Herein, we report the preparation and application of a new nano-structured monolithic nanocolumn based on modified graphene oxide using narrow fused silica capillary column (e.g., 50 μm internal diameter). The nanocolumn was prepared by an in situ polymerization using butyl methacrylate, ethylene dimethacrylate, and methacryloyl graphene oxide nanoparticles. Dimethyl formamide and water were used as the porogenic solvent. After polymerization, the obtained nanocolumn was coated with dimethyloctadecylchlorosilane in order to enhance the hydrophobicity. Both isocratic and gradient nano-liquid chromatographic separations for small molecules (e.g., alkylbenzenes) and macromolecules (e.g., intact proteins) were performed. Theoretical plates number up to 3600 plates/m in isocratic mode for propylbenzene were achieved. It was demonstrated that the feasibility of graphene oxide modified monolithic nanocolumn for high-efficiency and high-throughput nanoscale proteomics analysis. The high resolving power of monolithic nanocolumn yielded sensitive protein separation with narrower peak width while a high-resolution analysis of peptides from trypsin-digested cytochrome C could be obtained. Graphene oxide based monolithic nanocolumns are promising and can allow to powerful tools for trace proteom sample analysis.
A procedure is described for the extraction of dithiopyr [2-(difluoromethyl)-4-(2-methylpropyl)-6-(trifluoromethyl)-3,5-pyridine dicarbothioic acid, S,S-di methyl ester] from soil leachate and its analysis by isothermal electron-capture gas chromatography while using metribuzin [4-amino-6-(1,1-dimethylethyl)-1,2,4-triazin-5(4H)-one] as an internal standard during quantitation. Dithiopyr extraction from aqueous soil leachate by liquid-liquid extraction (LLE) with hexane, toluene, or ethyl acetate was compared with solid-phase extraction (SPE) using an LC-18 SPE tube. The influence of silanizing the glassware on dithiopyr recovery was tested. The LLE methods yielded approximately 56% recovery of dithiopyr from the aqueous solution. SPE with silanized glassware resulted in the highest dithiopyr recovery. This system yielded recovery levels near 99% dithiopyr from aqueous-solution volumes ranging from 4 to 80 mL, with the volumes containing dithiopyr concentrations ranging from 1 to 100 µg/L. The SPE soil-leachate samples were void of compounds that interfered with the gas chromatography of metribuzin and dithiopyr, and the electron-capture detector responded linearly to dithiopyr, in toluene, over a concentration range of 0.4 to 150 µg/L.
Peak compression is a unique feature of gradient elution and is non-existent in isocratic elution. Since the classical plate height equation, which is also called as van Deemter equation, is derived by assuming isocratic elution, it cannot be used to account for the effects of peak compression. As opposed to the case of isocratic elution, the retention factor (k) varies with the mobile phase composition (φ) under gradient elution, thereby complicating mathematical analysis. Herein, the research progress on peak compression in the past decade, especially the effect of the nonlinear feature of solvent strength models (i. e., expressions for ln k vs. φ), is reviewed. A general expression for the peak compression factor (G) is introduced, for which the variation in plate height (H) with φ is ignored. Based on this equation, the classical equation for G, which was first proposed by Poppe and assumes the linear solvent strength model (LSSM) and linear gradient elution, can be derived. The effects of pre-elution of the solute in the initial mobile phase on G, which are attributed to the dwelling time of the system, are included in the Poppe equation. When the solvent strength model is nonlinear, e. g., the quadratic solvent strength model (QSSM), the analytical expressions for G can also be obtained from the general expression. Under ideal chromatographic conditions, where H=0 and the adsorption isotherm is linear, the peak compression is determined by the ratio of the retention factor of the solute in the initial mobile phase to that at the eluted mobile phase composition.
Stationary phase gradients formed in a single column housing or by serially connected columns are the most recently explored subset of non-uniform chromatography. Such non-uniform columns have exhibited improved separation capabilities including better resolution and reduced analysis time, particularly when compared to a traditional uniformly modified column. When combined with a mobile phase gradient, peak focusing and improved resolution of the critical pair can result. In this review chapter, both continuous and discontinuous stationary phase gradients that specifically exhibit a change in chemistry along their length are described for the first time. Stationary phase gradients are explored for a variety of different substrates including thin-layer chromatography plates, silica and polymeric monolithic columns, and particle-packed columns as well as for different chromatographic modes such as reversed-phase and HILIC. This chapter begins with an introduction into discontinuous and continuous (single-component, multicomponent) gradients, how they are constructed and characterized, their chromatographic properties, and performance, and then ends with a discussion on the incorporation of gradient stationary phases into liquid chromatography simulations for method development. The promise of unique selectivity, improved efficiency, and reduced analysis time may make these types of stationary phases a new tool for the modern chromatographer.
