The practical lower limit of this test method is 1 ng of quinine sulfate per mL (one part per billion; 1 ppb). Quinine, C20H24N2O2, molar weight of 324.43 and is an alkaloid extracted from the bark of the cinchona tree, at the right. It has been used for many years as an antimalarial agent; although it does not cure malaria, it is effective in alleviating the symptoms of malarial attacks. The usual medicinal form is quinine dihydrochloride or quinine sulfate. Quinine is a strongly fluorescing compound, especially in dilute acid solution, and thus can be detected in very trace amounts.
Quinine in 0.05 M H2SO4 is said to have two analytically useful excitation wavelengths, 250 and 350 nm. Regardless of which excitation wavelength is used, the wavelength of maximum fluorescence intensity is 450 nm.
Preparation of Stock and Standard Solutions
1. Prepare a 1000 ppm stock solution of quinine by carefully weighing exactly 0.1207 g of quinine sulfate onto a glassine weighing paper, transferring into a 100-mL volumetric flask, and pipetting in 5.00 mL of 1 M H2SO4. (NOTE 1: Carefully dissolve all the quinine; a few squirts from a wash bottle may help to wash the solid material from the neck. Dissolve completely before diluting.) Dilute to volume with distilled water and mix thoroughly.
