Octyl-bonded phase

Bereznitski, Y., Jaroniec, M., Kruk, M. and Buszewski, B. Adsorption characterization of octyl bonded phases for high performance liquid chromatography, J. Liq. Chromatogr. Related Technol, 1996, 19, 2767-2784. 

Octadecyl (octyl) bonded phase with low percentage of free silanol groups ION-SUPPRESSION MODE methanol (acetonitrile) -water containing ca. 0.01 - 0.1 M phosphate buffer, ammonium carbonate or sodium acetate (pH 4-7). 

Figure 9. Concentration of bonded silane determined as a function of reaction time for the fluoride derlvatlzatlon of both monomeric and polymeric octyl bonded phases. The dashed line represents 100% recovery as determined by elemental analysis.

Figure 11. The structures of species identified by GC-MS in the HF digest of polymeric octyl bonded phases. The upper two products were the major components, comprising about 85T of the total silane.

Therefore, a multi-octyl-bonded phase was constructed to increase the contact surface area, as shown in Figure 6.23. The model phase consisted of 1237 atoms, 1296 bonds and 8471 connectors including 20 dimetho y-octylsilanes and 6 trimethylsilanes. The correlation coefficient was subsequently improved ... 

Figure 6.23 Docking of 4-hydro)y-3-metho3y-cinnamic acid on a modified octyl-bonded phase. White, light-gray, gray, and blaek balls represent hydrogen, earbon, silicon, and ojygen, respectively. The atom size is 20% of the original size for the phase for clear demonstration.

Berthod studied the adsorption of SDS on five different Hypersil stationary phases [11]. Five 10 cm x 4.6 mm i.d. LC columns were specially selected for the study. All experiments were done in a thermostatic bath at 30°C. The amount of SDS adsorbed on the phases was determined by crosschecking the results obtained with (i) the breakthrough method, (ii) controlling the concentration of the effluent solution, and (iii) by the stripping method. The 3 results were in agreement within an 8% experimental error. Figure 4.2 presents the SDS adsorption on bare Hypersil , CPS Hypersil , a cyanopropyl bonded phase, SAS Hypersil , a methyl bonded phase, MOS Hypersil , an octyl bonded phase and ODS Hypersil , an octadecyl bonded phase. The physicochemical properties of these phases are listed in Table 4.1. A very differing behavior was obtained... 

Variations in retention and selectivity have been studied in cyano, phenyl, and octyl reversed bonded phase HPLC columns. The retention of toluene, phenol, aniline, and nitrobenzene in these columns has been measured using binary mixtures of water and methanol, acetonitrile, or tetrahydrofuran mobile phases in order to determine the relative contributions of proton donor-proton acceptor and dipole-dipole interactions in the retention process. Retention and selectivity in these columns were correlated with polar group selectivities of mobile-phase organic modifiers and the polarity of the bonded stationary phases. In spite of the prominent role of bonded phase volume and residual silanols in the retention process, each column exhibited some unique selectivities when used with different organic modifiers [84],... 

Water/SPM Collect water samples on disposable octyl-bonded silica solid-phase extraction columns dry elute with hexane/ether SPM collected by continuous flow centrifugation extract with acetone/water/benzene GC/ECD GC/MS O.lpg/L (water) 0.1 mg/kg (suspended particulate) 83 (water) 82 (suspended particulate) Ritsema et al. 1989... 

The best octyl- and octadecyl-bonded silica gels should be chemically stable in highly basic solutions. The well-bonded phases give good results in the inertness... 

Several improved stationary phase materials have been synthesized for reversed-phase liquid chromatography. One material is vinyl alcohol copolymer gel. This stationary phase is quite polar and chemically very stable however, it demonstrated a strong retention capacity for polycyclic aromatic hydrocarbons.45 9 Although stable octadecyl- and octyl-bonded silica gels have been synthesized from pure silica gel50,51 and are now commercially available, such an optimization system has not yet been built. Further experiments are required to elucidate the retention mechanism, and to systematize it within the context of instrumentation. 

The nomenclature of the RP is not consequent. The RP most often used contains octyl (RP C8) or octadecyl (RP C18) groups. There is no differentiation even when two methyl groups are introduced additionally with the silane (as with monofunctional silanes) or only one (difunctional) or none (trifunctional silane). Some manufacturer use silanes with bulky side groups (e.g., isopropyl groups) to improve the hydrolytic stability of the bonded phases, but here also, only the longest alkyl group is used in nomenclature. RP C8 and RP C18 are the work horses in HPLC. Shorter chains (RP4) are used in protein separations, and special selectivity can be obtained with bonded phenyl, cyano, amino or fluoro groups. 

Several different analytical and ultra-micropreparative CEC approaches have been described for such peptide separations. For example, open tubular (OT-CEC) methods have been used 290-294 with etched fused silicas to increase the surface area with diols or octadecyl chains then bonded to the surface.1 With such OT-CEC systems, the peptide-ligand interactions of, for example, angiotensin I-III increased with increasing hydrophobicity of the bonded phase on the capillary wall. Porous layer open tubular (PLOT) capillaries coated with anionic polymers 295 or poly(aspartic acid) 296 have also been employed 297 to separate basic peptides on the inner wall of fused silica capillaries of 20 pm i.d. When the same eluent conditions were employed, superior performance was observed for these PLOT capillaries compared to the corresponding capillary zone electrophoresis (HP-CZE) separation. Peptide mixtures can be analyzed 298-300 with OT-CEC systems based on octyl-bonded fused silica capillaries that have been coated with (3-aminopropyl)trimethoxysilane (APS), as well as with pressurized CEC (pCEC) packed with particles of similar surface chemistry, to decrease the electrostatic interactions between the solute and the surface, coupled to a mass spectrometer (MS). In the pressurized flow version of electrochromatography, a pLC pump is also employed (Figure 26) to facilitate liquid flow, reduce bubble formation, and to fine-tune the selectivity of the separation of the peptide mixture. 

Derivatized-silica bonded phases are available in disposable 1-, 3-, and 5-mL columns as follows nonpolar phases (ethyl, octyl, cyclohexyl,... 

If solutes dissolve only in nonpolar or weakly polar solvents, the decision tree suggests that we try reversed-phase chromatography. Our choices include bonded phases containing octadecyl (CJg), octyl, butyl, ethyl, methyl, phenyl, and cyano groups. 

The advances in column and instrument technology have significantly enhanced HPLC performance in recent years. Results comparing the effects of various column packings on TG separation by RP-HPLC were presented by El-Hamdy and Perkins (87). Six commercially packed columns produced by different manufacturers were used PARTISIL ODS-1 and ODS-2 octadecyl-bonded silica of 10-/rm partical size, ZORBAK-ODS octadecyl-silica of 6-7-/rm diameter (250 X 4.6-mm ID), 5-/rm octyl-bonded spherical silica LC-8, 5-//m methyl-bonded spherical silica LC-1, and a 5-/rm octadecyl-bonded spherical silica LC-18 (150 X 4.6-mm ID). The mobile phase employed consisted of mixtures of methanol/acetone/isopropanol/acetonitrile ranging from l 0 3 4to 1 6 3 4. Triglycerides were solubilized in either THF or acetone at 100 mg/ml for each compound. 

El-Fizga (108) developed a simple, rapid method for the detection of oils high in linoleic acid in olive oil by RP-HPLC and a simple authenticity factor and a derived equation to determine the extent of adulteration with a one short chromatographic step, completed in less than 15 min. They used two 150 X 4.5-mm ID stainless steel columns packed with an octyl-bonded silica stationary phase (Supelcosil-LC 8) (Supelco. Bellefonte, PA, USA) and a differential reffactometric detector. The isocratic mobile phase was acetone-acetonitrile (70 30, v/v) (Table 5). 

The physical structure of the stationary phase depends on the compatibility of the solvent with the bonded n-alkyl chains. Compatible nonpolar solvents tend to promote extension of the chains, allowing full penetration by the solvent. Conversely, fairly polar solvents tend to promote collapse of the chains upon each other, allowing negligible solvent penetration. The stationary phase therefore has the ability to adjust itself to maintain a relatively nonpolar character (113). Retention on monomeric bonded phases with octyl (C8) or longer chains are dominated by a partitioning-like mechanism (114). 

---