Octadecyl-bonded stationary phase,

Oats, p-glucan quantitation, 753 (table) Octadecyl-bonded stationary phase, see C18 columns... 

Development of fast, accurate, and reproducible high-performance liquid chromatography (HPLC) methods has offset the use of traditional open-column and TLC methods in modern chlorophyll separation and analysis. A number of normal and reversed-phase methods have been developed for analysis of chlorophyll derivatives in food samples (unit F4.4), with octadecyl-bonded stationary phase (C]8) techniques predominating in the literature (Schwartz and Lorenzo, 1990). Inclusion of buffer salts such as ammonium acetate in the mobile phase is often useful, as this provides a proton equilibrium suitable for ionizable chlorophyllides and pheophorbides (Almela et al., 2000). 

Dubruc et al. reported the analysis of vincamine in plasma. Analysis was performed on an octadecyl bonded stationary phase. Large volumes (0.5 ml) of the sample in a non-eluting solvent (0.02 M aqueous potassium phosphate) were used for automatic operation of the analysis. 

Hoffman and Liao (160) observed apparent deviations from linearity in plots of the logarithm of retention factor versus carbon number for normal alcohols which yere chromatographed in acetonitrile-woici tnixiurcs rich in acetonitrile. The results have been explained in terms of normal phase" interactions with the otherwise nonpolar stationnry ph isc, i.e., the alcohols werei assumed to form hydrogen bonds or otherwise interact with residual silanols at the surface of octadecyl silica stationary phase. 

This protocol focuses on the analysis of chlorophyll a and b, and the more nonpolar derivatives, including pheophytins and pyropheophytins. An octadecyl-bonded, reversed-phase stationary phase is used with a methanol/water mixture and ethyl acetate mobile phases in a gradient elution to provide rapid and complete separation of the major chlorophyll derivatives in 25 to 30 min. This is coupled with traditional UV/visible spectrophotometric detection at 654 nm to selectively screen these photosynthetic pigments in food and plant tissues. 

Yu, J. El-Rassi, Z. Reversed-phase Uquid-chromatography with microspherical octadecyl-zirconia bonded stationary phases. J. Chromatogr. 1993, 631, 91-106. 

Recently, in HPLC separations of fullerenes, the octadecyl silica (ODS) bonded stationary phase has been extensively used. " Both polymeric and monomeric ODS phases have been used, and these provide a higher selectivity than other phases. Figure 32F-5 shows the preparative separation of whole soot extract and a higher fullerenes fraction on a polymeric ODS column. These were among the first separations of the individual higher fullerenes. Note the excellent resolution compared with the size-exclusion separation of Figure 32F-4. 

This chromatographic mode usually involves aliphatic chemically bonded stationary phases with, for example, octyl, octadecyl, or phenyl ligands. The mode of chromatography also is characterized by a mixed mechanism of solute retention Solute molecules interact specifically with the residual active sites of the silica matrix, whereas their interactions... 

Most commonly, liquid-liquid partition chromatography is conducted with a bonded stationary phase covalently attached to silanol groups on the silica surface (Table 22-3). The octadecyl (Cig) stationary phase is, by far, the most common in HPLC. The Si—O—Si bond that attaches the stationary phase to the silica is stable only over the pH range 2-8. Strongly acidic or basic eluents generally cannot be used with silica. 

Reverse phase HPLC describes methods that utilize a polar mobile phase in combination with a nonpolar stationary phase. As stated above, the nonpolar stationary phase structure is a bonded phase—a structure that is chemically bonded to the silica particles. Here, typical column names often have the carbon number designation indicating the length of a carbon chain to which the nonpolar nature is attributed. Typical designations are C8, C18 (or ODS, meaning octadecyl silane), etc. Common mobile phase liquids are water, methanol, acetonitrile (CH3CN), and acetic acid buffered solutions. 

Many manufacturers sell the same types of stationary phase materials,6 but the most popular stationary phase materials, e.g. octadecyl-bonded silica gels, from different sources, even from the same manufacturer, often demonstrate different retention capacities and selectivities. Such differences are due to the aggressive reactivity of the silylation method and the different bonding reactions that are used, as described earlier. 

Two types of system are used for ion-pair liquid chromatography. When polar stationary phase materials, such as silica gel, are used an ion-pair partition mechanism is applied. When non-polar stationary phase materials, such as octadecyl-bonded silica gel and polystyrene gel, are employed a paired-ion adsorption mechanism is involved. The former is called normal-phase ion-pair partition liquid chromatography, and the latter is called reversed-phase ion-pair liquid chromatography. 

Example 2 Chromatography of nitroaniline isomers. The elution order of the nitroaniline isomers was ortho, meta, and para in normal-phase liquid chromatography using H-butanol-w-hexane mixtures as the eluent, when the stationary phase material was either silica gel, alumina, an ion-exchanger, polystyrene gel, or octadecyl-bonded silica gel. The results indicate that the separation of these compounds can be performed on a range of different types of stationary phase materials if the correct eluent is selected. The best separation will be achieved by the right combination of stationary phase material and eluent.68... 

Enthalpy can be measured by liquid chromatography where enthalpy is a slope of the relationship between In k and the inverse value of the absolute temperature. A schematic diagram is shown in Figure 6.7. The slope depends upon the solutes being retained by the same liquid chromatographic mechanism. An example is given in Table 6.4. The results, measured on an octadecyl-bonded vinyl alcohol copolymer gel, did not show a simple linear relationship. This is due to a conformation change of the octadecyl-bonded vinyl alcohol copolymer gel stationary phase material, which has a phase transition point at about 33 °C. 

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. 

Ixmaceous bonded phases such asjoetyl and octadecyl silica are relatively stable in contact with most aqueous eluents having a pH value less than 8. Nevertheless, funJter improvement in the stability of such stationary phases would be highly desirable in order to obtain columns of extended operational life. 

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