Quantitative determination of conjugated linoleic acid isomers in beef fat

The amounts of 14 conjugated linoleic acid (CLA) isomers (t12t14, t11t13, t10t12, t9t11, t8t10, t7t9, t6t8; 12,14 c/t, t11c13, c11t13, t10c12, 9,11 c/t, t8c10, t7c9-18:2) in 20 beef samples were determined by triple-column silver-ion high-performance liquid chromatography (Ag+-HPLC). Quantitation was performed using an external CLA reference standard consisting of cis9,trans11-18:2,trans9,trans11-18:2 and cis9,cis11-18: 2. Linearity was checked as being r > 0.9999 between 0.02 × 10-3 to 2 mg/ml. The determination limit (5-fold signal/noise ratio) of the CLA reference was estimated to be 0.25, 0.50, 1.0 ng/injection for the cis/trans, trans,trans and cis,cis isomers, respectively. As expected, cis9,trans11-18:2 was the predominant isomer (1.95 ± 0.54 mg/g fat) in beef, followed by trans7,cis9-18:2 (0.19 ± 0.04 mg/g fat); cis,cis isomers were below the determination limit in most beef samples. Total CLA amounts determined by Ag+-HPLC were compared to total CLAs determined by gas chromatography (GC, 100 m CPSilTM 88 column). The amounts obtained by GC were generally higher than those determined by Ag+ -HPLC due to co-eluting compounds.