Preparation and evaluation of C18-bonded 1-µm silica particles for pressurized capillary electrochromatography

Submicron Nonporous Silica Particles for Enhanced Separation Performance in pCEC

Applications of submicron-scale particles are of rising interest in separation science due to their favorable surface-to-volume ratio and their fabrication of highly ordered structures. The uniformly dense packing beds in columns assembled from nanoparticles combined with an electroosmotic flow-driven system has great potential in a highly efficient separation system. Here, we packed capillary columns using a gravity method with synthesized nanoscale C18-SiO₂ particles having diameters of 300-900 nm. The separation of small molecules and proteins was evaluated in the packed columns on a pressurized capillary electrochromatography platform. The run-to-run reproducibility regarding retention time and peak area for the PAHs using a column packed with 300 nm C18-SiO₂ particles were less than 1.61% and 3.17%, respectively. Our study exhibited a systematic separation analysis of small molecules and proteins based on the columns packed with submicron particles combined with the pressurized capillary electrochromatography (pCEC) platform. This study may provide a promising analytical approach with extraordinary column efficiency, resolution, and speed for the separation of complex samples.

Silica spheres coated with C18-modified gold nanoparticles for capillary LC and pressurized CEC separations

Nonporous monodispersed silica spheres of 1.3 microm were coated with gold nanoparticles (AuNPs) and subsequently coated with n-octadecanethiol. By transmission electron microscopy analysis, the average diameter of the AuNPs on the silica spheres was determined to be 12 nm. The chromatographic and electrochromatographic properties of self-assembled n-octadecanethiol AuNP-coated silica microspheres (C18-AuNPs-SiO2) were investigated using a group of nonpolar PAHs. The stationary phase appears to display a characteristic reversed-phase behavior. Higher separation efficiency and shorter separation times were obtained using pressurized CEC (pCEC) compared with capillary LC (CLC). A maximum column efficiency of about 2.5x10(5) plates per meter and less than 18 min separation time for benzene were obtained in pCEC while only 66 507 plates per meter and an analysis time of nearly 100 min were observed in CLC mode. A chemical stability test of the C18-AuNPs-SiO2 stationary phase under extremely high and low pH conditions demonstrated that it is stable at pH 12 and 1 for at least 60 h. The results confirm that C18-AuNPs-SiO2 possesses a high rigidity to withstand high packing pressures and can be used as a good stationary phase for CLC and pCEC.

Pressurized capillary electrochromatographic assay of trimethoprim impurities using 1 microm particle-based columns

One micrometre silica particles, derivatized with C18, were electrokinetically packed into a 75-microm-i.d. capillary. The resulting column was evaluated for the separation of trimethoprim (TMP) and its impurities using pressurized capillary electrochromatography (pCEC), starting from a capillary liquid chromatographic (CLC) separation. These samples require gradient elution when separated by high performance liquid chromatography (HPLC), but with the new columns isocratic elution suffices for their separation by CLC or pCEC. Only 70,000 theoretical plates/m for impurity C were achieved using CLC mode at relative low pressure (78 bar) although very small particles were utilized. When a voltage above 2 kV (50 V/cm) was applied, unknown peaks appeared, which was assumed due to an electrophoretic effect with the unknown peaks resolving as a result of the applied voltage. In order to minimize these unfavorable contributions, only a low voltage was applied, still leading to higher separation performances and shorter separation times than in CLC. The optimal analyzing conditions in pCEC included a pressure of 78 bar, an applied voltage of 1 kV, and a mobile phase consisting of 80 mM sodium perchlorate (pH 3.1)/methanol (60/40, v/v). These conditions were used to separate and quantify four major impurities in TMP within 22 min. The obtained calibration curves were linear (r>0.9980) in concentration ranges between 0.005 and 0.1 mg/mL for impurities A and C; 0.02 and 0.10 mg/mL for impurity F; and 0.01 and 0.10 mg/mL for impurity H. The detection limits (S/N=3) for impurities A, C, F, and H were 0.52, 0.84, 3.18, and 2.41 microg/mL, respectively. The calibration curves were successfully applied to analyze spiked bulk samples, with mean recoveries ranging from 92% to 110%. The developed method can therefore be considered simple, rapid, and repeatable.