The reference method was performed by high performance liquid chromatography (HPLC). To analyze carotenoid type compounds, a Beckman liquid chromatograph consisting of a model Gold System 116 M pump, a model 165 variable wavelength UV-visible detector, and a model 340 organizer equipped with a 20 μl loop injector was used. The separation was performed on Chromsil-C18 (240x4.6 mm i.d.) 6 μm column using 2:1:1 acetonitrile-2-propanolmethanol as the mobile phase at a flow rate of 0.9-1.2 ml min-1 (Biacs and Daood, 1994). The column effluents were detected at 480 nm, and the signals were recorded by a Shimadzu C-R3A integrator. The samples were analysed in duplicate and standard error of laboratory (SEL) was calculated from these duplicates.