Silica argentation

Trans fatty acids in liver of rats Silica AgNOj argentation 15... 

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. 

Like tertiary amines, tertiary phosphines are readily transformed into the corresponding oxides, phosphine oxides. Tributylphosphine in di-chloromethane is oxidized with ozone adsorbed on silica gel at -70 "C to tributylphosphine oxide in 92% yield [113]. Other oxidants used to transform phosphines into phosphine oxides are potassium peroxysulfate [205], argentic oxide [381 ], manganese dioxide [813], and barium manganate [S55] (equation 537). 

Monocarboxylic esters were further separated by argentation TLC [silica GF254 impregnated with 5% (w/v) silver nitrate] into saturated esters (Rf = 0.51), monounsaturated esters (R = 0.42), diunsaturated esters (Rf = 0.29), and poly unsaturated esters (Rf < 0.29) by development in n-heptane-diethyl ether-methanol (90 10 1, v/v/v) (19). 

Argentation chromatography, in which silver is used as a 7T complexing metal on a silica gel support, is usually employed for the separation of organic compounds with electron-donor properties due to the presence of unsaturated groups in the molecule of the analytes (see Table 2). 

Petroleum ether (30-60 deg C) Counter current chromatogFaphy in hexane/nitromethane Preparative TLC on Silica gel argentation TLC IR,GC-MS. GC (27,47)... 

Porto et al. methylated a Clusia floral resin extract, chromatographed the derived compounds over silica gel, rechromatographed using preparative argentation TLC (5% silver nitrate), and thereby isolated seven polyisoprenylated benzophenones [68], Lokvam et al. [74] and de Oliveira et al. [59] also methylated a crude extract before isolating chamones I, II (119, 116), and nemorosone 11 (89) from Clusia species. 

It is also possible to modify the adsorption properties of the silica gel by incorporating substances such as bases or buffers enabling coatings with accurately defined pH to be prepared. In a like manner silver nitrate can be added this admixture changes the adsorptive properties to permit increased discrimination and separation of unsaturated compounds, especially alkenes. The technique is commonly known as Argentation TLC. 

Argentation HPLC using silver nitrate-loaded silica stationary phases has been used for the analysis of the positional isomers of triglycerides including 2-unsaturated-l,3-disaturated (SUS), 1-un-saturated-2,3-disaturated (USS), fully saturated (SSS) and fully unsaturated (UUU) isomers (Smith et al., 1980 Hammond, 1981). In these separations the silica was loaded with 10% silver nitrate and the mobile phases used were either benzene (Smith et al., 1980) or toluene (Hammond, 1981). The resolution of the lipid species was found to... 

Argentation chromatography using chloroform to elute a silver nitrate-loaded silica stationary phase has also been used to resolve the cis and trans isomers of sterol acetates, although column efficiencies were reported to be poor (Colin et al., 1979). 

In summary, for the chromatographic resolution of sterols it is advisable that reversed phase techniques should be tried initially and in the event of co-elution normal phase chromatography on silica should be tried as an alternative option. For very specific separations where the aforementioned techniques have failed, either NARP or argentation chromatography could be attempted although it is probable that if reversed phase or normal phase failed to elicit a separation, the remaining techniques would also be unsuccessful. 

There are a few techniques related to normal phase chromatography that are worth mentioning. Silica can be coated with the salts of heavy metals. This results in unique selectivities for analytes that form complexes with the metal ions. An example of this is argentation chromatography (20). The silica is coated with silver nitrate, which gives it a spedal sel ivity for compounds with al atic double bonds. The technique has acquired a broad range of applications. However, while Ag -coated TLC plates are commercially available, HPLC columns have to be prepared by the user. Other metal salts can be used in a similar manner for different applications. 

TLC is commonly used for the separation of different classes of wax components or for analysis of monomers from cutin and suberin depolymerization. A typical separation is shown in Fig. 6.12. By such methods, it is possible to separate hydrocarbons, wax esters, primary alcohols, secondary alcohols and /8-diketones from plant waxes (von Wettstein-Knowles, 1979). Products of hydrogenolysis from cutin can be separated by TLC into alkan-l-ols, alkane-a,ft>-diols, Cis triols, Ci6 triols and Cis tetrols (Kolattukudy, 1980). Unsaturated components can be resolved by argentation-TLC (Tulloch, 1976) and threo or erythro diastereoisomers separated by boric acid/silica gel TLC (Eglinton and Hunneman, 1968). Straight-chain compounds can be preferentially removed from branched compounds as their urea complexes (Kolattukudy, 1980). 

Silver ion TLC has also been used for separation of a variety of substituted unsaturated fatty acids such as epoxy, hydroxy and halohydroxy fatty acids, and these have been reviewed by Morris and Nichols (1972). Wax esters and steryl esters exhibit similar polarities and do not separate on column chromatography or normal silica TLC. Kiosseoglou and Boskou (1990) have separated the wax esters from steryl esters by using argentation TLC. They have used silica gel plates impregnated with 10% silver nitrate, with developing solvent hexane/chloroform (7 3 vol./vol.). 

Argentation TLC (on silver nitrate-impregnated silica gel plates) and hexane-ethyl acetate-diisopropyl ether (2 2 1, v/v/v) as mobile phase were used to separate vitamins Ki, K2, K3, ubiquinone-6, ubiquinone-9, and ubiquinone-10 (Rp values 0.71, 0.63, 0.57, 0.42, 0.21, and 0.28, respectively). ° ... 

Most of the prenyUipids, such as chlorophylls, carotenoids, and prenylquinones, as well as tocopherols and vitamin Ki, which occur in plant hpid extracts, can be separated by TLC using sUica gel plates or special mixtures of silica gel with other adsorbents. But the compounds with one double bond per isoprene and others with a partially or fiiUy unsaturated isoprenoid chain can be separated efficiently by argentation TLC. The Rp values of selected hpophUic vitamins, their provitamins, and related compounds separated by argentation TLC using different mobile phases are listed in Table... 

Long-chain alkylphenols present in natural cashew nut shell liquid have been chromatographed on argentated silica gel G (10% w/w silver nitrate) with diethyl ether-light petroleum-formic acid (30 70 1) as eluent for the separation of unsaturated constiments. 

Pyka and Bober " separated a- and y-terpinene as well as a- and 3-pinene on silica gel and on silica gel impregnated with various percentages of silver nitrate. It was stated that argentation thin-layer chromatography is an efficient technique for separation of isomeric a- and y-terpinene as well as a- and (3-pinene. The topological indices based on... 

Reference may be made here to a procedure for separating mixtures of mono-olefinic fatty acids and for subsequently determining the structure of the components [11]. This method utilises two-dimensional TLC on silica gel, impregnated with paraffin and silver nitrate a similar procedure is described in [225]. A scheme for determination of the position and configuration of the double bonds in polyolefinic acids [170] depends on partial hydrogenation with hydrazine, separation of the reduction products through argentation-TLC and determination of... 

Compound class Adsorp- tion TLC hRf Solvents for argentation-TLC (5% AgNOa in silica gel G) Solvents for partition TLC in reversed phase (paraffin oil on kieselguhr G)... 

Fig. 149. Separation of the triglycerides of sunflower oil (Helianthus annuus) by combination of argentatic i -TLC and reversed phase partition-TLC [89]. A pre-fractionation of the triglycerides through argentation-TLC adsorbent silica gel G-silver nitrate (25%) solvent benzene-diethyl ether (80 + 20) time of run 3.5 h spray reagent 2, 7 -dichlorofluorescein in ethanol (UV light) amount 20 mg B separation of fractions 1—9 (A) through reversed phase partition-TLC stationary phase paraffin on kieselguhr G solvent acetone-acetonitrile (80 + 20) (80% saturated with paraffin), developed twice time of run 40 min each time visualisation aqueous-alcoholic a-cyclodextrin solution-iodine vapour amounts ca 12 [xg of each of the fractions 1—9 and 20 jxg sunflower oil...

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