Quantification of total lipids based on the characteristic absorption of tetra-β-(2-octanyloxy)-substituted nickel phthalocyanine

Abstract

Total lipid content, as an important biochemical parameter for evaluating lipid-containing samples, needs to be determined more accurately and conveniently. Here, a UV-vis method was developed for the quantification of total lipids based on the characteristic absorption, namely the Q band at 675 nm, of tetra-β-(2-octanyloxy)-substituted nickel phthalocyanine chromophore molecules, which sensitively responded to the lipid content in solution with petroleum ether as a solvent. Particularly, when the lipid sample was no more than 0.5 mL in 3 mL of the detection solution, the absorption increased linearly with the lipids as the free chromophore molecules gradually increased from the aggregates to the solvent. When the concentration of this chromophore was set at 2.0 × 10⁻⁵ mol L⁻¹, a linear relationship between the changed characteristic absorbance (ΔA_{675 nm}) and the lipid concentration (c_o < 0.1533 g mL⁻¹), expressed as ΔA_{675 nm} = 4.92511c_o + 0.01478, could be obtained. To test the accuracy of this equation in the quantification of total lipids, it was used to analyze the lipid samples prepared via artificially mixing 10 g of edible oils with 20 mL of petroleum ether, and thereafter separating them from the solvent. The results showed that the UV-vis method produced an average error −0.20 g smaller than the 0.87 g given by the traditional gravimetric quantification. The analysis of the total lipid content of nut seeds and cereal grains also indicated that the UV-vis method could correct the gravimetric quantification and make the results more accurate, meaning the interference from residual solvent in the samples is avoided.