Grafting of Tetraphenylethylene on Silica Surface, Characterizations, and Their Chromatographic Performance as Reversed-Phase Stationary Phases

Adjustable chromatographic performance of silica-based mixed-mode stationary phase through the control of co-grafting amounts of imidazole and C18 chain

In this paper, three imidazole- and C18- bifunctional silica stationary phases (Sil-Im-C18) were prepared by adjusting introduction interval of octadecyltrichlorosilane (ODS) and 3-imidazol-1-ylpropyl(trimethoxy)silane (TMPImS), which can be used for reversed-phase liquid chromatography (RPLC) and ion exchange chromatography (IEC) with adjustable performance. The successful preparation of Sil-Im-C18 were confirmed by the characterizations of elemental analysis, infrared spectroscopy (FTIR) and contact angle (CA). Chromatographic performance of Sil-Im-C18 were evaluated by the separation of Tanaka test mixture, alkylbenzenes, linear PAHs and a set of analytes with different properties (uracil, phenol, 1,2-dinitrobenzene and naphthalene), and compared with commonly used C18 column. It was found that the chromatographic performance of Sil-Im-C18 changed significantly with the difference in bonding amount of imidazole and C18. Sil-Im-C18 demonstrated the excellent separation performance towards polycyclic aromatic hydrocarbons (PAHs), phenylesters, phenylamines, phenols and inorganic anions, and notably, nucleobases and nucleosides can be separated using pure water as mobile phases. The van Deemter plot showed that the column efficiency of Sil-Im-C18-3 was 64,933 plate·m-1 for naphthalene, indicated that Sil-Im-C18 was reasonably chromatographic columns. The RSD values of retention time were 0.22 %-0.61 % for 10 needles alkylbenzenes injected continuously at 50 °C to investigate thermal stability and repeatability, all the fluctuations of k of naphthalene were less than 2.3 % for Sil-Im-C18-1 during flushing 24 h with the mobile phase at different pH values (pH = 3 and 8), the retention time of alkylbenzenes were almost same for Sil-Im-C18-1 at different time, the RSD values of retention time of alkylbenzenes were 0.45 %-2.28 % for two batches Sil-Im-C18-1, revealing the excellent repeatability, thermal stability, durability and reproducibility of Sil-Im-C18, and implying a commercial prospect.

Core-shell silica@pyridyl conjugated microporous polymer as a stationary phase for high performance liquid chromatography

BACKGROUND

Because of the advantages of good selectivity, high sensitivity, and fast analysis, high performance liquid chromatography (HPLC) has become one of the modern analytical techniques in wide application range, such as biological analysis, environmental detection, pharmaceutical and food inspection, agriculture and other fields. The stationary phase greatly decides the chromatographic separation performance, so the development of novel stationary phase is most important for HPLC.

RESULTS

Pyridyl conjugated microporous polymers (P-CMP) with one to four layers were modified on the surface of amino silica to obtain a novel core-shell material (SiO2@P-CMP) by the layer-by-layer assembly strategy and Chichibabin reaction. The relationship between the structure of SiO2@P-CMP and chromatographic performance was carefully investigated, and the retention mechanism was revealed. The interactions including π-π stacking, hydrophobic effect and hydrogen bond gradually enhanced with the increase of P-CMP layers on the silica surface. Compared with C18 column, SiO2@P-CMP columns displayed better separation selectivity for polycyclic aromatic hydrocarbons (PAHs). According to the relative retention values (α), the separation performance of SiO2@P-CMP columns (α = 1.144-1.884) for PAH isomers and other analytes was obviously better than that of C18 column (α = 0.998-1.487). Furthermore, the SiO2@P-CMP column with four layers was selected to separate different types of analytes (eight PAHs, four bisphenols, four estrogens and nine phthalates), and the peak order of analytes was different from that on the C18 column due to the influence of hydrogen-bonding and π-π interactions. The relative standard deviations (n = 10) of retention time and peak area on SiO2@P-CMP column were between 0.28 % and 1.98 %.

SIGNIFICANCE AND NOVELTY

Pyridyl conjugated microporous polymer was introduced as the stationary phase for the first time in HPLC. The proposed column displayed better separation characteristics compared to Zorbax SB-C18 column. It provided a new idea for the separation of small molecules and the development of chromatographic packing or extraction material.

Synthesis and evaluation of aromatic stationary phases based on linear solvation energy relationship model for expanded application in supercritical fluid chromatography

In the last decade, the separation application based on aromatic stationary phases has been demonstrated in supercritical fluid chromatography (SFC). In this paper, four aromatic stationary phases involving aniline (S-aniline), 1-aminonaphthalene (S-1-ami-naph), 1-aminoanthracene (S-1-ami-anth) and 1-aminopyrene (S-1-ami-py) were synthesized based on full porous particles (FPP) silica, which were not end-capped for providing extra electrostatic interaction. Retention mechanism of these phases in SFC was investigated using a linear solvation energy relationship (LSER) model. The aromatic stationary phases with five positive parameters (a, b, s, e and d+) can provide hydrogen bonding, π-π, dipole-dipole and cation exchange interactions, which belong to the moderate polar phases. The LSER results obtained using routine test solutes demonstrated that the aforementioned interactions of four aromatic stationary phases were influenced by the type and bonding density of the ligand, but to a certain extent. Furthermore, the LSER data verified that the S-1-ami-anth column based on full porous particles silica had higher cation exchange capacity (d+ value), compared to the commercialized 1-AA column (based on the ethylene-bridged hybrid particles). The relationship between the d+ value and SFC additive was quantitatively proved so as to regulate electrostatic interaction reasonably. This value was greatly increased by phosphoric acid, slightly increased by trifluoroacetic acid and formic acid, but significantly reduced by ammonium formate and diethylamine. Taking the S-1-ami-naph column as an example, better peek shape of the flavonoids was obtained after the addition of 0.1 % phosphoric acid in MeOH while isoquinoline alkaloids were eluted successfully within 11 min after adding 0.1 % diethylamine in MeOH. Combined with the unique π-π interaction and controllable electrostatic interaction, the aromatic stationary phases in this study have been proven to have expandable application potential in SFC separation.

A C4-modified bipyridinium multi-mode stationary phase for reversed phase, hydrophilic interaction and ion exchange chromatography

A novel C4-modified bipyridinium stationary phase (Sil-DPC4) was prepared and characterized by elemental analysis (EA) and Fourier transform infrared spectrometry (FT-IR) and further investigated for multi-mode liquid chromatography. The chromatographic performances of Sil-DPC4 were evaluated by reversed-phase chromatography using polycyclic aromatic hydrocarbons (PAHs), phenylamines and phenols, hydrophilic interaction chromatography using nucleosides and nucleobases, and ion exchange chromatography using inorganic ions and organic ions. The effects of the acetonitrile content, salt concentration and pH value of the mobile phase on the retention of Sil-DPC4 were also investigated. Sil-DPC4 showed multiple retention mechanisms including π-π, hydrophobic and electrostatic interactions for PAHs, phenylamines and phenols compared with a dipyridine modified silica stationary phase (Sil-DP) and C18 in RPLC, faster separation for nucleosides and nucleobases compared with Sil-DP, and higher hydrophilicity than HILIC in HILIC, and stronger retention and better separation ability for inorganic ions and organic ions in comparison to Sil-DP in IEC. Besides, Sil-DPC4 was used successfully to detect iodide in artificial seawater and had the potential to analyze radionuclide iodine-131 in seawater. In conclusion, multiple retention mechanisms of Sil-DPC4 could make it have potential applications in complex samples.

Effect of stereoconfiguration of aromatic ligands on retention and selectivity of terphenyl isomer-bonded stationary phases

The structure of ligands has a significant influence on the separation properties of alkyl and aromatic phases in reversed-phase liquid chromatography. Compared with alkyl phases, the effect of stereoconfiguration of aromatic ligands on the retention and selectivity of stationary phases has rarely been addressed. To illustrate the issue, three terphenyl isomer-bonded stationary phases were prepared via the coupling chemistry of isocyanate with terphenyl amine isomers, 3,4-diphenylaniline, 2,4-diphenylaniline and 4-amino-p-terphenyl, respectively. The retention behaviors of stationary phases were assessed in terms of retention strength, selectivity, hydrophobic and π-π interaction by five kinds of solutes. It is found that the selectivity towards the solutes is slightly larger on the branched m-terphenyl-bonded phase (m-π³) than o-terphenyl-bonded phase (o-π³) but is significantly improved on the chain p-terphenyl-bonded phase (p-π³). The results can be interpreted by the ease self-adjustment of the conformation of the chain p-terphenyl ligand and the smaller steric effect of p-π³ towards the insertion of solutes into the ligand brushes. In addition, the p-π³ yields excellent selective separation towards aromatic solutes. These findings are of significance in the design of aromatic stationary phases.

Non-porous silica support covalent organic frameworks as stationary phases for liquid chromatography

A new strategy using non-porous silica (NPS) spheres as the support and covalent organic frameworks (COFs) as the porous functional shell for liquid chromatography was developed to ensure the independent effect of the COFs on the separation. As a proof of concept, NPS@TPB-DMTP was prepared for liquid chromatographic analysis using 1,3,5-tris(4-aminophenyl)benzene (TPB) and 2,5-dimethoxy-1,4-benzenedicarboxaldehyde (DMTP) as monomers by in situ polymerisation on the surface of NPS. It is a new way of developing COF-based stationary phases, which will be helpful in understanding what effect the COFs will have on separation.