Crude palm kernel oil was obtained from Chumporn Palm Oil
Industry Public Company Limited (Chumporn Province, Thailand).
Crude coconut oil was obtained from Thai Taksin Company
(Prachuap Kirikun Province, Thailand). Both oils were
used as received.
Potassium nitrate (KNO3) and zinc oxide (ZnO with purity
higher than 99.0% and stannous chloride (SnCl2) 98% were
purchased from Fluka. Zirconium oxide (ZrO2) and anhydrous
sodium sulfate 99.0% were received from Riedel-de Haen. ¨
Sulfuric acid (H2SO4) 96%, methanol 99.8%, HPLC grade acetonitrile
and acetone were purchased from Labscan (Bangkok,
Thailand). Ammonium hydroxide (28–30% A.C.S. Reagent)
was obtained from J.T. Baker.