed in the radial direction throughout the monolith. Also, in-line monitoring of chromatographic separations of small molecules at high flow rates is shown. The coupling of monolithic chromatography columns and NMR provides both real-time peak detection and chemical shift information for small aromatic molecules. These experiments demonstrate the unique power of magnetic resonance, both direct and remote, in studying chromatographic processes.

Covalently modified graphitic carbon-based stationary phases for anion chromatography.

Carbon-clad zirconia particles have been converted into ion-exchange media by in situ diazonium generation and thermal deposition. The surfaces prepared possess either a permanently charged anion-exchange site or a weak anion-exchange site. Surface modification optimization experiments were performed both on planar carbon surfaces and on non-porous 2 microm and porous 3 microm carbon-based particles. Modification by traditional electrochemical and thermal deposition were compared. Surface modification with the tertiary amine functionality, N,N-dimethyl-p-phenylenediamine, yielded a capacity of 6.5 microequiv./column, stable retention for >33,000 column volumes and retention reproducibility of <2% RSD. A quaternary amine functionality (strong base exchanger) was achieved by reaction of the tertiary amine phase with methyl iodide. Utilizing short columns (35 x 4 mm i.d.) mixtures of common inorganic anions were separated with efficiencies of 21,000 plates/m.

Surfactant coated graphitic carbon based stationary phases for anion-exchange chromatography.

Separations of common inorganic anions were carried out on three different surfactant coated media using carbonate/bicarbonate eluents with suppressed conductivity detection. Graphitic carbon columns (porous graphitic carbon and carbon-clad zirconia) packed with 3 microm particles have been converted into anion-exchange stationary phases by equilibration with the cationic surfactants: didodecyldimethylammonium bromide (DDAB); cetyltrimethylammonium bromide (CTAB); and cetylpyridinium chloride (CPC). Additionally, an ethylene-bridged silica column was studied with CPC coatings. Porous graphitic carbon (PGC) columns coated with DDAB exhibited pressure increases and loss of resolution at higher capacities. CPC coatings on PGC exhibited better repeatability and efficiencies of 5.0 x 10(4)plates/m. However, CPC coatings exhibited a 15% loss in retention factor with <1.2 x 10(3) column volumes on PGC. Conversely, the ethylene-bridged silica column showed complete failure in less than 8 h of use. As with PGC, carbon-clad zirconia coated with CPC showed an initial loss of capacity, but thereafter was stable for more than 1.7 x 10(3) column volumes (t(r) RSD<2%).

Agglomerated carbon based phases for anion exchange chromatography.

Carbon-clad zirconia particles have been converted into ion exchange media through addition of charged latexes after covalent modification of the carbon surface. A variety of methodologies were investigated to introduce a negative charge to the carbon surface in the form of either sulfonate or oxygen containing functionalities (e.g. hydroxyl or carboxylate). Short analytical sized columns (35 mm × 4 mm I.D.) were packed with modified 2 µm nonporous carbon clad zirconia. Addition of the latex particles after the initial packing produced almost double the efficiency for the system compared to adding the latexes before packing. The optimized system could separate mixtures of common inorganic anions with efficiencies greater than of 41,000 plates/m and retention reproducibility of <2% RSD.

Incorporation of carbon nanotubes in porous polymer monolithic capillary columns to enhance the chromatographic separation of small molecules.

Multiwalled carbon nanotubes have been entrapped in monolithic poly(glycidyl methacrylate-co-ethylene dimethacrylate) capillary columns to afford stationary phases with enhanced liquid chromatographic performance for small molecules in the reversed phase. While the column with no nanotubes exhibited an efficiency of only 1800 plates/m, addition of a small amount of nanotubes to the polymerization mixture increased the efficiency to over 15,000 and 35,000 plates/m at flow rates of 1 and 0.15 µL/min, respectively. Alternatively, the native glycidyl methacrylate-based monolith was functionalized with ammonia and, then, shortened carbon nanotubes, bearing carboxyl functionalities, were attached to the pore surface through the aid of electrostatic interactions with the amine functionalities. Reducing the pore size of the monolith enhanced the column efficiency for the retained analyte, benzene, to 30,000 plates/m at a flow rate of 0.25 µL/min. Addition of tetrahydrofuran to the typical aqueous acetonitrile eluents improved the peak shape and increased the column efficiency to 44,000 plates/m calculated for the retained benzene peak.

Developments in ion chromatography using monolithic columns.

The focus of this review is on current status and on-going developments in ion chromatography (IC) using monolithic phases. The use and potential of both silica and polymeric monoliths in IC is discussed, with silica monoliths achieving efficiencies upwards of 10(5) plates/m for inorganic ions in a few minutes or less. Ion exchange capacity can be introduced onto the monolithic columns through the addition of ion interaction reagents to the eluent, coating of the monolith with ionic surfactants or polyelectrolyte latexes, and covalent bonding. The majority of the studies to date have used surfactant-coated columns, but the stability of surfactant coatings limits this approach. Applications of monolithic IC columns to the separation of inorganic anions and cations are tabulated. Finally, a discussion on the recent commercialization of monolithic IC columns and the use of monolithic phases for IC peripherals such as preconcentrator columns, microextractors and suppressors is presented.