Hongdeng Qiu

Development of silica-based stationary phases for high-performance liquid chromatography

Stationary phases are the basis of the development and application of high-performance liquid chromatography (HPLC). In this review we focused on the development of silica-based stationary phases, including the synthesis of silica gel and the application of silica in hydrophilic interaction chromatography (HILIC), reversed-phase liquid chromatography (RPLC), chiral separation chromatography, and ion chromatography. New stationary phases, advances in ionic liquid-modified silica, silica-based core-shell materials, and silica-based monolithic columns for HPLC are introduced separately.

Investigation of π-π and ion-dipole interactions on 1-allyl-3-butylimidazolium ionic liquid-modified silica stationary phase in reversed-phase liquid chromatography

1-Allyl-3-butylimidazolium bromide ionic liquid [AyBIm]Br was prepared and used for the modification of mercaptopropyl-functionalized silica through surface radical chain-transfer addition. The obtained ionic liquid-modified silica (SiImBr) was characterized by elemental analysis, infrared spectroscopy, NMR spectroscopy, and thermogravimetric analysis. The selective retention behaviours of polycyclic aromatic hydrocarbons (PAHs) including some positional isomers were investigated using SiImBr as a stationary phase in reversed-phase liquid chromatography. The results showed that SiImBr presented multiple interactions including hydrophobic, pi-pi, and ion-dipole interactions during the separation of PAHs and dipolar compounds. However, it is proposed that pi-pi and ion-dipole interactions play important roles in the separation of PAHs and dipolar compounds. These results indicate that the ionic liquid-modified silica stationary phase is promising for future applications. A commercially available monomeric octadecylated silica (ODS) column and a custom-made poly(styrene)-grafted silica (Sil-St(n)) column were used as references.

New poly(ionic liquid)-grafted silica multi-mode stationary phase for anion-exchange/reversed-phase/hydrophilic interaction liquid chromatography

A new poly(ionic liquid)-grafted silica stationary phase was prepared and characterized. It was then applied for multi-mode chromatographies, including ion-exchange, reversed-phase, and hydrophilic interaction liquid chromatographies for the effective separation of anions, hydrophobic compounds, and small polar molecules, respectively.

New strategy for drastic enhancement of selectivity via chemical modification of counter anions in ionic liquid polymer phase

A new strategy for the design of a new chromatographic stationary phase via simple modifications of the counter anions in poly(ionic liquid)-grafted silica phase based on ionic self-assembly technology is proposed. The phase with methyl orange dye as counter anions exhibits ultra-high selectivity towards shape-constrained isomers.

On-line coupling of ionic liquid-based single-drop microextraction with capillary electrophoresis for sensitive detection of phenols

An ionic liquid-based single-drop microextraction (IL-SDME) procedure using IL as an extractant on-line coupled to capillary electrophoresis (CE) is proposed. The method is capable of quantifying trace amounts of phenols in environmental water samples. For the SDME of three phenols, a 2.40 nL IL microdrop was exposed for 10 min to the aqueous sample and then was directly injected into the capillary column for analysis. Extraction parameters such as the extraction time, the IL single-drop volume, pH of the sample solution, ionic strength, volume of the sample solution and the extraction temperature were systematically investigated. Detection limits to three phenols were less than 0.05 microg mL(-1), and their calibration curves were all linear (R(2) > or = 0.9994) in the range from 0.05 to 50 microg mL(-1). And enrichment factors for three phenols were 156, 107 and 257 without agitation, respectively. This method was then utilized to analyze two real environmental samples from Yellow River and tap water, obtaining satisfactory results. Compared with the usual SDME for CE, IL-SDME-CE is a simple, low-cost, fast and environmentally friendly preconcentration technique.

A new imidazolium-embedded C18 stationary phase with enhanced performance in reversed-phase liquid chromatography.

In this paper, a new imidazolium-embedded C(18) stationary phase (SiImC(18)) for reversed-phase high-performance liquid chromatography is described. 1-Allyl-3-octadecylimidazolium bromide ionic liquid compound having a long alkyl chain and reactive groups was newly prepared and grafted onto 3-mercaptopropyltrimethoxysilane-modified silica via a surface-initiated radical-chain transfer addition reaction. The SiImC(18) obtained was characterized by elemental analysis, infrared spectroscopy, thermogravimetric analysis, diffuse reflectance infrared Fourier transform, and solid-state (13)C and (29)Si cross-polarization/magic angle spinning nuclear magnetic resonance spectroscopy. The selectivity toward polycyclic aromatic hydrocarbons relative to that toward alkylbenzenes exhibited by SiImC(18) was higher than the corresponding selectivity exhibited by a conventional octadecyl silica (ODS) column, which could be explained by electrostatic π-π interaction cationic imidazolium and electron-rich aromatic rings. On the other hand, SiImC(18) also showed high selectivity for polar compounds, which was based on the multiple interaction and retention mechanisms of this phase with different analytes. 1,6-Dinitropyrene and 1,8-dinitropyrene, which form a positional isomer pair of dipolar compounds, were separated successfully with the SiImC(18) phase. Seven nucleosides and bases (i.e. cytidine, uracil, uridine, thymine, guanosine, xanthosine, and adenosine) were separated using only water as the mobile phase within 8min, which is difficult to achieve when using conventional hydrophobic columns such as ODS. The combination of electrostatic and hydrophobic interactions is important for the effective separation of such basic compounds without the use of any organic additive as the eluent in the octadecylimidazolium column.

Poly(1-allylimidazole)-grafted silica, a new specific stationary phase for reversed-phase and anion-exchange liquid chromatography

A new specific stationary phase based on poly(1-allylimidazole)-grafted silica has been synthesized and characterized, by infrared spectra, elemental analysis, thermogravimetric analysis and X-ray photoelectron spectroscopy. The results of test showed that poly(1-allylimidazole) can effectively mask the residual silanol groups and reduce the adverse effect of residual silanol. Using this stationary phase, phenol compounds, aniline compounds, and polycyclic aromatic hydrocarbons were successfully separated with symmetric peak shapes in the reversed-phase chromatography. Inorganic anions (IO(3)(-), BrO(3)(-), Br(-), NO(3)(-), I(-), SCN(-)) were also separated completely in the anion-exchange chromatography using sodium chloride solution as the mobile phase. The effects of pH and the concentration of eluent on the separation of inorganic anions were studied. The separation mechanism appears to involve the mixed interactions of hydrogen bonding, hydrophobic, pi-pi, electrostatic, and anion-exchange interactions.

A sulfonic-azobenzene-grafted silica amphiphilic material: a versatile stationary phase for mixed-mode chromatography.

A novel sulfonic-azobenzene-functionalized amphiphilic silica material was synthesized through the preparation of a new sulfonic azobenzene monomer and its grafting on mercaptopropyl-modified silica by a surface-initiated radical chain-transfer reaction. The synthesis was confirmed by infrared spectra, elemental analysis, and thermogravimetric analysis. This new material was successfully applied as a new kind of mixed-mode stationary phase in liquid chromatography. This allows an exceptionally flexible adjustment of retention and selectivity by tuning the experimental conditions. The distinct separation mechanisms were outlined by selected examples of chromatographic separations in the different modes. In reversed-phase liquid chromatography, this new stationary phase presented specific chromatographic performance when evaluated using a Tanaka test mixture. Seven dinitro aromatic isomers, four steroids, and seven flavonoids were separated successfully in simple reversed-phase mode. This stationary phase can also be used in hydrophilic interaction chromatography because of the existing polar functional groups; for this, nucleosides and their bases were used as a test mixture. Interestingly, the same nucleosides and bases can also be separated in per aqueous liquid chromatography using the same stationary phase. Three ginsenosides including Rg1, Re, and Rb1 were successfully separated in hydrophilic mode. There is the potential for more applications to benefit from this useful column.

A facile and specific approach to new liquid chromatography adsorbents obtained by ionic self-assembly

A new ionic-liquid monomer, 1-vinyl-3-octadecylimidazolium bromide ([C(18)VIm]Br), was prepared and polymerized on porous silica particles by means of a surface-initiated radical chain-transfer reaction. Further modification for functionalization was performed through the exchange of counteranions from bromide to methyl orange (MO). Two new silica-poly(octadecylimidazolium) (Sil-PImC(18)) hybrid materials (Sil-PImC(18)-Br and Sil-PImC(18)-MO) were synthesized and characterized by elemental analysis, thermogravimetric analysis, diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), and solid-state (13)C CP/MAS NMR spectroscopy. Sil-PImC(18)-MO presented ultra-high shape selectivity for constrained isomers of polycyclic aromatic hydrocarbons (PAHs) both in reversed- and normal-phase HPLC when used as the stationary phase. Fundamental aspects of the molecular shape selectivity were evaluated by using Standard Reference Material (SRM) 869b; the column selectivity test mixture for liquid chromatography. The impact of this phase was also demonstrated by the separation of SRM 1647e (16 priority pollutant PAHs) and several steroid isomers. Enhanced selectivity could be explained by the highly oriented arrangement between the octadecylimidazolium chain and a rigid segment of MO. These findings may open a new window of research for the design of materials used in chromatographic supports, solid extraction, catalysis, and electrolytes by simple modifications of the counterions in the poly(ionic liquid) analogous phase.

Enhancement of molecular shape selectivity by in situ anion-exchange in poly(octadecylimidazolium) silica column

This paper demonstrates that in situ anion exchange could be successfully applied as a new method for modifying the surface properties of a poly(octadecylimidazolium)-grafted silica stationary phase to tune and enhance selectivity. Specifically, the original stationary phase was prepared by surface-initiated radical chain-transfer polymerization of 1-vinyl-3-octadecylimidazolium bromide as an ionic liquid monomer; the Br(-) counter anion was then exchanged for methyl orange via an in-column process. As evaluated via the separation of constrained isomers of polycyclic aromatic hydrocarbons (PAHs), the in situ exchange enhanced the molecular shape-selectivity performance. Enhanced selectivity was also confirmed using Standard Reference Material (SRM) 869b (column selectivity test mixture) and SRM 1647e (16 priority pollutant PAHs). The reproducibility of new column was tested via the separation of pyrene, triphenylene, benzo[a]anthracene and chrysene with methanol as eluent at 10 °C and the RSD values (n=12) of the retention factors of them are within 0.27-0.77%.