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

Potentiometric response characteristics were evaluated for quinine selective sensors based on a lipophilic ion-exchanger potassium tetrakis[3,5-bis(trifluoromethylphenyl)]borate (PTFB) immobilized together with plasticizing solvents in polyvinyl chloride membranes. The use of dioctyl phthalate (DOP), 2-nitrophenyl phenyl ether (NPPE), and bis(2-ethylhexyl)adipate (BEHA) plasticizers produced good quality quinine sensors that were sensitive and fast responding, and exhibited near Nernstian responses when used as batch-sensors. These membranes were further tested in a wall-jet flow-through potentiometric flow injection analysis (FIA) detector. Quinine sensors containing BEHA were the most suitable membrane, with no noticeable differences in sensitivity 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 coefficients <-2.60) or as FIA detector. When the FIA method was used, recoveries 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 carbonated (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 samples.