Octadecyl-silica

Strong cationic-exchange extraction and reversed-phase extraction (eliminates ion pairing when used in place of octadecyl silica. 

RP-HPLC has been employed for the determination of flavonoids and other phenolic compounds in cranberry juice. The neutral and acidic analytes were preconcentrated octadecyl silica SPE cartridges conditioned with distilled water (neutral analytes) or with 0.01 M HC1 (acidic compounds). Hydrolysis of samples was carried out in aqueous methanol solution acidified with 6 M HC1 at 35°C for 16h. Chromatographic separation was performed in an ODS column (150 X 4.6mm i.d. particle size 5/.an). Solvents A and B were water-acetic acid (97 3, v/v) and methanol, respectively. The gradient started with 0 per cent B (flow rate, 0.9 ml/min), reached 10 per cent B in lQmin (flowrate, 1.0 ml/min) and increased to 70 per cent B in 40min (flowrate, 1.0 ml/min). Analytes were detected at 280 and 360 nm. Some typical chromatograms are presented in Fig. 2.71. The concentrations of flavonoids and phenolic acids are compiled in Table 2.69. It was stated that the SPE-HPLC procedure makes possible the simultaneous determination of phenolic compounds and flavonoids, therefore, it can be employed for the measurement of these classes of analytes in other fruit juices [188],... 

Hilhorst, M. J., Somsen, G. W., and de Jong, G. J. (2000). Capillary electrochromatography of basic compounds using octadecyl-silica stationary phases with an amine-containing mobile phase. /. Chromatogr. A 872, 315—321. 

Jia et al. (2005) developed a two-dimensional (2-D) separation system of coupling chromatography to electrophoresis for profiling Escherichia coli metabolites. Capillary EC with a monolithic silica-octadecyl silica column (500 x 0.2 mm ID) was used as the first dimension, from which the effluent fractions were further analyzed by CE acting as the second dimension. Multi-dimensional separations have found wide applications in biomedical and pharmaceutical analysis. 

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. 

The effect of pore size on the retentive capacity of various octadecyl silica bonded phases is illustrated in Fig. 10. Exclusion effects become appreciable when bulky octadecyl groups (see Table V) are attached to the pore wall in 6 nm pore diameter silica (Si-60). As a result, the stationary... 

Fio. 12. Graph illustrating the dependence of the logarithm of retention factor for aromatic hydrocarbons on the carbon load of octadecyl silica bonded phases prepared from Par-tisil with octadecyhrichlorosilane. Mobile phase methanol-water (70 30) eluitest A, benzene A, naphthalene , phenanthrene , anthracene O, pyrene. Reprinted with permission from Herndon t al. (70). 

Pio. 15. Graph illustrating the dependence of the reduced plate height on the carbon load of octadecyl silica uuionary phase, prepared Aom Fsitisil with octadecyltrichloro-silane. Ehieiit. methanol-water eiuite, polyey aromatic hydrocarbons retention foctor, 4. Prom the dM of Henman er of. (7 5). 

Fio. 26. Methylene group selectivity, ocn,i of several hydroorganic mobile phases when octadecyl silica stationary phase is used. The selectivity is the ratio of the retention factor of a member of a homologous series to that of another member which differs in having one less methylene group. The solvents shown here are (A) acetone, (B) acetonitrile, and (C) methanol. The dau were taken at ambient temperature and the selectivity values are plotted on a logarithmic scale. Reprinted with permission ftom Kaiger et al. (/4S).. ... 

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. 

Fio. 36. Vloi t Hoff plou of the retention bctors of aromatic acids in reversed-phase chromatography using octadecyl silica as the stationaiy phase and neat aqueous 30 taM NaHiPO buffer (pH 2.0) (open symbols), or the same buffer containing 696 (v/v) of aceloni ti (closed symbols) as the eluent. Column S imSpherisorbODS, 230 x 4.6 mm. Eluites 3.4xlihydroxymandelic acid (O. ) 4 hydroxymandelic acid ( , ) 4-hydroxyphenylacetic acid (7. ) 3,4-dihydroxyphenylacetic acid (A, A). Reprinted with permission from Me-lander tt at. U77). 

It is interesting to note that in all cases studied, the value of k of mixed solveiils showed a dependence on the solvent coiiiposilioii vei similai lo that illusiratcd by curve a in bigs. 39a and ii. The plots have been ob tained from analysis of RPC data with octadecyl silica as the stationary... 

Fio. 51. Dependence of retention of catecholamines on volume percent acetonitrile in hetaeric chromatography. The ehient is water-acetonitrile at the volume percent indicated containing 0.2% (v/v) sulfuric acid and 0.1% (w/v) sodium dodecyl sulfote. The catecholamines separated are noradrenaline (NA), adrenaline (A). L-3,4-dihydroxyphenylalanine (LD), normetanephrine (NMA), dopamine (DA), metadrenaline (MA), and 3-methoxytyramine (MDA). Column 5- tm octadecyl silica treated with triroethylchlorosilane, 125 X 5 mm i.d. Reprinted with permission from Knox and Jurand (223). ... 

Fio. 35. HPLC record of a standard mixture of retinol and retinyl esters. Conditions column, 10 un octadecyl silica flowrate, I ml/min . mobile phase, (A) CHtOH/58.9 x I0- Af [Ag ], (B) CHsOH/23.5 x lO" W [Ag ], (C) CH,OH detection 330 hm. Peak identity (I) retinol (2) retinyl propionate (3) retinyl linoleate (4) retinyl lauratc (S) retinyl oleate (6) retinyl myristate (7) retinyl palmitate and (8) retinyl stearate. Reprinted with permission from DeRuyter and DeLeenheer (242), Anal. Chem. Copyright 1979 by the American Chemical Society. 

Fio. 60. Chromatogram of acidified urine extract. Column 5>/ m octadecyl silica, 25 cm X 4.6 mm i.d., temperature, 70 C flowrate, 2.0 ml/min. Gradient elution from 0.1 M phosphaM buffer, pH 2.1, with acetonitrile to about 4096 (v/v) organic solvent concentration. Sample size 10 containing the extract of 100 of urine. Reprinted with permission from Horvath et al. (3S4). 

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