Quantification of total lipid based on characteristic absorption of tetra-β-(2-octanyloxy) substituted nickel phthalocyanine
The total lipid content as an important biochemical parameter for evaluating the lipid-containing samples, needs to be determined more accurately and conveniently. Here, a UV-vis method is developed for quantification of total lipid based on the characteristic absorption, namely Q band at 675 nm, of tetra-β-(2-octanyloxy) substituted nickel phthalocyanine chromophore molecules, which sensitively responds to lipid content in solution with petroleum ether as solvent. Especially, when lipid sample is no more than 0.5 mL in 3 mL of detection solution, the absorption increases linearly with lipid for that the free chromophore molecules gradually increase from aggregates to solvent. If the concentration of this chromophore is set at 2.0 × 10-5 mol L-1, a linear relation between the changed characteristic absorbance (∆A675 nm) and the lipid concentration (co, < 0.1533 g mL-1), expressed as ∆A675 nm = 4.92511co + 0.01478, can be obtained. To test the accuracy of this equation in quantification of total lipid, it was used to analyze the lipid samples prepared via artificially mixing 10 g edible oils with 20 mL petroleum ether and thereafter separating them from solvent. The results showed that the UV-vis method produced an average error -0.20 g smaller than 0.87 g given by the traditional gravimetric quantification. The analysis on total lipid content of nut seeds and cereal grains also indicates that the UV-vis method can correct the gravimetric quantification and made the results more accurate, for that the interference from solvent residual in samples is avoided.