Chapter 4 - Introduction - Page 115

     The ability of silver ion and some other metal ions to interact with double bonds (DBs) has been well known for a long time (Dobson et al., 1995; Guha and Janak, 1972; Devries, 1962; Morris, 1962).  Silver-ion chromatography, sometimes called argentation chromatography, is a separation technique based on the formation of weak reversible charge-transfer complexes between silver ions and DBs of unsaturated organic molecules.
     Silver-ion chromatography can be performed either in a planar arrangement (silver-ion thin layer chromatography, Ag-TLC) or on a column, which is often used for preparative purposes or silver-ion high-performance liquid chromatography (Ag-HPLC) used for high-resolution separations. Ag-TLC is a well-established technique in the analysis of lipids and other organic molecules containing DBs because it is cheap, simple to use, and applicable both on analytical and preparative scales. Ag-TLC has been reviewed in several works (Dobson et al., 1995; Momchilova and Nikolova-Damyanova, 2003); therefore it is not discussed in this chapter.
     The retention in Ag-HPLC is governed by the DB number. More DBs mean stronger interactions and therefore higher retention times. In practice, the situation is not as simple because other factors have to be taken into account, such as the DB geometry (cis vs. trans), distances among individual DB (from the conjugation to remote DB), overall molecular structures, the type of stationary phase, the composition of mobile phase, the gradient steepness, separation temperature, etc. The majority of Ag-HPLC applications are in the area of lipids, but this technique has a great potential also for other compounds containing DBs (Guha and Janak, 1972; Williams and Mander, 2001; Mander and Williams, 2016).
     This chapter focuses mainly on lipids, such as triacylglycerols (TGs), fatty acids (FAs), and fatty acid methyl esters (FAMEs), but other nonpolar lipids (e.g., sterols, wax esters, cholesterol esters, vitamins, carotenoids) can be analyzed as well (Vrkoslav et al., 2013; Joh et al., 1995; Kakela et al., 2002; Shan and Wilson, 2002).