Chromatographic silica, characterization

A soln of the substrate (a terminally protected Ser or Thr derivative) (5 mmol), CDI (0.81 g, 5 mmol) and TEA (0.7 mL, 5 mmol) in dry THF (20 mL) was stirred at rt for 4-6 h. The solvent was evaporated in vacuo and the residue chromatographed (silica gel 60, benzene/acetone 8 1) to give DHA derivatives which crystallized (EtOAc/petroleum ether or Et20/petroleum ether mixtures). Characterization of the DHA derivatives is given in Table 4. 

Z-SeC(Dpm)-ODpm (200 mg, 0.3 mmol) was dissolved in acetone (10 mL) and the soln cooled in an ice bath. To this soln was added 30% H202 (0.5 mL) and the mixture stirred for about 45 min at 4°C. Then, the mixture was diluted with ice-cold H20 (50 mL) and extracted with EtOAc. The organic layer was separated, washed with H20 (3 x 25 mL), dried and concentrated in vacuo. The resultant oil, revealed two compounds by TLC (benzene/EtOAc 9 1) and was column chromatographed (silica gel). Z-AAla-ODpm was eluted first (hexane/benzene 1 3), then the starting material was eluted (benzene). Z-AAla-ODpm crystallized from EtOAc/hexane and characterized by IR and NMR yield 74 mg (62%) mp 78.5-79.5 °C. However, if the mixture was kept at rt for 15 min in the presence of H202 prior to workup, the starting material was completely converted into the AAla derivative. 

High-Speed Size Characterization of Chromatographic Silica by Flow/Hyperlayer Field-Flow Fractionation, S. K. Ratanathanawongs and J. C. Giddings, J. Chromatogr., 467, 341 (1989). 

One important application of Fl-FFF is the determination of the particle size distribution of chromatographic silica for HPLC packings [226,428,429] which, in combination with S-FFF, allows characterization of the porosity of the samples and particle size distribution. 

In other work carried out on two different occasions, both sedimentation FFF (14) and flow FFF (15) were used to characterize the size distribution of chromatographic silicas having particles ranging up to approximately 20 pm in diameter. In more recent work (16), sedimentation FFF was used to obtain both the size and apparent density (thus giving the porosity and pore volume) of silica support particles used in chromatography. 

As suggested earlier, both sedimentation and flow FFF (in steric or hyperlayer modes) can be used to characterize particles in the size range of chromatographic silica. However, the two FFF approaches are more complementary than they are redundant. This is once again (as found also in the normal mode) a consequence of the different force laws that control particle behavior in the two systems. The driving force in flow FFF depends only on the Stokes diameter ds as shown in equation 8. Thus for spherical silica particles, the diameter and diameter distribution, and nothing more, is characterized by flow FFF. However, the force acting on particles in a sedimentation FFF channel is a function of both particle diameter d and the density difference Ap as shown by equation 7. Measurement by sedimentation FFF alone thus yields a mix of diameter and density information. However, if d can be established independently by other means (such as flow FFF or microscopy), then Ap can be obtained... 

In other work carried out on two different occasions, both sedimentation FFF [14] and flow FFF [15] were used to characterize the size distribution of chromatographic silicas having particles ranging up to approximately... 

As suggested earlier, both sedimentation and flow FFF (in steric or hyperlayer modes) can be used to characterize particles in the size range of chromatographic silica. However, the two FFF approaches are more complementary than they are redundant. This is once again (as found also in the normal mode) a consequence of... 

Pazourek, J. Chmelik, J. Characterization of chromatographic silica gel support particles by graviational field-flow fi ctionation. J. Microcol. Separ. 1997,9 (8), 611-617. 

Chromatographic Characterization of TTXs. The vast majority of reports have identified TTX and anhydro-TTX in bacterial cultures using HPLC, TLC, and GC-MS. Yasumoto et al. (30) showed that TTX-like substances extracted from a Pseudomonas sp. culture could bind to activated charcoal at pH 5.5 and be eluted with 20% ethanol in 1% acetic acid. In addition, HPLC analysis demonstrated TTX and anhydro-TTX-like fluorophors following strong base treatment. These compounds migrated on silica gel comparably to TTX and anhydro-TTX. Furthermore, when analyzed by electron ionization (EI)-MS and fast atom... 

De Jong, G. J. Optimization and characterization of silica-based reversed-phase liquid chromatographic systems for the analysis of basic pharmaceuticals./. Chromatogr. A 2000, 897,1-22. 

Lee et al. [30] described a micellar electrokinetic capillary chromatographic method for the determination of some antiepileptics including valproic acid. They used a fused silica capillary column (72 cm x 50 pm) and SDS as the micellar phase and multiwavelength UV detection. Reaction conditions, such as pH and concentration of running buffer were optimized. Solutes were identified by characterizing the sample peak in terms of retention time and absorption spectra. Recoveries were 93-105%. 

A variety of procedures were utilized to analyze this reaction mixture and to characterize a,10-diaminopolystyrene. Thin layer chromatographic analysis using toluene as eluent exhibited three spots with Rf values of 0.85, 0.09, and 0.05 which corresponded to polystyrene, poly(styryl)amine and a,w-diaminopolystyrene (see Figure 1). Pure samples of each of these products were obtained by silica gel column Chromatography of the crude reaction mixture initially using toluene as eluent [for polystyrene and poly(styryl)amine] followed by a methanol/toluene mixture (5/100 v/v) for the diamine. Size-exclusion chromatography could not be used to characterize the diamine since no peak was observed for this material, apparently because of the complication of physical adsorption to the column packing material. Therefore, the dibenzoyl derivative (eq. 5) was prepared and used for most of the analytical characterizations. 

The most common approach to chromatographic stationary-phase characterization is in terms of bulk-phase properties, such as percent carbon loading onto the silica substrate. This property together with the surface area of the substrate and the molecular characteristics of the bonded silane can be used to calculate the bonding density (A) of the chromatographic sorbent [60] ... 

Sandoval, I.E. and Pesek, I.I., Synthesis and characterization of a hydride-modified porous silica material as an intermediate in the preparation of chemically bonded chromatographic stationary phases, AnaZ. Chem., 61, 2067, 1989. 

Kazuhiro, K., Tanaka, N., Iwaguchi, K., Onishi, S., Jinno, K., Eksteen, R., Hosoya, K. and Araki, M., Chromatographic Characterization of Silica Cjg Packing Materials. Correlation between a Preparation Method and Retention Behavior of Stationary Phase./. Chromatogr. Sci., 27 721—728, 1989. 

Allen, D., and El Rassl, Z. (2004). Capillary electrochromatography with monolithic silica columns III. Preparation of hydrophilic silica monoliths having surface-bound cyano groups chromatographic characterization and application to the separation of carbohydrates, nucleosides, nucleic acid bases and other neutral polar species.. Chromatogr. A 1029, 239—247. 

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