Cyanopropyl-bonded silica

Haliclonacyclamine F (25), arenosclerin D (26), and arenosclerin E (27) have been recently isolated from the sponge Pachychalina alcaloidifera endemic in Brazil [26]. The alkaloids 25-27 were isolated from the cytotoxic, antibiotic, and antituberculosis MeOH crude extract of P. alcaloidifera by a series of separations on silica-gel and cyanopropyl-bonded silica-gel columns. The structures of compounds 25-27 were established by the same approach employed for the structural elucidation of haliclonacyclamine E (13) and arenosclerins A-C (14-16) [18], as well as by comparison with NMR data for this last series of alkaloids. The alkaloids 25-27 displayed moderate cytotoxic activity against SF295 (human CNS), MDA-MB435 (human breast), HCT8 (colon), and HL60 (leukemia) cancer cell lines. 

Smith and Sanagi reported the packed-column SFC of benzodiazepines (diazepam, lorazepam, lormetazepam, nordazepam, temazepam, es-trazolam, chlordiazepoxide, triazolam, cloxazolam, ketazolam, and lopra-zolam) with methanol-modified carbon dioxide as the mobile phase [29]. The effect of methanol concentration on separation was studied on three columns polystyrene-divinylbenzene, octadecylsilane, and cyanopropyl-bonded silica columns. They concluded that proportion of methanol has marked effect on the selectivity of compounds containing different functional groups. 

Cyanopropyl-bonded silica phases emanate from the early days of normal-phase liquid chromatography, although they can also be used in the RP mode. 

Due to the absence of hydrogen donor capabilities [31], cyanopropyl silica phases are less retentive in normal-phase liquid chromatography than under-ivatized silica or other NP packing materials. Therefore, very few applications have been reported that utilize cyanopropyl-bonded silica in the HILIC mode [32,33]. The limited number of applications may also be attributed to the mechanical instabiUty of cyanopropyl-bonded silica when operated with solvents of intermediate polarity. This instabihty is caused by a decrease in the adhesion of particles to each other that maintain the integrity of the column bed in either nonpolar or highly polar solvents [25]. Dinh et al. [34] performed a multivariate modeling of column selectivity by principal component analysis of chromatographic data from polar compounds of various structures on 20 commercially available HILIC columns and verified the low potential of cyanopropyl-bonded silica columns due to insufficient hydrophilicity. 

Table 7.3 Structural and technical properties of common cyanopropyl-bonded silica columns.

In the liquid chromatographic methods, separation of nitrofurans is generally carried out on nonpolar reversed-phase columns, the preferred sorbent being octadecyl bonded silica (Tabic 29.5). Polar columns containing cyanopropyl-based sorbents (164, 165) have also been used for die isocratic separation of nitrofuran residues isolated from edible animal products. A literature survey shows that there exists a clear preference for acidic mobile phases containing acetonitrile as the organic modifier (Fig. 29.5.1). 

Owing to their intrinsic basicity, marine invertebrate guanidine alkaloids have a rather polar behavior and their isolation from complex mixtures may be difficult. Isolation procedures frequently include chromatography on lipophilic Sephadex LH20, on reversed phase silica gel (such as Cig bonded, aminopropyl bonded, or cyanopropyl bonded), or even on ion-exchange resins. HPLC purification using acidic (TFA) or buffered eluents have frequently been employed. 

Polar bonded phases for normal-phase separations have recently gained popularity. These include the dihydroxypropyl propyl ether (diol), aminopropyl, cyanopropyl, and nitrophenyl bonded silicas. These phases are advantageous to silica because they are less active and, yet, produce similar interactions, require shorter equilibration times, and are influenced less by the water content of the mobile phase. The retention of most analytes upon the diol and amino phases is similar to that of the parent silica and alumnia, whereas the cyanopropyl and nitrophenyl phases generally show less retention. 

Chemically bonded silica gels with cyanopropyl, aminopropyl, and diol functional groups are also available and each has been used for normal-phase as well as reversed-phase separations. Polymeric-based packings are available and can be used for ion-exchange or reversed-phase applications. Cross-linked polymeric based packings have been combined with ion-exchange materials to create a more pH-stable stationary phase. Graphitized carbon... 

The most popular bonded phases for normal-phase chromatography are aminopropyl, cyanopropyl, and diol. Other polar bonded phases, including bonded ion exchangers, can be used as well. Another, less commonly used polar bonded phase is the nitrophenyl bonded phase. The retentivity of the aminopropyl and the diol phase is comparable to that of the oxides. Cyanopropyl and nitrophenyl phases have a smaller retentivity. Although differences in selectivity can be found for the different bonded phases, they are typically small. Thus the elution order of most members in a family of related compounds is largely preserved from packing to packing. For example, in a study of the elution behavior of steroids on silica, and aminopropyl and cyanopropyl bonded phases, an inversion in elution order was ol rved only for two to four pairs out of IS analytes (16). 

The structure of SDS and CTAB surfactant layer adsorbed on porous silica was studied by NMR. The C18 (Microsorb ODS, 5 pm, 6 nm pore diameter) and C8 (Microsorb) bonded phases were studied first [17]. The cyanopropyl bonded phase (Microsorb), was also studied [18]. Cross polarization (CP), magic angle spinning (MAS) with high power proton... 

Chemically bonded silicas include eilkyl-bonded (C4-C.18), perfluoropropyl-, cyanopropyl-, aminopropyl-, diolphenyl- and other radicals [4,... 

An ion-pair reagent was not wholly essential for separation, and it was found that the optimum concentration was probably below 0.01 mole/liter. Different chain lengths up to C4 caused minor modifications in the order of / / values for the eight acids. For the aromatic dicarboxylic acids in chloroform-tetrahydrofuran (2 1) on silufol (175), the Ry order found was isophthalic (0.5), terephtha-lic acid (0.49), 2-hydroxyethyl isophthalate (0.33), 2-hydroxyethyl-terephthalate (0.35), and the bis ester (both 0.18). Cj and cyanopropyl-bonded high performance silica gel layers in conjunction with ion-pair reagents have been employed to separate dihydroxybenzoic acids (175a,175b). 

Stationary phases SI silica gel, CN cyanopropyl silica, AM amino-bonded silica. DIOL diol-bonded silica, C-2 dimethyl silica. C-8 octyl silica, C-18 octadecyl silica, Ce cellulose. AL alumina, Po polyamide coated silica, I-P impregnated with paraffin. I-F impregnated with formamide, I-Ag impregnated with silver nitrate, 1-Pg impregnated with propylene glycol. 

Silica has often been modified with silver for argentation chromatography because of the additional selectivity conferred by the interactions between silver and Jt-bonds of unsaturated hydrocarbons. In a recent example, methyl linoleate was separated from methyl linolenate on silver-modified silica in a dioxane-hexane mixture.23 Bonded phases using amino or cyano groups have proved to be of great utility. In a recent application on a 250 x 1-mm Deltabond (Keystone Scientific Belief onte, PA) Cyano cyanopropyl column, carbon dioxide was dissolved under pressure into the hexane mobile phase, serving to reduce the viscosity from 6.2 to 1 MPa and improve efficiency and peak symmetry.24 It was proposed that the carbon dioxide served to suppress the effect of residual surface silanols on retention. 

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