Chromatography argentation

The first publications on the separation of lipids on silvernitrate-impregnated adsorbent columns [207] and layers [6, 136, 208] appeared in 1962/63. Argentation-TLC quickly gained favour and has become one of the indispensable aids in the chemistry of lipophilic natural products. 

The Brockmann rules (see p. 201) for pure adsorption chromatography are valid with certain limitations for argentation (adsorption) chromatography too. Hydrocarbons are eluted before esters and these before aldehydes, alcohols and acids (cf. Fig. 129, p. 375) the subfractionation of 

Saturated and unsaturated compounds are not separated on boric acid-containing adsorbents. From Fig. 142d it is seen that saturated and unsaturated dihydroxy-esters are fractionated according to both the number of double bonds and the configuration of the hydroxyl groups, when chromatographed on a silica gel G layer, impregnated with both silver nitrate and boric acid. 

On adsorbent layers containing silver nitrate, cyclic olefines and their derivatives migrate less far than the corresponding straight chain compoimds. Thus, for example, methyl chaulmoograte and methyl oleate can be clearly separated [124]. 

Lipids are applied to the layers as 0.1 or 1% solutions in hexane or hexane-diethyl ether (50 + 50). 

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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. 

Metal Ion-Solute. Transition metals have been loaded onto classical LC columns and used for selective separations (21). More recently successful attempts have been made to control selectivity by metal additions directly to the mobile phase (22). This is a form of ion pairing, but the pairing agent is polar instead of hydrophobic. Silver ion has been used for the separation of olefins (23). M. deRuyter and A. deLeenheer have employed argentation chromatography to resolve difficult mixtures of Retinyl esters (24). 

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). 

Synthesis has been useful in the chemistry of phenolic lipids for structural confirmation and in enabling structure/property correlations to be made by the general applicability of syntheses to a range of non-natural isomers. With the polyunsaturated constituents synthesis is valuable since either argentation chromatography or preparative HPLC have usually proved laborious on the natural products and on transformed isomeric compounds required for evaluatory purposes in structure/activity studies. 

Silver ion (or argentation) chromatography is based on the characteristic property of unsaturated organic compounds to form transient charge-transfer complexes with... 

Retention in argentation chromatography is thought to occur by two separate but simultaneous mechanisms firstly through interac-... 

Argentation chromatography has been used to obtain an efficient separation of a series of closely related C27 sterol precursors of cholesterol which differed by the number and location of carbon-carbon double bonds (Pascal et al., 1980). In this chromatographic separation the stationary phase was an alumina column which had been loaded with silver nitrate and the mobile phase contained an initial solvent mixture of hexane-toluene (9 1) which was changed to (6 4) approximately half-way through the separation (Pascal et al.,... 

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. 

Mobile phases for argentation chromatography. It is necessary to add acetonitrile to the CO2 to facilitate elution of TG. Acetonitrile modifies the interactions between the analytes and the stationary phase and also improves the solubility of the analytes in the mobile phase. Although the solubility of TG in supercritical CO2 is, in general, good, the presence of a polar modifier may diminish the risk of analyte precipitation as the pressure is released in the restrictor. 

Demirbiiker, M. and Blomberg, L. G. (1991) Separation of triacylglycerols by supercritical fluid argentation chromatography. J. Chromatogr., 550, 765-74. 

Schmid, H.H.O. Baumann, WJ. Cuhero, J.M. Mangold, H.K. Fractionation of lipids by successive adsorption and argentation chromatography of adjacent layers. Biochim. Biophys. Acta 1966,125, 189. 

For the isolation of constituents by argentation chromatography, a modified capillary mat system was used 8). For biosynthetic studies, seeds were grown in Petri dishes (9). 

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