Development of an automated photometric assay for serum lithium and use of binding equilibrium expressions to optimize results

Background: A spectrophotometric serum lithium assay and relevant reaction equilibria used in the assay development are described. Methods: The dye, 7-nitro-2,12-dicarboxyl-16,17-dihydro-5H,15H-dibenzo[b,i] [1,11,4,5,7,8]dioxatetraazacyclotetradecine reacts rapidly with Li+ in alkaline water/dimethylsulfoxide with an absorbance change. Sodium binds weakly, thus presenting potential inaccuracy from varying endogenous sodium. To minimize this problem, excess sodium ion was added from reagents. This diminishes the signal from lithium binding, thus lowering sensitivity, requiring selection of reagent sodium concentration for proper balance of performance characteristics. Absorptivities and binding constants were determined. Description of competing equilibria by an explicit cubic equation was used to predict sensitivity and effects of varying endogenous Na+ concentration as functions of added NaCl. Results: Concentration-absorbance data were satisfactorily predicted by the cubic equation, equilibrium constants, and absorptivities. Li+ binds 300 times more tightly than Na+. Interference was minimized, while sufficient sensitivity for needed precision was retained. A full evaluation is summarized separately in this issue. Conclusions: Explicit description of the competitive binding was successfully used to develop an automated liquid reagent assay for Li+ with acceptable accuracy and precision. [Copyright &y& Elsevier]