Extreme Chromatography: Chapter Six

$Text Box: SILVER-ION HIGH-PERFORMANCE LIQUID CHROMATOGRAPHY - MASS SPECTROMETRY IN THE ANALYSIS OF LIPIDS MICHAL HOLčAPEK AND MIROSLAV LiSA UNIVERSITY OF PARDUBICE, FACULTY OF CHEMICAL TECHNOLOGY, DEPARTMENT OF ANALYTICAL CHEMISTRY, PARDUBICE, CZECH REPUBLIC Introduction The ability of silver-ion and some other metal-ions to interact with double bonds (DBs) has been a well known property for a long time (Dobson et al., 1995; Guha & 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-ion 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 column [used mainly 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 lipid analysis because it is cheap, simple to use and applicable both in analytical and preparative scale. Disadvantages of Ag-TLC are mainly the contamination of fractions in the preparative isolation by silver and the limited resolution of TLC technique. Ag-TLC has been reviewed in several works (Dobson et al., 1995; Momchilova & Nikolova-Damyanova, 2003), therefore it is not discussed in this chapter. The retention in the 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 DBs (from the conjugation to remote DBs), overall molecular structures, the type of stationary phase, the composition of mobile phase, the gradient steepness, separation temperature, etc. The vast majority of Ag-HPLC applications is in the area of lipids, but this technique can be used for other compounds containing DBs as well (Guha & Janak, 1972; Williams & Mander, 2001). This chapter focuses solely on lipids, mainly triacylglycerols (TGs), fatty acids (FAs) and their simple derivatives such as fatty acid methyl esters (FAMEs).$