Polyvinyl chloride-based membranes for flow injection analysis of quinine in beverages

Potentiometric response characteristics were evaluated for quinine selectiv e sensors based on a lipophilic ion-exchanger potassium tetrakis[3,5-bis(tr ifluoromethylphenyl)]borate (PTFB) immobilized together with plasticizing s olvents in polyvinyl chloride membranes. The use of dioctyl phthalate (DOP) , 2-nitrophenyl phenyl ether (NPPE), and bis(2-ethylhexyl)adipate (BEHA) pl asticizers produced good quality quinine sensors that were sensitive and fa st responding, and exhibited near Nernstian responses when used as batch-se nsors. These membranes were further tested in a wall-jet flow-through poten tiometric flow injection analysis (FIA) detector. Quinine sensors containin g BEHA were the most suitable membrane, with no noticeable differences in s ensitivity even after 5 h of continuous exposure to solutions. Interference by foreign species such as alkali, alkaline earth metal ions, sugars, and sodium benzoate was minimal in either the batch-mode (log selectivity coeff icients < -2.60) or as FIA detector. When the FIA method was used, recoveri es averaged 99.70% from mineral water samples spiked with 16.22 and 162.20 ppm quinine. Results for determination of quinine in other drinks such as c arbonated (64.88 and 71.37 ppm) and bitter lemon drinks (55.15 ppm) agreed with those of the manufacturer's claimed values of < 68, < 81, and 51 ppm, respectively. With a sample throughput of 50 samples/h under optimized flow conditions, the method can be routinely used to monitor quinine in such sa mples.